Remote Sensing of In-Flight Icing Conditions: Operational, Meteorological, and Technological Considerations

Appendix A: Synopsis ofoperational information needs, state ofknowledge, strengths, Appendix C:Synopsis ofsensor technology needs, state ofknowledge, strengths, ILLUSTRATIONS Figure TABLE Table 1. Relationships between radar band, frequency, wavelength, and weather parameter NASA/CR--2000-209938 ii GLOSSARY AGATE: Advanced General Aviation Technology Experiment.

ceilometer: An instrument that remotely measures the height of cloud base above ground level.

disdrometer: Instrument for measuring the sizes of raindrops.

exceedance conditions: Conditions outside of the envelope of conditions defined by FAR 25, Appendix C.

FAR 25, Appendix C: FAA Federal Aviation Regulation 25, Appendix C, defining the range of liquid water, drop size, temperature, and exposure conditions for which aircraft are certified for flight in icing.

Free Hight: FAA concept of autonomous aircraft flight, with aircraft-ground sepa- ration provided by onboard aircraft sensors rather than by air traffic control (ATC).

pireps: In-flight pilot reports of flying conditions, including icing.

Detection of icing condition remotely; remote-sensing system predic- prediction detection: tions that icing conditions are in the flight path.

The distance between consecutive radar measurements along a radial, range gate: typically about 100 to 500 m, and controlled by pulse duration• reactive detection: Detection of icing conditions in situ; reaction of the in-situ instruments indicates that icing is occurring.

supercooled large drops (SLDs): Drops larger than typical cloud drops defined in FAR 25, Appendix C; typically drizzle drops ranging in diameter from 50 to 500 _m.

NASA/CR--2000-209938 iii Remote Sensing of In-Flight Icing Conditions:

Operational, Meteorological, and Technological Considerations

Charles C. Ryerson U.S. Army Corps of Engineers Hanover, New Hampshire 03755 1.0 EXECUTIVE SUMMARY In-flight icing is a significant aviation risk despite sities. The absolute range of cloud liquid-water content improving icing forecasts and onboard ice protection in clouds of various genera is generally well known, but its vertical and horizontal distribution within clouds systems. A remote-sensing system designed to detect and cloud masses is not well understood. The cloud gla- icing conditions in the flight path could allow aircraft to avoid and exit hazardous conditions. Ground-based ciation process is also imperfectly understood, as is the near airports or airborne, such systems would be most effect of mixed-phase conditions on aircraft icing and useful to low, slow-flying aircraft that frequently on cloud remote sensing. The shape of the drop-size encounter icing, such as turboprops and helicopters. spectrum is not well characterized in icing conditions, Development of an icing remote-sensing system especially in supercooled large drops, and the median requires consideration of the operational environment volume diameter does not often provide an adequate within which it is used, the meteorological environment description of the drop-size spectrum, especially if the it senses, and the technology available for sensing icing distribution is bimodal. Though it is generally known conditions.

that static temperature changes more rapidly vertically Operationally, pilots need information in the cock- than horizontally, there has been very little characteri- pit for making risk-management decisions. Displays must zation of temperature distribution in icing conditions.

evoke proper pilot decisions and provide clear, unam- The ability to detect temperature change ahead of an biguous warnings of severe conditions for avoidance. aircraft is critical, because the temperature within liquid Human factors and cockpit and aircraft integration water determines whether icing will occur. Character- issues must be developed in addition to avoid-and-exit ization of icing conditions is expensive, typically requir- protocol and training. Dispatchers and meteorologists ing research aircraft. More reliable and less costly need integration of icing remote-sensing systems into instrumentation is needed to replace first-generation opti- the weather system infrastructure. Cost, maintenance, cal probes, and to reduce cost, coordination is needed power, weight, and space are a.concern of manu- with other federal programs that make cloud microphys- facturers, operators, and regulators, as is the evalua- ical measurements. Such cooperation has started tion of aircraft flight envelopes in icing conditions. between the Canadian Atmospheric Environment Ser- An icing remote-sensing system detects conditions vice, Transport Canada, NASA, and the FAA.

The core of an icing avoidance system is the tech- conducive to icing, including cloud and precipitation liquid-water content, the drop-size spectrum, and tem- nology used to sense icing microphysical conditions.

Radar and microwave radiometers are the most viable perature. An icing metric algorithm would convert these measurements into an estimate of icing potential for technologies. Ranging capability makes radar an attrac- cockpit display. To develop specifications, the absolute tive technology for detecting liquid water, drop size, magnitudes of cloud microphysical conditions and the and possibly temperature. Multiple-band radars, such spatial and temporal variability of icing weather condi- as X and K a bands to retrieve liquid-water content using tions, must be understood at multiple scales. Icing cloud differential attenuation techniques and X, Ka, and W microphysics have been measured since the 1940s by bands to retrieve liquid water and drop size using neu- NACA, NASA, the FAA, NCAR, and several univer- ral nets, currently appear most viable. Information- NASA/CR--2000-209938 l retrieval algorithms, noise, and Mie scattering present requirements of the system, the environment to be major radar technological challenges. Although radar can sensed, and the technology available to accomplish the be used in a variety of orientations, scanning vertically task. This report identifies the state of knowledge, from the ground or horizontally from the air, its size, strengths, weaknesses, major issues, barriers, and oppor- tunities, and it identifies the research and investment weight, and power demands make ground-based radar a more viable near-term technology. needed to create a prototype icing remote-sensing Microwave radiometer development is less mature system.

than radar technology. However, the recent introduction 2.2 Scope of a radiometer that scans and profiles temperature, water This report provides a framework for the develop- vapor, and cloud liquid water, and experimentation with ment of a plan for creating in-flight icing-avoidance techniques to use radiometers in a horizontal sensing mode, in addition to the more traditional vertical or near- remote-sensing systems for use in the national airspace.

There are three strategic elements: vertical modes, are promising. Microwave radiometers are passive, an advantage to the military, but they lack • Identifying the operational needs of pilots, opera- absolute ranging capability--a disadvantage. Identifica- tors, manufacturers, and regulators as to functional tion of cloud glaciation and drop size with polarization requirements, system utilization, aircraft integra- is an additional useful radar and microwave radiometer tion, and human factors capability.

• Identifying sensing requirements Lidar is not considered a viable technology for remote • Identifying technologies, and their state of devel- sensing of icing conditions because it cannot sense deeply opment, for an integrated sensing system.

into optically thick clouds. Remote detection of temper- ature is possible with microwave radiometers and RASS 2.3 Goal from the ground, but perhaps only by using microwave This report is intended to provide background infor- radiometers from aircraft. Considerable development is mation to facilitate the creation of a development plan needed in this area.

to improve aircraft operational capabilities and safety The most critical needs in operational research are to in icing environments. This will be accomplished by assess cockpit and aircraft system integration, develop developing remote-sensing systems that provide pilots avoid-and-exit protocol, and assess the human factors with information about the location and intensity of in- in using remotely sensed icing information. In addition, flight icing hazards, giving them the ability to avoid remotely sensed icing information must be integrated and exit icing expediently. This will into weather and air traffic control infrastructures, and air- • Increase safety craft flight envelopes and the hazard of icing to aircraft • Reduce delays performance in these envelopes need better definition.

• Increase aircraft utilization Improved absolute and spatial characterization of cloud • Increase military readiness.

and precipitation fiquid-water content, drop-size spectra, and temperature are needed to develop remote-sensing One method of either avoiding or escaping icing is system specifications. An icing metric must also be devel- to sense remotely, either from the ground or from air- oped that will allow the sensed microphysical conditions craft, atmospheric icing potential (Ryerson 1996, 1997, to be converted into a measure of icing potential for air- 1998). This requires scanning the airspace ahead of an craft. Technology development requires refinement of aircraft for supercooled water and presenting that infor- inversion techniques for unambiguously retrieving mation to the pilot in a manner consistent with effi- liquid-water content, drop size, and temperature from cient cockpit risk assessment.

clouds and precipitation. These goals can be accom- Currently, no dedicated system exists for remotely plished with strong leadership and collaboration among sensing the icing potential in a projected flight path for federal agencies, including NASA, the FAA, the National an individual aircraft. A remote-sensing system that Center for Atmospheric Research, NOAA, and DoD. advises pilots of the icing risk ahead of an aircraft will Partnership between government and industry will bring be an information management system that senses the viable technologies to prototype and to market. environment, processes the Sensed information, and presents it in a useful form. This requires that the proper environmental parameters be sensed with an accuracy 2.0 INTRODUCTION suitable for providing useful information, that the infor- 2.1 Purpose mation be processed with sufficient speed to assist pi- Development of a remote-sensing icing-avoidance lots, and that information be presented in a manner that avionics system requires assessment of the operational aids pilots in making icing risk-management decisions.

NASA/CR--2000-209938 2 2.4 Relevance pilots in instrument meteorological conditions (IMC)-- Current methods of avoiding icing, including meteor- a pilot training problem--many of the accidents may ological and pilot reports, are extremely ineffective be preventable with onboard icing-avoidance systems (Bertorelli 1992). Even a VFR pilot may be able to avoid (Erickson 1997). Icing information is not provided with the detail, accuracy, and timeliness needed for commer- the most serious of icing conditions--freezing drizzle cial and private aircraft to avoid icing conditions effi- or freezing rain--with onboard remote-sensing icing avoidance capability. An instrument flight rule (IFR) ciently. As a result, either aircraft cannot fly or large pilot could avoid icing within IMC, or at least avoid areas of potentially flyable airspace must sometimes be conditions that tax aircraft ice-removal systems.

avoided because of inadequate spatial and temporal reso- Helicopters flying low-altitude missions to service lution of forecasts. Military aviators and civil aviators offshore oil rigs and in search-and-rescue operations, in the Far North where bush flying is common also need for example, are particularly vulnerable to icing. In addi- to be able to avoid icing autonomously because fore- tion, limited altitude capability, low speeds, rotating casts are often unavailable in operational areas (Owen components, and generally low power reserves make 1997). In addition, increased use of laminar flow air- them more susceptible to ice than fixed-wing aircraft foils and more efficient engine designs less tolerant of (Manningham 1991). However, helicopters' low speed contaminants make some aircraft more susceptible to icing.

and maneuverability may be an asset if they are As air traffic increases in volume, new aircraft designs are implemented, and new routes are established, more air- equipped with an icing-avoidance system because they have more flexible course- and altitude-changing capa- craft that are less tolerant to contamination may be exposed to icing. To increase aviation safety and bility than fixed-wing aircraft. As recently as 1995, only efficiency, and to increase military readiness and air su- one commercial helicopter was icing-certified, the Aero- periority, improved methods of avoiding and exiting spatiale Super Puma (AHS 1995).

icing are needed. In-flight icing is not generally considered a problem Aircraft flying at 400 knots or greater, which includes in Army aviation, despite problems in Bosnia, because most missions are flown in warm climates, missions most jets, generally do not have icing problems because they typically have heated leading edges and fly above are not flown if ice is predicted, and icing is so infre- most icing (Taylor 1991). However, jets on approach quent that readiness is little affected. However, about 9% and departure, 300-kt turboprops, piston aircraft, and of Army medevac flights in Alaska are canceled due to helicopters are all susceptible. Turboprops fly exclu- icing, and medevac commanders give icing avoidance sively at lower altitudes and are thus exposed to ice for a high priority. Mayer et al. (1984) found about 525 extended periods. Few light piston-engine aircraft have icing-related mishaps in the Navy between 1964 and 1984, with about 70% due to in-flight problems and deicing capability. Helicopters are probably the most threatened of all aircraft because of their unique aero- nearly all due to foreign-object damage from ice. Acci- dent reports in recent years suggest that the Navy has dynamics and mission requirements and because they had fewer icing problems, with more reports of hail- typically lack deicing capability.

Pilots, operators, and manufacturers typically do not impact damage than airframe ice accretion problems, know when most aircraft reach their performance limits but Lef et al. (1994) state that the Navy is concerned in icing (Erickson 1997). There are generally few clues about the icing threat to cartier-launched aircraft and that provided to the pilot that indicate how close an aircraft helicopter icing accidents are not infrequent. Air Force is to those limits. This is of particular concern for air- transport aircraft have also experienced icing problems, craft operation outside of FAA FAR 25, Appendix C, for example, in tropical cumulus clouds at high alti- design guidelines. The result is that many pilots may tudes. The Coast Guard reports problems with icing in search-and-rescue and enforcement missions (Yatto unknowingly operate their aircraft at or near safety limits 1997).

when in many icing situations, despite the availability Military and civilian unmanned aerial vehicles of onboard protection systems.

Fortunately, transport-category aircraft are rarely lost (UAVs) are special cases in need of icing avoidance sys- to icing, although there are reported incidents (Engel- tems. UAVs, especially high-altitude, long-endurance UAVs, may be required to seek routes through icing berg and Bryant 1995), but private general aviation does conditions autonomously (Siquig 1993). Onboard not fare as well. Aviation magazines carry many reports weather-sensing systems could be coupled with autono- of private general-aviation icing incidents and accidents.

mous controls to allow UAVs to avoid or minimize Between 40 and 60 private general-aviation accidents icing, which impacts them more severely than it does annually are attributed to in-flight structural icing, about conventional aircraft because of their low power and 50% of which are fatal (AVEMCO 1983, Taylor 1991).

Although about 50% of these are visual flight rule (VFR) high-efficiency airfoils.

NASA/CR--2000-209938 3 Inthe future, several changes inflightactivity could early in the research and development process on scen-

affect vulnerability toicing: ario and training aids, which may help establish the

direction of technology development.

• Implementation ofFree Flight may reduce airtraf-

Although it may not be absolutely necessary, devel-

ficcontrol surveillance ofaircraft routes, making

opment of an icing avoidance avionics system also pilots more responsible forweather avoidance.

requires an understanding of pilot decision-making pro-

• Increased commuter aircraft activity will create

cesses. The form in which information is delivered to

more flights bysmaller aircraft atlower altitudes

pilots may subconsciously affect their decision-making

andslower speeds, increasing vulnerability to

process (Hansman 1997). Understanding how pilots icing.

make decisions for avoiding or coping with in-flight haz-

• Increased military emphasis onhelicopters and

ards will affect the development of a decision-support

UAVs,bothuniquely vulnerable to icing,is

system. Appropriate paradigms for icing avoidance may expected.

be current fielded thunderstorm and wind-shear avoid-

• Increased airtrafficis also expected globally, so

ance systems. Pilots generally view any information

toreduce thenumber oficingaccidents theacci-

beyond that currently available in the cockpit as useful dent rate must decrease (Brown and Dorr1997).

(Erickson 1997), but the kind of information desired 2.5 Philosophy must be identified: too much or inappropriate informa- Icing remote-sensor development is driven essen- tion could confuse pilots.

tially by one question: What icing information does the The pilot's needs determine the information provided pilot need to make better risk-management decisions? by a remote-sensing system, but the atmospheric envi- A remote-sensing system that reports icing potential ronment and the physics of icing and aircraft flight deter- mine what information must be sensed to create the ahead of an aircraft to a pilot is a decision-support sys- information the pilot needs. Pilots are concerned about tem that provides information needed to make decisions flight safety and are thus concerned about the perfor- regarding flight safety. Pilots' needs drive the develop- ment process because they are the ultimate users of mance of the aircraft should it ice. Aircraft performance information generated by the system. The development changes in response to ice accretion on the airframe.

process, however, must work within the restrictions and Weather is the phenomenon that causes changes in air- craft performance by providing conditions conducive to opportunities provided by the aircraft, regulators, sens- ing technology, and meteorology. ice formation on an airframe. Thus, the ice accretion pro- Because pilot information needs are the primary dri- cess may be viewed as an input-process-response sys- vers of the process, an early development requirement tem (Fig. 1). Weather is processed by the aircraft to pro- should be to determine what pilots need to know. Do duce ice on the airframe, which, in turn, influences air- pilot information needs change with platform, mission, craft performance.

Aircraft Process Response Input Figure 1. Aircraft icing paradigm.

airspace class, mode of flight (i.e., approach, departure, Sensing requirements are independent of specific air- or cruise), or some other factor? Is a ground-based sys- craft, because different aircraft process identical weather tem sufficient, or would an aircraft-mounted sensing conditions in different ways: The same weather condi- system add significant value? What spatial and temporal tions may produce different icing conditions on a light resolutions are needed, and how do pilots prefer to view piston-engine aircraft than on a jet transport or a helicop- the data--as plan, profile, or perspective views, and as ter. And identical weather conditions may produce dif- individual temperature and liquid-water content maps, ferent icing conditions on an aircraft in different flight or as composite maps of icing potential? How should configurations depending upon power application, angle icing potential be expressed, and how should hazard of attack, skin temperature, and other factors. Although areas be identified? Though all of this information is the meteorological conditions may be identical, the pro- not needed to begin research in all areas, it does set the cessed information provided by the remote-sensing sys- stage and reduces the possibility of misdirecting tem should be aircraft-specific to allow the pilot to antici- research and development. It also allows work to begin pate potential aircraft performance changes due to icing.

NASA/CR--2000-209938 4 icing is most frequently encountered. Because wind- 2.6 Organization of this report shear alert systems have evolved since the development Broadly, research and development for remotely of cockpit resource-management concepts, they may detecting icing conditions can be placed in three cate- serve as useful analogs for designing an effective pilot gories: operations, meteorology, and technology. Op- erations includes the human/machine interface, interface for icing avoid-and-exit advisory systems.

Research is needed in this area. In addition, unlike wind regulatory issues, avoid/escape strategies, aircraft inte- shear, which is a directly sensed threat to aircraft, icing gration, training, and terminology. Meteorology involves atmospheric environment and characteristics that must does not occur until aircraft enter icing conditions, pro- be sensed. Technology refers to the remote-sensing cess cloud microphysical conditions, and create ice on systems that may be able to sense icing potential. Sub- the airframe (Fig. 1). Thus, icing potential is a virtual areas of research and development needed are identi- phenomenon, and the most appropriate methods for quantifying, analyzing, and displaying the virtual threat fied within each primary category. This report gives an overview of the state of the art, describes barriers and must be determined. The types of display, terminology, methods of indicating potential icing intensity, sensor opportunities to development, and recommend devel- range, resolution, accuracy, and refresh rate and warning opment directions.

time needed are a function of airspace class, aircraft type and configuration, and mode of flight. This mix of 3.0 OPERATIONAL REQUIREMENTS conditions needs to be considered in developing opti- mal pilot information systems, training protocol, and 3.1 Summary sensing systems.

Pilots are risk managers. When it is a question of flight safety, they want clear, unambiguous informa- Cost, weight, space, power, and maintenance are some of the concerns of aircraft manufacturers and tion about the location and intensity of weather threats before they enter them. A top-level requirement with operators. The aircraft most needing protection--com- muters, helicopters, and light aircraft---offer the fewest regard to icing is to provide pilots with a decision-support system specific to the remote sensing of icing potential of these resources, so for airborne sensing systems, the ahead of aircraft. Standoff guidance about icing poten- need is to provide the greatest benefit for the least tial could be provided from satellite and ground-based impact. The spatial and temporal threat of icing is gener- sensors uplinking information to the cockpit or from ally small when viewed annually, so it is probable that aircraft-mounted remote-sensing systems. However, icing remote-sensing systems will not be installed in satellites still do not have the capability of providing lieu of competing avionics or weapons systems. They high spatial- and temporal-resolution icing information. will probably be used only if mandated or required Ground-based sensing systems at airports would be because of market or extreme safety pressures.

most cost-effective per aircraft served, protect the most Remote sensors may provide pilots with the location critical phases of flight, and are systems for which and intensity of icing potential ahead of their aircraft.

However, pilots must also be able to determine how sensing technologies are most mature. Aircraft-based systems would be most costly per aircraft and the tech- this icing potential will affect safety, because a deci- sion to enter or avoid the sensed conditions is a func- nologies are least mature, but aircraft would be pro- tion of the aircraft's ability to operate in icing. Aircraft tected in all phases of flight, especially when arriving or departing from small, remote airports that do not have are certified for flight in icing conditions according to remote-sensing systems. atmospheric criteria specified in FAA FAR 25, Appen- Ground-based systems should be developed first, dix C, but pilots need to know how aircraft respond to conditions outside of Appendix C. They also need to because of their near-term technological maturity and know how much additional icing an iced aircraft can cost effectiveness as an operational test bed, and to pro- tolerate. That is, they need to know how much ice is tect congested airport approach and departure areas.

necessary to produce unsafe operating conditions. Air- Satellite-based sensors need continued development to craft may have to be tested outside of Appendix C con- supplement local sensing systems and to provide pro- ditions to determine their operational limits. In addi- tection during cruise flight where spatial and temporal tion, the development of smart aircraft-monitoring sys- detail may not be as critical. Aircraft-based systems need tems may also be needed to guide the pilot's decision the greatest development, but they offer the greatest to enter or avoid icing.

potential for providing the information pilots need.

Pilots need information, not data. Thus, remote- An important side benefit of onboard icing remote- sensing systems is the potential for downlinking weather sensing systems must provide clear, simple displays that information to other aircraft, air traffic controllers, and reduce and do not add to flight-management demands meteorologists. Downlinked weather information, from in critical approach and departure flight regimes where NASA/CR--2000-209938 5 3.3 Pilot needs and human factors both remote-sensing and in-situ sensors aboard aircraft, willimprove temporal and spatial accuracy beyond what Pilots have three concerns about the icing environ- isnow possible withpilotreports. Accurate downlinked ment (Vigeant-Langlois and Hansman 1999):

cloud liquid-water content, drop sizes, and temperature

• How to recognize that they are approaching or are

will improve icing forecasts and provide more accurate

in icing conditions hazardous to their aircraft

icing warnings forother aircraft. Up-and downlinking

• How to avoid icing

ofweather information toand from the cockpit are areas

• How to escape icing.

ofresearch being addressed bythe Advanced General

In-fright icing is a frequent topic in pilot safety briefs Aviation Technology Experiment (AGATE) program and popular literature (Buck 1988, Collins 1989, and by NASA Langley Research Center in the Aviation Schuyler 1989, Taylor 1991, Bertorelli 1992, Home Weather Information (AvWIN) program.

1994, and others). Over the years, roles of thumb, advice, Efficient, cost-effective methods for testing remote- and regulatory requirements have created ad hoc proto- sensing technology, testing display and information- cols for icing avoidance and escape. As a result, most management techniques, and developing avoid-and-exit pilots manage to avoid forecasted icing by not flying, protocol and training are necessary for development and or by making route deviations to avoid the conditions, certification. Sensors may be tested from ground-based test beds and airborne platforms. Ground-based test beds or they encounter ice and escape safely through good fortune. Tales of icing mishaps and escape are com- include wind tunnels, spray figs, and mountain-top test mon fare in winter aviation literature (Creley 1990, sites. The advantages of ground-based test beds include cost, accessibility, and control of test conditions. Moun- McClean 1992). Many pilots do not survive icing tain-top test sites allow less control over conditions, encounters because they did not recognize that they were in ice until too late, and they did not have the but they do provide the variability of natural icing. Wind ability or capability to escape once immersed.

tunnels and spray rigs cannot provide the spatial con- One of the larger causes of this avoidance and escape ditions necessary for testing remote sensors, but they can be used to test in-situ sensors and the environmen- problem is the quality of icing forecasts. Most pilots who want to avoid icing cannot, because icing fore- tal effects of icing conditions on sensors and airfoils.

casts do not have sufficient accuracy as to spatial, tem- Airborne platforms provide the best environment for poral, and intensity criteria (Erickson et al. 1996, Green testing aircraft-based remote sensors. Overall, a combin- et al. 1996, Stack 1996, Clark 1997). Forecasts are often ation of ground and airborne test beds will be necessary made conservatively, on the side of safety, to minimize to test both ground-based and airborne remote-sensing accidents and compensate for inadequacies in forecast systems.

procedure. However, this causes aircraft to not fly or to :].2 Introduction divert when it may not be necessary because large areas forecasted for icing may not have ice or even clouds.

Operations establish the functional requirements of As a result, the aviation system is less efficient, though a remote-sensing system designed to detect icing. The safer.

goal of system development is to improve the safety and efficiency of aircraft operations. However, opera- 3.3.1 Pilot needs tions is a complex, multifaceted problem. A compre- Pilots need information for making risk-management hensive review of general operational needs in icing decisions about in-fright icing. They want to be able to environments is presented by Brayton and Hakala (1996). determine whether they are in or about to enter icing, because icing clues are often not visible from the cock- One element, and ultimately the most important ele- pit windows (Erickson 1997). Pilots also need to know ment, of operations has to do with pilots and their needs the location of icing and how intense the icing might (Vigeant-Langlois and Hansman 1999). Although pilots be without dipping their wings into it, as is now neces- are on the leading edge of the icing problem because sary (Green et al. 1996). Onboard, in-situ ice detectors they are actually within the icing environment, they deal (reactive detection systems) are a solution to the prob- with more than icing. Systems from which pilots seek lem of determining exposure to icing for many fixed- icing guidance must he designed in a manner that best wing aircraft and a few helicopters (Bracken et al. 1996), suits the operational requirements of the cockpit envi- but even these systems require that the aircraft enter ronment and that effectively and efficiently helps pilots icing conditions before determining that they are in make management decisions. This includes the design hazardous conditions.

of the display and information delivery system and train- Pilots need clear, unambiguous guidelines as to when ing in its use.

severe conditions are entered or warning that severe NASA/CR--2000-209938 6 3.3.2 Human factors

conditions lieahead withsufficient lead timetoavoid

Development of icing-avoidance avionics is a

them (Erickson 1997). According toColeman (1996)

human-centered development process because the intent

ofthe Regional Airline Association, the answer tomain-

is to display information to pilots. Thus, information

taining safety inicing conditions istolocate severe icing

needs must be assessed, and perceptual issues vs.

accurately andthen avoid it. Because icingforecasts

display design must be considered (Hansman 1997, cannot provide the needed accuracy atthe present time, Vigeant-Langlois and Hansman 1999). A useful

remote detection orstandoff (prediction detection) sys-

approach to designing a human/machine interface for tems may (Green etal.1996).

icing avoidance is to review issues addressed in other

Standoff guidance oficingconditions ahead ofair-

weather-avoidance areas. Recent developments of

craft could beprovided in atleast three ways, allusing

onboard wind-shear alert systems and other weather-

a formof remote-sensing system. One method under

avoidance systems, especially since cockpit resource

development utilizes satellite remote-sensing to map

management has been recognized as a consideration in

icing potential anduplink information toaircraft (Lee

single- and multiple-pilot cockpits, may serve as reason- and Clark1995, Vivekanandan etal. 1996, Lee1997, able analogs. One important finding of cockpit resource-

Thompson etal.1997, Curry and Liu 1992). Satellite-

derived information can be used to delimit areas with management research has been that automation can cause human error as well as reduce it (Helmreich liquid water, subfreezing temperatures, and cloud cover.

1997). Because icing-avoidance systems are likely to Analyses could be accomplished in near real-time and be in greatest demand during the approach and depar- would provide a useful predictive detection capability.

ture phases of flight, balancing the distraction against Another method, also under development, utilizes the aid provided by an advisory system is critical to ground-based sensors at airports to map icing condi- its usefulness. A poorly designed interface may actu- tions in approach and departure areas (Gary 1983, Deck- ally produce a hazard to flight, so proper human/ er et al. 1986, Stankov et al. 1992). Remote sensors to machine interface design is nearly as crucial to final detect temperature profiles, cloud boundaries, liquid- success as is the ability to sense cloud microphysics water content, drop-size spectra, and cloud phase from accurately.

the ground are nearly available. Walter and Moynihan Icing is probably a more complex problem than wind (1997) even propose a mobile system for military use.

shear and convective turbulence because it is less com- An airport-based system would serve all aircraft, util- pact geographically, it is a hazard in all modes of flight, ize largely existing technologies or technologies that and there is significant variation in the ability of differ- are nearing maturity, and serve the phases of flight most ent aircraft to cope with the hazard. However, there may likely to experience icing. It would be a cost-effective be similarities. Wanke and Hansman (1991) evaluated approach, considering cost per aircraft served (Owen graphical displays of microburst alerts from both 1997).

ground-based and airborne detection systems. The A third system would be an in-flight, aircraft-mounted issues ranged from display clarity to pilot response to remote detection system (Sand and Kropfli 1991 ; Fourn- alerts. Questions addressed involved the visual clutter ier 1993; Siquig 1993; EWA 1996; Ryerson 1996, 1997, of adding alerts to existing navigational displays, val- 1998). An airborne system would require elements simi- ue of single- vs. multiple-level intensity display, value lar to a ground-based system, with the ability to detect of indicating the alert source (ground or airborne), and temperature, liquid-water content, and drop spectra.

effect of alert source on pilot procedural response.

However, the technologies may be quite different Active airline pilots were tested in a realistic transport- because of their use on a small moving platform, scan- aircraft flight simulator. The results showed that multi- ning primarily horizontally, and operating within level intensity displays are desirable, the source of infor- restricted power and weight limits.

mation was not important because confidence was Pilots need better information than is now available, placed in the alert whatever the source, and correlation and they require information that is easily understood with other information was not important. The study and provides options (Vigeant-Langlois and Hansman also suggested needed training areas, because pilots 1999). An ability to see through an icing weather sys- often responded to alerts with evasive action that was tem and map the extent of a threat area would be opti- inappropriate or not necessary.

mal, but any more information than is now avaiIable The amount of information displayed in the Wanke would be welcome (Clark 1997). A warning time of 1 and Hansman (1991) study did not appear to be an issue.

to 5 minutes, preferably integrated into an existing dis- That is not always the situation, however, and it may play system, of areas that are of risk to aircraft would be related to single- vs. multiple-pilot cockpit environ- be most useful, and even no warning time may be accept- ments. For example, Svensson et al. (1997) evaluated able (Erickson 1997).

NASA/CR--2000-209938 7 theeffects of information complexity on fighter pilot alert system is the warning time provided for pilot reac- performance in the Swedish air force. They found that tion. The warning time needed may be a function of even moderately complex information, measured by the airspace class, aircraft type, mission, and mode of flight.

amount of information provided, interfered with flight Though any warning time, no matter how short, may tasks. The critical flight measure was an ability to main- be helpful (Erickson 1997), there may be minimum rain altitude above undulating terrain. When informa- warning times that are more acceptable than others.

tion load increased to more than 8 to 10 items, pilots Anderson and Carbaugh (1993) address this problem could no longer integrate information and still fly the for wind-shear-alert systems and consider it a critical airplane. The authors indicate, through numerous exam- factor in how pilots judge the value of an alert system.

ples, that humans have severe limitations in what they Vigeant-Langlois and Hansman (1999) address warning can receive, process, and remember, and the 8 to 10 distances for icing, with commuter pilots reporting as little as 20 nautical miles as sufficient. All other classes information load items found in this study is consistent with the generally 7 information load items found in of pilots wanted longer warning distances, and thus many other studies. They conclude that modem technol- greater warning times.

ogy can provide large quantities of information that According to Hansman (1997), aircraft certified for often make the pilot feel more confident, but perfor- flight in icing have only to avoid severe icing, whereas mance is governed by human, not technological, limi- aircraft not certified for flight in icing require more deci- tations. Information overload can be a serious issue sion support for strategic and tactical planning, go/no-go because pilots will either fixate on one, perhaps trivial, decisions, and escape guidance. A simple pilot deci- problem or lose the ability to ably accomplish any tasks. sion structure has two options if icing is forecast, either Though flying in icing may not be as stressful as to not go--the risk-adverse path, or to go--the risk- terrain-following flight in a high-performance aircraft, tolerant path. There are three outcomes for the risk- icing often is most threatening in the busy, critical depar- tolerant path: ture and approach phases of flight.

• Encountering no ice Another issue for icing displays is the type of graphic • Encountering ice but having options for avoiding image provided to the pilot. Displays may be aircraft- its effects referenced or ground-referenced, and each may have • A catastrophic outcome.

plan, profile, or perspective Views. Though perspective views look realistic, plan and profile views are better In the decision-making process, the risk of making for decision making (Hansman 1997). Early MIT stud- a flight is weighed against the flight's value. For high- ies of terrain-avoidance displays indicated that the type risk flights, the incentive to make the flight must be of display affects behavior and thus the avoidance strat- high for it to occur, or options must be available to egy used by pilots, for example by avoiding terrain by reduce the risk. The decision to fly into potential icing climbing vs. turning. Aircraft performance characteris- involves having options for avoidance and escape, such tics, vertical and horizontal range, resolution, accuracy, as seeking dry or warm air or turning back or landing at alternates, to ensure a successful outcome. Other scan rate, and sensor limitations also affect the displayed information.

possible outcomes are to reach the destination without Pilots need information, not data, so the display must encountering ice or to have an ice-induced accident. A be a rendition of the icing environment that allows the remote-sensing ice-avoidance system may provide pilot to obtain the needed information unambiguously information for exercising options and reducing risk.

and in a form that promotes appropriate response. One Escape options are either vertical---finding warm or dry of the issues is 2-D vs. 3-D displays and the way pilots air, or lateral---finding dry air (Vigeant-Langlois and relate to each for different tasks. Cloutier (1997) pre- Hansman 1999). Research must be conducted on each of the risk paths described above into how operators sented two potential 2-D displays showing plan and profile views of icing potential for helicopter pilots. and pilots may use in-flight ice detection to make flight Boyer (1994) indicated that little research had been done decisions. Vigeant-Langlois and Hansman (1999) also evaluating the effectiveness of 3-D weather displays, indicate that pilots want escape guidance to be displayed and he addressed the benefits and costs of 3-D vs. 2-D in the cockpit.

displays and conducted an evaluation using student 3.4 Manufacturers and operators pilots for navigating around weather systems. He con- cluded that 2-D displays offer advantages for navigating Aircraft operators and manufacturers are concerned around weather, with few benefits attributable to the with the cost, weight, space, power, maintenance, and training requirements of placing additional avionics 3-D display.

A measure of effectiveness of a look-ahead weather packages on aircraft. They are also concerned about NASA/CR--2000-209938 8 advise the pilot whether conditions are threatening to

theimplications of a remote detection system being

safe flight. Systems could provide air traffic controllers

onboard, foritimplies that the aircraft cannot cope with

(ATCs) and meteorologists with icing intensity icing (Bond etal. 1997).

information and measured cloud microphysicai

Cost isalarge issue because aircraft that most need

parameters. They could upload weather and satellite

remote ice-detection systems canafford it least--the

information from the surface and integrate it with

regional airlines (Owen 1997). In addition, unless a

remote-sensor guidance (Bond et al. 1997). They could

system isextremely inexpensive, remote-detection sys-

also use information from onboard in-situ sensors to

tems will find little useonlight, privateaircraft

(Vigeant-Langlois and Hansman 1999). This isasignifi- corroborate remotely sensed information and integrate all sources of information into a comprehensive icing

cant problem inthe Far North where lightaircraft oper-

advisor 3 , system. Overall, aircraft icing remote-sensing

ate withnoiceprotection andwithfeworinadequate

weather advisories (Owen 1997). Even airlines operat- systems could aid pilots, meteorologists, air traffic controllers, dispatchers, and ultimately the public

inglarge transport-category aircraft are reluctant touse

through improved aviation safety.

avionics they perceive tobeoflimited value because,

beyond the initialcost, there isthe cost offlyingit (lost

3.5.2 Regulatory Issues payload) as wellasmaintenance and training costs.

Regulatory agencies are responsible for providing

Weight andspace areserious problems, especially

leadership and procedures for maintaining, improving,

forlightaircraft and onmany smaller civilian and mili-

and enforcing aviation safety. As a result, other than

tary helicopters. Although these aircraft may not beice-

requirements of individual operators and the military,

protected, they may beIFR-rated and thus require the

systems for remotely detecting icing may not be used additional security provided by remote icedetection.

on most aircraft without being required by regulators.

Single-engine lightaircraft have littlespace and weight

Regulatory needs for operating in icing environments

reserves and, in addition, there arefewlocations for

have been identified by Brayton and Hakala (1996).

sensor arrays since the engine and propeller dominate

Nearly aIl changes in the regulatory environment that thefrontofthefuselage. It may bepossible, however, they recommend would be affected by implementation

tooperate asensor through the propeller bysynchron-

of remote ice-detection systems, and all would need izing it withthe rotating propeller (Kirkpatrick 1970).

evaluation should onboard remote-sensing systems

Power is alsoa problem onmany aircraft. Larger

become available. The areas most affected, and requir-

civilianaircraft andmilitaryaircraft carrypower-

ing greatest study by regulators, would be weather demanding avionics and weapons systems. Asaresult, reporting procedures between aircraft and the ground;

if space, weight, orpower requirements arelarge fora

automated substitution for standard icing pilot reports;

remote-sensing system, tradeoffs between other avion-

handling procedures for aircraft wishing diversion;

icsoraweapons system and the remote-sensing system

flight crew, dispatch, and air traffic control (ATC) train-

must beconsidered. The icingremote-sensing system

ing; and icing severity terminology. Aircraft certifica-

may beavoided because ofthe small percentage oftime

tion to fly in icing conditions probably would not be an

that it may actually beused. Military users may also be

issue, because specific aircraft capabilities within icing

concerned about the signature provided bysystems util-

would not be affected by warning systems; only their izing active rather than passive remote sensors.

ability to avoid and escape would be changed.

A remote-sensing system designed to detect icing

conditions ahead of anaircraft mustbeinexpensive, 3.5.3 Incentives small in size, lowinweight, and require littlepower.

