244 WEATHER AND FORECASTING VOLUME 13

History of Operational Use of Weather Radar by U.S. Weather Services. Part II: Development of Operational Doppler Weather Radars

ROGER C. WHITON* AND PAUL L. SMITHϩ Air Weather Service, Scott Air Force Base, Illinois

STUART G. BIGLER National Weather Service, Washington, D.C.

KENNETH E. WILK National Severe Storms Laboratory, Norman, Oklahoma

ALBERT C. HARBUCK# Air Weather Service, Scott Air Force Base, Illinois (Manuscript received 14 March 1997, in ®nal form 19 February 1998)

ABSTRACT The second part of a history of the use of storm surveillance radars by operational military and civil weather services in the United States is presented. This part describes the genesis and evolution of two operational Doppler weather radars, the Next-Generation Weather Radar and Terminal Doppler Weather Radar.

1. Advances made by Doppler radar operational application of this capability. Thus, opera- meteorological research tional use of Doppler weather radar had to await the development of pulse-Doppler technology (that provid- The wartime Rad Lab investigators recognized the ed the range capability) for the extraction of moments possibility that radar systems could employ the Doppler such as mean radial velocity and spectrum width from effect to measure target velocities. This offered a po- tential for remote measurement of wind speeds. The pulse Doppler spectra and techniques for interpreting Weather Bureau followed up on this potential beginning the velocity patterns observable with a single radar. in fall 1956 and continued through 1960. This early Rogers (1990) reviews the history of early efforts to effort involved conducting tests on an experimental, 3- apply Doppler techniques in radar meteorology. cm, continuous-wave (CW) Doppler weather radar sys- In 1961, the Air Force Cambridge Research Labo- tem at Wichita Falls, Texas, and Wichita, Kansas. Al- ratories (AFCRL) put into operation a 5-cm pulsed though plagued by noisy magnetrons and attenuation by Doppler radar called Porcupine that was adapted for rain, these early systems were capable of detecting 205- meteorological measurements. A signal and data pro- mph (ϳ30 km hϪ1) winds near a tornado vortex (Holmes cessor called the plan shear indicator (PSI) (Armstrong and Smith 1958; Rockney 1960; Smith and Holmes and Donaldson 1969) was developed in connection with 1961). The inability of a CW radar system to determine the Porcupine Doppler to enable the Doppler data to be the range to the target was a serious impediment to displayed in real time. The PSI employed a coherent memory ®lter (CMF) to make coarse, real-time Doppler *Current af®liation: Science Applications International Corpora- spectral analyses over the entire range of the radar (Chi- tion, O'Fallon, Illinois. mera 1960; Atlas 1963; Groginsky 1965, 1966). Using ϩ Current af®liation: Institute of Atmospheric Sciences, South Da- the PSI, the ®rst mesocyclone detected by Doppler radar kota School of Mines and Technology, Rapid City, South Dakota. # Current af®liation: Amherst Systems, Inc., Warner Robins, Geor- was recorded on 9 August 1968 (Donaldson et al. 1969). gia. Donaldson (1970) investigated Doppler radar's ability to resolve vortices of different sizes and showed that the mesocyclone, which Fujita and others had linked Corresponding author address: Dr. Roger C. Whiton, SAIC, 619 W. Hwy 50, O'Fallon, IL 62269. with tornadoes, could be identi®ed at more distant rang- E-mail: [email protected] es than the smaller tornado vortex signature could.