Regulators determine what kinds of equipment

Though different types ofaircraft may use systems of

should be mandatory on aircraft in different categories

different capabilities toreduce theimpact ofsome of

of operation. Because of cost and complexity, unless

these factors, alldevelopment should focus onmini-

there are special needs of individual operators, regu- mizing these liabilities.

lators may have to mandate installation of remote- sensing equipment for detecting icing conditions on 3.5 Regulatory issues, weather forecasting, specific classes of aircraft. Such a mandate would be and traffic management preceded by a thorough evaluation of remote-sensing 3.5.1 Functional requirements system capabilities, with a focus on their ability to Above all, a system designed to detect icing conditions enhance safety. Mandates for use on aircraft, or simply remotely is a pilot decision-support system (Clark certification for those operators voluntarily using sys- 1997). It is a system that senses conditions ahead of an tems, would require that regulators consider issues of aircraft and translates it into an icing intensity index to system integration and protocol compatibility.

NASA/CR--2000-209938 9 3.5.4 National airspace impact tors could use the information to establish protocol and Regulators must determine the impact of remote- requirements. Since too much downlinked data could sensing systems on operation of the national airspace result in confusion rather than clarity and actually system, air traffic control, and the Free Flight concept.

decrease the quality of information subsequently pro- vided back to pilots, human-factors specialists should 3.5.5 Aircraft operational limits in icing work with pilot and meteorological interests to resolve Pilots need to know the potential intensity of icing these problems.

ahead of their aircraft. They also need to know how their aircraft responds to icing conditions, and they need 3.5.7 Training to know the limits of their aircraft with regard to icing.

Training is critical to successful use of automated FAR Part 25, Appendix C, defines icing cloud micro- systems. It is needed to provide familiarization with physics for aircraft design. However, if an aircraft is systems and procedures. This will be most important not designed specifically to fly in conditions beyond as remote-sensing systems are initially placed in the those described in Appendix C, it is not known whether field to assure that pilots, air traffic controllers, and it can safely operate in those exceedance conditions. If operators are aware of their operational characteristics.

aircraft are not tested in conditions beyond Appendix There is often a tendency to over-rely on technology C, perhaps they should not be sent into those condi- because of apparent belief in its accuracy and reliability tions (Hill 1997).

(Transport Canada 1996). As a result, cockpit technol- Icing risk varies with aircraft size, aircraft design, ogy tends to reduce vigilance and situational aware- and airfoil type. Aircraft manufacturers must identify ness, which, in an icing environment, could be fatal.

the icing conditions that are beyond the capabilities of Since remote-sensing technology may actually their aircraft. Consideration must be given whether to detach pilots from the icing threat because automation expand Appendix C conditions or create a new FAR to tends to increase confidence and reduce situational address these conditions. Presently, pilots do not know awareness, there is increasing need to promote train- if they are flying in conditions within which the air- ing. Training is needed on how the operational charac- craft was tested, and they do not know if the icing being teristics of remote-sensing systems operate and where experienced will take the aircraft to its limits (Bettcher their abilities and failings lie. This training should be et al. 1996, Parelon 1996, Erickson 1997, FAA 1997).

fed by studies about the characteristics of the system, Although it is not absolutely necessary to the function- perhaps through work that would have been done to ing of a remote icing-detection system, providing pilots verify system capability (Baum and Seymour 1980).

with information about their aircraft's operational limits In addition, there must be training on how to respond and being able to relate information provided by sens- when an icing warning is displayed. Human-factors ing systems to those limits would give pilots more con- research has addressed this issue for wind shear and fidence about decisions to avoid or fly through icing.

terrain-avoidance systems and recently for icing (Hans- man 1997, Vigeant-Langlois and Hansman 1999). How- 3.4.6 Weather downlinking ever, research is needed to determine the most appro- Onboard icing-sensing systems, through immediate pilate avoidance and escape procedures for various and continuous downlinking, could provide forecast- classes of aircraft in different types of airspace and ers and numerical models with objective and timely meteorological conditions. Establishing training stan- temperature, liquid-water content, and drop-size infor- dards and best management practices is a regulatory mation that is accurate in position. Goals of forecasters and operator responsibility.

at the NWS Aviation Weather Center are to better iden- tify where icing is occurring, identify areas of greatest 3.6 Test beds and platforms risk, and determine when icing conditions disappear Efficient, cost-effective methods of testing elements (Carle 1997). The military has similar concerns (Tucker of remote-sensing systems, and full systems, are needed 1983, Peer 1986, Goe 1997). Downlinking of informa- in the development stage under conditions representa- tion gathered onboard would indicate the magnitude tive of the operating environment. Ground-based test and location of icing potential, indicate where there is systems are generally less expensive than airborne plat- no icing potential (Vigeant-Langlois and Hansman forms, so their use should be encouraged at all stages 1999), and provide improved forecast verification. A of development until full testing on aircraft is required.

program should be organized to formulate standards Airborne platforms, spray tankers, and perhaps for integrating _ound, satellite, in-situ, and aircraft- mountain-top observatories should be used to test proto- based icing information and to establish protocol for types of individual sensors and of entire remote-sensing auto-reporting to ground and to other aircraft. Regula- systems. Remote-sensing systems intended for place- NASA/CR--2000-209938 10 instruments, from multicylinders that provide integrated

ment onaircraft must becapable ofsensing horizontally

measurements to current electro-optical systems that

ahead oftheaircraft and above, below, and tothesides

provide measurements with high temporal resolution.

oftheflightpath, sothey should betested inthesame

Thousands of research flight hours have resulted in a

position as they will beused aboard aircraft toprovide

general understanding of the magnitude of liquid-water

confident results (Ryerson etal.,2000). Mountain-top

contents, and their spatial patterns, that can be encoun-

facilities that have potential testing capabilities include

tered by aircraft. For example, it is understood that the Desert Research Institute's Storm Peak Laboratory; ElkMountain, operated bythe University ofWyoming; higher liquid-water contents are generally found in cumuliform rather than in stratiform clouds, that sum-

Whiteface Mountain Observatory, operated bythe State

mer supercooled liquid-water contents are highest, and

University ofNew York, Albany; and Mt.Washington

Observatory, N.H.(Ryerson etal.,inprep.). Anadvan- that liquid water is generally more "cellular" than homo- geneous over thousands of square kilometers. Studies

tage ofmountain-top facilities isthe availability ofnatu-

have also demonstrated that icing conditions, for liquid

ralicing conditions. Likewise, adisadvantage ofnatu-

water and drop size, are extremely variable and diffi-

ralicing isthe lackofcontrol over conditions. Research

aircraft available fortesting mayinclude theNASA cult to generalize. It is also recognized, though not nec- TwinOtter, theNRC TwinOtter and Convair 580, the essarily widely, that FAR 25, Appendix C, conditions are only representative values for engineering design

NCARKingAir,andaircraft fromtheUniversity of

purposes and are not intended to represent the actual

Wyoming, the University ofNorth Dakota, and avari-

character of the icing atmosphere as encountered by an

etyof private companies (Marcotte etal. 1996). The

aircraft.

AirForce tanker spray righas been removed fromser-

The characterization of the icing atmosphere has

vice, buttheArmystill operates itsHelicopter Icing

been accomplished somewhat randomly because of the Spray System (HISS) fromFortRucker, Alabama.

cost of airborne research projects. Each program has a

Flight simulators may also provide information use-

specific focus, so flight hours are typically consumed

ful forestablishing sensor system characteristics. For

trying to answer the primary research questions of the

example, pilots'abilities toreact within given warning

timesprovides information for establishing sensing project. A large-scale monitoring program dedicated to the characterization of icing conditions would allow range and update frequency.

large geographic areas, with weather conditions experi- enced by most of the nation, to be sampled consistently 4.0 METEOROLOGICAL SENSING and frequently to produce information that is statisti- REQUIREMENTS cally valid. Work by Cooper et ai. (1982) and Sand et al. (1984), by the Canadian Freezing Drizzle Experi- 4.1 Summary ment (CFDE), and by the NASA Glenn Research Center The development of systems to measure icing poten- tial remotely and in situ requires an understanding of during the winters of 1996-1997, 1997-1998, and 1998-1999 (Miller et al. 1998) have come closest to the medium being sensed: the atmosphere and its ther- the ideal of covering large geographic areas with mod- mal and liquid characteristics must be understood with em, carefully calibrated instrumentation. One way to regard to the absolute magnitude of conditions and their do this would be to instrument commercial or military spatial distribution. This information is needed to evalu- aircraft that fly large numbers of hours, as Perkins ate the feasibility of sensing and avoiding icing poten- (1952) did, enabling a representative sample of icing tial, to design instruments to sense conditions, and to conditions to be made nationwide and reported through develop methods of avoiding and exiting icing condi- a system such as ACARS (Aircraft Communications tions. The atmosphere must be carefully characterized and Reporting System). Ground-based remote-sensing with regard to icing potential to develop sensors and systems installed at airports to protect terminal areas training protocol, to create terminology for advising may also be able to provide characterization informa- pilots, and to provide better icing forecasts. Character- ization is needed at all scales from the submesoscale to tion similar to that of in-flight programs. This is another argument for accelerating airport-based remote-sensing the global scale, although the synoptic scale is proba- icing-avoidance systems. Finally, icing radiosondes are bly best understood with regard to icing.* available for measuring supercooled liquid water with Attempts to characterize the icing atmosphere, liquid-water content, drop size, and temperature have height within clouds (Hill 1994). Such radiosondes, been conducted since the 1940s with a large range of fielded nationally by the NWS, could improve icing forecasts and the characterization of supercooled cloud water.

* Personalcommunication, M. Politovich, National Center for Atmo- Cloud liquid water is generally better understood spheric Research, Boulder, Colorado, 1997.

NASA/CR--2000-209938 11

thanis dropsize. Nevertheless, themagnitude, distri-

ASOS (Automated Surface Observing System) observa- bution, and organization ofsupercooled liquidwater in tions at over 600 locations nationally (Ramsay 1997), 3-Dspace isstillonlygenerally understood, especially which observe freezing rain, could improve understand- withregard to theconditions thataircraft typically ing of the location, spatial patterns, frequency, and mag- nitude of SLDs.

encounter. A large component of theproblem within

supercooled clouds is glaciation. Though attempts have Proponents of in-flight remote-sensing systems have been made to model and measure glaciation to develop argued that outside air-temperature measurements made a better understanding of the process, it is still not possi- at the fuselage are adequate for temperature character- ble to predict accurately whether a given cloud is glaci- ization ahead of the aircraft. Though this may be gener- ated, when it will glaciate, and how much of the total water ally true in cruise at constant-altitude flight, it is not content is ice. Some remote sensors are sensitive primari- true where icing is most likely to occur: within storms ly to liquid water, such as microwave radiometers, and and in the climb-out and descent phases of flight. Air for users of these systems, mixed-phase clouds are of temperature changes most rapidly in the vertical and little concern. However, mixed-phase conditions may within storm systems. Storms and lower altitudes are enhance the ability of radar to detect liquid water, so also where supercooled water is more frequent, so the the glaciation process needs to be better understood. reliability of outside air-temperature measurements for Although less important than hquid-water content for predicting temperature ahead of the aircraft is least determining the amount of ice to form on an aircraft, where the need is greatest. Thermal lag also occurs as snow falls into warm air and melts, and as rain falls drop size, and especially supercooled large drops (SLDs), determine the location and shape of ice forma- into colder air and supercools, making drop tempera- tions. Thus, drop size may have a larger impact on iced ture unknown even if air temperature is known. In addi- aircraft aerodynamics than liquid-water content does. tion, evidence from a few studies suggests that temper- Drop size is also a more difficult parameter to measure ature does fluctuate considerably within cloud masses and from cloud to cloud and from clear to cloud. Air- than liquid water, and characterization is therefore less complete than for liquid water. Cloud droplet size varies temperature fluctuation, especially near 0°C, must be better characterized within clouds and near frontal sur- with cloud type, cloud dynamics, location within clouds, from cloud to cloud, with the season, air-mass origin, faces. Radiosonde observations and in-flight measure- and other factors. Drop sizes are often characterized by ments from existing flight programs can provide most of this information.

the median volume diameter (MVD), which assumes a unimodal drop-size distribution. This may not always The spatial structure and the size of icing areas have be the situation, especially when SLDs are present. not been characterized. Spatial patterns of icing must be The shape of the drop-size distribution must be care- characterized at all scales, from global to submesoscale, fully sensed and characterized. This is especially impor- but spatial patterns are perhaps best understood at the tant for SLDs. Instruments that count drops, such as synoptic scale. The horizontal extents of icing speci- optical array probes, have the best probability of suc- fied in FAR 25, Appendix C, do not imply the overall cessfully characterizing drop size. Characterizations of dimensions of icing cloud systems. Overall, little work has been conducted in this area, with the best character- drop size conducted concurrently with liquid-water measurements will provide relationships between the izations being by Cooper at al. (1982) and by the Cana- two and to atmospheric dynamics. Drops are usually dian CFDE program (Cober et al. 1996b).

smaller within stratiform than within cumuliform clouds, Cloud microphysics are a focus of several large fed- but more emphasis should be placed on explicit drop- erally funded research programs, and icing remote- size measurements and, at least, characterization of the sensing researchers should partner with these teams to drop-size spectra. accomplish objectives more efficiently. For example, The need to characterize SLDs is even more critical the DoE Atmospheric Radiation Measurement (ARM) than characterizing smaller drops because of the danger Program monitors cloud microphysics to determine the SLDs present to aircraft and because far less is currently effects of cloud cover, type, height, and phase on glo- known about them than about smaller drops. Flight pro- bal radiation budgets. The ARM program maintains grams to measure SLD characteristics, such asthe Cana- field sites in Oklahoma and on the North Slope of dian CFDE project and the NASA Glenn Research Cen- Alaska. Remote sensing of clouds is one of their tools, ter SLD program, should be continued and expanded. and NOAA ETL has been a participant in this capacity.

The Global Energy and Water Cycle Experiment Ground-based programs may also be useful for charac- (GEWEX), part of the World Climate Research Pro- terizing conditions aloft. For example, sleet, freezing gram, also has a cloud microphysics component that drizzle, and freezing rain at the surface are often accom- panied by freezing precipitation aloft. Utilization of may be of value to aircraft-icing remote-sensing research.

NASA/CR--2000-209938 12 siderably within clouds and from cloud to cloud. In addi- Finally, experiments arebeing conducted by federal, state, and private groups toaugment precipitation (cloud tion, temperature may change rapidly within frontal seeding). This often involves monitoring ofcloud micro- systems and in the vertical, as experienced by aircraft physics. Efforts should bemade tointeract withthese when changing altitude rapidly upon departing or approaching terminal areas.

groups and perhaps toconduct coordinated research.

Characterization requires reliable and accurate instru- Liquid-water drop-size spectra determine the loca- tion of drop impingement on airframe structures, the

mentation formaking in-situ measurements. Theideal

type of ice that forms, the shape of ice that forms, and

instrumentation for measuring cloudmicrophysics

the location of ice on the airframe as a result of run-

would besimilar incharacter to instruments currently

back. Runback is caused by supercooled large-drop

used onaircraft fordetermining airspeed andoutside

impingement and flow along the airfoil chord causing airtemperature: generally small, inexpensive, accurate, robust, maintenance-free, andunobtrusive. A focused freezing on areas of the airfoil unprotected by deicing effortis needed to simplifyandminiaturize current equipment. Runback has become a critical problem,

instrumentation, but efforts should also bemade tocom-

especially with respect to SLDs within the drizzle size pletely rethink cloud microphysics instrumentation and range (typically diameters of 50 to 500 p.m). Runback todesign and develop completely new concepts. and formation of an ice ridge immediately aft of the Finally, icingterminology needs improvement. This boot-protected leading edge is believed to be the cause of the ATR-72 crash in Roselawn, Indiana, in October

isbeing addressed inthe FAAInflight Aircraft Icing Plan

(FAA1997). Currently, icingreports and forecasts are 1994 (NTSB 1996). The overall characterization of notpurelymeteorological but include theaircraft. SLDs vs. smaller drop sizes is relatively poorly under- Though practical, because pilots observe howicingis stood, so special emphasis should be placed on charac- terizing the SLD environment.*

affecting theiraircraft, thecurrent terminology is not

Liquid water, drop temperature, and the drop-size

effective because itdoes not utilize purely meteorologi-

spectra are the most important indicators of in-flight

calinformation toevaluate icingintensity. Theaircraft

aircraft icing potential, and thus the most important

must beseparated fromweather toevaluate icingcon-

conditions to characterize for the development of the ditions objectively and unambiguously.

sensing needs of remote-sensing systems. However, it 4.2 Introduction may also be useful to sense conditions that are not criti- cal to icing potential but that may serve as surrogates Information required to assess in-flight aviation icing for the other conditions. For example, range-resolved hazard is derived from measurements of the atmospheric remote sensing of air temperature or droplet tempera- conditions that create ice on aircraft. In order of impor- ture may prove to be the most difficult remote-sensing tance, those conditions are cloud or precipitation liquid- challenge. Whether clouds are glaciated or partially water content, drop temperature, and drop size.* Of these glaciated, or mixed-phase, may provide an indication three conditions, liquid water is most important because it is the material that creates ice on the aircraft. Liquid- of whether liquid-water temperatures are warmer or colder than freezing, so the ability to detect ice within water magnitude varies widely, both spatially and tem- porally, so it must be measured continuously. .clouds may serve as a surrogate binary temperature indi- Droplet temperature is the second most important cation of above- or below-freezing conditions. How- atmospheric condition affecting icing; it determines in ever, little is known about the glaciation process and part whether liquid water will freeze on an aircraft struc- the probability of glaciation at given temperatures below ture. Air temperature (static and total temperature are freezing. Nevertheless, characterization of cloud glaci- not distinguished here) may serve as a surrogate for drop- ation with temperature, to determine if it would be a let temperature, especially if droplets are so small that meaningful surrogate for temperature, may be useful.

The magnitudes of liquid-water content, tempera- their fall speed allows them to maintain a temperature ture, and drop-size spectra must be characterized to nearly that of the surrounding atmosphere. However, snow or graupel failing into a warm layer may melt or provide specifications for remote-sensing technology.

partially melt, resulting in particle temperatures colder The spatial variability of these conditions must also be characterized. Characterization is needed at the mesos- than the air in the warm layer. If the particles or droplets cate (-104-106-m scale), synoptic scale (~ 106-m scale), then fall into colder air below, they will be warmer than the air. Air temperature may be relatively constant over and global scale, although for different reasons, depend- large horizontal distances, but it may also fluctuate con- ing on the scale. Characterization of spatial variability * Personal communication, M. Politovich, National Center for Atmospheric Research, Boulder, Colorado, 1997.

NASA/CR--2000-209938 13 tion of cloud type), allowing for larger liquid-water contents in shorter distances, Appendix C represents only integrated liquid-water contents over distances and does not address maxima or minima that can occur t within those distances. That is, "pockets" of liquid-water t content can be much larger or smaller than the inte- Flight ..-- grated values within a distance represented by Appen- Path f Proposed _ '_ dix C. According to Masters (1983), the original intent of Appendix C was to represent averaged liquid-water content values during exposure over varying distances, so Appendix C is only a starting point for remote- sensing system specification. Appendix C is effective, within its limits, for creating ice-protection system Sensor-Equipped Aircraft specifications as long as instantaneous values of liquid water are of no concern, but it is not as effective for Figure 2. Sensing range affects avoid-and-exit capability.

determining the maximum and minimum liquid-water values that may be experienced along a given route of flight. For this reason, it is necessary to review field may provide indications of the range and spatial reso- measurements within icing conditions to determine the lution needed by remote-sensing systems. The ability absolute range of conditions that can be experienced.

of a system to sense completely through typical icing In addition, the range of conditions with greatest impact storm areas, for example, would provide aircraft with on aircraft operations must be considered for studies of avoidance capability without the risk of entrapment the effect of icing on in-flight aircraft performance (Jeck (Kirkpatrick 1970) (Fig. 2). In addition, if icing poten- 1998).

tial is nearly uniform spatially at the submesoscale, then The National Advisory Committee for Aeronautics remote-sensing capabilities may not be practical for (NACA) Lewis Flight Propulsion Laboratory and Ames avoiding or escaping icing. Spatial characteristics at the Aeronautical Laboratory conducted many research synoptic scale will indicate what portions of storms flights within stratus and cumulus clouds from 1945 provide the greatest icing threat and indicate where through 1950 (Lewis et a]. 1947, Lewis and Hoecker remote-sensing systems are most needed. Global-scale 1948, Kline 1949, Kline and Walker 1951). Cloud patterns of icing indicate where icing threats and the liquid-water content was measured with rotating multi- needs for remote-sensing capabilities are greatest.

cylinders and a rotating-disk icing-rate meter. Rotating- 4.3 Characterization needs multicylinder and icing-disk measurements are inte- grated over periods of minutes, so absolute magnitudes Characterization of the dynamic range of liquid over shorter periods, and thus distances, are not well water, drop size, and temperature are needed to estab- identified. Nevertheless, one flight program provided lish sensing specifications for hardware development.

maximum liquid-water contents of 0.28 g m -3 and 0.76 The general ranges of conditions that could be observed g m-3 for stratus and cumulus clouds, respectively, and are generally understood (Pruppacher and Klett 1997, 90% of the measurements measured less than 0.5 g m -3 Rogers and Yau 1989, Fletcher 1962), but the absolute in stratus clouds and less than 1.2 g m -3 in cumulus ranges of conditions within icing clouds, and the ranges clouds (Lewis et al. 1947). Rights over the Great Lakes of conditions necessary to produce dangerous ice accre- measured liquid-water contents ranging from 0.05 g m -3 tions on aircraft under a full range of flight and design to 0.57 g m-3 with a median of 0.22 g m -3 and a mean of conditions, are not well known.

0.19 g m-3 (Kline 1949). Ninety percent of liquid-water contents were less than 0.40 g m-3, and 50% of all cases 4.3.1 Liquid-water content were less than O. 18 g m -3. Thirty-seven research flights FAR 25, Appendix C (FAA 1991), defines liquid- over most of the northern United States measured mean water content and mean effective drop diameter (similar to the median volume diameter [MVD]) from the sur- values of maximum liquid-water contents averaged over face to 6707 m for stratiform clouds and from 1220 m distances of 0.5, 3.0, 15, and 60 miles of 1.05, 0.63, to 6707 m for cumuliform clouds. This is the minimum 0.33, and 0.14 g m-3 for cumulus clouds, and 0.44, 0.27, standard to which all aircraft ice protection is designed 0.16, and 0.08 g m-3 for stratus clouds. Finally, flights in stratiform clouds yielded a maximum liquid-water and may be the minimum standard to which remote- content of 1.30 g m-3, with 90% of measurements less sensing systems should be designed. However, although horizontal extent is factored into Appendix C (as a func- than 0.54 g m-3 and 50% less than 0.30 g m -3 (Kline NASA/CR--2000-209938 14 different conditions experienced by light aircraft and

and Walker 1951). Large values were typically sought

helicopters below 3049 m. Supercooled liquid-water con-

during these flightsforestablishing engineering stan-

dards.

tents of up to 1.7 g m-3 were found, but 99% of values were less than 1.1 g m-3, and 95% were less than 0.6 g

Perkins (1952) instrumented withicingrate meters

fourUnited Airlines DC-4 aircraft thatflewfrom New"

m-3 for all cloud types. Larger liquid-water contents are theoretically possible below 3049 m, but values greater

York CitytoSan Francisco from January through May

than 2.2 g m-3 are not likely. Liquid-water values were

1951. Ofatotal of 1120 hours offlighttime ontypical

commercial routes and altitudes, icing was encountered largest in cumuliform clouds and behind cold fronts in maritime air.

1.5% ofthetime. Maximum liquid-water contents did

Jeck (1982) also sorted the database described above

notexceed 1.0 gm-3,and 80% ofmeasurements were

less than 0.4gm-3. (Jeck 1983) according to synoptic situation and air-mass

In March 1979, Jeck (1980) made in-cloud liquid- category. The resulting liquid-water contents represent water measurements overLakeMichigan andin the average values over uniform cloud intervals of more than 1 km, so peak liquid-water values are not represented.

vicinity of Lake Erie. He compared instrumentation

In general, modem data liquid-water contents were 1.1 used in research flights from the 1940s and 1950s to then-current instrumentation and discussed sources of to 1.2 g m-3 in cumulus clouds within lake-effect areas, modified continental air masses, high-pressure areas measurement error when using multicylinders to meas- without fronts, and in maritime air masses. Liquid-water ure liquid water. The most significant problem was run- contents were lowest, 0.2 to 0.4 g m-3, in warm frontal offdue to incomplete freezing of water impinging upon stratus clouds, occluded-front stratus clouds, cold-front the cylinders. This occurred when the proper combina- cumulus clouds, and upslope stratus clouds.

tions of air temperature and liquid-water content caused Jeck later expanded the database for icing conditions the ice temperature to remain at 0°C, the so-called "Ludlum" limit. Jeck mounted a Johnson-Williams hot- below 3049 m to include all altitudes and presented wire liquid-water probe that measured liquid water accu- seasonal analyses of liquid-water content for design rately only when drop sizes were smaller than 30 _tm purposes (Jeck 1989). Seasonal liquid-water magnitudes (due to design limitations) on a Lockheed Super Con- were isolated not by calendar date, but by grouping stellation aircraft. Measured liquid-water contents were measurements by the height of the freezing level, with somewhat smaller than historical measurements, in part lower freezing levels occurring during winter condi- because his measurements were in the lower portions tions. Freezing levels below 1524 m agl were used for winter conditions and above 3049 m for summer con- of stratus clouds, whereas earlier measurements sought the largest values typically encountered, near cloud tops. ditions. Nearly 85% of stratus cloud occurrences were On the Jeck flights, icing generally did not occur on the below 3049 m, with the largest liquid-water contents, aircraft when liquid-water contents were less than 0.08 maximizing near 0.9 g m -3, occurring near 1524 m for to 0.10 g m -3. In stratus clouds less than 1524 m agl, warm and cold seasons. Liquid-water content was less 95% of all liquid-water measurements were less than than 0.3 g m-3 90% of the time in the stratus. More than 50% of convective clouds occurred above 3049 m, and 0.6 g m -3.

cold-season convective clouds typically had liquid- In a comprehensive review, Cooper et al. (1982) and water contents of less than 2.0 g m -3, with this maxi- Sand et al. (1984) summarized five years of flights made mum occurring near 3659 m. Summer convective with modem instrumentation by the University of Wyo- clouds, however, had maximum supercooled liquid- ming King Air. Over 98% of summer and winter and water contents approaching 5.0 g m -3, but only above continental and coastal cloud liquid-water measure- 6098 m.

ments---423,787 seconds of measurements with a In a review of the state of knowledge of aircraft icing Johnson-Williams hot-wire liquid-water probe and a for- conditions from around the globe, Hoffman (1984) ward-scattering spectrometer probe (FSSP)--were less than 1.0 g m-3, and only 0.2% of samples exceeded 2.0 g stated that icing occurs within liquid-water contents of m-3. Liquid-water contents nearly as high as 3.0 g m -3 0.01g m-3 to 6.0 g m-3, though values larger than 2.5 g were encountered, but in less than 0.01% of all measure- m -3 are found only in tropical cumulonimbus clouds.

ments.

At any given altitude, liquid water within stratus clouds can vary between 0.01 to 1.0 g m -3, and in cumulus Jeck (1983) and Masters (1983) compiled a new clouds it can vary between 0.0i and 1.7 g m -3.

database of supercooled cloud properties up to 3049 m Twenty-five flights measuring liquid water in strato- from about 12,955 km of icing observations using a cumulus clouds in Germany with a Johnson-Williams mix of old and new measurement technology, from multicylinders to newer optical and hot-wire instrumen- probe indicated that liquid-water contents varied from tation. This database was constructed to address the 0.05 to 0.45 g m -3 (Hoffman et al. 1986). Integration NASA/CR--2000-209938 15 distances were 18.5 km,sothevalues donotrepresent true range ofsupercooled liquid-water contents that may

maxima encountered during flight. beencountered. Though general values are known, most

Telford (1988) analyzed the causes ofinstability and older measurements were made over rather long averag-

theloss ofaDesert Research Institute research aircraft

ingdistances. Nearly instantaneous measurements, on

theorder ofonemeasurement each second (150 mfor

measuring layered cloud properties inthe Sierra Nevada.

Extreme icing was associated withthe crash, and liquid- a 300-ktresearch aircraft), provide highresolution.

water contents were measured from 0.2 gm -3toamaxi- Though the granularity ofcloud liquid water isundoubt- mum of 1.4g m-3 priorto thecrash. Measurements edlyfiner, higher resolution wouldnotbenecessary

were instantaneous and were made withanFSSP and a

fordeveloping remote-sensor specifications.

Johnson-Williams probe. Liquid-water content has been measured during field

In adetailed study of a winter storm and shallow programs with modem instrumentation at as fine as 1- cold-front passage in theDenver area, Politovich and second intervals. Examples include ASTEX, the Atlan- Bernstein (1995) measured unusually high liquid-water tic Stratocumulus Transition Experiment; FIRE, the contents forthatarea instratiform clouds of0.6gm -3. First International Satellite Cloud Climatology Project Severe icing was also reported bythe research aircraft. Regional Experiment; the U.S. DoE ARM campaign, During thesecond Canadian Atlantic Storms Pro- Enhanced Shortwave Experiment; and many other gram (CASP), Cober etal.(1995) reported 3745 super- smaller programs. These data could be reanalyzed cooled liquid-water content measurements withinan for remote-sensing purposes if they could be acquired.

800-km radius ofHalifax, Nova Scotia, withinstrati- In addition, new flights should be made with better instrumentation.

formand "system" clouds such as through coldfronts,

warm fronts, and low-pressure areas. Clouds ofoceanic

and continental origin were included. Though few com- 4.3.2 Cloud drop-size spectra

parisons of liquidwaterin maritime vs.continental Liquid water is delivered to aircraft surfaces as dis- clouds have been conducted specifically withregard to crete drops varying in diameter from only a few microns aircraft icing, ingeneral, liquid-water contents aresim- at the smallest diameter to over 4000 pm in rain drops ilarforcontinental and marine clouds ofagiven gen- (Fletcher 1962, Pruppacher and Klett 1997, Rogers and Yau 1989_ Willis and Tattelman 1989). Drop size has

era(Rogers and Yau1989). Cober etal.(1996a) also

found that liquid water varied littlebetween cloud types, several important roles in aircraft icing.

except forlarger liquid-water-content standard devia- One effect of drop size on airframe icing is its influ- ence on the amount of water collected. The amount of

tions insystem clouds. Median supercooled liquid-water

content was 0.11 gm-3,withsupercooled liquid-water liquid water delivered to an airframe surface is a func- content exceeding 0.94 g m-3only0.0!%ofthe time, tion of the collection efficiency of that surface as similar toconditions measured by Sand etal.(1984) affected by the relative speed between the surface and

over theGreat Lakes and California inwinter. Stewart

the drop, the radius of the surface, and the drop diameter.

etal.(1996) measured supercooled liquid-water con- As relative wind speed increases, drop size increases, tents ofwarm and coldfronts offthe Nova Scotia coast.

and as surface radius decreases, droplet collection effi- Cloudtypes arenotprovided, butthemeasurements ciency increases. As a result, smaller drops are carried were made withmodem optical instruments. Maximum over an airfoil surface to impact aft of the leading edge, supercooled liquid-water contents were not greater than whereas larger drops impact closer to the leading edge.

0.9g m -3,and typically they were less than 0.3gm-3. Objects with a large radius are preferentially impacted Pruppacher and Klett(1997) summarize character- by large drops rather than by small drops. As a result, isticliquid-water contents found in clouds bygenera, the amount of liquid water delivered to a specific por- warning thatliquid-water content typicallyvaries tion of an airframe surface is a function of the liquid strongly fromcloud tocloud. Early-stage cumulus typi- water residing within that portion of the total cloud drop- callyhave 0.2to0.5g m-3,later-stage cumulus 0.5to size spectrum striking the surface. This ignores run- 1.0 g m -3,andstratus andStratocumulus 0.1to0.5g back and other effects that occur after drops impinge m -3.Cumulus withstrong updrafts have liquid-water upon the surface.

contents upto5.0g m -3.Though these measurements A second effect of drop size is upon the. type and weremade in warmandcoldconditions, thetopsof shape of ice that forms on the airframe surface (Hans- cumulus clouds typically have the highest liquid-water man 1985, FAA 1991, Shah et al. 1998). Depending contents, which, in thetropics, areoften supercooled upon a variety of factors, including the amount of liquid

and produce significant icing. water impinging on a portion of airframe over a unit of

It isevident that modem measurements taken atshort

time and the collection efficiency of the icing surface timeintervals arenecessary to evaluate properly the as a function of the drop-size distribution, the type and NASAJCR--2000-209938 16 the MVD alone does not describe the shape of the drop-

shape ofthe ice accretion will vary considerably. Low

size distribution, nor does it adequately describe where liquid-water contents at low temperatures and small collection efficiency effects will cause ice accretion on drop size tend to create rime ice, and larger liquid-water the airframe (Newton 1979, Cooper et al. 1982).

contents in warmer temperatures and larger drops tend Early measurements of drop-size spectra were made to produce clear ice (FAA 1991). Rime tends to pro- either with oiled or soot-covered slides or with the use duce an ice surface that conforms generally to the shape of the airfoil. Clear ice may create a smooth surface, or of rotating multicylinders. Slides were difficult to use in it creates "horns" near the leading edge that have a large high wind speeds, though they were occasionally used as late as the 1960s (Warner 1969). Usually, rotating multi- impact on drag and airfoil lift. As drop size increases within clear-ice conditions, the size of the accretion cylinders, developed in the early 1940s at Mt. Wash- ington Observatory, were used instead of slides, and increases, the impingement limits increase in area, and the horns tend to form farther back on the airfoil (FAA they are still in use (FAA 1991, Howe 1991). Multi- cylinders provide an indication of the shape of the drop- 1991). Overall, according to Sand et al. (1984) and Polito- size spectra by utilizing curves developed from theory vich (1989), drops larger than 30 _tm in diameter have by Langmuir and Blodgett (1946) using the collection a greater effect on flight than smaller droplets.

efficiency of various-diameter cylinders for given wind A third effect of drop size is runback. Although run- speeds and drop sizes. From these curves, and the back may occur over a large range of cloud drop sizes, amount of ice collecting on each cylinder, the MVD depending upon temperature and liquid-water content, runback becomes more serious when the drizzle-size can be estimated. However, serious errors in MVD esti- mation could occur with multicylinder use in large- regime is entered, at about 50/am. Here, all water does not freeze near its impingement location--some runs droplet situations, where MVDs approach 30 _tm or back and freezes beyond ice-protected areas of the lead- larger (Jeck 1980).

ing edge. This often creates an ice ridge or roughens Drop-size spectra were measured coincidentally with wing surfaces, significantly altering airfoil aerodynam- liquid-water content in most experiments. The database ics and aircraft performance. created by Jeck (1980) at the Naval Research Labs, in Cloud droplet size varies by cloud genera, from cloud cooperation with the FAA, is probably the most compre- hensive available. Jeck (1983) and Masters (1983) sum- to cloud, by season, and with location within clouds.

marized older and modern measurements both below For example, the largest drops in growing, nonprecipi- 3049 m and at all levels of the atmosphere. Jeck (1983) taring cumulus clouds typically occur near the center and top of the cloud within updrafts. Smaller drops are indicates that below 3049 m, average MVDs measured found near the cloud base and near the cloud perimeter with multicylinder and newer optical instruments, for supercooled layer clouds, are about 13 lam and for con- where dry air entrainment causes evaporation of drops (FAA 1991). Overall, drop size is controlled by evapo- vective clouds they are 18 _tm. MVD also shows temper- ration, collision-coalescence, curvature and solute ature dependence, with MVD increasing from 10 I.tm to about 30 _tm in stratiform clouds as temperature effects, the Bergeron process, and the number and type increases from -25°C to 0°C. Jeck (1982) also observed of cloud condensation nuclei present (Miller and Anthes 1980). As an example, maritime clouds of a given gen- that MVD generally increases with altitude in single- era typically exhibit broader drop-size spectra than do layer clouds below 3049 m.

continental clouds due to differences in the type, num- Jeck (1983) questions the use of a minimum MVD ber, and size of cloud condensation nuclei (Rogers and of 15 lam in FAR 25, Appendix C, considering analy- Yau 1989). ses of the database of cloud properties below 3049 m (Jeck 1980). Masters (1983) and Jeck (1983) both pro- Cloud drop-size spectra are typically characterized by the median volume diameter (MVD), the drop size vide diagrams from this database showing MVDs in where one-half of the spectrum's water volume resides icing clouds well below 15 lam.

A summary of five years of cloud measurements by within smaller-diameter droplets and the other half the University of Wyoming (Sand et al. 1984) showed resides within larger droplets. Internal cloud dynamics MVDs ranged from 5 to 40 _tm, with a characteristic may create bimodal drop-size distributions, observed in MVD of about 15 _tm. The smallest MVDs were meas- most cloud types in most climatic regimes (Pruppacher and Klett 1997, Politovich and Vali 1983). Bimodal dis- ured during the winter over the Great Lakes and the Great Plains, with the largest MVDs in the summer over tributions are not properly represented by a single MVD, the Great Lakes and Illinois and in the winter over Flor- however, which relies on a unimodal distribution. The ida. Droplets were smaller in the Great Lakes and Illi- average collection efficiency of a drop-size spectrum nois areas because of low liquid-water contents, accord- around a median volume diameter is generally quite ing to Sand et al. (1984). No relationship was found close to the collection efficiency of the MVD. However, NASA/CR--2000-209938 17 between icing effects on aircraft performance, a Beech sition Experiment (ASTEX), the First ISCCP (Interna- King Air, and the MVDs that produced the ice. Only tional Satellite Cloud Climatology Project) regional MVDs larger than 40 _tm, reaching into the supercooled experiment (FIRE), and the U.S. Department of Energy large-drop regime, affected aircraft performance. Atmospheric Radiation Measurement (ARM) program.