᭧ 1998 American Meteorological Society

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Based on this, Donaldson (1970) developed a set of the radar. Use of a second Doppler radar with a different minimum values of shear, persistence, and vertical ex- viewing aspect permits determination of the full wind tent as requisites for identi®cation of a mesocyclone vector (with some restrictions). Dual-Doppler radar ob- signature. Kraus (1973) identi®ed the Brookline, Mas- servations were conducted by Peace and Brown (1968) sachusetts tornado of 1972 based on its vortex signature. as early as 1967, by Lhermitte (1970) and Lhermitte Investigations by the Weather Bureau to determine and Miller (1970) in 1969, by the National Hail Re- desirable characteristics of a Doppler weather radar and search Experiment (Knight and Squires 1982) in the its utility in meteorology were continued in the 1960s 1970s, and by others. Scan procedures and methods for at the National Severe Storms Laboratory (NSSL) (e.g., deriving two- and three-dimensional velocity structures, Lhermitte and Kessler 1964). The initial experiments for regions containing precipitation echoes, were de- used an X-band system with a comb ®lter for spectral veloped. It was evident, however, that for operational analysis of the echoes; this system used the same trailer purposes improved means for displaying and interpret- and some of the hardware employed in the Smith and ing the velocity patterns observed by a single Doppler Holmes (1961) experiments (E. Kessler 1997, personal radar would be necessary. The fortuitous advent of af- communication). In 1971, NSSL put into operation their fordable microprocessors for digital signal processing ®rst S-band Doppler weather radar designed speci®cally capable of operating in a real-time environment and the for severe storm studies (Sirmans and Doviak 1973). later availability of high-resolution, graphical, color dis- NSSL added a similar system at Cimarron Field, 42 km plays, greatly facilitated these Doppler radar develop- northwest of NSSL, in 1973, with full dual-Doppler ments. Fundamental research using dual-Doppler data operations beginning in 1974. Using these radars, Bur- produced the understanding needed to interpret the sin- gess, Brown, and others frequently observed mesocy- gle-Doppler velocity patterns. clone signatures and produced some of the ®rst real- The early interest focused on the storm-scale velocity time Doppler displays. The ®rst reported tornado vortex patterns, which would provide clues to storm severity. signature was associated with the Union City tornado However, the color plan position indicator (PPI) velocity (Burgess et al. 1975). Keystone research in the structure displays also revealed intriguing patterns even in wide- of mesocyclones (Burgess 1976) and tornado vortices spread stratiform precipitation (Kraus and Donaldson (Brown and Lemon 1976), combined with an abundance 1976). They were identi®ed as re¯ections of the vari- of severe storms in Oklahoma, helped NSSL set the ation of the wind velocity with height; sometimes the stage for the Joint Doppler Operational Project (JDOP) patterns contained sharp gradients indicating the pres- by providing the fundamental knowledge of storm dy- ence of frontal boundaries (Wilson et al. 1980). Such namics, based on dual-Doppler information, that was information is useful in weather analysis and forecast- necessary before single-Doppler schemes could be test- ing. Wood and Brown (1986) provide a convenient sum- ed in JDOP. mary of the characteristics of these velocity patterns and In signal processing, important mathematical under- their interpretation. pinnings had been provided by Cooley and Tukey The increased sensitivity of weather radar systems, (1965), developers of the fast Fourier transform, and by due in part to the Doppler processing performed on the the real-time Doppler velocity processing schemes of signal, meant that some of the velocity patterns could Rummler (1968a,b,c) and Miller and Rochwarger even be observed in clear air, adding to the forecasting (1972). In 1974, based on Rummler's scheme, Grogin- value of the data. The velocity azimuth display (VAD) sky (1972), Lhermitte (1972), and Novick and Glover technique earlier developed by Lhermitte and Atlas (1975) placed into operation the ®rst multichannel pulse (1961) had been improved upon by Browning and Wex- pair processor (PPP) at the AFCRL. Work at NSSL (e.g., ler (1968). That fundamental underpinning would later Sirmans and Bumgarner 1975) led to improvements in be extended by Rabin and Zrnic (1980) to provide wind the PPP technique; using different pulse repetition in- information in the clear air using data from a single tervals (Sirmans et al. 1976; Doviak et al. 1978) pro- Doppler radar. Rabin and ZrnicÂ's work would later serve vided a means of resolving some of the range-Doppler as the fundamental basis of one of the Weather Sur- ambiguities troublesome even with S-band radars. The veillance Radar-88 Doppler's (WSR-88D's) most im- improved PPP technique was implemented on the NSSL portant operating modes, the clear-air or VAD-wind radars in 1975 by Sirmans and at the National Center mode. for Atmospheric Research (NCAR) by Gray et al. Based in part on the early echo isolation algorithms (1975). After the PPP made possible real-time Doppler developed in the late 1960s, Captain D. Forsyth, then velocity calculations, color monitors, then becoming an Air Force of®cer assigned to the Air Force Geo- commercially available, permitted real-time color dis- physics Laboratory (AFGL), was asked by K. Glover plays of the radar re¯ectivity factor, mean radial veloc- to develop an echo-tracking algorithm. Working with ity, and spectrum width (Gray et al. 1975; Jagodnik et his co-investigator Captain C. Bjerkaas, another Air al. 1975). Force of®cer assigned to the laboratory, Forsyth pro- A single Doppler radar measures only one component duced the tracking algorithm that was used with some of the wind velocity, namely, the component radial to success in JDOP (see section 3). From 1980 to 1982,