Roebber (1988), in a review of icing potential on The general characteristics of MVD by cloud genera helicopters and fixed-wing aircraft off the east coast of are understood. However, less is generally understood Canada, presents statistics of drop sizes encountered about cloud drop size than about cloud liquid-water con- during icing and reported by the Royal Canadian Air tent. Controls of drop-size spectra are not well para- Force. The MVDs of convective clouds were between meterized, although the general controls are believed to 18 and 21 lam and for layered clouds near 12 txm, but be understood. Changes in drop size over time within MVDs as large as 50 _tm were generally observed in con- storms, and diurnally, have been tracked and simulated, vective clouds, and as large as 40 ktm in layered clouds. but general theory explaining drop-size evolution over time is not mature.

Jeck (1989) summarizes MVDs from his FAA/NRL icing database for clouds at all altitudes. Mean MVDs Understanding of drop size has been hindered by the need for improved instrumentation, the three-dimensional are 13 p.m for layer clouds and 17 _tm for convective clouds. The range of MVDs within the database, by gen- complexity of liquid water within clouds, the need for eral cloud type, are 7-21 p.m for layer clouds and 10-26 observation flights focusing on drop-size measurements, _tm for convective clouds. and too much emphasis on reporting only MVDs instead Cober eta]. (1995) reported on 31 flights into Cana- of the full drop-size spectrum.

dian east coast winter storms over the North Atlantic 4.3.3 Supercooled large drops Ocean and created a high-quality database of those flights. Flights were made into fronts, tow-pressure areas, The existence of large droplets (>50-1am diameter) and stratus clouds. The average MVD for all clouds was was well known to early NACA investigators of the 18 ktm: 16 _tm for low-level stratus clouds and 20 _tm microphysics of icing clouds, but they were not included for "system" clouds. These measurements compare well in the FAR 25, Appendix C tables, which include drop sizes from only 15 to 40 _m (FAA 1991). Sand et al.

with earlier measurements in the area, according to Cober et al. (1995), and with measurements by Sand et (1984) and Politovich (1989) state that droplets larger al. (1984). than 30 _tm in diameter have a greater effect on flight Politovich and Bernstein (1995) investigated the pro- than smaller droplets. Hansman (1985) indicates that, duction and depletion of supercooled liquid water in a from model and wind-tunnel tests, large drops present a February 1990 winter storm in the Denver area. Strati- much larger threat to aircraft than small drops and that form clouds associated with a cold-front passage creat- even a small liquid-water content in large drops may be ed mean droplet diameters of 10-13 lam, with droplets a significant icing threat. Bragg (1996) attributes large- larger than 50 lam in diameter observed. droplet ice accretions, and the formation of ice ridges Small diurnal changes in drop-size spectra occur as aft of ice-protected areas, as a likely cause of flow sepa- ration, aileron snatch, and loss of roll control. Shah et a result of changes in cloud dynamics between night and day. Modeling of marine stratocumulus clouds by al. (1998) indicate that secondary ice shapes producing Considine (1997) demonstrated increases in MVD of a ridges spanwise along a wing can be created by super- few microns in the afternoon and decreases at night, with cooled large drops (SLDs), even with a heated leading minima in the morning. Much of the effect is due to edge. The larger drops also strike unprotected areas of daytime decreases in dr), air entrainment and increases the aircraft, such as the underside of the wing, increas- in entrainment at night. ing drag (Politovich 1989). Loss of a research aircraft An active area to watch for advances in information by the Desert Research Institute in icing conditions may regarding drop-size spectra, outside of aircraft icing, is have been caused by SLDs, typically drops in the 50- to climate change research, Measurements and models 500-1am-diameter size range (Telford 1988). Coffey characterizing cloud microphysical properties have (1995) describes the hazard of SLDs as observed from become critical for parameterizing the effects of clouds the cockpit of a research aircraft, with advice on how to avoid and exit SLD conditions.

on climate change. Radiative models used io simulate potential climate change and isolate the effects of green- Droplets larger than about 50 lain in diameter do not house gases are very sensitive to cloud drop-size distri- remain suspended in clouds by turbulence effects as do bution (Choularton and Bower 1993, Telford 1996). smaller droplets. Gravitational forces cause them to fall Experiments analyzing the roles of cloud microphysi- at greater speeds as drop size increases, producing pre- cal properties in climate change that are either in progress cipitation. Though long recognized as a hazard, these or completed include the Atlantic Stratocumulus Tran- large drops have been receiving more attention in recent NASA/CR--2000-209938 18

overall, and alarger drop spectral width. They attribute

years. The crash ofanATR-72 atRoselawn, Indiana, in

theformation ofdrizzle drops instratus clouds toareas

October 1994, focused theattention of aviation icing

where updraft velocities are greater, which causes differ-

researchers onSLDs and their unique hazard toaircraft

entpercentages of cloudcondensation nucleiatthe

(NTSB 1996, Broderick 1996).

cloud base tobeactivated. They suggest thatupdraft

Early research reports describing the results offlights

velocity can beused topredict drop concentration and

measuring themicrophysical properties oficing clouds

have mentioned supercooled drizzle drops, forexample, thewidth ofthedrop-size spectrum.

In one ofthemost ambitious drizzle measurement

Kline(1949), butrelatively few reports focused on

SLDs. Rodert (195 i) and Lewis (1951 )bothindicated programs todate, Cober etal.(1995; 1996a,b,c) describe

freezing drizzle measurements made in theCanadian

theimportance offreezing drizzle and freezing rainas

Freezing Drizzle Experiment (CFDE) off theNew-

aircraft icing hazards. However, most workuntilrecent

foundland coast. Freezing drizzle wasencountered in

years has been ]'nresponse toneeds tounderstand rain-

fourresearch flights within thick (~1000-m) stratiform

drop-formation mechanisms forcloud physicists, rather

clouds. Inthese fourencounters, liquid-water content

than foraviation needs (Fletcher 1962, Hobbs and Deepak

varied between 0.05 and 0.2g m-3when MVDs were

1981, Cotton and Anthes 1989, Rogers and Yau1989,

larger than40lam. MVDsaslarge as950lamwere

Houze 1993, Young 1993, Pruppacher and Klett1997).

measured infreezing rain below thecloud base. Within

Isaac and Schemenauer (1979) found supercooled

clouds, MVDs often exceeded 500 tam. When combined

large drops near the tops ofcumulus clouds nearYellow-

liquid-water contents and MVDs were compared toFAR

knife, NWT. Many cloud tops between 0°Cand -8°C

25, Appendix C,34of 147 data points felloutside the

had concentrations ofsupercooled drops larger than 70

_tm. About twice as many clouds had concentrations of envelopes. They conclude thatfreezing drizzle may be large water droplets as hadconcentrations oficecrys- afrequent phenomenon inEast Coast winter storms and tals. Large drops were associated withlowliquid-water asignificant aviation hazard.

Jeck(1996) published themostcomprehensive

contents, and droplets larger than150 lam never had a

concentration of more than1L-l. Theauthors could

review todate ofthestate ofknowledge about freezing

rain(ZR) and drizzle (ZL)withregard toaviation. He

notexplain whythedrops existed anddidnotrelate

indicates thatfewinstrumented aircraft have flownin

them toaircraft icingsince the purpose oftheresearch

ZLand ZR,and thatlittleisknown about themeteoro-

was related toprecipitation enhancement.

Politovich (1989) describes icing from large droplets logicalconditions andgeographic locations of SLD onaresearch aircraft flyinginCalifornia and Arizona. occurrence. Elevated ZLand ZR,encountered byair-

craft inflight, areahazard that may notbeexperienced

Eleven flightsarecharacterized within a narrow tem-

atthesurface if they freeze assleet before reaching the

perature range, between-5.5 and-9.4°C and drop con-

centrations ofgenerally less than100 cm -3.Conditions ground. Though techniques have been proposed for

detecting ZL andZRfromradiosondes, noreliable

hadthegreatest effect onaircraft performance when

fewer than 0.1 - 1cm -3droplets occurred inasize range methods ofprediction are available, especially forZL,

which can occur without the traditional warm layer often

from30to400_m.Politovich indicates thatthefre-

quency ofthese occurrences islowbutnotrare. Ample found in ZR.ZRandZLaretypically lower-altitude phenomena, withmost occurring below 3811 magl,

moisture and time, accompanied byrift,must beavail-

making them adistinct hazard tononpressurized air-

ableto create these largedrops. Shesuggests that

environments mostlikely to experience SLDsare craft, helicopters, and allaircraft onapproach and depar-

ture.Littleisknown about thefrequency, depth, and

orographic and upslope in warm fronts andwithin the

horizontal extent ofZRandZL layers, thecauses of

warm sector of cyclones where adequate moisture and

riftare available. ZL,and the fullrange ofmeteorological conditions asso-

ciated witheach. Jeck indicated that theuse ofMVDto

Feingold etal.(1996) argue, fromnumerical simu-

characterize dropspectra associated withSLDs is not

lations, thattheproduction of drizzle within clouds is

appropriate because theMVDprovides noindication

related todroplet residence time and within-cloud turbu-

thatSLDs exist.

lence. Vigorously growing clouds produce more driz-

Hobbs andRangno (1996) observed supercooled

zlebecause they allow longer in-cloud drop dwell times,

drizzle drops withvery highliquid-water contents off

prolonging thecollision-coalescence process. Their

the Washington coast. Thestratocumulus clouds were

arguments aresimilar tothatofPolitovich (1989).

trapped above aninversion, preventing cloud conden-

Hudson and Svensson (1995) measured drizzle-drop

sation nucleifromthemarine boundary layerbelow

concentrations offthe Southern California coast as part

the inversion from reaching the clouds. Asaresult, drop

oftheFIREexperiments andassociated drizzle drops

concentrations were low(- 500 L-l), liquid-water con-

closely withlower drop concentrations, larger droplets

NASA/CR--2000-209938 19

tents were high(upto0.8g m-3), and drops as large as It is evident that SLDs create uniquely hazardous 200 gm in diameter were present. The authors indicate in-flight icing conditions, yet little is known about the that supercooled layer clouds that form in clean mari- phenomenon: its characteristics, its climatology, or what time air (lacking cloud condensation nuclei) that is comprises an SLD condition (Shah et al. 1998). Hights decoupled from the surface could pose a significant threat during the winters of 1996-1997, 1997-1998, and to aircraft from supercooled drizzle or rain. 1998-1999 by NASA Glenn Research Center's Twin Cober et al. (1996b,c) report conditions off the east Otter aircraft into SLD should help answer some of the coast of Canada similar to those reported by Hobbs and remaining questions (Miller et al. 1998). Jeck's (1996) Rangno (1996). Freezing drizzle was observed in 1100- report addresses most of the weaknesses in knowledge m-thick stratiform clouds in temperatures between -11 °C about SLDs and is probably the most complete and suc- and -8°C. The maritime air was very clean, with con- cinct paper on the subject from an avi_on perspective.

densation nuclei allowing only a few drops to grow large 4.3.4 Temperature and coalesce. Though the MVD was 29 p.m, cloud drop- The thermal environment of an icing event deter- lets larger than 40 _m exceeded 300 L -l, and 500-_m- mines the type, amount, and location of ice formation diameter drops were measured near the cloud tops. This on an airframe (Cooper and Sand 1997). The thermal suggests one mechanism for ZL, that of isolating humid environment is controlled by radiative, convective, con- air with few condensation nuclei, allowing coalescence ductive, latent heat and advective processes of the atmo- and drop growth to occur.

sphere and the airframe and by the dynamics of the air- In reports exploring the causes ofZL offthe Canadian craft moving through the atmosphere. When isolated east coast, Isaac et al. (1996) and Cober et al. (1996b,c) from the airframe and the thermodynamics of the icing review the processes that could cause ZL and compare them with CFDE measurements. In Newfoundland, ZL processes, thermal processes within the atmosphere alone determine the temperature of air and of drops.

is associated with easterly and southeasterly winds and The "source" of cold also affects the amount, type, rarely with westerly winds. Only about 15% of ZR cases are nonclassical, but 60% of ZL cases are nonclassical. and shape of ice that forms. For example, droplets warmer than 0°C may freeze upon a cold-soaked air- Classical ZR and ZL result from overrunning, such as occurs within warm fronts. Nonclassical drizzle forma- frame, but supercooled droplets may not freeze effi- ciently on an airframe warmed aerodynamically above tion does not involve overrunning. Mechanisms may freezing. Supercooled drops impinging upon an air- include giant aerosol initiation of large drops, wind shear frame that is colder than 0°C will typically produce ice.

leading to entrainment, mixing and coalescence, long Of the thermal processes operating, the temperature of drop lifetimes in stratiform clouds that encourage drop the droplets, or the temperature of the atmosphere sur- growth, and high supersaturations. Eleven days of flights rounding the droplets, is typically most important in in both classical and nonclassical freezing precipitation determining whether ice will form on an airframe.

situations showed no consistency of mechanism, except for wind direction and the existence of inversions and According to Rodert (1951), it is tacitly assumed that cloud droplets are at the same temperature as the wind shear near the cloud top.

surrounding atmosphere. This may not always be true Climatologies of SLD accretions at the surface have for cloud or for precipitation drops, which typically cool been developed as a method of assessing where freezing to the wet-bulb temperature of the surrounding atmo- rain may be occurring aloft as a hazard to aircraft. Strapp sphere through evaporation (Cooper and Sand 1997).

et al. (1996), Robbins and Cortinas (1996), and Bern- Since the relative humidity within icing clouds is typi- stein and Brown (1997) completed independent climatol- cally near 100%, the dew point and air temperature will ogies of the frequency of SLD events in North America also be similar, especially within stratiform clouds of to assess where aircraft icing due to ZR and ZL may be stable air masses. Within cumulus clouds with active occurring with greater frequency. All maps indicate updrafts and entrainment of dry air, evaporation and freezing precipitation at the surface as being most com- subsequent cooling may be greatest near the outside of mon east of the Rocky Mountains, with frequency the cloud where entrainment is most active (FAA 1991).

increasing from the mid-Mississippi Valley to the North- Therefore, one will find warm cores in clouds with inter- east and Labrador, with an axis through the Great Lakes nal updrafts because of reduced evaporation and the Basin. Ahmed and Brown (1995) produced a climatolo- release of latent heat as drops grow. In general, cloud- gy of in-flight ZR globally, with seasonal detail in Great size droplets reach thermal equilibrium with surround- Britain and Europe from the U.K. Meteorological Office's ing air very rapidly, typically within 1 second (Borovi- numerical model output. Their model-derived climatolo- kov et al. 1963). Precipitation drops cool to the wet- gy suggests high frequencies of ZR over the Atlantic and Pacific Oceans. bulb temperature after they have fallen into dry air NASA/CR--2000-209938 20 level flight, so during approach and departure static

below cloud base and begin toevaporate, butlagtimes

outside air temperature (OAT) at the aircraft will not can beontheorder of 10s(Fletcher 1962).

be a reliable indicator of air temperature within the flight

Droplets may also becooler than thesurrounding air

path ahead of the aircraft. Vertical temperatures can

within warm tongues of airadvected over colder sur-

vary, from nearly isothermal over large vertical dis-

face airand below colder airaloft.Snow fallinginto

tances to changing by tens of degrees over a few hun-

these warm layers fromabove may partially orcom-

dreds of meters, especially when transiting inversions.

pletely melt. However, untilfullymelted theirtempera-

ture remains at0°C, so they remain colder than the warm As an example, Schroeder (1990) illustrates a winter temperature inversion over the Denver area of about

layeruntilall icemelts andthedrops begintoheat

22°C within a vertical distance of less than 500 m. Such

through convection andradiation. These areas are

rapid changes are not unusual during winter.

often identified onradar displays as"bright bands"--

This evidence suggests that OAT measured at the

zones where falling icecrystals melt and coalesce into

aircraft, though a general indicator, is insufficient for raindrops.

determining if liquid water in the flight path is super-

Temperature withinclouds mayfluctuate several

cooled. Confidence in the representativeness of OAT

degrees overdistances ofonlyafewmeters. Inaddi-

to predict temperature ahead of the aircraft varies with

tion,within-cloud temperatures canbeconsiderably

the meteorological conditions around the aircraft and

different fromoutside-cloud temperatures. Rapid and

with the mode of flight: ascent, level, or descent.

significant temperature fluctuation from cloud tocloud, Nonthermal parameters may be useful surrogates for

and within clouds, makes determination ofsupercool-

indicating temperature. Detection of glaciation within

ingdifficult.Forexample, datafromNCAR Winter

a cloud suggests that any liquid water within the cloud

IcingStorms Project (WISP) flights indicate that tem-

is supercooled. However, if ice crystals are not present,

perature fluctuations fromclear airtocloud can beas

the method is not effective because there is no physical

much as6°C(NCAR 1990). Flights inPoland witha

indication of supercooling.

rapid-response airborne thermometer show temperature

fluctuations withinclouds of 2°Cin distances of less In addition to detecting temperature within the flight than 150 m(Haman and Malinowski 1996). Time series path, range-resolved temperature must also be sensed above and below the aircraft to provide a potential route

of temperature through thecoreof a warmcumulus

cloud (Lawson andCooper 1990) showed a3 to4°C of escape from icing into warm air. Since air tempera- increase oftemperature upon entering thecloud, with ture varies more rapidly vertically than horizontally, especially within inversions, sensing temperature above

similarsubsequent cooling uponexit.Temperature

withinthecloud wasnearly constant. Penetrations of and below aircraft may be useful.

The accuracy of temperature measurement may also

supercooled stratiform clouds showed, depending upon

thethermometer observed since several were being be critical because of its effect, with liquid-water con- tent and drop size, on ice type, density, and shape on

tested, a0to 1 °Cdecrease intemperature when inside

leading edges (Wright 1995). Since very small changes

thecloud as compared withdryairaround thecloud. In

another case, butwithout identification ofcloud genera, in temperature may create large changes in ice accretion cooling of 3°Cwasobserved withinthecloud when amount and shape, it may be useful to measure temper- ature ahead, above, and below the aircraft with high

compared withsurrounding dryair.Lawson and Rodi

(1992) penetrated warm cumulus humilis clouds with accuracy.

fast-response thermometers and showed immediate 6°C

4.3. 5 Spatial structure

cooling when entering theclouds andimmediate 6°C

Spatial scales of icing conditions affect the utility of warming when exiting.

remote-sensing systems. Icing conditions that are spa- Temperature changes can also be large and rapid in tially homogeneous over thousands of square kilometers the horizontal when an aircraft transits fronts, though offer less potential for avoidance without climbing or not as rapid as upon entering or exiting clouds. For descending. The size of icing cells and storm areas also example, transiting a cold front in horizontal flight can affects the needed sensing range of remote-sensing sys- produce temperature changes of 0.2°C per kilometer or tems. Storms with small icing cells may be sensed by a more (Berry et al. 1945). Smaller changes are observed remote-sensing system sufficiently to allow an aircraft when transiting warm fronts in level flight. This is ignor- to progress iteratively through the system. Storms with ing turbulent mixing in the shear zone along frontal large icing cells may be too large to be sensed through, surfaces, which can cause more rapid localized tem- potentially trapping aviators (Fig. 2) (Kirkpatrick 1970).

perature changes.

The inability to sense completely through icing reduces The most rapid temperature changes, however, are avoidance options and may limit an aircraft to turning experienced during ascent or descent rather than within NASA/CR--2000-209938 21

back along itsoriginal route toavoid icing. Most icing

cal properties related to icing. The length of supercooled areas may bescanned completely through withadetec- liquid-water content patches was measured. A patch was tionrange of 80kmor more, assuggested byCurry defined as having supercooled liquid-water content of and Liu(1992).

at least 0.025 g m -3 for at least 0.5 km of flight. Patch- Relatively littleis "known about the horizontal extent es terminated when supercooled liquid-water content of icing conditions, though more is known today because was less than 0.025 g m -3 for 0.5 km. Average patch of modern research flights than was known in 1979 length was 4.3 km, with a mean liquid-water content of when Milton Beheim of NASA Lewis Research Center 0.13 g m -3, and the median patch length was 1.7 km.

indicated that the horizontal extent of the icing cloud About 90% of patches were less than 7 km in length, had not been adequately defined (Beheim 1979). He with less than 2% longer than 50 km. Frights were made also indicated that the fine-grain structure of the icing in low-level stratus clouds and within low-pressure areas cloud had not been well defined.

and through fronts.

FAR 25, Appendix C, tries to address icing spatial Politovich (1982) describes flights through super- cooled stratiform clouds over the Great Lakes and the scale by providing tables for continuous conditions within stratiform clouds and intermittent conditions Great Plains in 1981. A cloud extent started when the within cumulus clouds (FAA 1991). Jeck (I983) indi- aircraft was within supercooled liquid cloud for at least cates, however, that the horizontal extent of icing speci- 1 kin, and cloud elements "separated by less than the fied in Appendix C has no specification for the discon- element length were combined unless the gap was tinuity in icing and the size and frequency of any cloud greater than 6 km." The average icing encounter in Great gaps. He concludes that "horizontal extent," as indicated Lakes stratiforna clouds was 9 km long, and within Great in Appendix C, does not imply the overall dimensions Plains stratiform clouds the average encounter was 24 of icing cloud systems.

km long. Embedded cells of supercooled liquid water Jeck (1983) provides two methods of expressing the within bands of frozen clouds averaged 6 km in length.

horizontal extent of icing. For engineering-design pur- The larger extents were a result of large-scale lifting of poses, he indicates that the horizontal extent of icing air masses. Though isolated pockets of higher liquid- encounters, consistent with Appendix C, is the "dis, water content occurred, the clouds were generally fairly tance flown during a given icing encounter until a cloud uniform at a given flight level.

gap of some specified duration signals the end" of the Cooper et al. (1982) characterized distances of encounter. Of more use in determining the utility of liquid-water content encountered greater than specified remote sensing is the horizontal extent of individual thresholds, and the frequency and size of gaps between icing events, where an icing event is the actual distance icing encounters (where evaporation or sublimation of of icing, which ceases at a cloud gap of any length. ice accreted on an aircraft could occur). Data from 1083 Jeck reanalyzed NACA data by the horizontal extent flight hours in California, Montana, Utah, Florida, Kan- of the icing event and presented modern data in the same sas, Illinois, Michigan, and the Great Lakes in summer way. The analyses indicate, as is consistent with Appen- and winter conditions were used to compile the infor- dix C, that there is an inverse relationship between mation. In all seasons, flights were in icing conditions, liquid-water content and event horizontal extent. Hori- but at higher altitudes in summer than in winter. The zontal extent is about 33 km at a liquid-water content flights deliberately sought the most severe icing condi- of about 0.01 g m -3 and is about 5.5 km for the largest tions. Cooper and his colleagues present information observed liquid-water contents, about 1.5 g m-3. This indicating exposure distance in two ways: should not impiy, however, that liquid-water content is • Probability of exceeding a given liquid-water con- constant for these distances. These are average values, tent in a given distance and individual patches of larger or smaller liquid-water • Probability of exceeding a liquid-water content of contents can occur within these extents. An aircraft with 01i g m z3 for each region.

ice protection may be able to tolerate liquid-water con- tent to a given magnitude but may have to avoid larger As examples, when averaged over I km, liquid-water liquid-water contents. Thus, it may also be helpful to content exceeding 0. I g m-3 occurred about 5% of the know" the size of icing "patches" with larger than speci- time and exceeded 0.5 g m -3 about 1% of the time.

fied liquid-water contents.

When averaged over a distance of 10 km, liquid-water The size of liquid-water content patches may be content exceeded 0.5 g m -3 about 0.5% of the time.

ascertained from measurements during research flights. Viewed regionally, there are large differences in the In 1992, Cober et al. (1995) flew 31 missions, as part extent of liquid-water content greater than 0.1 g m -3.

The Great Lakes area, which has the lowest overall of the Canadian Atlantic Storms Project (CASP), into East Coast winter storms to measure cloud microphysi- liquid-water content, has the longest continuous icing NASA/CR--2000-209938 22 encounters, over80kmin thewinter. Kansas, Mon- Strapp et al. (1996) and Bemstein and Brown (1997) have created modem climatologies of the occurrence

tana, Illinois, andFlorida hadshort encounters, most

being less than 36kminlength. InKansas and Florida of freezing precipitation. Neither study, however, specu- there was about a 1%chance thatanicingencounter lates about the spatial extent of individual freezing pre- wouldextend more than10km.Cooper and hiscol- cipitation storms. Bemstein (1996) indicates implicitly that freezing precipitation does occur for hundreds of

leagues pointoutthatonlyabout 10% ofencounters

extended morethan5 km,andthemajority of icing kilometers, but broken in continuity. As an example, measured was in cumulus clouds.

he presents a map for 1800 hr on 6 March 1996, illustrat- ing freezing precipitation as extending in a broken band

Gap encounters areuseful forassessing theutility

from New York City through Missouri, with a maxi-

ofremote sensing toavoid icing, because aircraft could

mum width of about 250 km.

avoid icebynavigating gaps. Cooper etal,(1982) report

In general, spatial patterns of icing on sub-kilome-

that gaps are typically short, likeicing encounters, with

ter to tens-of-kilometers scales, and ieing's relationship

50%being less than5 kmin extent. A gap occurred

to synoptic and mesoscale weather, are only generally when liquid-water content wasless than0.01g m-3.

understood. The ability to avoid icing is a function of

Gerber (1996) reports liquid-water content gaps, or

minima (called turbules), ofamuch smaller scale em- its spatial distribution. Large cloud masses that are

bedded within marine stratocumulus clouds. Turbules

homogeneous with respect to icing are difficult to avoid.

aretypically afewhundred meters or less across.

Klineand Walker (1951) related icingto synoptic 4.3.6 Mixed-phase clouds

Supercooled liquid water freezes on aircraft struc-

patterns instratiform clouds during 22flights from1948

tures, whereas ice crystals within clouds, snow, and ice

through 1950. Inextratropical cyclones, most icing was

pellets typically do not adhere (Riley 1998). Never-

associated withpost-cold-frontal situations, withmost

theless, clouds composed of mixtures of ice crystals

icinginthesouthwestern and northwestern quadrants

and supercooled water are of interest for remote sens-

ofthestorm. Very littleicing was found inthe overrun-

ing of icing potential for several reasons. First, remote- ningportions ofwarm fronts east ofthestorm center.

sensing systems scanning clouds that are completely

Mosticingwastypically300-400 kmbehind cold

fronts, and north ofthecenter oflows,similar topat- glaciated or mixed phase must distinguish successfully between ice and supercooled liquid water and not be terns reported byRyerson (1990) atMt.Washington, compromised by the presence of ice crystals. A remote-

N.H.,and Mt. Mansfield, Vt. These patterns arecon-

sensing system must be capable of quantifying the

trarytoanalyses of pilotreports of icingreported by

amount of supercooled liquid water mixed with ice crys-

Politovich* thatindicate thatmost icingis ahead of

tals or of sensing beyond a frozen cloud in the fore-

warm fronts and near the center oflows, withleast icing

behind warm fronts and cold fronts. ground, for example, to a more distant supercooled

Littleis known about thehorizontal extent of ZL

liquid cloud.

The second concern for mixed-phase clouds is in

andZR,according toJeck (1996) in a summary of

reference to the need to range-resolve temperature to

knowledge about thephenomenon. Bennett's (1959)

determine if sensed liquid water is supercooled. A cloud

report indicated that most freezing rainoccurs inover-

made up of a mixture of liquid water and ice crystals is

running situations, so it isassociated withwarm fronts

likely to contain supercooled liquid water. As a result,

in many instances. Freezing raincanextend continu-

even if a method is not found for range-resolving tem-

ously orintermittently several hundred kilometers par-

perature, it may be possible to determine whether liquid

allelto a frontandshort distances perpendicular to

fronts. It canalso beassociated withcoldfronts, but wateris supercooled by sensing the presence of ice crys- tals mixed with the liquid water. Thus, mixed-phase then ittypically isofshorter extent than inwarm fronts.

clouds may serve as a surrogate for explicit tempera- Design values use160 kmasa representative extent.

ture measurement ahead of the aircraft. Mixed-phase There is no information for the extent of ZL, according to Jeck, who argues that extent should be related to the clouds, however, may be less of an icing hazard than time an aircraft must spend below 7000 ft, especially supercooled clouds without ice crystals (Guttman and on approach and departure. This agrees with Perkins' Jeck 1987, Riley 1998).

(1952) conclusions that over 50% of icing conditions Simply seeking ice crystals may not be a reliable solution to determining supercooling, however. Clouds in general are found during climb or descent.

may be composed completely of supercooled liquid water and still contain no ice crystals, or they may con- rain a concentration of ice crystals that is so small as to * Personal communication, M. Politovich, National Center for Atmo- be not detectable. The success of using mixed-phase spheric Research, Boulder, Colorado, 1997.

NASA/CR--2000-209938 23

clouds asanindicator of supercooling is thus depen- Additional information about the utility of using dent upon theprobability ofsupercooled liquidclouds mixed-phase clouds for indications of supercooling is containing detectable icecrystals. Parameterizing nucle- available from various field programs. Tremblay et al.

ationof water droplets in clouds, andtheglaciation (1996) observed mixed-phase clouds at 4461 points as part of CFDE in 1995 off the Newfoundland coast. Plots

process, isone oftheclassical problems cloud physics

of the proportion of liquid water vs. ice water within has yettosolve.

When acloud iscooled below 0°C, icecrystals could mixed-phase clouds showed a temperature dependence form.However, because there arerelatively fewice between 0°C and -l 0°C if liquid-water contents larger than 0.3 g m-3 are ignored. As temperature decreased,

nuclei inthe atmosphere when compared withconden-

the proportion of ice increased. However, the relation-

sation nuclei, nucleation often does not begin until drop-

ship is also proportional to the cloud liquid-water con-

lets coolto-10°C (Rogers and Yau 1989). Observations

tent, with the proportion of cloud water nucleating

of258 clouds byHobbs etal.(1974, as cited byRogers

andYau 1989) showed that glaciation typically does not increasing, at a given temperature, with cloud total begin untilcloud-top temperatures cooltoabout -4°C, liquid-water content. In flights through summer cumu- after whichthepercentage of clouds containing ice lus in southern Missouri, Koenig (1963) found glacia- increases to100% atacloud-top temperature ofabout tion related to drop size, with clouds with large drops -20°C.Rogers and Yau(1989) state thatit isimpossi- rapidly forming high concentrations of ice crystals bletodetermine atwhat cloud-top temperature any indi- regardless of the availability of ice nuclei.

vidual cloud will begin toglaciate or toestimate how Bower et al. (1996) surveyed frontal and maritime much glaciation willoccur. Thus, ingeneral, clouds with convective clouds from the United Kingdom and con- tinental convective clouds from New Mexico and Mon-

tops warmer than about -5°Careicefree, and clouds

tana to refine parameterization schemes for global circu-

withtops colder than -20°C are virtually guaranteed to

lation models. Detailed analyses were done for all have icecrystals (Riley1998).

clouds that had been measured for glaciation activity

The firstcrystals toappear inacloud must form on

using aircraft-mounted instrumentation. Continental and

icenuclei (Rogers and Yau1989). Additional crystals

areformed fromsecondary processes such asthe frac- maritime frontal clouds had very rapid glaciation, begin- tureoficecrystals and theshattering or splintering of ning at 0°C, with total glaciation typically ocurring at temperatures of-10 to -15°C. Continental and mari-

drops as they freeze. These crystal fragments then strike

time convective cloud glaciation was much slower and liquid-water droplets, causing them to freeze though less complete, with glaciation beginning at 0°C, but at contact nucleation, or the fragments simply serve as deposition nuclei (Houze 1993). -3°C, typical clouds were only about 40% glaciated.

At -15°C, some clouds were still 90% supercooled Overall, glaciation is difficult to predict because it liquid water.

depends upon cloud type, cloud age, liquid-water con- tent, and geographical location--especially as related It is not clear from these studies whether glaciation begins in earnest at 0°C. Characteristics of mixed-phase to air mass type and availability of icing nuclei (Rogers clouds must be better defined to determine the proba- and Yau 1989). Since the probability of glaciation bility of ice crystals at temperatures below 0°C. The increases with decreasing temperature, it would be most current and thorough review of mixed-phase expected to find a monotonically increasing percent- clouds and aircraft icing is by Riley (1998).

age of cloud water to be frozen at lower temperatures.

This does not appear to occur, however. Instead, once 4.4 In-situ instrumentation clouds begin to glaciate, freezing occurs rapidly, and the final ice particle concentration is not proportional In-situ measurements of cloud microphysics are to temperature (Pruppacher and Klett 1997). needed to support and augment the remote sensing of in-flight icing conditions, and in-situ instrumentation is Mixed-phase clouds are also not necessarily uni- needed for improved characterization of icing cloud formly glaciated. On the basis of a large number of microphysics. Drop-size distributions are often not cor- soundings of nimbostratus clouds in Russia, Borovik- ov et al. (1963) report that three different types of mixed rectly represented by current measurement methods, and cloud structure can occur: clouds can consist of rela- ice crystals and drops can be confused by the coarse- ness of sensing systems. Two measurements by the same tively uniform mixtures of crystals and water through- out, successive layers of water droplets and ice crys- model of instrument often do not agree when used on the tals, or three or four layers of warm water, supercooled same aircraft, which indicates repeatability or calibration water, mixed conditions, and ice. The relative frequency problems. The dynamic range and sensitivity of instru- of each was observed 52%, 28%, and 20% of the time, ments is often insufficient, and gaps in size distributions synthesized from combinations of instruments often occur.

respectively, in Russia.

NASA/CRI2000-209938 24 In-situ instrumentation would beuseful onboard all and the reverse-flow thermometer were both accurate within clouds, and the Rosemount was not (Lawson and

aircraft forsensing when icing conditions have been

entered and for near-real-time calibration of onboard Cooper 1990).

remote sensors. Instrumentation intended for use on all Marillier et al. (1991) constructed an ultrasonic thermometer that measured temperature ahead of an air- aircraft may not require the accuracy of research instru- ments. Ease of use, maintenance, cost, and size are more craft to avoid cloud-wetting effects and to obtain temper- important factors in these applications. ature at 100 Hz. Measurements were typically accurate Most current aircraft-mounted instrumentation suited to within a few tenths of a degree Celsius of a colocated for cloud microphysical measurements is intended for Rosemount probe.

Lawson and Rodi (1992), Friehe and Khelif (1993), research applications. Very few instruments, notably air- and Haman and Malinowski (1996) have constructed temperature measurement devices, are sufficiently inex- very fast response thermometers in an attempt to match pensive and robust for general field use. The following is a brief review of available instrumentation and their the response time of a forward-scattering spectrometer probe (FSSP). These thermometers are very delicate, general applicability. This review does not discuss with either platinum wires 12.5 ]am in diameter (Law- instrumentation for detecting ice on aircraft surfaces, son and Rodi 1992), tungsten wires 2.5 _m in diameter, either preflight or in-flight. A review of in-flight ice or small thermistors (Friehe and Khelif 1993, Haman detectors is available in the FAA Aircraft Icing Hand- and Malinowski 1996). Both wire probes are accurate book (FAA 1991) and in an SAE document (SAE 1995).

but delicate and broke easily when stressed by high- Cloud microphysics measurement instruments are typically designed to measure temperature, liquid-water speed airflow and precipitation. All three instruments content, and elements of the drop-size spectrum. Early are too delicate for general operational use, and the ther- mistor instrument had calibration difficulties.

instrumentation was manually operated, but most mod- em instruments are electronic.

4.4.2 Liquid-water content measurement 4.4.1 Temperature measurement The first common instrument for liquid-water meas- Temperature cannot be measured with a standard urement was the rotating multicylinder, developed at outside air-temperature probe because, in addition to Mt. Washington Observatory between 1940 and 1945 accuracy and exposure problems, it is wetted by cloud (Lewis et al. 1947, 1953; FAA 1991; Howe 1991). In water. A wet thermometer measures the wet-bulb tem- the 1930s, a single rotating cylinder was used. The rotat- perature. Lawson and Cooper (1990) provide a detailed ing multicylinder, still in use at Mr. Washington Observ- analysis of the problem. Thus, the most important task atory but no longer used on aircraft, is used to deter- is to protect the thermometer from cloud droplets with- mine cloud liquid-water content and the shape of the drop-size spectrum. The multicylinder typically con- out disturbing the measurement.

A common temperature measurement instrument is sists of six cylinders, stepped in diameter. The shape of the Rosemount total temperature probe (Haman et al. the drop-size spectrum is determined by comparing the 1997). It is used by the U.S. Air Force (Glass and Gran- mass of ice on each cylinder to curves of expected ice tham 1981), by NCAR (Sand et al. 1984) on its King accretion on each cylinder for a given shaped drop spec- Air, and on the Canadian Convair (Cober et al. 1996b). trum, after theory developed by Langmuir and Blodgett The instrument measures the resistance of a platinum (1946). Cloud liquid-water content is related to the mass wire in a bridge circuit. Accuracy is claimed to be from of ice on the cylinder; the smallest cylinder with the 0.5°C (Sand at al. 1984) to +I°C (Cober et al. 1996b), highest collection efficiency is typically used for the calculation.

but wetting in clouds is a problem.

Rotating multicylinders are manually operated and, NCAR and the AES also use reverse-flow tempera- depending upon cloud liquid-water content and rela- ture probes. According to Cober et al. (1996b), they are as accurate as the Rosemount and prevent wetting of tive wind speed, are exposed from a few minutes to more than 20 minutes (Kline 1949, Howe 1991).

the probe.