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Forsyth continued developing the algorithm after mov- WSR-57 without need for participation by the Depart- ing to NSSL. The improved algorithm was used in the ment of Transportation or DOD (Johannessen and Kes- Boston area Next-Generation Weather Radar (NEX- sler 1976). RAD) demonstration in 1983±84. The algorithm was AWS in 1976 began considering the operational use later incorporated into the WSR-88D as the storm series of air traf®c control radars as an alternative to acquiring of segment identi®cation, centroid location, tracking, a follow-on weather radar. A feasibility study showed position forecasting, and structure analysis routines that the air traf®c control alternative was not very ca- (Forsyth et al. 1981). Other algorithms for mesocyclone, pable, but the alternative was inexpensive and therefore tornado vortex, and hail detection were developed later attractive. Captain T. Linn at AWS headquarters be- and ®elded with NEXRAD. lieved strongly that a single, national Doppler weather Operational radar meteorologists, except those at a radar system was affordable and certainly a better al- few uniquely equipped radars, did not have access to ternative than using air traf®c control radars. In frus- this advanced technology; however, they could see that tration over not being able to advance what he believed the results being achieved in the research community to be the best alternative, Linn ®nally wrote his con- might have the potential for a great payoff if they could gressman recommending a single national system be be afforded on operational weather radar systems. developed. A Congressional inquiry was made in 1976, and in order to develop a response, a triagency meeting was held on the feasibility of converging on a single 2. Limitations of operational weather radars and development effort. As to whether the target system identi®cation of the need for follow-on, would be a Doppler radar, it was recommended that a improved weather radar systems study be made on the technical characteristics and ben- Beginning in 1971, Air Weather Service (AWS) rec- e®ts of an operational Doppler weather radar system. ognized a need for a follow-on weather radar, a suc- cessor to the aging FPS-77. Limitations in the FPS-77's 3. Joint Doppler Operational Project (JDOP) designÐparticularly its limited quantitative capabilities and lack of associated data processing and communi- The two principal laboratories involvedÐthe De- cationsÐeven then drove AWS to consider alternatives partment of Commerce's NSSL and the air force's for a successor system. Because at that time it took 10 AFGLÐagreed to combine their capabilities under yr to build the requirements documentation, fund, pro- NSSL's leadership to make the project a success. This duce, and ®eld a major sensing system that would be agreement was the genesis of the Joint Doppler Oper- maintainable by the Department of Defense (DOD), ational Project. As part of the preparation before JDOP, AWS stated a requirement in the 1970s for such pro- a working group included several members from AWS cessing as a part of what was then called the Weather headquarters, including Captain R. Bonesteele, one of Radar of the 1980s program. At that time, there was no the present authors (ACH) and D. Sirmans of NSSL. consensus (and no budget) for a Doppler weather radar; One of the purposes of that working group was to de- Doppler systems were considered expensive research velop some of the requirements for quantitative mea- tools whose operational bene®ts had not yet been quan- surements using JDOP radars and displays. The group's ti®ed. Early requirements statements for the follow-on work focused on alignment capability, calibration pro- system did not call explicitly for a Doppler capability cedures, and basic testing requirements. but did not exclude it either (Barad et al. 1973). Re- During 1977, the Federal Committee for Meteoro- search in the 1970s pointed strongly toward the feasi- logical Services and Supporting Research established bility of an operational Doppler weather radar system an interagency working group for the Next-Generation that might be expensive, although affordable. In 1976, Weather Radar. R. Bonesteele laid the planning ground- Dr. D. Atlas, a leader in radar meteorological research work for the JDOP effort but moved to the Pentagon and president of the American Meteorological Society, in 1978, where he established research and development produced testimony on severe storm identi®cation tech- funding lines for the air force portion of what would niques before the U.S. House of Representatives Sub- later be called NEXRAD. Other members of the working committee on Environment and the Atmosphere, rec- group are listed in the JDOP Final Report (JDOP Staff ommending that the nation's next operational weather 1979). JDOP itself began in 1977 and ended in 1979, radar be a Doppler system. Atlas was joined by Dr. E. with major participation from both NSSL and AFGL. Kessler of NSSL and other leading meteorologists in Federal Aviation Administration (FAA) participation in endorsing a ®eld experiment to test Doppler radar's po- JDOP began in late 1978. The project was as much an tential for operational use. operational effort as a research effort, and its objectives The nation's operational weather agencies, at the time, were operationally focused. Operational weather organ- had their own, separate programs for follow-on radar izations such as AWS and NWS as well as the research systems. In 1976, the National Weather Service (NWS) laboratories were well represented in JDOP. One of the and NSSL announced their intention to design, acquire, present authors (KEW) served as overall project coor- and ®eld a Doppler weather radar to replace the aging dinator, as NSSL had been selected as the location of