There has been considerable effort to improve the Jeck (1980) thoroughly reviews problems with the multicylinder method, especially with regard to runoff accuracy, resolution, and response time of thermome- ters to measure small-scale cloud features. Lawson and near the Ludlum limit. The advantage of the multi- cylinder method is that, with a skilled operator, it is Cooper (1990) analyzed the Ophir radiometric thermo- cheap, easy to use, and reasonably accurate in colder meter to improve reliability within clouds and to temperatures, smaller drop sizes, and moderate liquid- improve response time. However, the sample volume water contents.

is quite large (10 m in depth), and the instrument is The only other early method of measuring liquid- large, expensive, and complex. The Ophir thermometer NASA/CR--2000-209938 25

water content thatreceived much usewastheicing-

serviced. King et al. (1978) reported a sensitivity of rate meter, ahorizontal rodwithholes facing intothe 0.02 g m-3, a response time better than 0.05 s, and an relative wind(Perkins 1952). Iceplugging theholes accuracy of 5% at t .0 g m-3. Baumgardner (1983) could would besensed withapressure transducer, triggering draw no conclusions about the King probe, other than adeicing cycle. Assumptions about icedensity and the that it was promising and deserved more study. Cober

meter's collection efficiency allowed rough estimation

et al. (1996b) use the King probe on the Canadian Con- ofliquid-water content. Electronic devices have since vair and claim accuracies of _+0.02 g m -3 for liquid-

replaced multicylinders. water contents of less than 0.2 g m -3. In a range of

Today, theprimary electronic devices formeasur- liquid-water contents between 0.1 and 1.25 g m-3, and ingliquid-water content are the Rosemount icedetector with MVDs between 10 and 40 p.m, Ide (1996) found andtheKingandJohnson-Williams hotwireprobes accuracy for the King probe to be within +0.1 g m-3 of (FAA1991, Knollenberg 1981, Glass andGrantham the calibrated NASA Glenn Research Center Icing

1981). The Rosemount icedetector isastandard instru-

Research Tunnel (IRT). In general, wind-tunnel testing ment onmost icing research aircraft and onthe ground, has shown the King probe to be generally accurate to and itmay also beused tocompute liquid-water content 5% at 1.0 g m-3, and it is generally superior to but more byrelating thedeicing rate of thedetector torelative fragile than the Johnson-Williams probe (FAA 1991).

Another hot-wire-based instrument is the Nezorov windvelocity (Brown 1981, FAA1991, Claffey etal.

1995). A6-mm-diameter by25-mm-long probe vibrates probe developed in Russia (Korolev et al. 1996). Cur- axiallyatitsresonant frequency of40,000 Hz. As ice rently being flown on the Canadian NRC Convair and the NASA Glenn Research Center Twin Otter, it has accretes on the probe, its frequency drops until, at a preset frequency, a heater deices the probe. Liquid water the unique ability to quantify both the supercooled may be computed if the mass of ice, the exposure time, liquid-water content and the ice-water component of the relative wind velocity, and the collection efficiency clouds (Miller et al. 1998). Similarly to the Johnson- of the probe are known. The detector is reasonably accu- Williams probe, a reference heater corrects for convec- rate, within the range of conditions found in most moun- tive heat losses. Though details are not available, it tain and aircraft applications, at moderate liquid-water appears that liquid- and ice-water components are sepa- contents (Claffey et al. 1995). The typical liquid-water rated by the lag caused by phase changes as water performance range is 0.05 to 3.0 g m-3 (FAA 1991).

vaporizes within the instrument (but ice particles may The Johnson-Williams probe exposes a hot wire to break away, causing negligible heat loss--and error). The the droplet-laden air flow, and a second "compensat- exact process is not clear. Comparisons with the King ing" wire is protected from liquid water but exposed to probe in CFDE flights show less than 10% disagree- the air flow (Knollenberg 198 !, FAA 1991). The second ment, with the Nezorov showing better performance in SLD environments. Wind-tunnel tests demonstrated wire compensates for variations in air speed, altitude, and air temperature. The resistance of the wires changes better stability than the King probe at low temperatures as they warm and cool, and the change of resistance is and the ability to measure the frozen component of measured through a Wheatstone Bridge circuit. The mixed-phase clouds. Verification in snow has not been instrument has an absolute liquid-water range from 0.0 possible because of a lack of standards.

to 1.5 g m-3 (Jeck 1980) to 6.0 g m -3 (FAA 1991). Per- Liquid-water content may also be measured opti- sonne et al. (1982) found undermeasurement of liquid- cally, typically utilizing the interaction of laser-based water content in large-drop environments for that por- collimated light and droplets. Gerber (1991, 1996) has tion of the liquid-water content in droplets larger than developed an instrument, the particle volume monitor 30 mm in diameter. A +20% error limit is often assumed (PVM), that measures cloud liquid-water content, inte- for the probe, but it can be smaller with wind-tunnel grated particle surface area, and effective cloud droplet radius. All measurements are made simultaneously in calibration (Baumgardner 1983, Sand et al. 1984).

The CSIRO, or King, hot-wire probe measures liquid a large, 1.25-cm 3 sample volume. The instrument oper- water by maintaining a copper wire coil nominally 1.5 ates by passing droplets through a laser beam, which mm in diameter exposed to the air stream at a constant then forward-scatter laser light through a lens and a temperature (King et al. 1978, Knollenberg 1981, FAA variable transmission filter onto a detector. Output from 1991). The electrical energy necessary to maintain a the detector is mathematically inverted to derive liquid- constant temperature under the cooling influence of the water content and effective drop radius. The instrument resembles a class of instruments called "laser-diffraction air stream and impinging water droplets is related to liquid-water content after corrections are made for air particle-sizing instruments" (Gerber 1996). Compari- sons of the PVM with other instruments in environ- temperature and wind speed. The probe requires no wind-tunnel calibration, is rugged, and is easily field mental chambers, on mountain tops, and on aircraft have NASA/CR--2000-209938 26 4.4.3 Drop-size measurement

been encouraging. Theinstrument hasbeen demon-

Measurement of cloud drop-size spectra has been

strated tobereliable inlarge-drop environments and in

important since early aircraft icing research, primarily

large liquid-water contents. It alsodoes notappear to

because of their impact on ice shape, ice type, collection

bedisturbed byicecrystals, butindependent tests must

bedone before thefull accuracy and reliability of the efficiency, and runback. However, the increased inter- instrument isknown.

est in SLDs in recent years has placed renewed emphasis The forward-scattering spectrometer probe (FSSP), on drop-size measurement. Fewer instruments are avail- developed byKnollenberg (1981) and manufactured by able for measuring drop-size spectra than for measuring liquid-water content, but many of the instruments

Particle Measuring Systems (PMS), is themost com-

described above for measuring fiquid-water content have

monly used optical probe; it is found onnearly all

research aircraft (Sand etal.1984, Cober etal.1996b, dual uses and measure both. In the case of optical instru- ments, liquid-water content is typically derived from the Baumgardner etal.1993, Thomas and Marwitz 1995).

measured drop-size spectrum and drop concentration.

Intended formeasuring drop sizes from2 to47jam in

Except for oiled or soot-covered slides, the rotating

diameter (94 jam inextended mode), cloud liquid-water

multicylinder described earlier was the first widely used

content canbecomputed byintegrating thespectrum

instrument for obtaining the shape of the drop-size spec-

ofdrop sizes. However, liquid-water measurements are

trum and MVD (Lewis et al. 1947, 1953; FAA 1991;

prone tolarge error due toover- orundersizing ofdrop

sizes. Theinstrument operates by forward-scattering Howe 1991). The drop-size spectrum shape is deter- lightthrough cloud droplets as they pass through anar- mined by, in effect, fitting the accreted ice weights of rowlaser beam to adetector thatrecords adropsize the six cylinders to a series of curves, each represent- proportional totheflash oflight.Drops areclassified ing a different droplet spectrum shape. The rotating multi- intofifteen 3-jam-wide bins.Sources of errorin the cylinder method provides only a general indication of instrument are asmall sample volume, false sizes from the breadth of the droplet size distribution, in part icecrystals, iceaccretion andfogging of theoptics, because it can be fit to only a finite set of curves and because some clouds have bimodal or multimodal dis-

blockage ofairflow through the instrument byice,and

saturation of theinstrument's electronics athighair- tributions. Howe (199 l) states that accuracy in deter- mination of liquid-water content and droplet size is speeds and large particle concentrations (FAA1991).

Inaddition, large drops are typically incorrectly meas- better than +10% when cloud drop-size distributions ured.Asa result of drop-size measurement errors, are narrow or moderately broad. When drop-size distri- butions are extremely broad, accuracy is reduced to liquid-water contents typically have up to 34% error about +20%.

(FAA 1991, Baumgardner 1983). Ide (1996) found Gerber's PVM (1991, 1996; Gerber et al. 1994) pro- liquid-water contents computed from an FSSP to be vides the effective drop radius of clouds. Few compari- overestimated by 50% in MVDs up to 60 jam, and by sons have been made with other instruments, but a com- 100 to 150% in larger MVDs at NASA's Glenn Research parison with the FSSP (Gerber 1996) shows a linear Center Icing Research Tunnel. Baumgardner et al. (1993) and Brenguier (1993), however, have successfully relationship between the two instruments, though not a modified the FSSP to measure the microstructure of 1:1 relationship. Gerber suggested that the mismatch, clouds at the centimeter scale.

with the FSSP providing smaller drop sizes than the PVM, was due to errors in the FSSP.

The Phase Doppler Particle Analyzer (PDPA), devel- oped by Aerometrics, Inc. with assistance from NASA The FSSP was described in the liquid-water discus- Glenn Research Center (NASA 1997), measures drop sion above, where some if its problems of measuring diameters from 0.7 to 125 jam but can be extended to drop size were also discussed. Overall, the instrument tends to broaden the drop-size spectra and, in drops 2000 jam (Aerometrics 1997). Droplet sampling is made in a small sample volume at the intersection of two laser larger than about 45 Jam, measurements may not be beams. Droplets passing through the beams create an inter- trustworthy (FAA 1991).

ference fringe pattern that is projected into several detect- Optical array probes (OAPs), manufactured by Par- ticle Measuring Systems (PMS), measure drop size by ors. The detectors produce a Doppler signal proportional imaging (Knollenberg 1981, Oldenburg and Ide 1990a, to the droplet's velocity and size. Droplet number density, and thus liquid-water content, are also computed. Cali- FAA 1991). OAPs image droplet shadows onto an array bration is not necessary. Models of the instrument have of photodiodes by allowing drops to flow through a laser beam. The loss of light on an individual array ele- been developed for both wind-tunnel and aircraft use.

ment is detected by a logic circuit that measures the However, the PDPA typically does not appear on equip- shadow size. The drop size is a function of the shadow ment lists of primary cloud research aircraft.

NASA/CR--2000-209938 27 size and theoptical elements between thedrop and the development for retrieving Iiquid-water content and detector array. If themeasurement is made along one drop-size spectra.* The second instrument, the CDS, dimension ofthe particle, the probe iscalled a1-D probe. measures the angular resolution of forward-scattered Only shadows fullywithin the array are accepted, so large light from an ensemble of cloud drops. A 256-photo- droplets--droplets not fully within thearray--are diode array measures the scattered light, and liquid- rejected. The range of drop sizes measured is typically water content is computed from the angular measure- 20 to 300 _tm, or 300 to 4500 _am; and other ranges are ments of the forward-scattered light, which expresses the collective drop sizes of the drop ensemble. Liquid- available (Knollenberg 1981, FAA 1991).

water content measurements have been successfully OAP 2-D probes are similar to I -D probes optically, but enhanced signal processing speed allows the photo- compared with other instruments in a wind tunnel, and drop-size spectra measurements have been tested in the diode array to be scanned faster, retrieving, in effect, a laboratory, aboard an aircraft, and at Mt. Washington shadow of particles as they traverse the laser beam. The Observatory. No data have been published in the open result is an image of particles indicating the shape of literature demonstrating the CDS's ability to measure ice crystals and the size of drops. Particle sizes are binned into 64 size classes. Two-D probes have been liquid water or the drop-size spectrum.

configured to detect particles up to 6400 ktm in diame- ter (Marcotte et at. 1996). The accuracy of OAPs has 4.5 Terminology been scrutinized in recent years with the renewed inter- A remote-sensing system designed to detect and map est in SLDs. Problems of aircraft speed and location of icing potential within a projected flight path wilI sense the drop within the imager depth of field can cause the meteorological conditions that create ice on an air- OAPs to miss smaller drops and oversize large drops craft: liquid-water content, temperature, and drop size.

(Lawson et al. 1996, Morrison et al. 1997). These prob- Then the remote-sensing system may utilize expert sys- lems are detected when FSSP and OAP ranges overlap tem or fuzzy logic to create information for a cockpit display that the pilot can use if icing is entered. Since and when OAPs are compared with other instruments.

ice cannot occur on the aircraft until the aircraft enters New array-processing techniques and algorithms have been suggested to correct these problems (Hobbs et al. the icing conditions, only an explicit numerical model 1996, Korolev et al. 1996). operated With the remote-sensing system, a set of guide- lines that relate icing potential to expected performance, A comparison of the PDPA with the FSSP and a PMS OAP (Oldenburg and Ide 1990a,b) indicates that all would solve the problem of how to relate meteorolog- ical conditions to the pilot. Bragg et al. (1998) propose three instruments generally agree well. Disagreements occurred in drop sizes smaller than about 10 _tm because a smart icing system that recognizes how aircraft sys- smaller droplets were suspected of freezing, and each tems should respond to icing and advises the pilot after instrument treats ice crystals differently because of the the aircraft has entered icing. A similar system could differing technologies, causing mismatched sample sta- be activated before an aircraft enters icing and act in tistics. They also disagreed for MVDs larger than 30 response to remotely sensed icing conditions ahead of the aircraft.

[am because of the configuration of the PDPA, which could be recortfigured to detect larger drop sizes. The Current terminology used to categorize or classify PDPA has the advantage of being a smaller instrument icing intensity, or severity, is often inconsistent among than the PMS probes, and it has the capacity of sensing government agencies, difficult to interpret and, at times, a wide range of drop sizes with one instrument, Whereas contradictory (Erickson et al. 1996; Green 1995; Auld the PMS probes require two instruments, the FSSP and 1989; Newton I977, 1979). Currently, the National an OAE Weather Service defines icing intensity with descrip- Lawson et al. (1996, 1998) and Lawson and Cor- tions that are related only to aircraft and contain no mack (1995) describe new optical probes that solve meteorological criteria (Auld 1989, Newton 1979).

According to Newton (1979), NWS definitions of trace, problems with the FSSP and OAPs. The new instru- ments, which are in near-production stage for both light, moderate, and severe are only reporting defini- ground and airborne use, are the cloud particle imager tions and contain no meteorological information that (CPI) and the cloud droplet spectrometer (CDS). The can be Used to forecast icing.

CPI creates images of cloud particles at a rate of 30 s-l, Newton (1979) also indicates that the National but at a potential rate of 240 s -1. Image detail as small Weather Service definitions are not related to FAA icing regulations (FAA 1991). Uniformly understood icing as 2 lam is possible, with maximum possible size limited by sample volume. Shadowgraph-type images of ice crystals can be made of nearly photographic quality. Im- * Personal communication, R. Paul Lawson, Stratton Park Engineer- ages can be obtained in real-time, and techniques are in ing Corp., 1999.

NASA/CR--2000-209938 28 definitions areneeded that are defined in standard terms More work is needed to perfect existing information- retrieval methods and to make them more robust. Dop- (Auld 1989, Newton 1979). A graduated, parametric pler and polarimetry techniques may be able to provide method of describing icing severity should be developed information about drop-size distributions. Most cloud that is meaningful to meteorologists, aircraft designers, microphysical work occurs in the X, K a, and W bands, regulators, and operators (Erickson et al. 1996).

A remote-sensing system must, at minimum, supply most ground-based precipitation measurement occurs in the S and C bands, and in X band from aircraft. A detailed information to the pilot with regard to the icing poten- tial of a given volume of atmosphere in the flight path. sensitivity analysis is needed to find the optimal mix of requirements for detecting liquid-water content and A cockpit display must relay to the pilot information that drop-size information from ground-based and airborne enables a risk-management decision to be made. This means the display must either provide information that systems.

Radar has shown modest success at measuring atmo- is uniformly applicable to all aircraft, but to which the spheric precipitation and cloud water content--both liquid pilot can reference his aircraft, or the system must directly and frozen. Rainfall rates may be estimated with S- and indicate the hazard potential to that particular aircraft.

C-band radars. NWS NEXRAD radars are an example According to Newton (1977, 1979), the most tenable solution with current definitions is to use a form of the of a system with some rainfall rate capabilities that may be useful for detecting freezing rain and perhaps freezing old NACA icing intensity curves. The FAA is address- drizzle conditions around airports. Although not practi- ing terminology problems as part of the FAA Inflight cal on aircraft because of size, using S band to detect Aircraft Icing Plan (FAA 1997).

freezing precipitation, X- and Ka-band differential atten- uation techniques to detect cloud water, or multiband 5.0 TECHNOLOGY DEVELOPMENT X-, Ka- and W-band neural network techniques may be 5.1 Summary optimal combinations at airports. In addition, Doppler and A remote-sensing system, operated either from the polarization techniques may provide some indication of ground or from an aircraft, must sense the three- the drop-size spectra and the presence of ice crystals.

dimensional fluctuation of atmospheric liquid-water con- A possible problem of radar, especially of the differ- ential attenuation methods using two or more wave- tent, drop size, and temperature both from outside of icing conditions and from within icing conditions. In addi- lengths, is the need to know droplet temperature to extract an accurate liquid-water content measurement tion, the sensing system must have a range, resolution, and refresh rate sufficient to satisfy pilot needs. The capa- because of the dependency of backscatter on drop tem- bilities of different technologies are often vastly differ- perature (Rinehart 1997). In addition, in military appli- cations radar could place an aircraft that is sensing icing ent, depending upon whether scanning is toward space, toward the earth's surface, or in the horizontal plane conditions at tactical disadvantage. Radars with stealth- ahead of an aircraft. In addition, it is likely that no sin- like capabilities should be investigated.

In general, for ground-based systems, the focus gle sensing technology is capable of providing all sens- should be on S- or C-band systems for determination of ing needs--a fusion of technologies will be necessary.

Radar is the most viable technology for providing precipitation rate and precipitation liquid-water content estimations and on X, Ka, and W bands for determin- range-resolved cloud liquid-water content through large ation of cloud water content. Doppler and polarization expanses of cloud. Its scan rate is rapid, orientation is techniques are available and practical in ground-based generally not a limitation (unless observing below the systems, but for airborne systems they may not be use- horizon where earth reflections may be a problem), and ful. For general aviation aircraft, (relatively) inexpensive there are a variety of bands from which to choose. Radar K a- or W-band radar could be used to simply indicate is a reasonably mature technology, and its basic back- the location of cloud bases and tops if pointed to zenith scatter capabilities may be augmented by Doppler and and nadir--a useful tool to escape icing.

polarization techniques.

Microwave radiometry has the potential to remotely An important issue that must be addressed in the use sense icing conditions and, like radar, has been proposed of radar for remotely detecting icing conditions is to and used for several icing studies. Microwave radiome- develop accurate and robust techniques for retrieving ters are capable of integrating or profiling liquid-water liquid water, drop size, and temperature information from content and water vapor and profiling temperature. These radar backscatter. Developments in this area include the capabilities have been demonstrated for zenith and in- differential attenuation method of liquid-water retrieval termediate angles.

demonstrated by NOAA-ETL (Martner et al. 1991; There are several ways radiometers could be used to 1993a,b) and the neural net retrieval of liquid water and detect icing conditions. Ground-based airport systems drop size by Quadrant Engineering (Mead et al. 1998).

NASA/CR--2000-209938 29

could use scanning radiometers tomeasure liquid water

and some aspects ofthe drop-size spectra and cloud phase

andtemperature profiles withheight. Integrated water composition using polarization. However, lidar's inher- could bedistributed through theatmosphere todeter- ent liability, its inability to penetrate cloud of large opti-

mine volumetric cloud water content if combined with

cal depth more than a few hundred meters, is severe.

cloud base, top,and layer information fromaKaradar. Lidar may be a useful, inexpensive technique for allow- However, thiscannowbeaccomplished without radar ing aircraft, especially those flying night VFR (visual flight bythe Radiometrics profiling radiometer. The ability of rules), to avoid icing, but it cannot provide guidance to radiometry toscan liquidwater atairports is nearly a escape icing. Its potentially low cost, high scan-rate capa- mature technology, withthe greatest concerns being the bility, and small size may make it a practical tool for need tokeep sensors freeof moisture andimproving small helicopters and light aircraft, especially aircraft scanning rates. Long scan time isoneofthemost seri- operating in the Far North with no ice protection in often ousradiometer problems because theyarepassive limiting weather and few winter hours of daylight. Devel- devices. Atairports, this can beovercome byusing multi- opment along these directions is being made in Canada.

pleradiometers, each assigned toadifferent sector of Canadian needs for this VFR cloud and icing avoidance sky, butit isagreater problem forairborne systems. capability may be great in the northern territories.

Airborne radiometry ofcloud liquidwater does not Temperature measurement is needed to determine if appear tohave been tried fromaircraft. It istheoretically cloud water or liquid precipitation is supercooled. It is also needed for radar retrieval of cloud water content.

possible forairborne radiometers tomeasure integrated

liquidwater scanning in thehorizontal, and thatfeasi- Two mature technologies are available for sounding bilityisbeing analyzed using measurements made from temperature from the ground: RASS (radio acoustic thesummit of Mt.Washington attheMt.Washington sounding systems) and microwave radiometers. RASS IcingSensors Project (MWISP). However, it maybe provides greater temperature resolution and thus accu- more feasible at85GHzthanatthemore commonly racy, which is especially needed during the winter when used 37GHz because ofgreater sensitivity tocloud water inversions are common. However, RASS's range is gen-

at85GHz. Thisneeds tobeexplored. erally limited to altitudes of less than 3 km agl. RASS

Asanalternative tohorizontal scanning, or inaddi- and microwave sounders could be used together at air- tiontoit, radiometers might beable tosense vertically, ports to provide the resolution needed at lower altitudes

atzenith and atnadir, fromanaircraft, asis nowdone

and temperatures above the terminal airspace.

fromtheground andfromsatellites, respectively. The At the present time, there is no explicit capability to amount of liquidwater above andbelowtheaircraft range-resolve temperature ahead of an aircraft. Radio- could thenbedetermined, and radar mightbeused to metry is the most promising possibility. The NASA Jet simply determine cloud tops and bases. Theslow inte- Propulsion Lab scans air temperature at high altitudes gration time ofradiometers, however--many seconds-- ahead of an aircraft, with temperature provided at a rel- wouldprovide onlyanaveraged or integrated liquid- atively fixed distance. Though not range-resolved, air- water content. Thismaybeuseful information, espe- craft motion effectively range-resolves the temperature ciallyin levelflightwithinrelatively uniform clouds, and thus may provide an interim solution. The efficacy butduring climb-out and descent, and withinfluctuat- of this system needs additional exploration for icing ingclouds such ascumuliform, thistechnique may not applications; its performance within clouds is unknown.

beasuseful because ofthelongintegration times and RASS has been found not to be practical for sensing tem- limited spatial resolution ofmicrowave radiometers. In perature from aircraft.

addition, nadir-viewing aircraft radiometers would have There is a poss_ility of range-resolving air tempera- ture above and below aircraft, at zenith and nadir, with

the complexity ofradiation from the earth's surface, and

varying altitude above thesurface, making cloud-water microwave radiometers using the same techniques that retrieval moredifficult.Thissuggests thatalthough are used by ground-based and satellite-based microwave ground- andsatellite-based radiometers may beable to radiometer temperature sounders. The scan times measure integrated liquidwater successfully now, air- required may cause smearing of temperature due to air- borne systems are presented withgreater difficulties that craft motion, approach and descent altitude changes may willrequire more retrieval technique modeling and field- prevent accurate temperature retrieval because of nec-

work. Inaddition, there is noindication thatradiome-

essary scan times, and ground radiation may cause prob- ters candetect characteristics of thedrop-size spectra lems when sensing to nadir. A zenith/nadir sensing sys- or detect ice,though it has been theorized thatdrizzle- tem does not indicate temperature ahead except through size drops may beidentified bypolarization effects. extrapolation, although some scanning capability may Lidar, especially multiple-field-of-view techniques, be possible. However, it would indicate where warm tem- haspromise formeasuring cloud liquid-water content peratures may exist for escape from icing.

NASA/CR--2000-209938 30 5.2 Introduction However, capabilities of some technologies are either A remote-sensing system designed to detect icing enhanced or severely limited by the direction they are potential ahead of aircraft would consist of three com- sensing, either horizontally or vertically.

ponents: Remote sensing of aircraft icing conditions is devel- oping along two parallel paths, depending upon the tech- • A suite of detectors to measure conditions that cause nology. One requirement is to create an ability to icing remotely detect icing conditions ahead of aircraft from • A processing system to integrate information, as it is sensors mounted on the aircraft. However, since more received from the sensors, to assess the icing hazard aircraft are exposed to icing in the departure and • An information display system to provide pilots with approach phases of flight, another requirement is to timely, useful maps of the icing hazard.

develop ground-based systems that are capable of scan- The sensing system, which acquires the information ning the airspace around airports. Airport-based sens- to provide a measure of icing potential with distance ing systems are likely to be developed before airborne ahead of the aircraft, must accomplish several tasks. systems because greater technological development has First, it must detect, either directly or through the use occurred with ground-based systems, and because they of surrogates, cloud and precipitation liquid-water con- present fewer weight, power, and size restrictions.

tent, temperature of the droplets (or the existence of It is currently unlikely that one technology will be supercooling), and some measure of the breadth of the able to sense liquid water, temperature, and elements drop-size spectrum. It must map the magnitude of these of the drop-size spectrum, so a remote-sensing system, conditions ahead of the aircraft for avoidance purposes, whether ground-based or airborne, will probably con- and it must locate where icing conditions do not exist sist of multiple technologies to obtain all the necessary ahead of the aircraft for escape purposes if the aircraft information. Technologies under consideration include is immersed in icing. In addition, the location of each radar, lidar, passive microwave radiometers, and radio condition must be measured in three-dimensional space acoustic sounding systems (RASS).

often enough to provide a continuously updated image 5.3 Radar of conditions to the pilot. The system must also scan a Radar is the most mature technology under consid- sufficiently large volume of atmosphere, to a great eration. Initially developed in the 1930s by the British, enough distance and with sufficient detail, to provide it was first used for detecting weather phenomena imme- pilots with avoidance and escape options. Since remote sensing is a stand-off technique, sensing systems typi- diately after World War II (Battan 1973, Toomay 1982).

cally detect and measure phenomena without being Weather radar operates in the shorter wavelengths of immersed them. To effectively assist escape from icing, the microwave spectrum and into the centimeter wave- however, they must also be able to sense that icing con- length spectrum, with longer wavelength (lower fre- ditions do not exist in a volume of air ahead of the air- quency) radars used to detect larger drops such as rain craft while sensing from within icing conditions. and shorter wavelengths used to detect cloud droplets.

A variety of technologies are available for remotely Radar wavelengths for atmospheric sensing can be spec- sensing atmospheric properties (Westwater and Krop- ified by band, frequency, or wavelength (Table 1).

As wavelength shortens, the abifity to detect smaller fli 1989). Current technologies are designed to operate from ground positions to zenith or from satellites to drop sizes improves, but range decreases and attenua- nadir. Some sensors can scan the atmosphere at inter- tion by precipitation and other atmospheric constitu- mediate angles, and some can scan in the horizontal. ents such as water vapor increases as well (Houze 1993).

Table 1. Relationships between radar band, frequency, wavelength, and weather parameter sensed.

Nominal frequency Nominal Meteorological Band (GHz) wavelen_h condition sensed S 3 GHz 10 cm Precipitation (NEXRAD) C 6 GHz 5 cm Precipitation (ships) X 10 GHz 3 cm Precipitation-clouds (aircraft) Ku 15 GHz 2 cm Precipitation-clouds K 30 GHz 1 crn Precipitation-clouds K a 35 GHz 8.7 mm Cloud droplets W 94 GHz 3.2 mm Cloud droplets NASAJCR--2000-209938 31 TheNational Weather Service's WSR-88D Doppler between drop size and backscatter complex and diffi- radar system isSband; this isthemost common wave- cult to predict (Battan 1973, Toomay 1982, Rinehart

length forland-based weather radars because ofitsabil-

1997). In addition, backscatter from ice particles vs.

ity to detect precipitation-size particles (Crumand

liquid drops reverses due to differences in the real part Alberty1993, Houze 1993). C-band radar is a com- of the complex index of refraction between water and

monshipboard radar, and Xband isthemost common

ice, with ice particles providing about 10 times the back-

airborne weather radar. Bands withshorter wavelengths

scatter energy of liquid drops of a given size (Battan than X band suffer fromprecipitation attenuation, but 1962).

theshorter wavelengths may alsoallowdetail tobe

Doppler radar measures the motion of drops or ice

retrieved about precipitation. Mostcloud microphysi-

crystals along the axis of the radar beam (Battan 1973).

calradar workiscurrently intheX,Ka,and Wbands, The fall speed of water drops is related to their size, and

but disadvantages ofthe milfimeter wavelengths include

the fall speed of ice crystals is related to their size and increased absorption by water vapor andoxygen and shape (Rogers andYau 1989). A vertically oriented radar strong extinction by cloud droplets, drizzle, andrain can distinguish drop sizes, and distinguish drops from

(Klugmann and Judaschke 1996). ice crystals, by their fall speed. Shorter-wavelength

Radar detects droplets andicecrystals intheatmo- radars have a greater ability to distinguish between fall sphere because of signal backscatter fromdroplets. speeds and thus are better able to resolve drop sizes.

Backscatter occurs as either Rayleigh orMiescattering. Because of the large volume a radar beam senses, and Rayleigh scattering occurs when droplet diameters are the large number of drops within a given volume of air, significantly smaller than the radar wavelength (Battan a spread of the Doppler spectrum typically occurs, which 1962). After some absorption by the droplets, the makes the Doppler signal difficult to interpret. Causes amount of energy backscattered to the radar is propor- of spread include spread in terminal velocities of the tional to the sixth power of the drop diameter (Battan drops or ice crystals within a sensing volume, turbu- 1962). Backscatter is also inversely proportional to the lence, and wind shear across the radar beam (Battan fourth power of the wavelength. As drop diameter 1973). Although Doppler radar is useful for determining approaches wavelength in size, backscatter increases, drop-size spectra from ground-based radar, it is likely so the strongest backscatter from small droplets occurs that Doppler techniques will be difficult from aircraft- from the shortest radar wavelengths (Battan 1962). At mounted radars because most drops will be fairing ortho- wavelengths greater than 3 cm, droplets of 2-nun diam- gonally to the radar beam, making a Doppler shift less eter and smaller are Rayleigh scatterers, and at wave- detectable. However, turbulence can cause a Doppler lengths of 10 cm, nearly all drops are Rayleigh scatterers shift because small drops, which are more influenced (Battan 1973). Small frozen drops and small ice crystals by turbulence than large drops, can be carried toward or backscatter about 20% as strongly as liquid drops of away from the aircraft, allowing their identification.

the same size (Battan 1962). In all cases, the tempera- The polarization of transmitted and received radar ture of droplets must be determined to evaluate fully energy can be used to determine the mean values and dis- the amount of water contributing to the backscatter, tributions of particle size, shape, and spatial orientation because attenuation from droplets also has a tempera- and to determine their phase(Houze 1993, Zmic 1996).

ture dependency (Battan 1973, Rinehart 1997).

Radar signals may be linearly or circularly polarized.

Attenuation by scattering also increases as wave- Linearly polarized radars transmit and receive energy length decreases, because more energy is scattered by in horizontal and vertical planes, principally because intervening precipitation and cloud droplets. These are falling drops typically shorten in the vertical axis and conflicting factors to consider, because as wavelength lengthen in their horizontal axis (Houze 1993). Differ- is decreased to detect cloud drops, attenuation increases, ential reflectivity and the linear depolarization ratio are which limits range in high drop concentrations and high computed from horizontal and vertical polarization.

liquid-water contents (Battan 1962, Rinehart 1997). Differential reflectivity, the ratio of the horizontal trans- As drop diameter approaches radar wavelength or mitted-to-received energy to the vertical transmitted-to- becomes larger, Rayleigh scattering no longer applies. received energy, typically indicates the oblateness of Instead, Mie scattering occurs, which produces a less falling drops--a measure of drop size. Drops smaller predictable backscatter due to complex interactions than 300 I.tm are typically spherical, but as size increas- between energy reflected within the droplet and energy es, oblateness and differential reflectivity both increase.

waves traveling along the droplet Surface (Toomay Ice crystals typically show no differential reflectivity.

1982). Depending upon the exact ratio of the drop size The linear depolarization ratio, the ratio of horizontally to the wavelength, these reflections and traveling waves transmitted to vertically received energy, indicates how may be additive or subtractive, making the relationship much of the transmitted signal is depolarized. Wet ice NASA/CR--2000-209938 32 was not a satisfactory proven method for estimating

particles produce less depolarization than water drops

rain rate from radar. That situation seems to have

ordryicecrystals do,soitisauseful method forlocat-

changed little; NEXRAD needs to utilize rain gauges ingmelting layers (Houze 1993, Rinehart 1997).

for correction (Crum and Alberty 1993, Houze 1993,

Circular depolarizing radars transmit a signal that

Rinehart 1997).

rotates one complete revolution orthogonal tothe beam

axisperradio frequency cycle(Toomay 1982, Houze

5.3.1.2 C-band radar. C-band radars have also been

1993). The circular depolarization ratioistheratioof

used experimentally to obtain rainfall rates, but again

theparallel (transmitted) component tothe orthogonal

they are too large for aircraft use, though they could be

(received) component andindicates thesphericity of

used at airports (Gorgucci and Sarchilli 1996a,b; Tian

particles. Polarization maybeuseful fordetermining

and Srivastava 1997). Doviac and Zrnic (1984) argue

some elements ofthedrop-size spectra, especially from

that this ability to measure rainfall rates, along with

airborne radar that isscanning drops and crystals ortho-

Doppler detection of wind shear, would be a useful tool

gonaltotheirfallingdirection andthusmaximizing

for predicting freezing rain at the surface and aloft.

shape deformation, or long-axis orientation, to the

horizontal. 5.3.'1.3 X-band radar. X-band radar is small enough

to be carried aboard aircraft and often is. It is useful for detecting precipitation, although with less accuracy than 5.3.1 Detection of liquid water longer wavelengths (Rogers and Yau 1989). X band is 5.3.1.1 S-band radar.The ability to detect raindrop- typically considered a precipitation radar, but it is capa- sized particles may be needed in a remote ice-detection ble of detecting cloud liquid water. For example, Paluch system because freezing rain is a serious aircraft icing et al. (1996), comparing the use of X- and Ka-band threat, although it is typically not considered as dan- radars for detecting cloud liquid-water content, found gerous as freezing drizzle. Raindrop sizes begin at about a close, consistent correlation between radar reflectiv- 500 _m diameter and extend to about 5 or 6 mm in ity and cloud liquid-water content in summer cumulus thunderstorms (Pruppacher and Klett 1997). It is pos- in Florida. However, they indicated that Bragg scatter- sible to experience icing in very large drops near the ing--the susceptibility of longer wavelengths to detect tops of towering cumulus in the tropics or in the mid- "angels" caused by turbulence--was a potentially latitudes in the summer months. However, freezing rain- greater problem with X-band radar (White et al. 1996).

drops will usually be found at the smaller end of the They also indicated that there is typically a strong rela- size spectrum, typically no larger than 3 ram, because tionship between drop size and reflectivity, which is a turbulence is small in most freezing rain (Jeck 1996).

source of error in radar liquid-water measurements. In According to Battan (1973), S-band 10-cm radar can the Florida observations, however, there was a strong successfully estimate rainfall rates, and thus liquid- reflectivity-liquid-water content relationship because water content, for long distances. The National Weather most of the liquid water was concentrated within the Service's NEXRAD, for example, is a 10-cm radar with large end of the drop-size spectrum. This suggests that a range of about 460 km for reflectivity measurements.

broad drop-size spectra will produce a larger rainfall NEXRAD does not provide explicit precipitation liquid- rate estimate error than narrow spectra.

water content at the present time, but it does provide 5.3.'1.4 Millimeter-band radar. Millimeter-wave vertically integrated liquid water from rainfall contained radars, principally the Ka and W bands, are the current in a 4 km by 4 km grid (Crum and Alberty 1993). Thus, choices for detecting cloud microphysical properties and precipitation liquid water may be estimated near air- precipitation (Mead et al. 1994, Kropfli and Kelly 1996).

ports from a ground-based remote detection system.

Since the backscattering cross-section of a drop is Doviac and Zmic (1984) indicate that accurate estimates inversely proportional to the fourth power of the wave- of rainfall rates and rainfall liquid water require a knowl- length, long-wavelength radars, despite their great range edge of the raindrop size distribution, which, if unknown, and power, are at a disadvantage for detecting cloud can cause rainfall rates to differ by a factor of four. In their review of the extensive work that has been done droplets. They can compensate with larger antennas and more power, but at high cost (Martner and Kropfli estimating rainfall rates from radar, they indicate that 1993), and, though they may be useful at airports for spatially detailed measurements of precipitation liquid water require knowledge of drop sizes unless the drop scanning for icing conditions, they do not fit on air- sizes are reasonably uniform. Curvature of the earth craft. Kropfli and Kelly (1996) indicate that millimeter- also causes radar to observe different portions of storms wavelength radars are sensitive to small hydrometeors, have excellent spatial resolution, minimal ground clut- with distance and thus different drop-size distributions.

Doviak (1983), in a review of rain rate estimation ter problems (which allows observation of weakly reflecting cloud with high resolution), and are easily methods using radar, indicated that at that time there NASA/CR--2000-209938 33 portable. However, because ofattenuation inmoderate by stating that they are severely attenuated by rainfall,

toheavy rainfall, they are best used innonprecipitating

but they provide fine-scale measurements of nonpreci-

clouds. Icecrystals can also cause non-Rayleigh scatter-

pitating and drizzle clouds because the amount of energy

ing,which causes lossof signal. If combined witha

reflected by' cloud droplets and ice crystals sharply longer-frequency radar, such asX band, differential increases as wavelength decreases.

attenuation canbeused toextract cloud liquid-water

There have been many applications of Ka-band radar

content (Martner etal. 1991). According toMead and

to measuring cloud microphysics, it appears to be the

hiscolleagues, asof 1994, onlyfiveuniversities and

most useful of the millimeter wavelengths for aircraft

onegovernment laboratory had operating millimeter-

icing. K a band, when compared with the shorter-wave- wave radars. Applications include studying internal length W band, can penetrate optically thick clouds with circulations of cumulus clouds, remotely measuring high liquid-water contents, it can detect clouds above rainfall drop-size distributions, and studying drizzle in light precipitation and multiple cloud layers, and Mie stratus clouds (Mead etal. 1994).

scattering is relatively uncommon (Kropfli et al. 1995).

Bragg scattering caused byatmospheric refraction, White et al (1996) indicate that Ka-band radar can be which is a problem forlonger-wavelength radars, is used to obtain the reflectivity, Doppler velocity, and negligible formillimeter-wavelength radars (Kropfli Doppler spectral width of drizzle drops. Directly related and Kelly1996). However, theshort wavelengths often to reflectivity are drizzle liquid-water content, flux, and restrict theapplicability ofRayleigh scattering. Thus, number concentration. Doppler velocity and spectral forKa-band radar, Miescattering begins forwater drops width provide estimates of the magnitudes and shape

atabout 2.7 mm diameter, and forW-band radar atabout

of the drop-size spectrum. Measurements made during 1mm diameter, which increases the complexity ofinter- the Atlantic Stratocumulus Transition Experiment pretation. Miescattering isaneven more difficult prob- (ASTEX) by Ka-band radar provided liquid-water con- lemforicecrystals and snowflakes, where Mietheory tents from 0.01to 0.14 g m-3, drop diameters from 20 to 320 p.m, and drop concentrations from 0.3 to 700 L -1.

has notbeen developed (Kropfli and Kelly1996). High

humidity is alsoa problem withtheshortest wave-

Kropfli et al. (1995) further demonstrated in ASTEX lengths, such asWband, where itsattenuation effects that Ka-band radar detected ice masses as low as 0.003 g m -3 in cirrus clouds at 7-km range. They also demon- may reduce sensitivity tosmall drops.

Doppler radar inthemillimeter wavelengths excels strated retrieval of integrated liquid water at zenith, as inmeasuring dropfallspeeds because ofitshighfre- may be applied at airports, and compared it with radio-

quency, whichallows highprecision. Doppler tech-

meter-derived integrated liquid water (Martner and niques, as indicated earlier, may behelpful forairport- Kropfli 1993). Radar and radiometer-integrated liquid-

based sensing systems, butthey may notbeuseful for

water estimates compared well except in drizzle.

aircraft-mounted systems unless theyarescanned up

The University of Massachusetts has developed a anddown atlarge angles (KropfliandKelly1996). dual-wavelength, ground-based cloud-profiling radar

Polarization measurements may also beused, especial-

system operating at Ka and W bands (Sekelsky and ly withKa-band radar, todetermine particle shape and Mclntosh 1996). A system with a l-m-diameter anten- orientation (Kropfli and Kelly1996). Forexample, the na is used to make polarimetric and Doppler measure- circular depolarization ratiohasbeen used todistin- ments of Clouds at both wavelengths. The authors state

guish plate-like crystals fromaggregates inColorado

that dual-wavelength millimeter-wave particle sizing

clouds using aground-based Ka-band radar, and future

(drops and ice crystals) has the potential for more accu-

capabilities mayallowdistinguishing between other

racy than single-wavelength Doppler methods. In addi- crystal types.

tion, they indicate that MVD and the shape of drop- Ka-band radar can determine cloud base and top, and size spectra can be determined more accurately with thus thickness, and cloud structure, which may beuse- both wavelengths. They speculate that it also may be

ful foravoiding icing(Politovich et al. 1995) when

possible to discriminate glaciated from liquid clouds

combined witha microwave radiometer capable of

and to estimate particle sizes in fully glaciated clouds.

measuring vertically integrated liquidwater. Vertical W-band radars, operating at about 95 GHz, are

cloud liquid-water profiles may bemapped byadjust-

becoming increasingly popular for cloud microphysics ingdropconcentration to force thetwosignals tofit. work (Mead et al. 1994, Kr0pfli and Kelly 1996). They Kropfliand Kelly (I996)state that,though W-hand offer even better_ize, weightl and power advanta-ges radars are_superior_for airborne Use because:of dleir than Ka-band radars, but they also suffer more severely smaller size, theKaband is lessattenuated by water from attenuation in large drops, humidity, and precipi- vapor and Cloud vcater, making ii more use_I..Kropfli tation. They are used to measure cloud structure and excel at observing drizzle, which is a subject of intense and Kelly (1996) summarize millimeter-wave radars NASA/CR--2000-209938 34 5.3.1.5 Liquid-water content retrieval techniques.

study because it is believed toregulate thethermody-

namic structure andradiative coupling of themarine

5.3.1.5.1 Single-band retrieval techniques. Frisch et

boundary layer(Frisch etal. 1995). Lhermitte (1987)

al. (1995) develop theory and demonstrate, using the

developed theory forW-band radar anddemonstrated

NOAA ETL radar (Manner and Kropfli 1993), the

some ofitscapabilities as aDoppler radar, buthedidnot

retrieval of drizzle and cloud droplet parameters with a measure cloud liquid-water content.

Ka-band radar and a radiometer for measuring integrated

AUniversity ofMassachusetts 95-GHz dual-polarized

liquid water. Doppler techniques were used to measure

radar for ground-based andairborne usefliesonthe

the drizzle drop-size spectra, and drizzle and cloud

University ofWyoming King Airresearch aircraft (Mead

liquid-water content were measured from reflectivity.

etal.1994). It has ademonstrated range of0.1to2.9 km

Cloud liquid-water content was computed by using and has observed 1-to2-mm graupel withsome rimed, Doppler velocities of drizzle and cloud to parse the two

branched crystals, crystal aggregates upto4 mm diame-

liquid-water contents. Overall, by using Ka-band radar,

ter,and needles upto 1 mmdiameter. In a study with

Doppler features, and a simple drizzle model, the

thisradar, a 30-km segment ofshallow stratus produc-

authors were able to extract drop number, size distribu-

ingfreezing drizzle was flown. The radar beam, point-

tion, liquid-water content, and mean liquid-water flux.

ingvertically, observed detailed cloud structure at30-m

They indicate that there is a potential for ground-based

resolution, and radar backscatter was compared tocloud

remote sensors to do long-term monitoring of cloud and

parameters measured within-situ instruments. The radar-

drizzle parameters, such as at airports. This radar sys-

enhanced interpretation ofin-situ measurements was not

tem in a scanning mode, together with a RASS for

itselfused to measure specific cloud physical parame-

measuring temperature profiles, may be an adequate

ters. A similar radar, builtbytheUniversity of Massa-

airport-based system.

chusetts and the NASA Jet Propulsion Lab, the Airborne

Liao and Sassen (1994) have also developed a tech-

Cloud Radar, flies onaNASA DC-8. More than 50hours

nique, although only as modeled theory, that allows

oftesting has occurred in cirrus, stratus, andcumulus

extraction of cloud liquid-water content by linking

clouds, and melting layers have been observed (GEWEX

liquid-water content and reflectivity, assuming a drop 1996).

concentration of 100 cm -3. The model applies for esti-

Inother W-band applications, Klugmann and Judasch-

mating liquid-water content in nonprecipitating cumulus

ke (1996) have developed aW-band Doppler radar in

and stratocumulus clouds. They also developed theory

Germany tomeasure vertical velocities within clouds by

for extracting ice water contents from clouds with

trackingdropspeeds. Clothiaux et al. (1995) also

Ka-band reflectivity.

explored theuseofW-band radar combined withother

5.3.1.5.2 Differential attenuation and dual-band tech-

remote sensors. Theyindicate that,when pointed at

niques. Combining two radar frequencies and analyz-

zenith, W-band radar isvaluable formapping cloud base

ing cloud liquid-water content using differential atten-

and top, butbase isoften indicated as toolowbecause of

uation has become a preferred method of measuring

precipitation. Though methods were notdeveloped to

range-resolved cloud liquid water. According to Mart-

compute cloud liquid-water content from the radar, in-situ

measurements of liquid-water content were compared ner et al. (1993a), using two radar wavelengths with

withtheradar calibration, and calibrations were consis-

significantly different liquid-water attenuation coeffi- cients to measure cloud water and ice content was first tentwithin-situ measurements.

Sassen andLiao(1996) developed theory formeas- theorized by Atlas (1954). Martner et al. (1991) and uring thecontents of iceand water clouds from W-band Gosset and Sauvageot (1992) independently developed field tests and additional theory, concluding that the best

radars. They indicate that formost cloud drops, Rayleigh

scattering applies inWband, and they provide algorithms radar wavelengths were in the X and Ka bands. Sand forcomputing cloud liquidand icecontent fromreflec- and Kropfli (1991) patented the concept. According to Martner et al. (1993a), as the radar beams enter a cloud,

tivities. Fox and Illingworth (1997a,b) computed W-band

the K a band is attenuated more rapidly than the X band.

reflectivity andcloud liquid-water content frommore

Assuming Rayleigh attenuation, the range derivative

than 4000 kmofflightin-situ drop spectra measurements

of the difference of the reflectivities is proportional to

byaircraft. They computed theprobability ofdetecting

the liquid-water content. Both water and ice contribute

various values ofliquid-water content asafunction of

to the reflectivity. However, according to Martner et al.

radar sensitivity. Computations were complicated bythe

(1993b), only the liquid water generates the differen- predominance ofdrizzle drops inmarine stratocumulus.

tial attenuation needed to compute liquid-water content.

They concluded thatahighly sensitive space-based ra-

darcould detect 100% of allmarine and stratocumulus Thus, the attenuation due to ice is the same for both bands--very small.

clouds, butit was notclear if cloud liquid-water content

According to Gosset and Sauvageot (1992), the dual- could bedirectly measured.

NASA/CR--2000-209938 35 wavelength differential attenuation method allows dis- radiometer or aircraft support. The February and March crimination between thesolidandliquidphases in a cases gave highly variable results/Martner et al. 1993b).

nonprecipitating cloud and anestimation ofwater and Measurements were made within upslope clouds in one icecontents. Theattenuation oficeisnegligible com- March case with aircraft-measured liquid-water contents pared withtheattenuation ofwater. Thus, after correc- of up to 0.4 g m-3, a cloud droplet MVD of about 15 tionforthetemperature oftheiceandwater particles _m, and no ice crystals. However, the radar measured and the coefficients ofattenuation ofwater and ice, liquid no liquid-water content. In another case with drizzle, liq- water canbecomputed fromthedifference between uid-water contents were occasionally negative, perhaps thereflectivities. They claim thattheory indicates that because drops fell outside of the Rayleigh regime. In the the technique allows the quantity and location ofsuper- April tests, liquid-water contents of 0.2 to 0.6 g m-3 were measured, but no in-situ verification was available.

cooled water tobeestimated precisely and easily. An

Martner et al. (1993a,b) have evaluated their meas- analysis ofpaired X and Ka bands, and KaandW bands, concluded thattheX- andKa-band pairwasbest for urements extensively. They identify several meteorologi- bothair-andground-based systems because attenua- cal conditions that cause problems and radar deficiencies tionofthe Wband istoohigh. There are problems with that may be responsible for problems. If non-Rayleigh Miescattering, however, if water drops are large. Gosset scattering occurs due to large water drops or ice crys- tals, there is a reversal of the X- Ka-band reflectivity

and Sauvageot (1992) indicated that, withaknowledge

difference trends, producing negative liquid-water con-

ofdrop temperature, thetechnique isuseful in mixed

tents. Measurements of liquid-water content may also

clouds and lightprecipitation. Since liquid-water con-

be particularly poor in regions of large aggregate snow-

tent atanyrange gate isproportional tothelocal range

derivative ofthe reflectivity difference ofthe two radars, flakes and just below the melting layer where liquid- covered ice crystals occur. In addition, false positive and

precise absolute calibrations oftheradar reflections are

notneeded because onlyachange intheirrelative val- negative liquid-water contents can occur at locations ues withrange isimportant (Martner etal.1991). within clouds where large changes in drop size occur-- Martner etal.(1991; 1993a,b) have done themost the boundaries between smaller drops and larger drops.

definitive fieldtesting ofdual-wavelength X-band and For example, if droplets change from small to large, Ka-band radars formeasuring supercooled liquidwater. X-band reflectivity may increase rapidly and Ka band During the1991 NCAR Winter IcingStorms Project does not--producing a large false positive liquid-water content. The reverse can occur as the beams move from (WISP), radars wereinstalled northeast of Boulder, Colorado, and liquid water was measured intermittently large to small droplets, causing a negative liquid-water content.

fromFebruary toApril.A steerable microwave radio-

meter tomeasure integrated liquid water and occasional Clouds composed of small droplets, typically those research flightsthrough theradar beams wereavail- of continental origin, are difficult to detect, especially at able toverify radar measurements. The radars and radio- long range and if liquid-water content is low. Ice crys- meter were scanned at an angle of 7.5 ° above the tals within the clouds can help make them visible, but horizon, and the research aircraft flew up the radar then Rayleigh problems may occur (Martner et al.

beam. Liquid-water content was analyzed with a least- 1993a,b). Higher-power radars may solve the problem.

squares fit between the X- and Ka-band reflectivities Overall, Martner et al. (1993a,b) were encouraged by for a 4-km window of gates (53 gates of 75 m each). the promise of dual-wavelength differential attenuation Computed liquid-water content was assigned to the to obtain cloud liquid-water content. Most of the prob- lems can be solved, and they indicate that more field center range gate of the 53 gates and then was shifted out one gate, and the computation was repeated. This trials should be performed with improved hardware and in more favorable weather conditions (Martner et al.

was repeated for each gate along the beam from 2.0 to 22.6 km from the radar (Martner et al. 1993a). Cloud 1993a).

Foumier (1993) also proposed a dual X- and Ka-band temperature, necessary for liquid-water content com- putations, was either estimated or available from the radar as a terminal aviation weather-sensing system to research aircraft.

estimate cloud parameters at distances from 20 to 30 km. He indicates that the radar would be capable of esti- Test results during WISP were mixed because of radar design deficiencies due to the low-budget nature mating, through differential attenuation, cloud type, visi- of the tests, weather conditions, and inability to obtain bility, wind fields, median drop diameters, ice vs. liquid- water content, and light and moderate precipitation.

complete in-situ measurements of the radar-measured cloud conditions. Seven cases were analyzed from Development of this system did not continue beyond the WISP91, five cases in February and March with radio- proposal stage, however, because of funding cuts at meter and aircraft support and two in April without Transport Canada.

NASA/CR--2000-209938 36 cussed earlier, Doppler techniques may be used to deter- 5.3.1.5.3 Multiple-band retrieval of liquid-water con- tent. Considerations for multiple-wavelength radars mine droplet size from the fall speeds, but the radar must include the use of more than two wavelengths and rela- attain a high angle scan, near zenith or nadir, to obtain a reliable velocity measurement. Since the range of drop tionships beyond differential attenuation. For example, speeds in the drop sizes of interest is small, use of Dop- Jameson (1994) develops theory necessary to measure pler techniques may be difficult. Cloud drops all fall at rainfall rate, rain water content, and mass-weighted mean less than 27 cm s-l, and drizzle falls from about 27 cm drop diameter from satellite and airborne radars using s-l to 2.06 m s-I (Rogers and Yau 1989). Larger drops multiple-wavelength radars. He claims that simple differ- fall faster.

ential attenuation can be used to determine water content.

Polarization relies upon the oblateness of larger drops However, three wavelengths--38, 25, and 13 GHz--are as they fall and distort in shape due to aerodynamic drag.

necessary to measure rain rate, rain water content, and Since drops smaller than 300 p.m in diameter are typi- mass-weighted mean drop diameter. In his summary, cally spherical, drop shape cannot be used to determine Jameson states that two frequencies are needed to meas- ure one parameter, and the measurement of two or three droplet size in the cloud drop range (Pruppacher and Klett 1997). Depending upon the wavelengths, especially parameters requires at least three, and ideally four, fre- quencies. Some of these concepts might be applied to for multiwavelength radars, it may also be possible to measuring icing potential. In a somewhat different use Rayleigh vs. Mie scattering to sort drops by size.

approach, Srivastava and Tian (1996) theorize that two 5.3.2.1 Doppler radar techniques. Thomson and List radars of the same or nearly the same wavelength, but (1996) developed a new method to determine rainfall physically located apart, could be used to improve com- drop-size spectra with a vertically pointing X-band Dop- putations of rainfall through simultaneous use of each pler radar. Errors in raindrop fall speeds with Doppler radar's attenuation and reflectivity.

radar occur from vertical wind, turbulence, pressure A neural-network-based retrieval technique has been dependence of terminal fall speed, and deviations from developed at Quadrant Engineering and at the Univer- Rayleigh scattering. Disdrometer measurements of drop sity of Massachusetts based upon backscatter informa- size during radar measurements in the Canadian Atlantic tion from three radar bands: X, Ka, and W (Mead at al.

Storms Project indicated that the power spectrum of the 1998, Koenig et al. 1999). A neural network was trained raindrop velocities was related to the drop-size spectra.

to estimate cloud temperature, liquid-water content, and Vertical wind effects were removed, and the drop spec- drop mean volume diameters (MeanVD) and mean radar trum was calculated from reflectivity. Good agreement reflectivity diameters (MZD) from the backscatter power between measurements and radar calculations of drop measurements from one, two, and three bands. Range spectra were found in two test cases.

resolution was 2 km, and cloud parameters were syn- Gossard (1994) proposed a method for extracting thetically created for a wide range of conditions found cloud droplet-size spectra information using Doppler in precipitating and nonprecipitating stratiform and radar in the K a band. Doppler radar typically cannot cumiliform clouds. The neural net was trained with detect cloud droplet spectra from fall speed because the 10,000 cases and tested with 200 cases. Temperature settling velocity of cloud droplets allows them to be car- retrievals were not theoretically possible because of radar ried by updrafts and downdrafts. Gossard developed a noise. However, with three radar bands, liquid water was technique, tested on a long-wavelength wind profiler retrieved with less than 0.17 g m-3 error, and MeanVD Doppler radar, that allows extraction of the cloud drop- and MZD with less than 16% error. The neural net is size spectrum by measuring the spectrum shape parame- currently being evaluated with actual cloud information ter independently of updrafts and downdrafts. He pro- to determine prediction accuracy.

poses that the method is effective in detecting drop growth to the drizzle-size range in stratus clouds. The 5.3.2 Detection of mixed.phase conditions method cannot be used in precipitation because the rain- and drop.size spectra drops overwhelm the cloud signal, and there is too much In addition to cloud liquid-water content, detection error in the technique to determine cloud liquid-water of icing conditions requires information about the loca- content, even with no precipitation. The technique may tion of liquid water vs. ice particles, and about the size be applicable at airports with ground-based radars, and distribution of water drops. The latter is particularly it may be useful for airborne radars if high-elevation important for detecting whether drops are within the scanning is performed on clouds above and below the drizzle or raindrop size ranges.

aircraft. Gossard et al. (1997) expand on the technique The technique most useful for detecting some of these and indicate that errors in liquid-water content are pos- parameters, but especially drop size, depends upon sible within a factor of two and that relative liquid water whether the radar is ground-based or airborne. As dis- and rainfall flux through clouds should be accurate.

NASA/CR--2000-209938 37 Lhermitte (1987) uses W-band Doppler radar to raindrops. They conclude that the combination of fall extract information about the rainfall drop-size spectra speed and the circular depolarization ratio will yield more inclouds. Using Doppler velocity measurements ofdrop information than either can individually.

fallspeeds, and Rayleigh and Miescattering, drop sizes

5.3.2.3 Polarization radar techniques. Reinking et

were detected infieldtests inFlorida and Colorado. He

al. (1996) proposed and tested a method of differentiat-

proposes that combining several radar wavelengths, such

ing drizzle from rain and ice crystals using elliptical

as W,Ka,and X bands, would provide thecapability of

depolarization ratios (EDRs) and linear depolarization

detecting thefullrange ofraindrop sizes using Doppler

ratios (LDRs) with the NOAA ETL scanning Ka-band

techniques. Lhermitte (1988) also presents amethod of

radar. Drizzle drops are spherical and do not polarize

backing outairvertical velocities fromraindrop fall

the signals, raindrops are nonspherical and will depolar- speeds to improve measurement of drop-size spectra.

ize the radar signal, and depolarization by ice crystals

Thetechnique relies upon Miebackscattering oscilla-

depends upon their shape and orientation. Scattering cal-

tions around Miescattering maxima and minima caused

culations, and measurements during WISP in 1993, indi-

byraindrops ofvarious sizes (Lhermitte 1987). The tech-

cate that EDR provides a good capability to distinguish

nique would beuseful in providing vertical profiles of

between ice crystals of various habits, drizzle, and rain.

drop-size spectra within clouds and could beapplied to

If the requirement is simply to distinguish drizzle from S-band radars such as NEXRAD.

ice, then LDR is better and could be applied to NWS 5.3.2.2 Doppler and polarization radar techniques.

NEXRAD radars for use at airports. Matrosov et al.

Wilson et al. (1997) developed a method of determining (1996) and Reinking et al. (1997) have further differen- parameters of the drop-size distribution of rainfall to tiated ice crystal types using EDR and LDR with drizzle estimate rainfall rates. This is accomplished, using S- drops as a reference. They are able to discriminate hydro- band radar, by measuring differential Doppler velocity meteor types within cloud systems, which will help deter- (DDV), the difference between Doppler velocities at mine the presence of cloud ice and drizzle.

vertical and horizontal polarization. DDV is indepen- Pazmany et al. (I 994) described the development of dent of turbulence and most shear. It is used with reflec- a new W-band dual-polarized Doppler radar at the Uni- tivity and differential reflectivity to produce a three-para- versity of Massachusetts. The radar flies on the Univer- meter gamma fit to the drop-size distribution. The radar sity of Wyoming King Air, as described earlier, and oper- beams must have elevation angles of 10° to 40 ° for accu- ates pointing either horizontally, along the flight path, rate measurements. The technique is not affected by drop or vertically. Early tests indicated the ability to detect oscillations and can detect ice crystals and determine melting bands and hydrometeor type. Flights with the the location of melting layers. Because of the radar wave- radar in 1992 and in WISP94 provided airborne in-situ length, the technique would be usable only with ground- and radar measurements of snowstorms, needle ice crys- based radars, such as the NEXRAD.

tals, and melting layers. Most observations were made Takahashi et al. (1996) utilized a Doppler and dual- at vertical incidence to clouds above the aircraft. The polarized X-band radar system to detect the drop-size paired radar and in-situ measurements will be used to distribution of rainfall in isolated cumulus clouds in develop relationships between the two.

Japan. Drop-size distribution was computed from ter- minal fall velocities of the drops and the differential 5.3.2.4 Neural net and other radar techniques.

Mead and Pazmany (Mead at al. 1998, Koenig et al.

reflectivity and horizontal polarization, Drop sizes larger 1999) proposed using a three-band radar consisting of than 140 lam were detectable. The linear depolarization ratio and the correlation between the vertical and hori- X, Ka, and W to detect liquid-water content and elements of the drop-size spectrum, using a neural net for post- zontal polarized waves were used to determine mixed- processing the radar returns. This technique, described phase layers, such as the bright band. Two parameters earlier, shows excellent potential for estimating elements of an assumed exponential drop-size distribution were returned by the radar. Measurements made of rainfall in of the drop-size spectrum, but it cannot detect mixed- isolated cumulus clouds were plausible, but no in-situ phase conditions.

verification measurements were made.

Using C-band radar, Huggel et al. (1996) improved estimates of rainfall rates by more accurately estimating In one of the first modern uses of a Ka-band radar, drop-size distribution. They argue that most liquid pre- Kropfli et al. (1982) used a Doppler radar scanning at a cipitation is formed in a bright band where falling ice high elevation angle, and circular depolarization, to dis- crystals melt and coalesce into raindrops. By develop- tinguish falling precipitation forms. Fall speed distin- ing a relationship between reflectivity within the "bright guished graupels, aggregates, and dendrites into five cat- band" and measured drop sizes with disdrometers, they egories within a squall line, and the circular depolari- zation ratio was able to distinguish frozen particles from were able to predict drop-size distributions below the NASA/CR--2000-209938 38 melting layer in moderate intensity rainfall, from1to spheric paths over a frequency range of I to 300 GHz

10mmhr-1,in Switzerland. is performed with RADTRAN or similar radiative trans-

fer models* (Falcone et el. 1982). RADTRAN is a 5.4 Passive microwave radiometers design tool to assess potential environmental impacts on microwave sensors. In effect, it allows reasonably Microwave radiometers operate by receiving thermal complete modeling of the atmospheric radiation envi- energy emitted and scattered by the earth's atmospheric ronment due to gases and clouds, including polariza- constituents (Grody 1997). They are passive instruments, tion, prior to actually building a radiometer.

receiving natural radiation and actively emitting none of their own. Radiative energy is emitted, scattered, and 5.4.1 Detecting liquid-water content absorbed by the atmosphere, and radiometers act as ther- Cloud liquid water and rainfall rates have been mometers to measure a narrow spectral portion of this observed with radiometers over oceans and over land energy. Passive microwave radiometers are used to from the ground, from the air, and from satellites.

measure many atmospheric and surface characteristics, Observing from the ground up to clouds, or from aloft but their ability to measure atmospheric temperature, down to clouds over water, is least difficult because cloud liquid-water content, and attributes of cloud and clouds contrast well with the thermally cold background precipitation constituents such as phase is needed for of space or water surfaces. Sensing cloud water from estimating icing hazards. Microwave measurement is aloft toward the ground is more difficult, as indicated based on the brightness temperature of atmospheric con- above, because of the radiative diversity of land sur- stituents. The radiation intensity observed by a radio- faces. Airport-based radiometers scanning for icing meter is a function of the temperature, reflectivity, trans- conditions will scan upward toward the cold of space.

missivity, and emissivity of the emitter and attenuation The situation may be more difficult for aircraft-mounted by constituents between the emitter and the radiometer radiometers because they will scan ahead, above, and at the specific wavelength of interest.

below the aircraft flight path and, depending upon the Each atmospheric constituent--gas, liquid, and solid- aircraft's flight altitude and attitude, they may be look- ----has a unique absorption spectrum. The atmosphere ing at the horizon, toward space, or toward the earth's in general absorbs in several narrow wavelength bands surface, which could be either land- or water-covered.

and allows radiation to be transmitted through several Thus, sensing cloud water along flight paths may be broad windows due to the atmosphere's gaseous com- considerably more complex and difficult than most position. The primary absorbers are oxygen and water radiometer sensing to date. In general, cloud liquid water vapor. Oxygen absorbs and re-emits in the 50-60-GHz can be sensed with microwave radiometers, but ice is region and at 118 GHz and is used for temperature pro- difficult to detect. Thus, cloud-water sensing is not com- filing. Water vapor has peak absorption and re-emis- plicated by the presence of ice, but the presence of ice sion at 22, 37, and 183 GHz (Grody 1997). Liquid-wa- cannot be used to determine whether cloud water is ter peak absorption and emission occurs near 37 GHz supercooled.

and 89 GHz.

Passive microwave detection of cloud water and pre- 5.4.1.1 Sensing upward from ground. Westwater (i978) reviewed the theory and assessed the accuracy cipitation is practical from the ground because the back- of determining cloud liquid water from upward-look- ground of space has a low brightness temperature, pro- ing radiometers. His analysis yielded a two-wavelength riding high radiative contrast with clouds. Satellites can detect cloud water and rainfall rates over water bodies system, operating at 30 GHz (I .0-cm- wavelength) and 21 GHz (l.4-cm wavelength) to detect liquid water and because water also provides a low brightness tempera- ture, but land masses are warmer and more radiatively water vapor, respectively. Two wavelengths are needed because, ifa single-frequency radiometer is used, varia- complex, making interpretation difficult. Thus, obser- vation direction is more critical for passive radiometry tions in water vapor cause apparent changes in cloud than for radar, because the latter creates the energy that liquid water (Hill 1991b), though Hill disputes the need it observes and so contrast is typically more adequate. for water vapor measurements during the winter (Hill Radiometers are also capable of measuring polar- 1991 a). Westwater indicates that knowing cloud temper- ized energy. Polarization describes brightness tempera- ature would improve the accuracy of water vapor and cloud water estimates.

ture (radiance) along either a horizontal plane between the emitter and the radiometer--vertical polarization, Hogg et el. (1983a,b) describe a dual-frequency or along a plane orthogonal to that path---horizontal polarization. Cloud and precipitation drops are usually * Statement madeby T.Lines, Raytheon Corp., Denver,Colorado, at Inflight Remote Sensing Icing Avoidance Workshop, Meteorologi- vertically polarized.

cal Panel, 2 April 1997.

Analysis of microwave attenuation for typical atmo- NASA/CR--2000-209938 39 radiometer atNOAA ETLthat measures both integrated icing pilot reports. Westwater and Kropfli (1989) cite water vapor (20.6 GHz)and cloud liquidwater (31.6 Fotino's work as one among several demonstrating the GHz). Accuracy wasdetermined by comparing with utility of scanning microwave radiometers at airports liquid-water estimates derived bymicrowave transmis- for detecting aircraft icing conditions. They stress the sions froma COMSTAR satellite through clouds. A importance of the 2 I-GHz and 3 I-GHz frequencies for scatter plotcomparing the twomethods showed anearly measuring integrated liquid water. They demonstrate 1 :1relationship, withscatter in the 1:1relationship how the radiometers can be used to continuously map increasing as liquidwater increased. They also demon- integrated liquid water, and they indicate that 90.0 GHz

strate azimuth scans oftheradiometer atanelevation

is also a frequency that can be used to detect liquid angle of 12.5 °, showing liquid-beating cloud fluctua- water. It is six times more sensitive than 31 GHz, and

tionsdepending upon theantenna direction. In heavy

can measure integrated liquid per unit area ranging in rains, such as 80mmhr -l, the radiometer saturates due magnitude from 0.03 to 5.0 ram, which translates to toexcessive radiation. Signals were not affected bywater 0.06 to 10.0 g m-3 of cloud water. This higher sensitiv-

or snow ontheantenna. Theauthors suggest thatan

ity is confirmed by Grody (1997), who indicates, how- airbome system could scan from the zenith toanyangle ever, that scattering from precipitation droplets is a seri- ous problem at 85-90 GHz.

forward along thelineof flight,suggesting thathori-

Hill (1991a, 1992) made some of the first measure- zontal sensing may bepossible.

Gary (1983) describes amicrowave system designed ments comparing radiometer-measured supercooled tomonitor aircraft icingconditions thatwasdemon- liquid water with in-situ aircraft measurements. The strated atBuffalo International Airport. Temperature pro- Utah State University radiometer, a copy of the NOAA files, water vapor, and cloud liquid water were meas- ETL scanning radiometer discussed above, operates at 20.6 and 31.65 GHz. Field tests were done at Sodus ured with radiometers. Water vapor and liquid-water measurements were made from radiometers operated Point, N.Y. A research aircraft carrying a Rosemount at 22.23 and 31.7 GHz, respectively. Water vapor and ice detector to measure supercooled liquid water was temperature were verified with radiosondes, but intended flown in ascending and descending spirals centered over overflights to verify liquid water did not occur. How- the radiometer. Measured liquid-water contents were ever, forecasts of aircraft icing and pilot reports of icing low, near the limits of the radiometer resolutions, so did compare well, suggesting that the liquid-water meas- completely valid comparisons could not be made. Seven urements were reasonable. This study is the first example validation tests were made, with two producing spuri- of a system explicitly designed and tested, using remote- ously high readings by the radiometer, potentially attrib- utable to cloud-entrained snowfall that did not adhere sensing devices, for detecting aircraft icing conditions.

A workshop about remote detection of aircraft icing, to the ice detector and a melt layer along an inversion.

sponsored by the University of North Dakota (Smith Hill (1991b) also compared the ability of a dual- 1985), concluded that passive microwave radiometers frequency radiometer, the Utah State instrument cited would be useful for determining cloud liquid water. above, and a single-frequency radiometer operating at However, since radiometers only provide integrated 31.65 GHz to measure cloud water. A dual-frequency liquid water, cloud top and base would also have to be unit corrects cloud water content by accounting for measured to provide an estimate of cloud liquid-water changes in water vapor. However, according to Hill, content in mass/volume units. They recommend that these changes are very small during the winter and intro- radiometers be used in scanning mode but indicate that duce little error if ignored. A comparison between the scanning is slow and would take about 5 min per 360 ° two radiometers during winter tests indicated a nearly scan at one elevation angle. A solution to this may be to 1:1 relationship, suggesting that the winter water vapor use multiple radiometers, each scanning assigned sec- correction is not needed. This would simplify cloud- tors and elevation angles. water measurements from aircraft, reducing hardware Fotino et al. (1986) related radiometer-derived zenith and computational requirements.

measurements of temperature and liquid water near Stankov et al. (1992) describe the use of four micro- Denver to pilot reports of icing. Frequencies of 20.6 wave radiometers to measure supercooled liquid water GHz and 31.65 GHz were use to measure water vapor in the Denver and Boulder area during WISP91. Though and liquid water, respectively. Measurements were inte- the radiometer estimates of liquid water were not explic- grated over 2-min periods. Comparisons with pilot itly compared with aircraft measurements, they con- reports were difficult because they are often inaccurate clude that the radiometer's ability to measure liquid in time and location, and pilots avoid icing upon hear- water was excellent. The only reported problem with the radiometers involved keeping snow from adhering ing reports of the conditions. They found overall strong correlations between the radiometer measurements and to the sensor windows.

NASA/CR--2000-209938 40 Martner etal.(1993a), aspart oftheLake Ontario theproduction of icecrystals andwater in risingair.

Winter Storms project, installed Doppler radar, wind The process operates onlyinclouds withupdrafts. Air

temperature andDopplerradarmeasurements of

profilers, and microwave radiometers tomeasure inte-

updrafts areused todescribe therate of production of

grated liquid water onthe eastern shore ofLake Ontario.

water vapor inexcess ofsaturation withrespect toice.

A freezing rainstorm on15February 1990 was contin-

The radar reflectivity isinverted todetermine the profile

uously monitored bythe microwave radiometers at20.6,

of ice-particle sizedistribution. From theice-particle

31.65, and90.0GHz.Vertically pointing scans and

size distribution, therate thatwater vapor can becon-

scans ata 7.5 ° elevation angle were made. Theradio-

meter could notobserve snow and ice,butit immedi- sumed bypuredeposition is determined. Thediffer-

ence between therate of water vapor production and

atelysignaled theonset of melting aloftatthebegin-

ningof liquidprecipitation. Some overestimates of deposition onicecrystals is theproduction of super- liquidwater may have occurred because water-coated cooled liquid water ateach level within the cloud. This snowflakes appeared tothe radiometer tobecompletely technique was simulated, butnofieldstudies h.ave been conducted todemonstrate itsviability.

fiquid water as they melted. Inaddition, high emission

Solheim and Godwin (1998) describe thedevelop-

during rainfalloccasionally saturated theradiometer

ment ofapassive microwave radiometer thatprofiles

signal. The radiometers were valuable fortheirability

toobserve theimmediate onset offreezing precipita- atmospheric temperature, water vapor, and liquid cloud tionaloft.

water. Operating attwofrequencies, the radiometer pro-

fileswater vapor byfrequency scanning near 22GHz

Huggins (1995) describes the useofadual-wave-

and temperature and liquid-water profiles byscanning

length radiometer system (20.6 and 31.65 GHz), similar

near 60GHz. Initialuse demonstrates radiometer pro-

totheETLsystem butmounted onatruck formobile

filesusing avariety ofretrieval algorithms compared

use. Thesystem was used tomonitor the spatial distri-

withradiosonde profiles. Theinstrument, although it

bution ofliquid water over the Wasatch Plateau ofUtah

needs additional testing, holds promise forproviding

forcloud seeding. Aunique spinning mirror system was

temperature andliquid-water profiles atairports to

used tokeep snow and rainoffthe radiometer mirror.

assess aircraft icing conditions.