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JDOP. R. Bonesteele served as principal agency rep- TABLE 1. Pioneers, contributors and supporters, and JDOP resentative for the air force, while Major T. Sieland had participants (D. Forsyth 1996, personal communication). the same responsibility for AWS. Coinvestigators from Contributors and AWS included Captains J. Bonewitz (JDOP during Pioneers supporters JDOP participants 1977), D. Forsyth, M. Mader, and M. Snapp. The AFGL Graham Armstrong Glen Anderson Ron Alberty Weather Radar Branch was represented by K. Glover. Dave Atlas Rick Anthes Bob Allen His research team included C. Bjerkaas and R. Donald- Dan Barczys Ken Banis Carl Bjerkaas son. NSSL's principal investigator was D. Burgess, with Louis Battan Stanley Barnes Ray Bonesteele warning coordination assistance provided by D. Devore, Ed Brandes Harold Baynton Dave Bonewitz Jim Brantley Al Bishop Norm Chaney a forecaster from the Oklahoma City forecast of®ce. Rodger Brown Bill Bumgarner Chuck Clark With the progress made in Doppler radar signal and data Keith Browning Rit Carbone Ray Crooks processing, AFGL was able to develop real-time signal Don Burgess John Carter Bob Davies-Jones analysis, data processing, and display software (includ- Tony Chimera Al Chmela Don Devore ing but not limited to echo tracking) that was the air Ralph Donaldson Ken Crawford J.T. Dooley Dick Doviak Rosemary Dyer Bob Elvander force's most signi®cant contribution of weather radar Ken Glover Cal Easterbrook Doug Forsyth technology to JDOP. Grant Gray Jim Evans David George JDOP bridged the gap between research and opera- Herb Groginsky Ted Fujita Izzy Goldman tions, as it considered operational details such as the Dave Holmes Speed Geotis Doug Greene type of displays most usable by operational meteorol- Ed Kessler Ken Hardy Jack Hinkelman Mike Kraus Larry Hennington Karl Johannessen ogists. Many of JDOP's results pointed strongly to de- Les Lemon Ted Linn William Klein sign speci®cs that a successful operational Doppler sys- Roger Lhermitte Jay Miller Al Koscielny tem would require. For example, during JDOP's use of Gene Mueller Allan Pearson Jean Lee AFGL's C-band Porcupine Doppler to observe the Wich- Bob Peace Bob Saf¯e Mike Mader ita Falls storm of April 1979, microwave attenuation Rod Rogers Mike Schmidt Mary McCoy caused by precipitation from an intervening storm Dale Sirmans Bob Sera®n Dick Mitchem Robert Smith Buzz Shinn Pio Petrocchi caused the Wichita Falls storm to disappear completely John Theiss Bill Smith Peter Ray from the displays of the C-band radar, whereas the S- Ray Wexler Paul Smith Chuck Safford band NSSL radar was able to observe it with no problem Dusan Zrnic Gene Walker Ken Shreeve (Allen et al. 1981). That ®nding later became useful in Roger Whiton Tom Sieland successfully combating the challenge that the S-band Jim Wilson Mike Snapp Merritt Techter WSR-88D was too expensive and should be replaced John Weaver by a more economical C-band Doppler system. Ken Wilk JDOP concluded that Doppler radar is superior to Allen Zahrai conventional radar and storm spotters; it increases warn- Dave Zittel ing lead time for tornadoes from about 2 min to 20 min, reduces false alarm ratios for tornadoes and severe thun- derstorms, and improves probability of detection for se- of the system that should be built. The interagency vere thunderstorms. JDOP further concluded that a working group wrote, in July 1979, a NEXRAD Concept Doppler radar with a suitably narrow beamwidth can Report. In the same year, J. Bonewitz and Lieutenant identify potentially severe thunderstorms by detecting Colonel C. Tidwell, then working out of the Of®ce of the mesocyclone signature at long ranges (230±350 km), the Federal Coordinator for Meteorological Services and whereas Doppler's capability to separate tornadic from Supporting Research, performed the cross-cut (multi- nontornadic storms is limited to closer ranges (less than agency) studies that obtained Of®ce of Management and 230 km). JDOP speci®cally recommended that the next- Budget support for NEXRAD. Triagency funding lines generation weather radar have a Doppler capability were then established. The stage was set for formation, (JDOP Staff 1979). in January 1980, of a Joint System Program Of®ce From 1960 through the end of JDOP, the pioneers, (JSPO). Assigned as the initial Air Force members of contributors, and supporters, and JDOP participants a JSPO that then consisted of only eight people were shown in Table 1 were instrumental in bringing the na- C. Tidwell, DOD deputy program manager, and J. Bo- tion's operational Doppler weather radar system into newitz, radar meteorologist. The program manager was operation (D. Forsyth 1996, personal communication). A. Hansen, NWS. S. Williamson later served as deputy If you know of anyone whose name was inadvertently program manager, NWS. J. Sowar was deputy program left off this list, please send the name and achievements manager, FAA. The JSPO's engineer was F. Blake, to D. Forsyth at NSSL. NWS. Differing agency requirements and program man- agement methods, ®scal pressures, and some distrust 4. Joint System Project Of®ce (JSPO) and Interim among the agencies made the ®rst year dif®cult. A. Dur- Operational Test Facility (IOTF) ham, the second JSPO program manager, was able to JDOP strongly demonstrated the usefulness of an op- cut through some of the problems and move the program erational Doppler weather radar and indicated the nature in a multiagency direction. The leadership of Dr. E.