Mostmicrowave radiometers measure integrated

Theeffects of rainfallonintegrated cloud liquid-

liquidwater; they provide noindication ofthedistribu-

watermeasurements alsodemand attention because

tionofliquidwater vertically through acloud. The dis-

rainfall causes cloud liquidwater tobeunderestimated

tribution ofliquid water within acloud layer, and among

in the20-90-GHz range. Sheppard (1996) examined

layers if there ismore than one layer, isneeded foraccu-

theeffect ofrainfall rate onvertically pointing micro-

rate aircraft icing estimates. Stankov etal.(1995) melded

waveradiometer measurements of integrated liquid

climatological anditerative techniques thathad been

water. Usinga radiative transfer model, theamount

used unsuccessfully in thepast intoa new technique

of errorin estimated cloudliquidwaterincreased

using Ka-band radar, windprofilers, andceilometers.

withrainfall rate. Thetheory wastested against mea-

Their goals were todetermine cloud base height, cloud

surements made in CASP. Sheppard indicates thata

top,and cloud layers. Then, using temperature profiles

futurestudyshould investigate theeffects of solid

and three different assumptions about thedistribution

precipitation.

of water withinclouds, theyparsed integrated water

measured bythe radiometers through the clouds. These

5.4.1.2 Sensing from satellites or aircraft over

experimental techniques were compared withaircraft

water or land. Passive microwave sensing of cloud measurements aspartof WISP. Results weremixed: liquid water from a satellite or aircraft over land or water

some profiles compared wellwithaircraft measure-

is more difficult than viewing toward zenith from be-

ments, andothers compared poorly withinthesame

low clouds. When observing toward the earth's surface,

storm. Stankov and hiscolleagues indicated thatmore

as explained earlier, the brightness temperature of water comparisons are needed. This work demonstrates afun- and land surfaces must be considered. When viewing

damental need formethods ofdistributing liquid water

toward space, background temperature is near absolute

through clouds aftertotalintegrated values aremeas-

zero (~3°K) and clouds emit strongly in contrast. When

ured.Thisis important for determining theconcen-

viewing toward water bodies from above clouds, the trations ofsupercooled liquid water ineach cloud profile.

water bodies appear cold, with brightness temperatures of about 140°K at 37 GHz (this can vary by 15°K or

Sauvageot (1996) has developed amethod forcon-

more), because the emissivity of water bodies is about

structing vertical profiles of liquidandicewater in

0.5 (Jones and Vonder Haar 1990, Greenwald et al.

mixed-phase clouds using amicrowave radiometer to

1993). Land surfaces typically have larger brightness

determine total integrated liquidwater, aDoppler radar

temperatures than water because land surfaces are often

todetermine updraft velocities, and theory toexplain

NASAJCR--2000-209938 41 effects prior to sensing clouds. A two-stage algorithm,

warmer, theiremissivity is close to 1,surface rough-

ness and vegetation contribute tovariability, and, most developed by Jones and Vonder Haar (1990), estimates importantly, soilmoisture varies widelyandradiates ground transmittance on clear days, and then, after clouds strongly (Jones and Vonder Haar 1990). This larger and move over, removes the effects of ground transmittance.

more variable emmitance overland requires different Greenwald et al. (1997b) further advanced the technique retrieval techniques than overwater. Theabove expla- by using the polarization differences of the brightness nation alsosuggests why,in principle, sensing cloud temperature effective over some land surfaces. The ad- liquidwater fromtheground should be,and is,more vantage of polarization is that it improves the ability to accurate, withvalues typically beingcorrect within estimate the liquid-water content of low-lying clouds.

15%, as demonstrated byHill (1992). Comparisons with ground-based radiometer measure-

ments in Colorado were generally good. This method 5.4.1.2.1 Liquid-water retrieval over water. Retrieval would be difficult to apply to in-flight detection of cloud over water is relatively direct, and techniques for retriev- liquid water because a priori ground radiance informa- ing cloud liquid water over the oceans have been in use tion would be difficult to obtain.

for about 20 years with some of the original techniques developed by Grody (1997). Schemes to retrieve liquid 5.4.1.2.3 Liquid-water retrieval from the horizontal.

water from microwave emissions are statistical, semi- Savage et al. (1999) have developed a technique for statistical, and semiphysical (Greenwald et al. 1993).

locating and estimating cloud liquid-water content using Greenwald and his colleagues developed a simple physi- two frequencies, 37 and 89 GHz, and three viewing cal technique that is accurate and has been rigorously angles. A radiometer placed on the nose of an aircraft verified, unlike many other methods. Verification was would scan horizontally ahead of the aircraft and 2 ° against ground-based microwave radiometers looking above and below the flight path. In a clear-sky condi- skyward detecting cloud liquid water at four oceanic tion, the +2 ° beam sees colder temperatures, observing locations. Relative errors in the algorithm range from toward cold space, than does the -2 ° beam observing 20 to 40%, and occasionally to 50%, with the largest toward the warmer surface of the earth. As the aircraft errors in areas with low fiquid-water content and thin approaches a cloud, the temperature of both beams con- clouds. The algorithm is also only valid for nonprecipi- verges toward that of the horizontal beam. During this tating clouds, because liquid precipitation radiates process, the horizontal beam provides an estimate of strongly. Frozen precipitation does not cause problems the cloud temperature. An estimate of liquid-water con- because ice is typically not visible at microwave wave- tent magnitude is obtained by comparing the bright- lengths.

ness temperatures of the 37- and 89-GHz beams in the Lee et al. (1994) applied the Defense Meteorological +2 ° orientation. Since the 37-GHz beam penetrates far- Satellite special sensor microwave/imager (DMSP ther than the 89-GHz beam, it will be colder than the SSM/I) in an attempt to predict aircraft icing over ocean 89-GHz beam if there is little liquid water, because it areas. They utilized a statistical retrieval algorithm to can detect the cold of space through the water. As liquid- convert transmittance to cloud water using the 37-GHz water content increases, the +2 ° 89- and 37-GHz bright- and 85-GHz bands. Though heavy precipitation con- ness temperatures converge as cold space is obscured.

taminates 37-GHz retrievals, according to Lee and Clar- Savage et al. (1999) also believe that the presence of ke supercooled water and significant precipitation are drizzle-size drops can be detected by sensing polarized mutually exclusive and thus reduce the magnitude of radiation scattered from the earth's surface by large the problem. Most analyses, however, were performed drops. The Savage techniques are being evaluated using with the 85-GHz band because it has higher resolution.

information gathered during the MWISP field project.

Aircraft icing was expected when liquid water was These examples of microwave radiometer capabili- greater than 0.2 kg m-2 and temperatures were between ties over land and water surfaces represent only a small 0°C and -20°C.

portion of all work accomplished, but cloud water 5.4.1.2.2 Liquid water retrieval over land Microwave retrieval over land is new and is not yet available opera- retrievals over land are possible if emittance from the tionally.* Precipitation retrieval from satellites over land surface can be accounted for. In addition, the water and land has been developed even more than cloud water retrieval; it is presented in papers by Spen- retrieval wavelength is changed over land to 85.5 GHz, which is more sensitive to cloud liquid water than are cer et al. (1989), Petty and Katsaros (1992), Vivekanan- other microwave channels, and surface effects become dan et al. (1993), and Ferraro and Marks (1996).

less important as cloud liquid water increases and atmo- spheric attenuation obscures the ground at this frequency.

Jones and Vonder Haar (1990) and Greenwald et al.

* Personal communication, J. Vivekanandan, National Center for (1997a) developed methods of subtracting surface Atmospheric Research, Boulder, Colorado, 1997.

NASA/CR--2000-209938 42 to icing, they may be penetrated, but the hazard is that 5.4.2 Drop-size spectra and cloud phase clouds beyond cannot be sensed if there is no cloud- The only research found that described detection of free space ahead of them, so aircraft flying IFR cannot cloud drop size with microwave radiometers was that use lidar to maximum advantage. Lidar could indicate of Savage et al. (1999), described above. Generally, to night VFR aircraft whether cloud lies ahead, and it cloud ice particles cannot be detected by microwave could indicate whether liquid precipitation that could radiometers because ice is transparent to microwaves.

be freezing also lies ahead below clouds, giving the However, Wu (1987) applied four channels of NASA's Advanced Microwave Moisture Sounder, flown on aircraft advance warning. Thus, lidar appears to have the greatest utility to night VFR pilots who have multi- high-altitude aircraft, to the problem of detecting the pie reasons to avoid clouds and may need to avoid pre- ice-water content of clouds in the microwave frequen- cipitation. Lidar is of greatest potential utility for avoid- cies of 92-, 183- (+2), 183- (_+5), and 183- (+9) GHz bands. A microwave radiativex transfer routine was ing icing and of least potential utility for escaping icing.

Lidar can operate over a range of wavelengths, it developed that allows detection of the ice-water con- centration through mixed-phase clouds by observing can be polarized, and it can be used in single-scattering the changes in brightness temperature of each frequency. and multiple-scattering modes where multiple-field-of- view lidars can utilize the information. Its most typical Some success was claimed by comparing computed ice- water contents with observations made in the near- cloud uses are for determining liquid-water content, infrared during the Cooperative Convection Precipita- phase, drop number, mean drop size, and optical thick- ness. Lidar is also widely used to determine ceiling tion Experiment. The study also showed that cloud height, though there is often difficulty with optically brightness temperature at each frequency depended not thin clouds, virga, and precipitation.

only on total ice-water content of a cloud, but also on its distribution within the cloud. Further developments Using an infrared-wavelength CO2 lidar operating at 10.6 lam, Eberhard (1993) developed theory and of Wu's technique have not been published in the last demonstrated retrieval of the mean radius of cloud drop- decade, so it is not clear whether the technique is fully size distributions. The lidar determines the extinction- viable.

to-backscatter ratio, which is then fitted to a variety of 5.5 Lidar expected drop-size distributions until a fit is obtained.

The method is valid for distributions with drop sizes Lidar, or light detection and ranging, is the optical falling between 1 and 17 ktm. Data from fair-weather equivalent of radar, operating in the visible and infra- cumulus clouds at Cape Kennedy provided reasonable red wavelengths. Unlike radar, however, wavelengths results, although no in-situ measurements were avail- used for lidar suffer rapid extinction in optically thick able for validation. Eberhard indicated that an 11-Jam clouds, so their use for sensing cloud properties is lim- wavelength may provide better results.

ited. Cloud scattering rapidly attenuates the signal, pre- Bissonnette and Hutt (Hutt et al. 1994; Bissonnette venting most lidars from penetrating dense clouds for and Hutt 1995a,b) at the Defence Research Establish- more than a few hundred meters. However, multiple ment at Valcartier, Quebec, Canada, have used the back- scattering of lidar returns from clouds can be used to scattered power from a 1.06-pro multiple-field-of-view advantage for interpreting elements of cloud composi- tion. (MFOV) polarized lidar to characterize cloud, fog, and aerosols. The system measures the backscatter from a Overall, lidar may be able to contribute to aircraft central beam with 1.5-m-long pulses and multiscattered icing avoidance by remotely sensing cloud conditions, but only in very specific and limited ways because of return signal intensity at three or more coaxial fields of view with a maximum of ten possible fields of view.

the extinction problem. An ideal supercooled liquid- The amount of scatter returned is proportional to the water sensor will range-resolve liquid water and drop size many kilometers ahead of an aircraft, even if the number density of drops in the cloud. Fitted to a multiple aircraft is flying within clouds. A lidar operates effec- scattering lidar equation, the measurements provide a tively only when the aircraft is flying within a nearly scattering coefficient and a droplet effective radius.

From this, and an assumed gamma drop-size distribu- cloud-free atmosphere. Lidar can sense through cloud- free atmosphere to the nearest clouds and determine tion, liquid-water content and extinction coefficients the properties of the first few hundred meters of those are computed. Range-resolved droplet size distribution (1 to -100 lam), liquid-water content to 1.0 g m-3, and clouds, but it cannot penetrate them to determine what extinction coefficient measurements have been made lies beyond. Therefore, lidar can only help aircraft avoid icing conditions by determining whether there are at ranges to 1000 m and verified in situ (Bissonnette et al. 1998). The linear polarization ratio, between paral- clouds in the immediate fright path and if they are con- lel and perpendicular polarization, is about 35 to 40% ducive to airframe icing. If the clouds are not conducive NASA/CR--2000-209938 43 when clouds arecomposed prinaarily of icecrystals.

liquid-water content and drop-size distribution. How-

The1.06-1am-wavelength isnoteye-safe, butaneye-

ever, because Raman systems are large and expensive

safe1.54- or2.0-_tm lidarcould bebuiltwith1.5- to

and can only be used at night, their application to air- 3.0-m resolution anda 100-Hz pulse repetition rate.

craft icing is unlikely.

Temperature atthecloud could also bedetermined by

In general, lidars have limited use for remotely ranging withthe lidarand sensing temperature withan detecting aircraft icing conditions because of their lim- IRradiometer. Evidence ofsupercooling could also be ited ability to penetrate clouds. Many ground-based and assessed using polarization techniques todetermine if airborne lidars are currently used for boundary-layer thecloud is mixed phase. research and wind-shear monitoring (Target al. 1991, Benayahu etal. (1995) developed a method for Hannon and Henderson 1995). Canadian organizations retrieving drop number and drop-size distribution from interested in aircraft icing have proposed a remote- clouds, and potentially liquid-water content, assuming sensing system for detecting icing conditions that may multiple scattering occurs atalltimes. A totalscatter use lidar as a principal component (EWA 1996).

signal andmultiple scattering signal, simultaneously Therefore, though not as promising for penetration of received from twoseparated receivers, contained infor- clouds, lidar does have demonstrated capabifity and may mation about theshape ofthe drop-size distribution and be a candidate technology for limited and specialized the mean droplet radius. Afieldtestwas conducted on applications.

amarine stratus cloud offthe coast ofIsrael withconcur-

rentlidar and in-situ aircraft measurements, withgood 5.6 Temperature measurement

agreement between themeasurements. Drop temperature, or a surrogate for drop tempera-

Eberhard (1995), working withaCO2 laser, argues ture, is necessary to determine if liquid cloud water is thatdepolarization in longer wavelengths cannot be supercooled. In most cases, drop temperatures will be used todiscriminate icefrom water inclouds. However, nearly the same as the air temperature, neglecting radi-

theratioof backscatter between twodifferent wave-

ative exchanges, but droplets can be considerably dif- lengths canindicate thepresence ofice.Seven wave- ferent from air temperature in several situations. Snow lengths between 10.4 and11.5 /am were selected and falling into warm air, such as in overrunning, will melt.

paired to test the theory, and the backscatter ratio The temperature of snow and ice crystals will rise to between water and ice was shown to range from 2 to 5 O°C until melting is complete, and then they will con- depending upon the frequency pairs chosen. The method tinue to warm. These drops may then fall into colder air below and remain warmer than the air until conduc- demonstrated the feasibility of detecting ice vs. water without polarization and estimated an approximate pro- tion, convection, radiative exchange, and evaporation portion of ice vs. water. No field tests were conducted cools them to the dew-point temperature.

to verify the method, though they were planned. The There likely will be little choice whether to measure technique is simple and eye safe. drop temperature or air temperature, because some sens- When a laser beam is incident upon water droplets, ing technologies may preferentially sense water drops most of the energy is scattered away from the drop with- or ice crystals and others may preferentially sense air out change but scatters a small portion of the light at temperature. A surrogate for air-temperature measure- different wavelengths. The scattering of light at differ- ments is the detection of ice crystals. If a cloud is mixed ent wavelengths is Raman scattering, actually an phase, then it is likely that any liquid water is super- exchange of energy between a photon and a molecule cooled. Ice crystals may be detected by radar and lidar.

with a resulting change in the energyiand thus wave- Ice detection methods were discussed above, so they will not be dealt with here.

length---of the photon (Carey 1987). Raman scattering has become a useful technique for atmospheric sensing There are at least four types of sensors for measur- of nitrogen, oxygen, and water vapor and for tempera- ing temperature profiles: ture profiling. Melfi et al. (1997) report on a potential • Microwave radiometers use of Raman scattering for measuring the liquid-water • Infrared radiometers content of clouds. During water-vapor measurements • Radio acoustic sounding (RASS) with a XeF laser centered at 0.35 _tm, they detected • Raman lidar.

two thin cloud layers as they passed over the lidar. Melfi et al. (1997) indicate that Raman lidar techniques hold Radiative techniques are infrared and millimeter promise as a new method for remotely measuring cloud wave, acoustic systems are based on tracking the speed of sound by radar, and lidars detect Raman scattering.

The purpose of this review is to indicate how tempera- * Personal communication, L. Bissonnette, Defence Research Estab- lishment, Valcartier. Quebec, Canada, 1997. ture is measured remotely in the atmosphere and to indi- NASA/CR--2000-209938 44 square error (rms) in the lowest 3000 m of the atmo-

cate which methods may bemost viable forremote sens-

sphere. Radiometers still do not have sufficient accu- ingofaircraft icingconditions.

racy or resolution with height to replace radiosondes.

5.6.1 Infrared radiometers When comparing the accuracy of radiosondes, RASS, and radiometers for temperature profiling, Schroeder Infrared radiometers are not sounders; temperatures (1990) determined that the three devices compared well obtained are an integration of the cloud boundary layer, in the summer when temperature changes with height were depending upon cloud optical depth, and the depth from not rapid, but during the winter the RASS clearly pro- which contributing radiating energy originates. Exam- vided better resolution.

ples of infrared thermal imagers are 10.8- and 11.8-p.m Decker et al. (1978) constructed a radiometer system sensors of the NOAA Advanced Very High Resolution operating around 60 GHz that scanned to an elevation Radiometer used to determine cloud-top temperatures angle of 45 ° from zenith. They indicated that temper- (Giraud et al. 1997, Lee 1997). Bissonnette* suggests ature retrievals are possible from elevation angles other that cloud temperature could be determined in a stand- than zenith, though they chose not to report on them.

off situation using an infrared radiometer and range The Solheim and Godwin (1998) microwave radiometer determined by lidar. This method would not be viable discussed earlier profiles temperature, water vapor, and within cloud, however, because the temperature pro- liquid water at zenith and at low elevation angles.

vided would represent an integration of the cloud mass Gary et al. (1992) report on an automatic tempera- only a few hundred meters ahead of the aircraft, depend- ture profiler operated on the NASA ER-2 high-altitude ing upon cloud optical depth.

aircraft. The radiometer, operating at 60 GHz, scans in 5. 6.2 Microwave radiometers 10 angular steps from 50 ° below the flight path to 60 ° above the flight path ahead of the aircraft. Brightness Microwave temperature sounders detect temperature temperatures from 15 distinct altitudes span from 2 km changes with altitude by either sensing from the ground below the aircraft to 3 km above it. Temperature retrieval to zenith or from satellites to nadir. Oxygen absorbs and thus re-emits in the 60-GHz region (+10 GHz) and at accuracy is a function of distance from the aircraft, with I 18.75 GHz. About 45 absorption lines centered on 60 greater distance causing greater uncertainty. Measure- ments are not range gated. However, as the aircraft flies, GHz are used to determine temperature with height in if scanning is rapid enough, temperatures measured sev- the atmosphere by pressure broadening (Grody 1997).

eral kilometers ahead of the aircraft can be assembled The absorption lines found in the + 10 GHz region around to create a composite temperature map. Such maps have 60 GHz result from decreasing atmospheric pressure been assembled by the authors to create altitude tem- with altitude (Elachi 1987). Oxygen molecular collisions perature profiles and horizontal profiles of temperature.

are frequent enough that at given pressures they reach a This profiling technique may be promising for detect- local thermodynamic equilibrium. This results in a shift ing temperature ahead of aircraft in an icing environ- in the wavelength of emission around 60 GHz as pressure ment. It deserves further exploration, but the instrument's changes with altitude, with lines of maximum emission capabilities within clouds are unknown.

shifting closer to 60 GHz with altitude. That is, the spec- trum of wavelength from which oxygen emits radiation 5.6.3 Radio acoustic sounding systems (RASS) near 60 Hz narrows as pressure decreases with altitude.

RASS is used operationally by NOAA; it operates As a result, temperature can be retrieved with altitude by by directing acoustic waves, typically at 900 Hz, verti- using the expected wavelengths of emission of oxygen cally into the atmosphere (Schroeder 1990, Matuura et al.

at given altitudes. Measurements are typically made on either side of 60 GHz. 1986, May et al, 1989). Compression and rarefaction by the sound wave alters the air's dielectric constant, allow- Microwave sounders routinely retrieve temperatures ing radar reflection. A strong reflection is obtained when with height (Westwater and Grody 1980, Westwater et the acoustic signal is matched to half of the radar wave- al. 1983, Askne 1987, Gary 1989, Solheim and Godwin length, creating Bragg scattering (May et al. 1988).

1998). The primary problem with satellite and ground- NOAA uses 404-MHz Doppler radar to track the acous- based radiometers is their inability to detect rapid tic wave, the same radars that are used for wind profiling changes in temperature with altitude, such as inversions, (Westwater 1997).

in the lower few kilometers of the atmosphere (West- RASS is generally immune to cloud effects, but there water 1997). However, overall accuracy of ground-based are other sources of error. Humidity changes the speed radiometers is typically better than 1.2°C root mean of sound and, if not considered, can cause errors of up to 2.2°C, and vertical wind velocities of 3 m s -1 can * Personal communication, L. Bissonnette, Defence Research Estab- produce a nearly 7°C temperature error (North et al.

lishment, Valcartier. Quebec, Canada, 1997.

NASA/CR--2000-209938 45

1973). The NOAA windprofilers arecapable ofmeas-

Though research and development may be accom-

uring temperature accurately inhorizontal and vertical

plished through partnering, purchasing, and in-house

windby averaging overa longtimeperiod, about 6

work, managers must acquire expertise to understand

min,and byusing multiple acoustic sources. Vertical

and properly direct activities. They must have the under-

wind errors have been experimentally reduced toas low

standing, focus, and comprehensive vision to complete as0.1 °Cusing 6-minaveraging times (Angevine and the project.

Ecklund 1994). Overall RASS accuracies arecompar- Adapt means to take technology and skills and adapt able toradiosondes, withoverall error consistently about it to needs (Deffeyes 1996). A full understanding of the 1 °Crms, withaltitudes of3.5kmaglreached 50% of needs, operations, environment, and technologies avail- thetime(Westwater 1997). Experimentally, RASS able allows developers to adapt existing or developing

measurements have been made to15kmaltitude and

technologies and techniques to the requirements of the more (Matuura etal. 1986).

product. This means, for example, adapting microwave RASS iscertainly aviable technique forsensing tem- radiometers or differential attenuation radar to operate perature from the ground during icing conditions around on aircraft with other sensors as a system to satisfy the airports. Itsonlylimitation atairports may beanocca- icing information needs of operators and pilots.

sional inability toreach needed altitudes. Use ofRASS

Adopt means to acquire new ways of thinking and in airborne applications, especially sensing ahead of of doing business (Deffeyes 1996). This will be neces- the aircraft, has been assessed tobeimpractical because sary for system developers, as well as for regulatory

ofproblems withaircraft pitch and yaw and cross winds

bodies, manufacturers, operators, and pilots. Adopting causing loss ofsignal (Mead etal.1998).

new techniques may improve efficiency and safety, but it requires willingness to change. For example, FAR 5.6.4 Raman lidar 25, Appendix C, has been the standard for aircraft design Raman lidar techniques, discussed above, are also criteria in icing conditions. As new information is used to measure atmospheric temperature profiles (Gill acquired characterizing the icing environment, users et al. 1979, Evans et al. 1997). Raman lidar uses a variety of Appendix C may be required to change the range of wavelengths, with examples at 0.55 and 0.35 Bm of conditions for certifying aircraft for flight in icing conditions.

(Evans et al. i997, Vaughan et al. 1993). Raman lidar is typically operated only at night because the signal is A logical initial location to provide icing protection overwhelmed by solar radiation. Long integration times using remote-sensing technology is at airports. The need of 10 min are often necessary to obtain accurate temper- for ice protection is greatest in the approach and depar- atures (Evans et al. 1997). In addition, it suffers from ture phases of flight because aircraft are operating at the typical extinction problems suffered by all lidars in lower altitudes, lingering in conditions for longer clouds. As a result, Raman lidar would not be practical periods at slower speeds, and operating closer to maneu- for ground or aircraft-mounted remote sensing in icing vering limits. Protecting airports would also provide conditions.

the greatest benefit for the least cost.

There are few known remote temperature-measuring The ability to sense cloud microphysical properties methods suitable for operation from airborne platforms remotely from the earth's surface is mature in some in icing conditions. The best possibilities lie with scan- technology areas. For example, RASS is a mature tech- ning microwave radiometers, but they scan slowly and nology for measuring temperature profiles in the lower may be difficult to use from a moving platform. Lidar atmosphere. Passive microwave radiometers are mature, methods are not practical, and infrared radiometers but have less resolution than RASS for measuring tem- operated in the 3.8- to ! 6.8-!am region have a short range perature profiles from the surface to midtroposphere in clouds because of reduced optical depth. For ground- altitudes. Integrated liquid-water measurements may be based systems, RASS and radiometers are proven and made from the earth's surface using microwave radio- thus offer the best prospects of success. meters, and the technology is mature. Liquid water may be distributed among clouds with lidar ceilometers used to determine cloud base and Ka- or W-band radar to 6.0 RECOMMENDATIONS determine cloud base and top locations, the locations Development of a remote-sensing system to detect of multiple cloud layers, and, if needed, cloud phase, icing potential requires developers and users to be adept, or the new radiometer that profiles temperature, water to adapt, and to adopt (Deffeyes 1996). Adept means that vapor, and cloud liquid water may be a viable option. A developers have a sufficient understanding of the oper- system composed of this hardware, driven by expert ational, meteorological, and sensor technology issues system logic, and perhaps supplemented with satellite to develop a coherent product that addresses user needs. information, would make a usable prototype airport- NASA/CR--2000-209938 46 safety in six areas, including weather. The weather

based remote-sensing system formapping icing poten-

requirements include the need to detect icing condi-

tial.A prototype ground-based demonstration system

tions remotely.

wouldallowoperational andtechnological problems

tobeidentified andresolved before airborne systems

7.2 FAA

reach asimilar stage ofdevelopment. Airborne systems

are much further from prototype system demonstration The FAA is the U.S. government agency responsible because little promising technology is near maturity. for aviation operations and safety. It implemented the The following recommendations for research are FAA Inflight Aircraft Icing Plan in April 1997 after three presented for operations, meteorological sensing needs, meetings examining causes, and solutions to the causes, of the October 1994 ATR-72 crash.

and technology.

In-flight icing is recognized as a significant hazard to both civilian and military aviation. In May 1996, the 6.1 Operational needs research FAA held an International Conference on Aircraft Icing • Assess human factors issues of remote-sensing in Springfield, Virginia, attended by over 400 civilian systems, assess cockpit and aircraft integration and military participants from 20 countries. As a result issues, and develop avoid-and-exit protocol and of the conference, the development of in-flight ice detec- training.

tion emerged as a goal to "accelerate development of • Assess integration into the weather system infra- airborne technologies that remotely assess icing condi- structure for other aircraft, air traffic controllers, tions by working with groups that already are support- and meteorologists.

ing research in this area." In response, the NASA Glenn • Identify aircraft flight envelopes in icing condi- Research Center, the FAA Technical Center, and tions and characterize the icing hazard.

CRREL organized a cooperative research program to accelerate development of systems for remotely detect- 6.2 Meteorological needs research ing icing conditions in the flight path.

• Characterize absolute magnitudes of the cloud microphysical conditions that produce icing.

7.3 NCAR • Assess the spatial and temporal variability of icing The National Center for Atmospheric Research, weather conditions at multiple scales.

funded by UCAR, which is in turn funded by the • Develop an icing metric algorithm to convert National Science Foundation, is the nation's preemi- liquid-water content, temperature, and elements of nent meteorological research organization. With NASA, the drop spectra into a measure of icing potential.

the FAA, and Canada's Atmospheric Environment Ser- vice, NCAR has led the nation in aircraft icing weather 6.3 Technology needs research research for several decades and will continue in that • Assess the feasibility of remote-sensing technol- role. NCAR excels in developing forecast ability, instru- ogies to provide liquid-water content, drop size, mentation, and atmospheric characterization. In addi- and temperature information.

tion, NCAR operates airborne research platforms well- • Develop methods of assessing feasibility studies suited for proving instrumentation concepts.

through measurements with hardware.

• Develop prototype technologies for field testing.

7.4 NOAA ETL The National Oceanographic and Atmospheric 7.0 GOVERNMENT ROLES, MISSIONS AND Administration's Environmental Technology Labora- COLLABORATIVE ACTIVITIES tory in Boulder, Colorado, is a pioneer developer of 7.1 NASA remote-sensing systems for detecting atmospheric phen- NASA is the U.S. government agency primarily omena using radar, microwave radiometers, and RASS.

ETL has developed some of the finest research radars responsible for aviation research. NACA, NASA's pre- and radiometers available and has played an important decessor, conducted pioneering icing research in the 1940s that continues today, principally at the Glenn role in several icing research programs, such as the Research Center at Lewis Field in Cleveland, Ohio. In 1989-1994 Winter Icing and Storms Project in Colo- rado, and the 1999 Mt. Washington Icing Sensors February 1997, President Clinton released the recom- mendations of the White House Commission on Avia- Project in New Hampshire. ETL often partners with NCAR in atmospheric research and will play an impor- tion Safety and Security for improving aviation safety.

tant role in atmospheric characterization and remote- In response, NASA implemented the Aviation Safety Program, a $500-million program to improve aviation sensing technology development.

NASA/CR--2000-209938 47 9.0 LITERATURE CITED 7.5 Department of Defense Aviation has been the nation's first line of defense Aerometrics (1997) ADA- 100 Icing Probe. Product bro- for 50 years. Each of the military services operates a chure.

fleet of aircraft best suited to their mission requirements.

Ahmed, M., and R. Brown (1995) Results from the Only the Army, however, directly addresses cold- freezing rain global climatology. Unpublished internal weather problems because of its close association with report, U.K. Meteorological Office.

winter weather conditions on and near the ground. The AHS (1995) Tape transcription of hzternational Icing Army's Cold Regions Research and Engineering Lab- Symposium '95 Workshop, American Helicopter Soci- oratory has taken the DoD lead in developing remote- ety, Montreal.

sensing systems for avoiding aircraft icing conditions.

Anderson, C., and D. Carbaugh (1993) Flight crew Working hand in hand with NASA and the FAA, intelface aspects offol3vard-looking airborne windshear CRREL is assisting in the management and develop- detection systems. NASA Contractor Report 191437.

ment of icing avoidance capabilities to improve mili- Angevine, W., and W. Eeklund (1994) Errors in radio tary readiness.

acoustic sounding of temperature. Journal of Atmo- spheric and Oceanic Technology, 11: 837-842.

8.0 IMPLEMENTATION Askne, J., G. Elgered, H. Nordius, G. Skoog, E. Win- berg, A. Hagard, E. Andersson, N. Gustafsson, J. Svens- Two tasks are involved in the implementation of a son, and I. Carlsson (1987) The ONSAM Experiment: remote-sensing ice-avoidance capability for aviation.

Remote sensing techniques for vertical sounding of the The first is the development of a coherent and cooper- atmosphere. Journal of Atmospheric and Oceanic Tech- ative research and development plan, led by the federal nology, 4" 180-190.

government but coordinated with industry and univer- Atlas, D. (1954) The estimation of cloud parameters by sities. The second is development of requirements, radar. Journal of Meteorology, 11: 309-317.

either voluntary or mandated, by the FAA to place Auld, H. (1989) Airframe icing: Some classification remote-sensing systems at airports or aboard aircraft, problems. Canadian Aeronautics and Space Journal, as is now common with weather-avoidance radar and 35(3): 152-154.

wind shear alert, collision-avoidance, and terrain-avoid- AVEMCO (1983) Ice: As unwelcome a sight as there ance systems.

is. AVEMCO Pilot Bulletin, Flying Safety Update No.

Research and development must be led by the federal 60.

government. The government is mandated the respon- Battan, L. (1962)RadarMeteorology. Chicago, Illinois: sibility of enforcing aviation safety, and it has the pub- University of Chicago Press.

lic trust. Development of icing avoidance capability Battan, L. (1973) Radar Observation of the Atmosphere.

with onboard remote sensors is a high-risk venture that Chicago, Illinois: University of Chicago Press.

is not likely to be undertaken by industry. In addition, Baum, R., and 1".Seymour (1980) In-flight evaluation the White House Commission on Aviation Safety and of a severe weather avoidance system for aircraft. Flight Security requires that the government take a leadership Dynamics Laboratory, Air Force Wright Aeronautical role in improving aviation safety. Weather, including Labs, Wright-Patterson Air Force Base, AFWAL-TR- icing, is related to roughly 27% of all general aviation 80-3022.

accidents and 33% of all commercial aviation accidents.

Baumgardner, D. (1983) An analysis and comparison In response to accidents, the FAA Inflight Aircraft Icing of five water droplet measuring instruments. Journal of Plan was developed. The NASA Aviation Safety Pro- Climate and Applied Meteorology, 22:891-910.

gram was also implemented in response to the report Baumgardner, D., B. Baker, and K. Weaver (1993) A of the White House Commission on Aviation Safety.

technique for the measurement of cloud structure on The Department of Defense is responding to a need to centimeter scales. Journal of Atmospheric and Oceanic improve military safety and readiness.

Technology, August, 10: 557-565.

The result is a need for federal leadership and coor- Beheim, M. (1979) Executive summary of Aircraft Icing dination in the development and implementation of a Specialists Workshop. In Proceedings of Aircraft Icing, capability for remotely detecting and avoiding in-flight Cleveland, Ohio, 19-21 July 1978, NASA Conference icing conditions. In response, NASA, the FAA, and Publication 2086, FAA-RD-78-109, p. 1-16.

CRREL have signed cooperative agreements to assess Benayahu, Y., A. Ben-David, S. Fastig, and A. Cohen the need and encourage the development of a remote- (1995) Cloud-droplet-size distribution from lidar sensing capability. This report supports that goal.

multiple-scattering measurements. Applied Optics, 34(9): 1569-1578.

NASA/CR--2000-209938 48 Bennett, I. (1959) Glaze, its meteorology and climatol- Royal Meteorological Society, 122:1815-1844.

Boyer, B. (1994) 3-D weather displays for aircraft cock- ogy, geographical distribution, and economic effects.

pits. Department of the Air Force, DTIC ADA289759.

Quartermaster Research and Engineering Center, Tech- Bracken, J., S. Green, J. Bettcher, and S. Erickson nical Report EP-105.

(1996) An operational perspective on ice detection and Bernstein, B.C. (1996) The stovepipe algorithm: Identi- protection systems research and development. In Pro- fying locations where supercooled large droplets are ceedings of the FAA InterT_ational Conference on Air- likely to exist. In Proceedings of the FAA International craft lnflight Icing, Springfield, Vh'ginia. vol. I, Work- Conference on Aircraft Inflight Icing, vol. II, Working Group Papers, Springfield, Virginia, p. 353-364. ing Group Papers, p. 137-148.

Bernstein, B.C., and B.G. Brown (1997) A climatolo- Bragg, M. (1996) Aircraft aerodynamic effects due to gy of supercooled large drop conditions based upon large droplet ice accretions. In Proceedings of 34th surface observations and pilot reports of icing. 7th Aerospace Sciences Meeting & Exhibit, 15-18 Janu- ar3; Reno, Nevada. AIAA-98-0932. Reston, Virginia: Conference on Aviation, Range and Aerospace Meteor- American Institute of Aeronautics and Astronautics.

ology, Long Beach, California, 2-7February, p. 82-87.

Bragg, M, W. Perkins, N. Sarter, T. Baser, P. Voul- Berry, E, Jr., E. Bollay, and N. Beers (1945) Hand- garis, H. Gurhacki, J. Melody, and S. MeCray (1998) book of Meteorology. New York: McGraw-Hill Book An interdisciplinary approach to inflight aircraft icing Company.

safety. In Proceedings of 34th Aerospace Sciences Meet- Bertorelli, P. (1992) Icing: Assessing the risk. IFR, ing & Exhibit, 15-18 Januar3; 1996, Reno, Nevada.

November, p. 6-9.

Betteher, J., S. Green, S. Eriekson, and J. Bracken AIAA-98-0095. Reston, Virginia: American Institute of Aeronautics and Astronautics.

(1996) A review of certification for flight in icing con- Brayton, R., and K. I-lakala (1996) Working Group ditions and recommendations for necessary improve- on Operational Regulations and Training Requirements.

ments. In Proceedings of the FAA International Confer- In Proceedings of the FAA International Conference on ence on Aircraft lnflight Icing, Springfield, Virginia, vol.

Aircraft bzflight Icing, Springfield, Virginia. vol. I, Plen- I, Working Group Papers, p. 437--446.

Bissonnette, L., and D. Hutt (1995a) Lidar remote ary Sessions, p. 217-222.

Brenguier, J. (1993) Observations of cloud microstruc- sensing of cloud and fog properties. In Proceedings of ture at the centimeter scale. Journal of Applied Meteor- European Symposium on Optics for Environmental and Public Safety, Munich, June 19-23 1995, Proceedings ology, 32: 783-793.

Broderick, A. (1996) Conference charge. In Proceed- no. 2506, paper no. 62. Sponsored by Commission of ings of the FAA International Conference on Aircraft the European Communities, Directorate General for Science, Research, and Development; Air and Waste lnflight Icing, Springfield, Virginia. vol. I, Plenat 3" Ses- Management Association; Deutsche Forschungsanstalt sions, p. 3-5.

Brown, D., and L. Dorr (1997) NASA, FAA and DoD ftir Luft- und Raumfahrt eV; European Optical Society; challenged to achieve White House Commission goal and International Society for Optical Engineering.

to reduce aircraft accidents. NASA News Release 97- Bissonnette, L., and D. Hutt (1995b) Multiple scat- tered aerosol lidar returns: Inversion method and com- 26, 12 February.

Brown, E. (1981) An evaluation of the Rosemount ice parison with in situ measurements. Applied Optics, 34: 6959-6975.

detector for aircraft hazard warning and for undercooled cloud water content measurements. National Center for Bissonnette, L., G. Roy, and J. Th_riault (1998) Lidar Atmospheric Research, Technical Note, October.

monitoring of adverse weather. Defense Research Estab- lishment, Valcartier, Qurbec, DREV-R-9710. Buck, R. (1988) Weather Flying. New York: Macmill- Bond, T., A. Reehorst, and C. Ryerson (Ed.) (1997) an Publishing Company.

Carle, W. (1997) Comments in meteorological panel NASA-LeRC/CRREL/ FAA Inflight Remote Sensing summary. In NASA-LeRC/CRREL Inflight Remote Sens- Icing Avoidance Workshop, Summaries and Presenta- ing Icing Avoidance Workshop, Summaries and Presen- tions, 1-2 April, Ohio Aerospace Institute, Cleveland, Ohio.

tations, 1-2April, Ohio Aerospace Institute, Cleveland, Ohio.