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Friday Jr., then director of NWS, and N. Blake, then at a computer-display subsystem called the principal FAA's deputy associate administrator for engineering user processor (PUP). The user has limited interaction and development, was responsible for keeping the WSR- with the radar and more substantial interaction with the 88D program going during the troublesome ®rst years. database, displays, and algorithm products. Many ca- Despite problems and disagreements, the JSPO authored pabilities have been gained (e.g., continuous coverage a joint operational requirements (JOR) document and a of the full surveillance volume, time-lapse sequences of NEXRAD technical requirements (NTR) document in the radar echo patterns, and machine-assisted interpre- 1981. A research and development (R&D) plan was tation of the data) but some have been lost. As an ex- written and a technical advisory committee established ample, although a low-resolution, synthetic range-height to guide the R&D program. indicator (RHI) display can be constructed using volume The JSPO Interim Operational Test Facility (IOTF) scan data, the operator can no longer perform vertical was established in Norman, Oklahoma in 1981 to con- sector scans using the radar to obtain a high-resolution tinue to make improvements and test concepts for NEX- RHI. RAD, under the guidance of one of the authors (KEW) WSR-88D products are summarized in Klazura and assisted by D. Zittel. In 1982, the DOD selected D. Imy (1993). Archival datasets produced by the WSR- Forsyth as deputy director (DOD) of the IOTF, soon 88D system are described in Crum et al. (1993). followed by IOTF members T. O'Bannon and M. Istok. The ®rst operational WSR-88D system (Twin Lakes, After that the operational NEXRAD system was com- Oklahoma, near Norman) was installed in fall 1990. The petitively acquired and ®elded as the successful WSR- last of 158 operational WSR-88Ds was installed in 88D. Many of the concepts implemented in the WSR- northern Indiana in late June 1997. 88D were originated by the IOTF. During development In the context of the WSR-88D, the concept of a of the WSR-88D, an extensive set of operating manuals ``radar'' can be considered to include both the radar data was published as Federal Meteorological Handbook-11 acquisition and the radar product generation units; the (FMH-11). Acquisition and ®elding included training PUP, used for display, is considered separately. WSR- courses for the meteorologists who would be users of 88Ds not readily accessible or otherwise dif®cult to the system and the electronics technicians who would maintain are installed in a redundant con®guration with maintain it. an operational backup ready to continue to supply crit- ical radar weather data if the primary unit fails. For that reason, the total number of radar systems exceeds the 5. WSR-88D total number of radar installations. NWS has now ac- The WSR-88D system, manufactured by Sperry and cepted 117 operational systems (three more will be ac- successor ®rms, is described in Crum and Alberty cepted in the future), FAA 12, and DOD 26; six radar (1993). It is an S-band system with Doppler capability. systems across all the agencies have been accepted for Other important improvements over the WSR-57, WSR- use in maintenance and training. The total across all 74C, and S, FPS-77, and FPQ-21 systems it replaces agencies for all purposes is 161 WSR-88D radar sys- include the following: tems, including the redundant or backup units. Equally important are the PUPs, the unit at which the operational R Matched-®lter receiver design, a narrow, 1Њ beam- meteorologist works. NWS has accepted 193 PUPs, width, and signal processing features that act together FAA 26, and DOD 188, for a total of 407, including to enhance sensitivity (the radar can detect radar re- those used for maintenance and training. ¯ectivity factors as low as Ϫ16 dBZ at a range of 50 All agencies with substantial weather radar programs km in the clear-air mode and Ϫ8dBZ at the same are now using the same radar as their principal radar range in the storm mode), improve spatial resolution, system, so modi®cations to the radar or improvements and enhance the radar's utility as a clear-air sensor; in operational techniques for its use, such as the inter- R Digital signal processing and extensive online data pretation algorithms, can be used advantageously by all processing to provide products (such as maps of echo the agencies, with associated cost savings. Some using tops or vertically integrated liquid) in addition to the agencies (i.e., NWS) intend to replace their PUPs with base data; forecaster workstations of the Automated Weather In- R Computer algorithms to identify signi®cant echo fea- formation Processing System; the ¯exibility of the tures (e.g., mesocyclones) and track echoes, still using WSR-88D's modular design is such that this can be done the nondynamical, centroid-tracking algorithm; easily without replacing the basic radar components. A R Quantitative calibration and measurement capabilities, continuing triagency NEXRAD product improvement and program is in place to provide ongoing upgrades to the R Color displays with many user-selectable features. system's hardware and software (Saf¯e and Johnson A major difference is that the operator no longer ma- 1997). nipulates the radar manually. Instead, the radar contin- The maintenance concept for the WSR-88D involves ually updates the three-dimensional database and frees organizational maintenance provided by the using agen- the operator for full-time data interpretation performed cy and depot-level maintenance provided by the NWS