Borovikov, A., I. Gaivoronskii, E. Zak, V. Kostarev, I. Mazin, V. Minervin, A. Khrgian, and S. Shmeter Care)', E (1987) Raman spectroscopy. In Encyclopedia of Physical Science and Technology. Orlando, Florida: (1963) Cloud Physics. Jerusalem: Israel Program for Scientific Translations.

Academic Press, vol. 2, p. 1-17.

Choularton, T.W., and N. Bower (1993)A model of Bower, K., S. Moss, D. Johnson, T. Choularton, J.

Latham, P. Brown, A. Biyth, and J. Cardwell (1996) the development of droplet effective radius in convec- A parameterization of the ice water content observed in tive cloud. Quarterly Journal of the Royal Meteoro- frontal and convective clouds. Quarterly Journal of the logical Society, 119: 443-456.

NASA/CR--2000-209938 49 Claffey, K, K. Jones, and C. Ryerson (1995) Use and Cooper, W., M. Politovich, and W. Sand (1982) Nature calibration of Rosemount ice detectors for meteorologi- of icing conditions encountered during meteorological cal research. Atmospheric Research, 36: 277-286. research. Unpubfished contract report. Martin, Pringle, Clark, J. (1997) Introduction to the Operations Panel Oliver, Triplett and Wallace, Wichita, Kansas.

discussions. NASA-LeRC/ CRREL/FAA Inflight Remote Cotton, W., and R. Anthes (1989) Storm and Cloud Sensing Icing Avoidance Workshop, Summaries and Dynamics. San Diego, California: Academic Press.

Presentations, 1-2 April, Ohio Aerospace Institute, Creley, J. (1990) Ice capade. Flying, June, p. 134-135.

Cleveland, Ohio.. Crum, T., and R. Alberty (1993) The WSR-88D and Clothiaux, E., M. Miller, B. Albrecht, T. Ackerman, the WSR-88D Operational Support Facility. Bulletin of J. Verlinde, D. Babb, R. Peters, and _; Syrett (1995) the American Meteorology Society', 74(9): 1669-1687.

An evaluation of a 94-GHz radar for remote sensing of Curry, J., and G. Liu (1992) Assessment of aircraft cloud properties. Journal of Atmospheric and Oceanic icing potential using satellite data. Journal of Applied Technology, 12(2): 201-229. Meteorology, 31:605-621.

Cloutler, C. (1997) Presentation. NASA-LeRC/CRREN Decker, M., E. Westwater, and F. Guiraud (1978) FAA Inflight Remote Sensing Icing Avoidance Work- Experimental evaluation of ground-based microwave shop. Summaries and Presentations, 1-2 April, Ohio radiometric sensing of atmospheric temperature and Aerospace Institute, Cleveland, Ohio. water vapor profiles. Journal of Applied Meteorology, 17: 1788-1795.

Cober, S., G. Isaac, and J. Strapp (1995)Aircraft icing Decker, M., P. Fotino, and J. Schroeder (1986) Remote measurements in east coast winter storms. Journal of Applied Meteorology, 34: 88-100. detection of aircraft icing conditions using microwave radiometers. NOAA Technical Memorandum ERL Cober, S., G. Isaac, and J. Strapp (1996a) Aircraft WPL-137.

icing from supercooled drizzle. In Proceedings of the 12th h2ternational Conference on Clouds and Precipi- Deffeyes, R. (1996)Adept, adapt, adopt: Three hurdles tation, 19-23 August, Zurich, Switzerland. International for new products. IEEE AES @stems Magazine, Octo- ber, p. 3-5.

Commission on Clouds and Precipitation (ICCP) of Doviak, R. (1983)A survey of radar rain measurement International Association of Meteorology and Atmo- spheric Sciences (IAMAS) of International Union of techniques. Journal of Climate and Applied Meteorol- ogy, 22(5): 832-849.

Geodesy and Geophysics (I'UGG), p. 1318-1321.

Cober, S., G. Isaac, J. Strapp, and D. Marcotte Doviak, R., and D. Zrnic (1984) Doppler Radar and Weather Observations. San Diego, California: Academic (1996b) Analysis of aircraft icing environments asso- Press, Inc.

ciated with supercooled drizzle. In Proceedings of the Eberhard, W. (1993) CO2 lidar technique for observ- FAA International Conference on Aircraft lnflight Icing, ing characteristic drop size in water clouds. IEEE Trans- Springfield, Virginia. vol. II, Working Group Papers, p. 1-12. actions on Geoscience and Remote Sensing, 31: 56-63.

Cober, S., J. Strapp, and G. Isaac (1996c) An example Eberhard, W. (1995) Theory for dual-wavelength CO2 of supercooled drizzle drops formed through a collision- lidar method to distinguish ice, mixed-phase, and water coalescence process. Journal of Applied Meteorology, clouds. Journal of Atmospheric and Oceanic Technol- December, 35: 2250-2260. ogy, 12: 130-140.

Coffey, R. (1995) Big fat drops of ice. IFR, October, p. Eiachi, C. (1987) Introduction to the Physics and Tech- 6-8.

niques of Remote Sensing. New York: John Wiley and Sons.

Coleman, W. (I 996) Presentation by the Regional Air- line Association. In Proceedings of the FAA Interna- Engeiberg, S., and A. Bryant (1995) Lost chances in tional Conference on Aircraft Inflight Icing, Springfield, making a commuter plane safer. The New York Times, Virginia. vol. I, Plenal 3, Sessions, p. 217-222. 26 February, p. 1, 24-26.

Collins, R. (1989) Ice. AOPA Pilot, December, p. 43- Erickson, S. (1997) Air line pilots. NASA-LeRC/ 47.

CRREL/FAA bflight Remote Sensing Icing Avoidance Considine, G.D. (1997) Modefing the diurnal variabil- Workshop, Summaries and Presentations, 1-2 April, ity in cloud microphysics in boundary layer clouds. Ohio Aerospace Institute, Cleveland, Ohio.

Journal of Geophysical Research, January, 102(D2): Erickson, S., S. Green, J. Bettcher, and J. Bracken 1717-1726.

(1996) Difficulties in the operation of aircraft in icing conditions In Proceedings of the FAA International Con- Cooper, W., and W. Sand (1997) Threshold conditions for ice accretion. In 35th AIAA Aerospace Sciences ference on Aircraft Inflight Icing, Springfield, Virginia.

Meeting, 6--l O Januar); Reno, Nevada. AIAA-97-0307. vol. I, Working Group Papers, p. 575-584.

Reston, Virginia: American Institute of Aeronautics and Evans, K.D., S.It. Melfi, R.A. Ferrare, and D.N.

Astronautics.

Whiteman (1997) Upper tropospheric temperature NASA/CR--2000-209938 50 measurements withtheuseof a Raman lidar. Applied wave temperature profiler during the airborne Antarctic ozone experiment. Journal of Geophysical Research, Optics, 36( 12): 2594-2602.

94(D9): 11,223-11,231.

EWA (i 996) Review of in-flight ice detection and meas- Gary, B., R. Denning, and S. Walter (1992) Airborne urement technologies. Document No. 1185-000-D000, Electronic Warfare Associates-Canada, Ltd., Ottawa. radiometric temperature profiling. In Proceedings of the Specialists Meeting on Microwave Radiometty and FAA (1991) A ircraft Icing Handbook. U.S. Department Remote Sensing, 14-16 January Boulder, Colorado.

of Transportation, vol. 1, DOT/FAA/CT-8818-1, DTIC ADA 238 039. Gerber, H. (1991) Direct measurement of suspended particulate volume concentration and far-infrared extinc- FAA (1997) FAA inflight aircraft icing plan. United tion coefficient with a laser-diffraction instrument.

States Department of Transportation, April.

Falcone, V., L. Abreu, and E. Shettle (1982) Atmo- Applied Optics, 30(33): 4824-4831.

spheric attenuation in the 30- to 300-GHz region using Gerber, H. (1996) Microphysics of marine stratocumu- RADTRAN and MWTRAN. Proceedings of the Soci- lus clouds with two drizzle modes. Journal oftheAtmo- ety of Photogrammetric Optical lnstm_mentation Engi- spheric Sciences, 32(12): 1649-1662.

Gerber, H., B. Arends and A. Ackerman (1994) New neering, 337: 62--66.

Feingold, G., B. Stevens, W.R. Cotton, and A.S. Frisch microphysics sensor for aircraft use. Atmospheric Research, 31: 235-252.

(1996) The relationship between drop in-cloud residence GEWEX (1996) New airborne cloud radar data.

time and drizzle production in numerically simulated GEWEX News, 6(3): 16.

stratocumulus clouds. Journal of the Atmospheric Sci- ences, 53(8): 1108-1122. Gill, R., K. Geller, J. Farina, and J. Cooney (1979) Meas- urement of atmospheric temperature profiles using Raman Ferraro, R., and G.F. Marks (1995) The development lidar. Journal of Applied Meteorology, 18: 225-227.

of SSM/I rain-rate retrieval algorithms using ground- Giraud, V., J.C. Buriez, Y. Fouquart, and F. Parol based radar measurements. Journal of Atmospheric and (1997) Large-scale analysis of cirrus clouds from Oceanic Technology, 12: 755-770.

AVHRR data: Assessment of both a microphysical index Fletcher, N. (1962) The Physics of Rainclouds. Cam- and the cloud-top temperature. Journal of Applied bridge: University Press.

Fotino, P., J. Schroeder, and M. Decker 1986) Ground- Meteorology, 36: 664-674.

Glass, M., and D. Grantham (1981) Response of cloud based detection of aircraft icing conditions using mi- crowave radiometers. IEEE Transactions in Geoscience microphysical instruments to aircraft ice conditions. Air and Remote Sensing, GE-24(6): 975-982. Force Geophysics Laboratory, Report AFGL-TR-81- 0192.

Fournier, G. (1993) Transport Canada proposed R&D Goe, D. (1997) Comments in meteorological panel sum- program for the development of a multi-parameter dual X-K a band doppler radar for aviation meteorology appli- mary. NASA-LeRC/CRREL/ FAA Inflight Remote Sens- cations. In Proceedings of the 5th International Confer- ing Icing Avoidance Workshop, Summaries and Presen- ence on Aviation Weather Systems, Vienna, Virginia. tations, 1-2April, Ohio Aerospace Institute, Cleveland, Ohio.

American Meteorological Society, p. 146-147.

Gorgucci, E. and G. Scarchilli (1996a) Error structure Fox, N., and A. lllingworth (1997a) The retrieval of of radar rainfall measurement at C-band frequencies with stratocumulus cloud properties by ground-based cloud dual polarization algorithm for attenuation correction.

radar. Journal of Applied Meteorology, 36: 485-492.

Journal of Geophysical Research, 101(D21): 26, Fox, N., and A. Illingworth (1997b) The potential of a 461-26,471.

spaceborne cloud radar for the detection of stratocumulus Gorgueci, E., and G. Scarchilli (1996b) Operational clouds. Journal of Applied Meteorology, 36: 676-687.

Friehe, C.A., and D. Khelif (1993) Fast-response air- monitoring of rainfall over the Arno River Basin using dual-polarized radar and rain gauges. Journal of Applied craft temperature sensors. Journal of Atmospheric and Meteorology, 35:122 l-1230.

Oceanic Technology, 9: 784-795.

Frisch, A., C. Fairall, and J. Snider (1995) Measure- Gossard, E. (1994) Measurement of cloud droplet size ment of stratus cloud and drizzle parameters in ASTEX spectra by Doppler radar. Journal of Atmospheric and with a Ka-band Doppler radar and a microwave Oceanic Technology, 11: 712-726.

Gossard, E., J. Snider, E. Clothiaux, B. Martner, J.

radiometer. Journal of the Atmospheric Sciences, 52: 2788-2799.

Gibson, R. Kropfli, and A. Frisch (1997) The poten- Gary, B. (1983) Microwave monitoring of aviation icing tial of 8-ram radars for remotely sensing cloud drop size clouds. Report AFGL-TR-83-0271. Air Force Geophysics distribution. Journal of Atmospheric and Oceanic Tech- nology, 14: 76--87.

Laboratory, Hanscom Field, Massachusetts.

Gary, B.L. (1989) Observational results using the micro- Gosset, M., and H. Sauvageot (1992) A dual-wave- NASA/CR--2000-209938 51 length radar method forice-water characterization in Hill, G. (1991a) Comparison of simultaneous airborne mixed-phase clouds. Journal of Atmospheric and and radiometric measurements of supercooled liquid Oceanic Technology, 9: 538-547. water. Journal of Applied Meteorology, 30: 1043-1046.

Green, S. (1995) Inflight certification working paper. Hill, G. (1991b) Measurement of atmospheric liquid In Proceedings of the International Icing Symposium, water by a ground-based single-frequency microwave 18-21 Septembel; Montreal, p. 401-411. radiometer. Journal of Atmospheric and Oceanic Tech- Green, S., J. Bettcher, and S. Erickson (1996) Inflight nology, 8: 685-690.

structural icing: An operational analysis and global Hill, G. (1992) Further comparisons of simultaneous approach. In Proceedings of the FAA htternational airborne and radiometric measurements of supercooled Conference on Aircraft b_ight Icing, Springfield, Vir- liquid water. Journal of Applied Meteorology, 31: 397-401.

ginia, vol. I, Plenary Sessions, p.129-164.

Greenwald, T., G. Stephens, T. Vonder Haar and D.

Hill, G.E. (1994) Analysis of supercooled liquid water measurements using microwave radiometer and vibrat- Jackson (I 993) A physical retrieval of cloud liquid wa- ter over the global oceans using special sensor micro- ing wire devices, Journal of Atmospheric and Oceanic wave/imager (SSM/I) observations. Journal of Geo- Technology, 11:1242-1252.

physical Research, 98(D I 0): 18,471-18,488. Hill, G. (1997) Regulatory and national air space issues.

Greenwald, T, C. Combs, A. Jones, D. Randal, and NASA-LeRC/CRREL/FAA lnflight Remote Sensing Ic- T. Vonder Haar (1997a) Further developments in esti- ing Avoidance Wor_hop, Summaries and Presentations, mating cloud liquid water over land using microwave 1-2 April, Ohio Aerospace Institute, Cleveland, Ohio.

and infrared satellite measurements. Journal of Applied Hobbs, P., and A. Deepak (1981) Clouds, Their For- Meteorology, 36: 389--405. marion, Optical Properties, and Effects. New York: Academic Press.

Greenwald, T., S. Christopher, and J. Chou (1997b) Hobbs, P., and A. Rangno (1996) Precipitation from a Cloud liquid water path comparisons from passive microwave and solar reflectance satellite measurements: maritime cloud layer with very low droplet concentra- Assessment of sub-field-of-view cloud effects in micro- tions, Atmospheric Research, 40:99-107.

wave retrievals. Journal of Geophysical Research, Hobbs, R., B. Morrison, R. Ashenden, and R. Ide 102(D16): 19,585-19,596.

(1996) Comparison of two data processing techniques Grody, N. (1997) Remote Sensing from Satellites Using for optical array probes In Proceedings of the FAA Microwave Radiomett3,. NOAA, microwave radiome- International Conference on Aircraft Inflight Icing, try short course text at 1997 American Meteorological Springfield, Virginia. vol. II, Working Group Papers, Society Meeting, Long Beach, California. p. 69-90.

Gnttman, N.B., and R.K. ,leek (1987) Aircraft icing Hobbs, P.V., L.E Radke, R.R. Weiss, D.G. Atkinson, environment in low ceiling conditions near Washing- J.D. Locatelli, K.R. Biswas, F.M. Turner, and C.E.

ton, D.C. Weather and Forecasting, 2:114-126. Robertson (1974) Structure of clouds and precipita- tion over the Cascade Mountains and their modifica- Haman, K., and S. Malinowski (1996) Temperature measurements in clouds on a centimetre scale prelimi- tion by artificial seeding (1972-73). Research report 8, nary results. Atmospheric Research, 41: 161-175. Department of Atmospheric Sciences, University of Haman, K., A. Makulski, and S. Malinowski (1997) Washington, Seattle.

A new ultrafast thermometer for airborne measurements Hoffman, H. (1984) On the state of knowledge on in clouds. Journal of Atmospheric and Oceanic Technol- meteorologically caused icing of aircraft in clouds.

ogy, 14:2 i7-227. Translation of Deutsche Forschungs- und Versuchsan- Harmon, S., and S. Henderson (1995) Wind measure- stalt fiir Luft- und Raumfahrt--Mitt. 83-12, European ment applications of coherent lidar. Review of Laser Space Agency Technical Translation ESA-TT-861, June 1984.

Engineering, 23(2): 124-130.

Hansman, R.J., Jr. (1985) Droplet size distribution ef- Hoffman, H., R. Roth, and J. Demmel (1986) Results fects on aircraft ice accretion. Journal of Aircraft, 22(6): of icing flight in winter 1983/84: Dependence of ice 503-508.

accretion thickness on cloud physics parameters as well I-Iansman, R.J., Jr. (1997) Overview of human cen- as object and cloud parameters. Translation of Deutsche tered design considerations for inflight remote sensing Forschungs- und Versuchsanstalt fiir Luft- und Raum- icing avoidance. NASA-LeRC/CRREUFAA b_ight Re- fahrt--FN-85-39, European Space Agency Technical mote Sensing Icing Avoidance Workshop, Summaries Translation ESA-TT-969, April 1986.

and Presentations, 1-2 April Ohio Aerospace Institute, Hogg, D.C., M.T. Decker, F.O. Guiraud, K.B. Earn- Cleveland, Ohio. shaw, D.A. Merrett, K.P. Moran, W.B. Sweezy, R.G.

I-lelmreich, R. (1997) Managing human error in avia- Strauch, E.R. Westwater, and C.G. Little (1983a) An automatic profiler of the temperature, wind and humid- tion. Scientific American, May, p. 62--67.

NASA/CR--2000-209938 52 Physics, 15-18 November; Chicago. American Meteor-

ityinthetroposphere. Journal of Climate and Applied

ological Society, p. 408-411.

Meteorology, 22:807-831.

Jeck, R. (1983) A new data base of supercooled cloud Hogg, D., E Guiraud, J. Snider, M. Decker, and E.

variables for altitudes up to 10,000 feet AGL and the Westwater (1983b)A steerable dual-channel micro- implications for low-altitude aircraft icing. Department wave radiometer for measurement of water vapor and of Transportation, FAA/CT-83-21, NRL Report 8738.

liquid in the troposphere. Journal of Climate and Jeck, R. (1989) Selecting representative test points for Applied Meteorology, 22: 789-806.

icing conditions using the new FAA/NRL database.

Home, T. (1994) Tailplane icing. AOPA Pilot, March, Presented at the 27th American Institute of Aeronau- p. 96-100.

tics and Astronautics Aerospace Sciences Meeting, Houze, R. Jr. (1993) Cloud Dynamics. San Diego, California: Academic Press. Reno, Nevada. Reston, Virginia: American Institute of Aeronautics and Astronautics.

Howe, J. (1991) Rotating multicylinder method for the Jeck, R. (1996) Representative values of icing-related measurement of cloud liquid-water content and drop- variables aloft in freezing rain and freezing drizzle. In let size. USA Cold Regions Research and Engineering Proceedings of the FAA International Conference on Laboratory, CRREL Report 91-2.

Aircraft Inl"light Icing, Springfield, Virginia. vol. I, Hudson, J.G., and G. Svensson (1995) Cloud micro- Working Group Papers, p. 57-68.

physical relationships in California marine stratus. Jour- Jeck, R. (1998) A workable, aircraft-specific icing nal of Applied Meteorology, 34: 2655-2666.

severity scheme. 36th American Institute of Aeronau- Huggel, A., W. Schmid, and A. Waldvogel (1996) tics and Astronautics Aerospace Sciences Meeting, Raindrop size distribution and the radar bright band.

12-15 Januar); Reno, Nevada, AIAA-98-0094. Reston, Journal of Applied MeteoroIogy, 35: 1688-1701.

Virginia: American Institute of Aeronautics and Huggins, A.W. (1995) Mobile microwave radiometer Astronautics.

observations: Spatial characteristics of supercooled Jones, A., and Vonder Haar, I". (1990) Passive micro- coud water and cloud seeding implications. Journal of wave remote sensing of cloud liquid water over land Applied Meteorology, 34: 432--446.

Hutt, D., L. Bissonnette, and L. Durand (1994) Mul- regions. Journal of Geophysical Research, 95(D10): 16,673-16,683.

tiple field of view lidar returns from atmospheric aero- King, W., D. Parkin, and R. Handsworth (1978) A sols. Applied Optics, 33(12): 2338-2348.

hot-wire liquid device having fully calculable response Ide, R. (1996) Comparison of liquid water content characteristics. Journal of Applied Meteorology, 17: measurement techniques in an icing wind tunnel. In 1809-1813.

Proceedings of the FAA International Conference on Kirkpatrick, G.M. (1970) General aviation weather Aircraft hlflight Icing, Springfield, Virginia. vol. I, avoidance sensor study. NASA Report NAS 12-2032.

Working Group Papers, p. 91-103.

NTIS No. N70-36981. Electronics Research Center, Isaac, G., A. Korolev, J. Strapp, S. Cober, A. Trem- Cambridge, Masssachusetts.

blay, and R.A. Smart (1996) Freezing drizzle forma- tion mechanisms. In Proceedings of the 12th Interna- Kline, D.B. (1949) Investigation of meteorological conditions associated with aircraft icing in layer-type tional Conference on Clouds and Precipitation, 19-23 clouds for 1947-1948 winter. Technical Note No. 1793, August, Zurich, Switzerland. International Commission on Clouds and Precipitation (ICCP) of Internatinal National Advisory Committee for Aeronautics.

Kline, D.B., and J°A. Walker (1951) Meteorological Association of Meteorology and Atmospheric Sciences analysis of icing conditions encountered in low-altitude (IAMAS) of International Union of Geodesy and Geo- stratiform clouds. Technical Note No. 2306, National physics (IUGG), p. 11-14.

Advisory Committee for Aeronautics.

Isaac, G.A., and R.S. Schemenauer (1979) Large Klugmann, D., and R. Judaschke (1996) A 94-GHz particles in supercooled regions of northern Canadian FM-CW doppler radar profiler using a solid state trans- cumulus clouds. Journal of Applied Meteorology, 18: 1056-1065. mitter. Beitr. Physica Atmospherica, 69(1): 239-246.

KnoUenberg, R. (1981 ) Techniques for probing cloud Jameson, A. (1994) The estimation of rainfall para- microstructure. In Clouds, Their Formation, Optical meters using spaceborne and airborne nadir-pointing Properties, and Effects (E Hobbs and A. Deepak, Ed.).

radars. Journal of Applied Meteorology, 33: 230-244.

New York: Academic Press, p. 15-91.

Jeck, R. (1980) Icing characteristics of low altitude, Koenig, L.R. (1963) The glaciating behavior of small supercooled layer clouds. U.S. Department of Trans- cumulonimbus clouds. Journal of the Atmospheric portation, Interim Report No. FAA-RD-80-24.

Sciences, 20: 29-47.

Jeck, R. (1982) 5500 miles of liquid water and drop- Koenig G., C. Ryerson, J. Mead, and A. Pazmany size measurements in supercooled clouds below 10,000 (1999) Multi-frequency radar technique for detecting feet AGL. In Proceedings of the Conference on Cloud NASA/CR--2000-209938 53 cloud parameters forpredicting aircraft icing potential. icing. In Compendium of Meteorology (T. Malone, Ed.).

In Proceedings of the American Meteorological Soci- Boston: American Meteorological Society, p. 1197- 1203.

ety 8th Conference on Aviation, Range and Aerospace Meteorolog3, 12-15 Januar 5. Dallas, Texas. Paper Lewis, W., and W.H. Hoecker (1948) Observations of icing conditions encountered in flight during 1948.

P13.13, p. 502-506.

Korolev, A., J. Strapp, and G. Isaac (1996) On the Technical Note 1904, National Advisory Committee for Aeronautics.

accuracy of PMS optical array probes. In Proceedings Lewis, W:, D. Kline, and C. Steinmetz (1947) A fur- of the FAA hztetT_ational Conference on Aircraft b_ight Icing. Springfield, _qrginia. vol. I, Working Group ther investigation of the meteorological conditions Papers, p. 105-113. conducive to arcraft icing. Technical Note No. 1424, Kropfli, R., and R. Kelly (1996) Meteorological National Advisory Committee for Aeronautics.

Lewis, W., P. Perkins, and R. Brunn (1953) Proce- research applications of mm-wave radar. Meteorologi- cal and Atmospheric Physics, 59: 105-121. dure for measuring liquid-water content and droplet Kropfli, R., M_ Moninger, and E. Pasqualucci (1982) sizes in supercooled clouds by rotating multicylinder method. Research memorandum, NACA RM E53D23, Cloud particle identification near the meeting layer with dual polarization K-band doppler radar. In Proceedings National Advisory Committee for Aeronautics.

of American Meteorological Society Conference on Lhermitte, R. (1987) A 94-GHz Doppler radar for Cloud Physics, 15-18 November, Chicago, Illinois, p. cloud observations. Journal of Atmospheric and 412-415.

Oceanic Technology, 4: 36-48.

Lhermitte, R. (1988) Observation of rain at vertical Kropfli, R., S. Matrosov, T. Uttai, B. Orr, A. Frisch, K. Clark, B. Bartram, R. Reinking, J. Snider, and incidence with a 94-GHz Doppler radar: An insight on B. Martner (1995) Cloud physics studies with 8-mm Mie scattering. Geophysical Research Letters, 15: 1125-i 128.

wavelength radar. Atmospheric Research. 35:299-313.

Langmnir, I., and K. Blodgett (1946) A mathemati- Liao, L., and K. Sassen (1994) Investigation of rela- cal investigation of water droplet trajectories. In Col- tionships between Ka-band radar reflectivity and ice and liquid water contents. Atmospheric Research, 34: lected Works of lra,ing Langmuir. Elmsford, New York; 231-248.

Pergamon Press, vol. 10, p. 335-393.

Mannlngham, D. (1991) Rime and reason. Business Lawson, R., and W. Cooper (1990) Performance of some airborne thermometers in clouds. Journal of Atmo- and Commercial Aviation, p. 82-85.

Marcotte, D., J. Strapp, S. Cober, and G. Isaac (1996) spheric and Oceanic Technology, 7: 480--494.

Lawson, R., and R. Cormack (1995) Theoretical The NRC Convair-580 research aircraft: Configuration in the Canadian freezing drizzle experiment. In Proceed- design and preliminary tests of two new particle spectro- meters for cloud microphysics. Atmospheric Research, ings of the FAA International Conference on Aircraft 35: 315-348.

Inflight Icing. Springfield, l/irginia, vol. I, Working Lawson, R., and A. Rodi (1992)A new airborne ther- Group Papers, p. 125-135.

Marillier, A., M. Cabane, and D. Cruette (1991) Pre- mometer for atmospheric and cloud physics research.

Part I: Design and preliminary flight tests. Journal of liminary tests of an ultrasonic thermoanemometer for Atmospheric and Oceanic Tecimoiog_,, 9:-556-574. aircraft measurements. Notes and Correspondence, 8: 597-605.

Lawson, R., A. Korolev, T. I-luang, L. Angus, K.

Weaver, J. Strapp, G. Isaac, and S. Cober (1996) Martner, B., and R. Kropfli (1993) Observations of Some instrumentation effects on airborne measurements multi-layered clouds using Ka-band radar. Presented at American Institute of Aeronautics and Astronautics 31st of the drop size distribution in freezing drizzle. In Pro- Aerospace Sciences Meeting, 11-14 Januar), Reno, ceedings of the FAA International Conference on Air- Nevada.

craft Inflight Icing. Springfield, Virginia. vol. I, _brk- Martner, B., R. Kropfli, L. Ash, and J. Snider (1991) ing Group Papers, p. 115-123.

Lawson, R.P., A.¥. Korolev, S.G. Cober, T. Huang, Progress report on analysis of differential attenuation J.W. Strapp, and G. Isaac (1998) Improved measure- radar obtained during WISP-91. NOAA Technical Memorandum ERL WPL-215.

ments of the drop size distribution of a freezing drizzle Martner, B., R. Kropfli, L. Ash, and J. Snider (1993a) event. Atmospheric Research, 47-48:181-191.

Lef, T.F., J.R. Clark, and S.D. Swadley (1994) Poten- Dual-wavelength differential attenuation radar measure- ments of cloud liquid water content. In Proceedings of tial applications of the SSM/I cloud liquid water para- the 26th International Conference on Radar Meteorol- meter to the estimation of marine aircraft icing. Weather ogy, Norman, Oklahoma, Preprints. American Meteo- and Forecasting, 9:173-182.

Lewis, W. (1951) Meteorological aspects of aircraft rological Society, p. 596-598.

NASA/CR--2000-209938 54 NASA (1997) Advanced probe fights icing and corro- Martner, B., R. Kropfli, L. Ash, and J. Snider (1993b) sion. NASA Tech Briefs.

Cloud liquid water content measurement tests using dual- NCAR (1990) 1990 winter icing storms project (WISP90) wavelength radar. NOAA Technical Memorandum ERL ETL-235. data catalog. National Center for Atmospheric Research, Research Applications Program, Boulder, Colorado.

Masters, C.O. (1983) A new characterization of super- Newton, D.W. (1977) An integrated approach to the cooled clouds below 10,000 feet AGL. Department of problem of aircraft icing. In Proceedings, AIAA Aircraft Transportation, Final Report, FAA/CT-83-22.

Systems and Technology MeethTg, Seattle, Washington, Matrosov, S., R. Reinking, R. Kropfli, and B. Bar- 22-24 August, New York. Reston, Virginia: American tram (1996) Estimation of ice hydrometeor types and Institute of Aeronautics and Astronautics.

shapes from radar polarization measurements. Journal Newton, D.W. (1979) A review of the icing situation of Atmospheric and Oceanic Technology, 13: 85-96.

from the standpoint of general aviation. In Proceedings Matuura, N, Y. Masnda, H. Inuki, S. Kato, S. Fukao, T. Sato, and T. Tsuda (1986) Radio acoustic measure- of Aircraft Icing, Cleveland, Ohio, 19-21 July 1978.

NASA Conference Publication 2086, FAA-RD-78-109, ment of temperature profile in the troposphere and strato- p. 31-38.

sphere. Nature, 323: 426-428.

North, E., A. Peterson, and H. Parry (1973) RASS, a May, P., R. Strauch, and K. Moran (1988) The alti- remote sensing system for measuring low-level temper- tude coverage of temperature measurements using RASS ature profiles. Bulletin, American Meteorological Soci- with wind profiler radars. Geophysical Research Let- ters, 15: 1381-1384. ety, 54(9): 912-919.

NTSB (1996) In-flight icing encounter and loss of con- May, P., K. Moran, and R. Strauch (1989) The accu- trol: Simmons Airlines, d.b.a. American Eagle Flight racy of RASS temperature measurements. Journal of 4184. Avions de Transport Regional (ATR) Model 72- Applied Meteorology, 28: 1329-1335.

212, N401 AM, Roselawn, Indiana, October 31 1994.

Mayer, P., R. Jeck, and P. Drake (1984) An analysis of National Transportation Safety Board Aircraft Accident icing-related mishaps on U.S. Navy aircraft from 1964 Report, Volume 1: Safety Board Report, NTSB/AAR- to 1984. Navy Research Lab Memorandum, Report 5418.

96/01.

McClean, P. (1992) Ice over Lake Erie. AOPA Pilot, Oldenbnrg, J., and R. Ide (1990a) Comparison of drop January, p. 83-84.

size distributions from two droplet sizing systems.

Mead, J., A. Pazmany, S. Sekelsky, and R. MeIntosh NASA Technical Memorandum 102520.

(1994) Millimeter-wave radars for remotely sensing Oldenburg, J., and R. Ide (1990b) Comparison of two clouds and precipitation. Proceedings IEEE, 82(23): droplet sizing systems in an icing wind tunnel. NASA p. 1891-1906.

Technical Memorandum 102456.

Mead, J., A. Pazmany, and M. Goodberlet (1998) Owen, T. (1997) A regulating agency's operational per- Evaluation of technologies for the design of a prototype spective. NASA-LeRC/ CRREL/FAA Inflight Remote in-flight remote aircraft icing potential detection system.

Sensing Icing Avoidance Workshop, Summaries and DOT/FAA/AR-98172 report.

Presentations, 1-2 April, Ohio Aerospace Institute, Melfi, S.H., K.D. Evans, J. Li, D. Whiteman, R. Fer- Cleveland, Ohio.

rare, and G. Schwemmer (1997) Observation of Raman Palueh, I., C. Knight, and L. Miller (1996) Cloud scattering by cloud droplets in the atmosphere. Applied liquid water and radar reflectivity of nonprecipitating Optics, 36(15): 3551-3559.

cumulus clouds. Journal of Atmospheric Sciences, Miller, A., and R. Anthes (1980) Meteorology: Colum- 53(11): 1587-1603.

bus, Ohio: Charles E. Merrill Publishing Company, 4th edition. Parelon, E. (1996) JAA Certification Policy NPA 25F- 219, Flight in icing conditions: Acceptable handling Miller, D., T. Ratvasky, B. Bernstein, 17.McDonough, characteristics and performance effects. In Proceedings and W. Strapp (1998) NASA/FAA/NCAR supercooled of the FAA International Conference on Aircraft bzflight large droplet icing flight research: Summary of winter Icing, vol. I, Working Group Papers, Springfield, Vir- 96-97 flight operations. In 36th AIAA Aerospace Sci- ginia, p. 413-436.

ences Meeting, 12-15 January, Reno, Nevada. AIAA- 98-0577. Reston, Virginia: American Institute of Aero- Pazmany, A.L., R. Melntosh, R. Kelly, and G. Vail nautics and Astronautics. (1994) An airborne 95-GHz dual-polarized radar for cloud studies. IEEE Transactions on Geoscience and Morrison, B.J., J. Jung, and R.L. Rose (1997) Mobile Remote Sensing, 32(4): 731-739.

aircraft icing forecast system. In Proceedings of the Peer, R. (1986) Tips on avoiding low-level icing. 5th Conference on Cloud Impacts on DoD Operations and Systems, 23-25 September, Newport, Rhode Island, Weather Wing Forecaster Memo.

Perkins, P.J. (1952) Preliminary survey of icing condi- p. 235-238.

NASA/CR--2000-209938 55

tionsmeasured during routine transcontinental arline

ological Society Conference on Weather Analysis and

operation. National Advisory Committee for Aeronaut-

Forecasting, 19-23 August, Norfolk, Virginia.

ics. Research Memorandum.

Rodert, L. (1951) Physical and operational aspects of Personne, E, J. Brenguier, J. Pinty, and Y. Pointin aircraft icing. In Compendium of Meteorology (T. Mal- (1982) Comparative study and calibration of sensors for one, Ed.). American Meteorological Society, Boston, the measurement of the liquid water content of clouds p. 1190-1196.

with small droplets. Jour77al of Applied Meteorology, 21: Roebber, P. (1988) Helicopter and airframe icing poten- 189-196.

tial off the Canadian east coast. Atmospheric Environ- ment Service, Downsview, Ontario, Canadian Climate Petty, G., and K. Katsaros (1992) Nimbus-7 SMMR precipitation observations calibrated against surface Center Report No. 88-5.

radar during TAMEX. Journal of Applied Meteorology, Rogers, R., and M. Yau (1989) A Short Course in Cloud 31: 489-505.

Physics. Oxford: Pergamon Press.

Politovich, M. (1982) Characteristics of icing conditions Ryerson, C, (1990) Atmospheric icing rates with eleva- in wintertime stratiform clouds. In Proceedings of Con- tion on northern New England mountains, U.S.A. Arc- ference on Cloud Physics, 15-18November, Chicago, Illin- tic and Alpine Research, 22(1): 90-97.

ois. American Meteorological Society, p. 404-407. Ryerson, C. (1996) Remote detection and avoidance of Politovich, M. (1989) Aircraft icing caused by large inflight icing. In Proceedings of the FAA International supercooled droplets. Journal of Applied Meteorology, Conference on Aircraft h_ight Icing, vol. II, Working 28: 856-868.

Group Papers, Springfield, Virginia, p. 179-190.

Politovich, M., and B.C. Bernstein (1995) Production Ryerson, C. (1997) Prospects and problems of remotely of depletion of supercooled liquid water in a Colorado detecting inflight icing potential. In Proceedings of the winter storm. Journal of Applied Meteorology, 34: American Meteorological Society 7th Conference on 2631-2648.

Aviation, Range and Aerospace Meteorology, 2-7 Feb- Politovich, M., and G. Vali (1983) Observations of liquid ruar); Long Beach, California, p. 138-143.

water in orographic clouds over Elk Mountain. Journal Ryerson, C. (1998) Aircraft icing remote sensing. In of the Atmospheric Sciences, 40(5): 130(O 1312. 36th AIAA Aerospace Sciences Meeting & Exhibit, Politovich, M., B. Stankov, and B. Martner (1995) 12-15 January 1998, Reno, Nevada. AIAA-98-0578.

Determination of liquid water altitudes using combined Reston, Virginia; American Institute of Aeronautics and Astronautics.

remote sensors. Journal of Applied Meteorology, 34: 2062-2075.

Ryerson, C., M. Politovich, K. Raneourt, G. Koenig, Pruppaeher_ H., and J. Klett (1997) Microphysics of R. Reinking, and D. Miller (2000) Overview of Mt.

Clouds and Precipitation. Dordrecht • Kluwer Academic Washington Icing Sensors Project. 38th AIAA Aerospace Publishers.

Sciences Meeting, 10-13 Januar); Reno, Nevada. AIAA- Ramsay, A. (1997) Freezing rain detection and report- 2000-0488. Reston, Virginia: American Institute of Aeronautics and Astronautics.

ing by the Automated Surface Observing System (ASOS). In Preprints of the 1st Symposium on Integrated SAE (1995) A ircrafi Ice Detectors and Icing Raw Meas- Observing Systems. American Meteorological Society, uring Instruments. AIR 4367. Warrendale, Pennsylva- paper Jl0.13, p. J65-J69. nia: Society of Automotive Engineers.

Reinking, R., S. Matrosov, B. Martner, and R. Kropfli Sand, W., and R. Kropfli (1991) Icing Hazard Detec- (1996) Differentiation of freezing drizzle from ice hydro- tion for Aircraft. United States Patent 5,028,929, July 2, 1991.

meteors and freezing rain with dual-polarization radar.

In Proceedings of the FAA bzternational Conference on Sand, W., W. Cooper, M. Politovich, and D. Veal Aircraft hTflight Icing, vol. II, Working Group Papers, (1984) Icing conditions encountered by a research air- Springfield, Virginia, p. 331-338. craft. Journal of Climate and Applied Meteorology, 23(10): 1427-1440.

Reinking, R., S. Matrosov, R. Bruintjes, and B. Mart- Sassen, K., and L. Liao (1996) Estimation of cloud ner (1997) Identification of hydrometeors with ellipti- cal and linear polarization Ka-band radar. Jounzal of content by W-band radar. Journal of AppliedMeteorol- Applied Meteorology, 36: 322-339. ogy, 35: 932-938.

Riley, J. (1998) Mixed-phase icing conditions: A review. Sauvageot, H. (1996) Retrieval of vertical profiles of DOT/FAA/AR-98/76 report. liquid water and ice content in mixed clouds from Dop- Rinehart, R. (1997) Radar for Meteorologists. Grand pler radar and microwave radiometer measurements.

Forks, North Dakota: Rinehart Publications.

Journal of Applied Meteorology, 35:14--23.

Robbins, C., and J. Cortinas, Jr. (1996) A climatology Savage. R., R. Lines, J. Cole, and G. Koenig (1999) A of freezing rain in the contiguous United States: Prelim- passive microwave icing avoidance system (MIAS). In inary results. In Preprints of the 15th American Meteor- Proceedings of the American Meteorological Society 8th NASA/CR--2000-209938 56 and Oceanic Technology, 12:488-510.

Conference on Aviation, Range and Aerospace Meteo- Stewart, R., R. Crawford, K. Szeto, and D. Hudak rolog3, 12-15 January, Dallas, Texas. Paper P12.9, (1996) Horizontal aircraft passes across 0°C regions p. 457-461.

within winter storms. Atmosphere-Ocean, 34(1): Schroeder, J. (1990) A comparison of temperature 133-159.

soundings obtained from simultaneous radiometric, ra- dio-acoustic, and rawinsonde measurements. Journal Strapp, J., R. Stuart, and G. Isaac (1996) A Canadi- an climatology of freezing precipitation, and a detailed of Atmospheric and Oceanographic Technology, 7: 495-503. study using data from St. John's, Newfoundland. In Proceedings of the FAA International Conference on Schuyler, N. (1989) The iceman cometh. Private Pilot, Aircraft It_ight Icing, vol. II. Working Group Papers, November, p. 31-37.

Final Report, p. 45-55.

Sekelsky, S., and R. Mclntosh (1996) Cloud observa- Svensson, E., M. Angelborg-Thanderz, L. Sjrberg, tions with a polarimetric 33-GHz and 95-GHz radar.

and S. Olsson (1997) Information complexity--Mental Meteorological and Atmospheric Physics, 59:123-140.

workload and performance in combat aircraft. Ergo- Shah, A., M. Patnoc, and E. Berg (1998) Droplet size nomics, 40(3): 362-380.

distribution and ice shapes. In 36th Aerospace Sciences Takaliashi, N, H. Uyeda and K. Kikuchi (1996) Evo- Meeting, 12-15 Januar); Reno, Nevada. AIAA-98-0487.

Reston, Virginia: American Institute of Aeronautics and lution process and precipitation particles of an isolated Astronautics.

echo observed with dual-polarization Doppler radar near Sheppard, B. (1996) Effect of rain on ground-based Sapporo on July 9 1992. Journal of Facilities Science, microwave radiometric measurements in the 20- to Hokkaido Universit); Ser. VII (Geophysics), 10(I): 135-153.

90-GHz range. Journal of Atmospheric and Oceano- graphic Technology, 13:1139-1151. Targ, R., M. Kavaya, R. Huffaker, and R. Bowles (1991) Coherent lidar airborne windshear sensor: Siquig, R. (1993) Weather and unmanned aerial vehi- Performance evaluation. Applied Optics, 30(15): cles. Report NRL/MR/75432-93-7206. Naval Research 2013-2026.

Laboratory.

Taylor, R. (1991)Aviation Weather. Greenwich, Con- Smith, P. (1985) Summary of Workshop on Remote necticut: Belvoir Publications.

Detection of Potential Aircraft Icing Conditions. Pre- pared for Center for Aerospace Sciences, SDSMT/IASI Telford, J.W. (1988) An example of the behavior of an R-85-06. aircraft with accumulated ice: Latent instability. Jour- Solheim, Fo, and J. Godwin (1998) Passive ground- nal of Applied Meteorology, 27:1093-1109.

based remote sensing of atmospheric temperature, water Telford, J.W. (1996) Clouds with turbulence: The role vapor, and cloud liquid water profiles by a frequency of entrainment. Atmospheric Research, 40:261-282.

synthesized microwave radiometer. Meteorologische Thomas, M., and J. Marwitz (1995) Droplet spectra in freezing rain. In Proceedings of the American Meteor- Zeitschrift., N.E 7, p. 370-376.

ological Society 6th Conference on Aviation Weather Spencer, R., H. Goodman, and R. Hood (1989) Pre- cipitation retrieval over land and ocean with the SSM/I: Systems, 1.5-20 Januar); Dallas, Texas, p. 253-256.

Identification and characteristics of the scattering signal. Thompson, G., R. Bullock, and T. Lee (1997) Using Journal of Atmospheric and Oceanic Technology, 6: satellite data to reduce spatial extent of diagnosed icing.

254-273.

Weather and Forecasting, I2: 185-190.

Thomson, A., and R. List (1996) Raindrop spectra and Srivastava, R., and L. Tian (1996) Measurement of attenuation by a dual-radar method: Concept and error updraft determination by combining Doppler radar and disdrometer. Journal of Am_ospheric and Oceanic Tech- analysis. Journal of Atmospheric and Oceanic Technol- ogy, 13: 937-947. nology, 13: 465-476.

Tian, L., and R. Srivastava (1997) Measurement of Stack, D. (1996) Inflight icing--The critical need for attenuation at C band in a convective storm by a dual- improved forecasts and indexing of the weather. In Pro- radar method. Journal of Atmospheric and Oceanic ceedings of the FAA International Conference on Air- craft Inflight Icing, vol. 17, Working Group Papers, Technology, 14: 184-196.

Toomay, J. (1982) Radar Principles for the Non- Springfield, Virginia, p. 575-584.

Specialist. London: Lifetime Learning Publications.

Stankov, B., E. Westwater, J. Snider, and R. Weber Transport Canada (1996) Human Factors for Avia- (1992) Remote measurements of supercooled integrated tion: Advanced Handbook. Transport Canada Safety and liquid water during WISP/FAA aircraft icing program.

Journal of Aircraft, 29(4): 604-715. Security, TP 12864 (E).

Tremblay, A., S. Cober, A. Glazer, and G. Isaac Stankov, B., B. Martner, and M. Politovich (1995) (1996) Observations and parameterization of mixed- Moisture profiling of the cloudy winter atmosphere us- phase clouds during the Canadian freezing drizzle ing combined remote sensors. Journal of Atmospheric NASA/CR--2000-209938 57

experiment. In Proceedings of the 12th bzternationaI

Westwater, E. (1997) Remote sensing of tropospheric Conference oli Clouds and Precipitation, Zurich, temperature and water vapor by integrated observing Switzerland.

systems. Bulletin of the American Meteorological Soci- et3; 78(9): 1991-2006.

Tucker, W. (I 983) Current procedures for forecasting aviation icing: A review. USA Cold Regions Research Westwater, E.R., and N.C. Grody (1980) Combined and Engineering Laboratory, Special Report 83-24. surface- and satellite-based microwave temperature pro- Vaughan, G., D.P. Waretng, S.J. Pepler, L. Thomas, file retrieval. Journal of Applied Meteorology, 19: 1438- i444.

and V. Mitev (1993) Atmospheric temperature meas- urements made by rotational Raman scattering. Applied Westwater, E., and R. Kropfli (1989) Remote sens- Optics, 32(15): 2758-2764. ing techniques of the wave propagation laboratory for Vigeant-Langlois, L., and R. Harts'man (1999) Human- the measurement of supercooled liquid water: Applica- centered design considerations for in-flight remote sens- tions to aircraft icing. NOAA Technical Memorandum ERL WPL-163, NOAA Environmental Research Labo- ing icing avoidance. Presented at 4th hztemational Remote ratories.

Sensing Confelence and E_hibitiord21st Canadian Sym- posium on Remote Sensing, 21--9,4 June, Ottawa, Ontario, Westwater, E.R., M.T. Decker, A. Zachs, and K.S.

Canada.

Gage (1983) Ground-based remote sensing of temper- Vivekanandan, J., J. Turk, and V. Bringi (1993) Com- ature profiles by a combination of microwave radiom- parisons of precipitation measurements by the advanced etry and radar. Journal of Climate and Applied Meteor- microwave precipitation radiometer and multiparame- ology, 22: 126-133.

ter radar. IEEE Transactions on Geoscience and Remote White, A., C. Fairall, A. Frisch, B. Orrm, and J.

Sensing, 31(4): p. 860-870. Snider (1996) Recent radar measurements of turbulence Vivekanandan, J., G. Thompson, and T. Lee (1996) and microphysical parameters in marine boundary layer Aircraft icing detection using satellite data and weather clouds. Atmospheric Research, 40:77-221 forecast model results. In Proceedings of the FAA Inter- Willis, P., and P. Tattelman (1989) Drop-size distri- national Conference on Aircraft Inflight Icing, vol. I, butions associated with intense rainfall. Journal of Plenaly Sessions, Springfield, Virginia, p. 365-374. Applied Meteorology, 28:3-15.

Walter, S., and P. Moynlhan (1997) Mobile aircraft Wilson, D., A. lllingworth, and T. Blackman (1997) icing forecast system. In Proceedings of the Confer- Differential Doppler velocity: A radar parameter for ence on Cloud Impacts on DoD Operations and Sys- characterizing hydrometeor size distributions. Journal tems, 23-25 September, Newport, Rhode Island. of Applied Meteorology, 36: 649-662.

Phillips Laboratory, Geophysics Directorate; Office of Wright, W. (1995) Capabilities of LEWICE 1.6 and the Oceanographer of the Navy; and Army Research comparison with experimental data. In Proceedings, Laboratory. Hampton, Virginia: Science and Technolo- International Icing S)vnposium "95, American Helicop- gy Corporation, p. 213-216. ter Society, Montreal, p. 349-361.

Wu, M-L (1987) Determination of cloud ice water con- Wanke, C., and R. Itansman, Jr. (1991) Alert genera- tion and cockpit presentation for an integrated cockpit tent and geometrical thickness using microwave and microburst alert system. Joint University Program for infrared radiometric measurements. Journal of Climate Air Transportation Research 1990-1991, Langley and Applied Meteorology, 26: 878-884.

Research Center. p. 29-39. Young, K. (1993) Microphysical Processes in Clouds.

Warner, J. (1969) The microstructure of cumulus New York: Oxford University Press.

cloud. Part I. General features of the droplet spectrum. Zrnic, D. (1996) Weather radar polarimetry--Trends Journal of the Atmospheric Sciences, 26: 1049-1059. toward operational applications. Bulletin of the Ameri- Westwater, E. (1978) The accuracy of water vapor and can Meteorological Society, 11(7): 1529-1534.

cloud liquid determination by dual-frequency groundbased microwave radiometry. Radio Science, 13(4): 677-685.

NASA/CR--2000-209938 58

APPENDIX A: SYNOPSIS OF OPERATIONAL INFORMATION NEEDS, STATE OF KNOWLEDGE, STRENGTHS, AND WEAKNESSES INFORMATION NEEDS Pilot needs and human factors 10. Create regulatory policy development plan.

1. 11. Consider system integration, protocol compati- Determine pilot information needs.

2. bility, and certification.

Design information displays conducive to effec- 12. Determine impact on NAS, ATC, and Free Flight.

tive risk assessment and cockpit resource man- 13. Develop standards to integrate ground, in-situ, agement.

and remotely sensed information.

3. Understand pilot decision-making process for 14. Develop auto-reporting standards for ground accurate hazard/risk assessment.

(ATC and weather) and other-aircraft linkages.

4. Develop avoid/escape strategies.

15. Develop testing standards for remote-sensing sys- 5. Determine effect of aircraft performance on tems.

avoid/exit strategies vs. information needed.

6. Determine an acceptable warning lead time.

Test beds and platforms 7. Determine optimal training requirements.

I.

Develop training and simulation protocols.

8. Develop unambiguous indications of icing, and 2.

Locate test beds and platforms for sensor, infor- an ability to sense through icing.

mation, avoidance, and escape procedure testing.

9. Improve operational guidelines for flight in icing.

.

Develop testing standards.

Operators and manufacturers STATE OF KNOWLEDGE 1. Minimize remote-sensing system cost, size, Pilot needs and human factors power, and weight and maximize maintainability, simplicity, and reliability. I. Pilots have no indication when they are in icing conditions that exceed aircraft limitations.

2. Establish aircraft space, weight, and power equip- ment constraints.

2. Pilots often do not know how to avoid or escape 3. Develop incentives to use in-tlight remote-sensing icing.

icing-avoidance systems. 3. Pilots want to know if icing conditions lie ahead.

4. Establish aircraft operational limits in icing.

4. The stated icing information needs of a few pilots, and of ALPA pilots as a union, are known.

Regulatory issues, weather forecasting, 5. Pilot reports are weak; spatial, temporal accuracy, and traffic management and terminology standardization are lacking.

1. Establish aircraft icing performance limits. 6. It is known how pilots use weather radar and wind 2. Develop simple, scientific, standardized scale for shear alert systems, which may serve as analogs.

reporting icing potential. 7. It is known how information displayed affects 3. Develop regulatory operational concept and func- pilot reaction to terrain and wind shear alerts.

tional requirements. 8. An effective icing display needs to be designed.

4. Develop training standards and requirements. 9. Pilots want a simple icing display, but other infor- 5. Develop clear, unambiguous guidelines of when mation may be needed because of airframe and mission differences.

severe icing conditions are entered.

6. Consider status of remote-sensing information-- 10. Avoid/exit strategies should be identified and whether it should be an advisory or warning sys- tested--perhaps in a simulator.

tem.

11. Effects of aircraft performance on avoid/exit strat- 7. Create incentives for operators and manufactur- egies and on information needs by pilots are not known.

ers to install remote-sensing systems.

8. Expand FAR 25, Appendix C, to include freez- 12. Limitations of aircraft performance in avoid/es- ing drizzle. cape strategies are known through experience.

9. Develop ground-based icing remote-sensing sys- 13. Pilots do not know aircraft limits in icing.

tems for terminal areas.

NASA/CR--2000-209938 59 Operators and manufacturers aircraft may require processing through a ground 1. Reliability, cost, size, power, and weight estimates station. This would be more difficult for military for remote-sensing systems can be estimated, aircraft, which may not be able to benefit from such services.

though poorly, from existing onboard systems.

2. Room, power available, and weight allowable for Experimental test beds remote-sensing systems are unknown for most 1. Experimental test beds for training and develop- aircraft, except when newly delivered from manu- ment of avoid/exit procedures could be developed facturers. After aircraft delivery, operators may provide information. using flight simulators.

2. Test beds for air traffic control and pilot training 3. Simplicity, maintainability, and reliability of icing are needed.

remote-sensing systems are currently unknown except for extrapolation of records from other weather-avoidance sensors.

OPERATIONAL INFORMATION KNOWLEDGE Strengths Regulatory issues, weather forecasting, and traffic management 1. General operational concept of icing remote- 1. Operating limits of aircraft in icing conditions out- sensing system.

side of FAR 25, Appendix C, are not known. 2. General ideas of pilot information needs.

3. Use of current onboard radar and wind shear alert 2. Information needed to create a standardized icing potential scale is available. as analogs.

3. An operational concept of a remote-sensing icing- 4. Use of remote-sensing systems in regulated air avoidance system needs further development. space.

5. Information display problems from radar and 4. Functional requirements of remote-sensing icing- wind shear alert systems.

avoidance system need development.

6. Weather information needed by meteorologists.

5. Regulations currently do not address icing condi- tions beyond conditions defined in FAR 25, Weaknesses Appendix C.

1. Lack of standardized, objective icing rating ter- 6. There is little incentive for operators to place an minology.

icing remote-sensing system on aircraft.

2. Unknown limits of aircraft capabilities in icing 7. There are no ground-based icing remote-sensing conditions outside Appendix C.

systems, though they are under development.

3. Poor knowledge of how to avoid and escape icing.

8. There is no regulatory policy with regard to 4. Lack of incentives for manufacturers and opera- remote-sensing icing-avoidance systems. How- ever, onboard weather radar and wind shear alert tors to use remote-sensing systems.

5. Definition of beyond FAR 25, Appendix C, con- systems are analogs for aircraft-based systems, ditions.

and ground-based wind shear alert systems are 6. Undeveloped functional requirements (specifica- analogs for ground-based icing-avoidance sys- tems. tions) of a remote-sensing icing detection system.

7. Use of remote-sensing systems in Free Flight.

9. Remote-sensing icing-avoidance systems may 8. Development of effective display for icing con- complicate Free Flight and ATC operations in ter- minal areas. ditions.

10. Weather forecasters do not have reliable indica- 9. Effects of aircraft performance on avoid/exit strat- egies and information needs.

tions of icing conditions. Only pireps provide information, which is often inaccurate as to posi- 10. Location of effective experimental test beds for pilots, air traffic controllers, and meteorologists.

tion, time, and intensity.

11. Reliability, cost, size, power, and integration into 11. Weather forecasters know the information they airframe.

want to improve icing forecasts.

12. Quality and type of information that will be pro- 12. Integrating aircraft and ground information is not vided to meteorologists and ATC from remote- well understood, though some work is being done sensing systems.

in the AGATE program.

13. Regulatory hurdles.

13. Integration ofin-situ, remote-sensing, and ground 14. Integration of ground, in-situ, and remotely information may be difficult because of spatial sensed information.

and temporal inconsistencies.

15. Training standards.

14. Reporting remotely sensed information to other NASA/CR--2000-209938 60 3. Investigate concerns and limitations of airframe 16.Establishing pilotneeds.

manufacturers.

17.Establishing remote-sensing system range, scan

4. Investigate operator needs and limitations.

rate, resolution, and warning time tomeet pilots'

needs.

5. Develop specifications for remote-sensing sys- tem.

6. Investigate information dissemination prob- GENERAL GOALS lems-to ground, to other aircraft, information needed by ground, information aircraft can pro- 1. Assess the needs of a greater variety of pilots.

vide (remote and in situ).

Study information needs of pilots in greater cross- platform/cross-mission variety. For example, 7. Investigate feasibility of using simulation to de- ALPA, military, helicopter vs. turboprop vs. jet velop pilot interfaces, and avoid-and-exit strate- gies.

vs. general aviation.

8. Investigate integration of remotely sensed and in- 2. Investigate pilot decision-making process. Inves- situ measurements for assessing proximity to air- tigate what information, to what distance, reso- lution, and detail and types of displays pilots need. craft operational limits.

9. Develop simulation and training aids.

NASA/CR12000-209938 61

APPENDIX B: SYNOPSIS OF SENSING NEEDS, STATE OF KNOWLEDGE, STRENGTHS, AND WEAKNESSES SENSING NEEDS General Characterization 1. Determine what characteristics of clouds are criti- 1. Good climatologies of icing conditions in all syn- cal to flight from flight tests, tunnel tests, and nu- optic situations. merical models, in a spectrum of meteorological 2. Synoptic, continental, and global icing patterns conditions from a wide variety of aircraft.

to determine system utility. 2. A meteorology-based icing intensity standard.

3. Fully characterize supercooled large droplet cli- 3. Determine the critical technical capabilities for a matology. remote-sensing system, such as range needed to 4. Characterize liquid-water content, drop size, and observe through most icing conditions, scanning temperature conditions in all icing synoptic situa- rate/resolution, accuracy. Information needed is tions.

primarily meteorological, but also operational.

4. Weather forecasters and numerical models need 5. Determine the variability of cloud characteristics (such as liquid-water content, droplet size, and downlinked objective, timely temperature, liquid- temperature) within 3-D space (vertical and hori- water content, and drop-size information that is zontal). accurate in position.

6. MVD or equivalent may not be acceptable because they poorly represent "nonstandard" (i.e., STATE OF KNOWLEDGE non-Gaussian) distributions of drop sizes Characterization • observed in clouds with drizzle drops.

7. Drop-size distributions are often not correctly 1. 3-D organization of icing patches is poorly under- represented by current instrumentation, and ice stood with regard to usefulness of a remote- crystals or drops can confuse sensing systems.

sensing system.

Better instrumentation is needed.

2. Range and scale of liquid-water content, drop size, 8. More research flights specifically planned to and temperature variability is not well understood measure information needed to characterize the in 3-D space, especially at the submesoscale, and icing environment with regard to remote-sensing especially for supercooled large drops.

systems--with better instrumentation.

3. Climatology of supercooled large drops is poorly understood.

9. Rework existing flight data.

10. Measure cloud microphysical properties and 4. Submesoscale, continental, and global scales of resulting ice on aircraft with in-situ sensing sys- aircraft icing are poorly understood.

tems to calibrate remote-sensing system dynam- 5. Drop-size distributions are not well characterized ically. by current instrumentation--especially for larger 11. Improve characterization of freezing rain aloft. drops.

12. Characterize droplet temperature variations. 6. Ranges of liquid water content are reasonably well 13. Characterize mixed-phase clouds with tempera- understood, but not in relation to drop size and ture.

temperature.

7. Synoptic scale of icing is reasonably well under- In-situ instrumentation stood.

1. In-situ instruments with better dynamic range and 8. General climatologies of aircraft icing exist and sensitivity. are reasonably well understood.

2. Small, accurate, and inexpensive in-situ technol- 9. Research flight information exists that can be reanal- ogies. yzed at little cost for better characterization.

3. Improved SLD measurement instrumentation. 10. Characterization of freezing rain and freezing driz- zle aloft poorly understood.

11. Characterization of temperature pooflyunderstood.

NASA/CR--2000-209938 63 In-situ instrumentation Weaknesses 1. Accuracy and dynamic range of in-situ instru- 1. Characterization of the 3-D scale of cloud physi- ments need improvement, especially for SLD.

cal properties at the submesoscale, continental, 2. Small, accurate, and inexpensive in-situ instru- and global scales, including temperature, liquid- ments are unavailable.

water content, and drop sizes.

3. 3-D remote-sensing resolution, range, and angu- 2. Characterization of SLD.

lar scanning area need specification.

3. Accurate, reliable, inexpensive in-situ instrumen- 4. Characteristics of clouds critical to flight in icing tation.

on many airframes in many conditions are poorly 4. There is no objective, weather-based icing index.

understood.

5. Further testing of airfoils needed under a variety of weather and operating conditions.

General 6. Specifications must be developed for remote- 1. We can currently only speculate about the ideal sensing systems.

remote sensor scanning range needed---distance 7. Characterization of test beds needed.

and angular, accuracy, and 3-D sensing resolu- 8. FAR 25, Appendix C, must be extended.

tion needed.

2. There is no index of icing conditions by intensity General goals as a function of weather condition alone, inde- 1. Define conditions within which clouds are mixed pendent of aircraft type.

phase, that is, have ice crystals, because mixed- 3. Characteristics of clouds critical to flight from phase situations indicate that supercooled liquid flight tests, tunnel tests, and numerical models, water exists.

in a spectrum of meteorological conditions from 2. Characterize 3-D spatial scales of icing by icing wide variety of aircraft, are not available.

potential and microphysical properties.

4. There is no objective system for reporting icing 3. Assess continental-scale and global-scale icing potential independent of aircraft type.

patterns, frequencies, and intensities to determine 5. There is currently no objective system of report- needs for commercial aviation and military avia- ing areas of icing or no icing that is accurate in tion in potential operational theaters.

intensity, position, and time.

4. Characterize SLD conditions with regard to fre- 6. Weather forecasters and numerical models cur- quency, range of conditions, synoptic situations, rently do not have available objective, timely tem- and continental and global patterns.

perature, liquid-water content, and drop-size 5. Reanalyze old flight data with improved instru- information that is accurate in position.

ment-correction algorithms.

7. A meteorology-based icing intensity standard is 6. Fly new research flights to develop specifications needed.

for remote-sensing systems.

8. Need to expand FAR 25, Appendix C to include 7. Develop small, turnkey, inexpensive in-situ SLD.

instruments for aircraft.

8. Investigate feasibility of dynamically calibrating SENSING REQUIREMENTS KNOWLEDGE remote-sensing systems from in-situ sensors.

Strengths 9. Develop sensing specifications for remote-sensing 1. General climatologies of icing are available.

systems.

2. Information desired to be downlinked to forecast- l0. Develop meteorologically indexed icing intensi- ers is known.

ty scale.

3. There is considerable data and confidence in FAR 11. Assess synoptic meteorological conditions within 25, Appendix C, icing-condition characterization.

which remote sensing would be most practical.

4. General research instrumentation is generally ade- 12. Identify appropriate test beds.

quate.

5. Appropriate test platforms are available.

NASA/CR--2000-209938 64

APPENDIX C: SYNOPSIS OF SENSOR TECHNOLOGY NEEDS, STATE OF KNOWLEDGE, STRENGTHS, AND WEAKNESSES SENSOR TECHNOLOGY NEEDS wavelengths allows liquid-water content to be retrieved.

l. Establish sensing needs for phenomena to be 8. Millimeter-wavelength radars most suited for sensed, including range and resolution.

cloud-water sensing.

2. Develop inversion theory to determine most appropriate technologies for sensing 3-D spatial 9. Widerranges of drop sizes in precipitating clouds increase the need for a multiple-wavelength (more structure of liquid-water content, drop-size spec- tra, and temperature ahead of aircraft. than two) radar system.

10. Differential attenuation techniques may require 3. Develop prototype hardware for testing theory for each selected component of remote-sensing droplet temperature for accurate assessment of liquid-water content.

system.

4. Select test bed for proving concepts developed 11. Gossett and Sauvageot (1992) theoretically demonstrated that 3.2-cm (X) and 0.87-cm CK a) in theory, develop test plans, and test.

5. Develop airborne prototype system component bands are the best for detecting water in clouds, but that other wavelength pairs are possible, prototypes.

6. Test airborne prototypes on flight platform and depending upon desired range and the existence verify capabilities. of hydrometeors.

7. Develop integrated system comprising all compo- 12. Millimeter-wavelength radars are suited to air- borne cloud studies because of their small size, nents, with processing and display system com- patible with onboard systems and protocol. low ground-clutter susceptibility, high resolution, 8. Test integrated prototype system on civilian and and sensitivity to small hydrometeors.

military flight platforms. 13. It may not be possible to uniquely define cloud 9. Certify system. drop-size distributions with radar. Only parame- ters of a distribution may be available.

STATE OF KNOWLEDGE Passive radiometers Radar 1. Operationally used to measure zenith and nadir 1. Ground (vertical scanning) and airborne (hori- temperature profiles, with thermal and spatial reso- .................. with distance from radiometer.

zontal scanning) systems are technically possible.

2. Integrated liquid-water path sensed in vertical at 2. Range-resolved liquid water measurements have 31.6 GHz with scanning possible.

been acquired from clouds.

3. Polarization techniques can be used to detect ice 3. Integrated liquid water might be sensed in hori- vs. water. zontal to provide integrated water that an aircraft could intercept.

4. Doppler techniques to detect droplet sizes may 4. Modeling and experiments needed at test beds.

not be possible with horizontally scanning sys- tems. 5. Precipitation can cause difficulties--modeling is needed.

5. Longer wavelengths (lower frequencies) cannot 6. Cloud phase may be possible with polarimetry.

detect smaller droplets but have longer range and 7. Scanning liquid-water radiometer under devel- can operate within Rayleigh regime to larger drop opment provides range resolution.

sizes. Useful for detecting precipitation.

8. Passive technology advantage for cost, size, 6. Shorter wavelengths (higher frequencies) detect weight, power, general aviation, and military ap- smaller drop sizes but have shorter range and plications.

cannot operate within Rayleigh regime in larger 9. Model with RADTRAN or its successors.

drops.

7. Differential attenuation of two or more radar 10. Radiometer scanning is slow.

NASA/CR--2000-209938 65 Lidar 4. A scanning liquid-water radiometer under devel- opment provides range resolution.

1. Scatter proportional to number density of drops.

5. Passive technology has an advantage for cost, 2. High spatial resolution.

size, weight, power, general aviation, and mili- 3. Multiple-field-of-view lidars can indicate effec- tive drop diameter, but distribution must be assumed tary applications.

6. Model with RADTRAN or its successors.

4. Liquid-water content can be measured.

7. Radiometers can be small and use little power.

5. Can estimate relative amount of ice crystals from amount of polarization.

Lidar 6. Current MFOV lidars not eye-safe, but could be 1. Pulses can be only 2 to 3 m long, yielding very at 1.54 or 2.0 gm with 1.5- to 3-m resolution.

7. Could be placed on an aircraft. high spatial resolution.

2. Multiple field-of-view lidars can indicate effec- 8. Detect to clouds through clear ,'fir, but cloud extinc- tion allows only few hundred meters penetration. tive drop diameter and liquid water content.

3. Can retrieve relative amount of ice crystals from Temperature measurements amount of polarization.

4. Multiple field-of-view lidars could be eye safe at 1. RASS profiles temperature to radiosonde accura- 1.54 or 2.0 mm with 1.5- to 3-m resolution.

cy to over 3.5 km, even inversions.

2. RASS has not been used on moving vehicles or in 5. Five-watt power demand, 6- to 8-in. receiving lens, the horizontal. 100 pulses s-l, l-m 3 volume, could be placed on an aircraft.

3. Pitch, roll, and yaw and aircraft speeds prevent RASS use on aircraft. 6. Lidar currently used for operational onboard wind shear alert.

4. RASS acoustic source could be aircraft engine noise, but turbulence and relative wind could cause 7. Small and inexpensive.

8. Rapid scanning possible.

loss of signal for up to 1 min depending upon air- craft speed and heading.

Temperature measurement 5. Radiometers may sense temperature in horizon- 1. RAgS sounds temperature with radiosonde accu- tal. Might try tunable system operating in the vicin- racy, even through inversions.

ity of the oxygen absorption band, with tuning 2. Radiometers are used operationally to create ver- providing range resolution.

tical temperature profiles.

61 Radiometers provide lower-resolution vertical 3. Radiometers may sense temperature in horizon- temperature profiles.

tal. Might try tunable system operating in the 7. Lidar not applicable.

vicinity of the oxygen absorption band, with tun- ing providing range resolution.

TECHNOLOGY KNOWLEDGE STRENGTHS Radar TECHNOLOGY KNOWLEDGE WEAKNESSES 1. Ground (vertical scanning) and airborne (horizon- Radar tal scanning) systems are possible.

1. Doppler techniques to detect droplet sizes not 2. Successful attempts have been made to acquire possible with horizontally scanning systems.

range-resolved liquid water from clouds.

2. Wider ranges of drop sizes in precipitating clouds, 3. Polarization techniques can be used to detect ice vs. water. for example, increases the need for a multiple- wavelength (more than two) radar system.

4. Millimeter-wavelength radars are suited to air- borne cloud studies because of their small size, 3. Dual-wavelength differential attenuation tech- niques require temperature for accurate liquid- low ground-clutter susceptibility, high resolution, water content.

and sensitivity to small hydrometeors.

4. Gossett and Sauvageot (1992) theoretically dem- 5. Radar scans rapidly.

onstrated that X and K a bands are the best for Passive radiometers detecting water in clouds, but that other wave- length pairs are possible. Modeling is needed.

1. Operationally used to measure zenith and nadir 5. It may not be possible to uniquely define cloud temperature profiles, and possibility in horizontal.

drop-size distributions with radar.

2. Integrated liquid-water path sensed in vertical with 6. Dual-frequency radar at X and Ka bands cannot scanning possible.

detect small drop diameters to a long range, nor 3. Cloud phase may be possible with polarimetry.

NASA/CR--2000-209938 66 Temperature measurement

fiquid-water content smaller than 0.2 gm -3(Mart-

1. RASS is operational from the ground and has not

neratal.1993)

been used on vehicles or in the horizontal.

7. W-band radar (95GHz) may besuitable butwith

2. Pitch, roll, and yaw and aircraft speed prevent

short range and great attenuation bylarge liquid-

RASS use on aircraft.

water content, large drops, and water vapor. Mod-

3. Radiometers cannot sense temperature in hori- eling is needed.

zontal using traditional inversion methods.

Passive radiometers 4. Radiometers scan slowly.

5. Lidar not applicable.

1. Liquid water might be sensed in horizontal to pro- vide integrated water that an aircraft might inter- General goals cept.

1. Perform feasibility studies of ability of technolo- 2. Temperature cannot be sensed in horizontal using conventional inversion methods.

gies to provide liquid-water content, drop size, and temperature information needed.

3. Modeling and experiments needed at test beds.

2. Develop methods of assessing feasibility studies 4. Precipitation can cause difficulties at some wave- through measurements with hardware.

lengths; modeling is needed.

3. Develop prototype technologies for field testing.

5. Cloud phase cannot be detected with polarimetry.

Lidar 1. High extinction in clouds.

2. Eye safety.

NA SA/CR---2000- 209938 67 REPORT DOCUMENTATION PAGE Form Approved OMB No. 0704-0188 Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for re'dewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reporls, 1215 Jefferson Davis Highway, Suite 1204, Adington, VA 22202-4302, and to the Office of Management and Budget, Paperwork Reduction Project (0704-0188), Washington, DC 20503.

1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED March 2000 Final Contractor Report 4. TITLE AND SUBTITLE 5. FUNDING NUMBERS Remote Sensing of In-Flight Icing Conditions: Operational, Meteorological, and Technological Considerations WU-548-21-234)0 Customer Order Number 6. AUTHOR(S) C-73343-E Charles C. Ryerson 8. PERFORMING ORGANIZATION 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) REPORT NUMBER U.S. Army Cold Regions Research and Engineering Laboratory E-12186 72 Lyme Road Hanover, New Hampshire 03755-1290 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSORING/MONITORING AGENCY REPORT NUMBER National Aeronautics and Space Administration John H. Glenn Research Center at Lewis Field NASA CR--2000-209938 Cleveland, Ohio 44135-3191 ERDC-CRRELM-4)0-1 11. SUPPLEMENTARY NOTES Project Manager, Andrew L. Reehorst, Turbomachinery and Propulsion Systems Division, NASA Glenn Research Center, organization code 5840, (216) 433-3938.

12a. DISTRIBUTION/AVAILABILITY STATEMENT 12b. DISTRIBUTION CODE Unclassified - Unlimited Subject Categories: 03 and 32 Distribution: Nonstandard This publication is available from the NASA Center for AeroSpace Information, (301) 621-0390.

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13. ABSTRACT (Maximum 200 words) Remote-sensing systems that map aircraft icing conditions in the flight path from airports or aircraft would allow icing to be avoided and exited. Icing remote-sensing system development requires consideration of the operational emfronment, the meteorological environmem, and the technology available.

Operationally, pilots need unambiguous cockpit icing displays for risk management decision-making. Human factors, aircraft integration, integration of remotely sensed icing information into the weather system infrastructures, and avoid-and-exit issues need resolution. Cost, maintenance, power, weight.

and space concern manufacturers, operators, and regulators. An icing remote-sensing system detects cloud and precipitation liquid water, drop size, and temperature. An algorithm is needed to convert these conditions into icing potential estimates for cockpit display. Specification development requires that magnitudes of cloud microphysical conditions and their spatial and temporal variability be understood at multiple scales.The core of an icing remote- sensing system is the technology that senses icing microphysical conditions. Radar and microwave radiometers penetrate clouds and can estimate liquid water and drop size. Retrieval development is needed; differential attenuation and neural network assessment of multiple-band radar returns are most promising to date. Airport-based radar or radiometers are the most viable near-tern1 technologies. A radiometer that profiles cloud liquid water, and experi- mental techniques to use radiometers horizontally, are promising. The most critical operational research needs are to assess cockpit and aircraft system integration, develop avoid-and-exit protocols, assess human factors, and integrate t_mote-sensing infom_ation into weather and air traffic control infrastruc- tures. Improved spatial characterization of cloud and precipitation liquid-water content, drop-size spectra, and temperature are needed, as well as an al- gorithm to convert sensed conditions into a measure of icing potential. Technology development also requires refinement of inversion techniques. These goals can be accomplished with collaboration among federal agencies including NASA, the FAA, the National Center for Atmospheric Research, NOAA, and the Department of Defense. This report reviews operational, meteorological, and technological considerations in developing the capability to remotely map in-flight icing conditions from the ground and from the air.

14. SUBJECT TERMS 15. NUMBER OF PAGES Aircraft icing; DoD; Drop size; FAA; Human factors; In-flight icing; Lidar; Liquid water 16. PRICE CODE content; NASA; Operations; Radar; Remote sensing; Temperatures A05 20. LIMITATION OF ABSTRACT 17. SECURITY CLASSIFICATION 18. SECURITY CLASSIRCATION 19. SECURITY CLASSIFICATION OF REPORT OF THIS PAGE OF ABSTRACT Unclassified Unclassified Unclassified Standard Form 298 (Rev. 2-89) NSN 7540-01-280-5500 Prescribed by ANSI Std. Z39-18 298-102