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Kansas City Logistics Facility. The WSR-88D contains spheric Research, NWS, and AWS established the Co- facilities for both automatic and manual calibration; operative Program for Operational Meteorology, Edu- when automatic calibration adjustments exceed certain cation, and Training (COMET), which includes several thresholds, manual maintenance is performed. The NEX- modules on radar meteorology and Doppler weather ra- RAD Operational Support Facility (OSF) provides op- dar applications. erational and technical support for the WSR-88D and can perform radar calibration visits or maintenance 7. TDWR training visits when requested to do so. It has not been considered necessary to schedule routine calibration vis- The TDWR emerged from shortfalls of the NEXRAD its to WSR-88D sites. The bene®ts of the national net- program and the Joint Aviation Weather Studies con- work of these radars are thus made available at all the ducted by NCAR. In 1985, the FAA asked the Martin operational weather stations and weather centers that Marietta Air Traf®c Control Division to investigate al- need the information to perform their jobs effectively. ternatives for detection of wind shear affecting aircraft In ®elding the WSR-88D, it was realized that some on approach to airports. Two alternatives were inves- requirements cannot be met or are mutually incompat- tigated: the Low-Level Wind Shear Advisory System ible. When the shortfall requirements are so serious they (LLWAS) and the TDWR. The FAA had at ®rst decided cannot be met by modifying the ®elded system, they on the former because of its lower life cycle cost. The can form the basis for acquisition of a separate system 1985 crash of Delta Flight 191 at Dallas±Fort Worth that speci®cally meets the unful®lled needs. Such was airport caused the FAA to reopen the decision and to the case with the FAA requirement for rapidly updated request Martin Marietta expand on the advantages of surveillance of the weather close to airports. Those re- the TDWR and provide a re®ned cost estimate. Using quirements, which could not be met by the volume- the new analysis, the FAA was able to convince a re- scanning WSR-88D, formed the basis for the Terminal ceptive Congress to fund both the TDWR and the Doppler Weather Radar (TDWR) (see section 7). LLWAS (C. Tidwell 1997, personal communication). Under a contract awarded in November 1988 to the Raytheon Manufacturing Company, FAA is acquiring 6. Training 47 TDWRs, including two to be used for training and An important aspect of the WSR-88D program has testing. The TDWR is now being installed, principally been an expanded training program to ensure more ef- at large commercial airports that are vulnerable to low- fective use of the system's capabilities. level wind shear and microbursts. The TDWR has a The air force short course in radar meteorology was narrow beamwidth for improved spatial resolution and suspended between 1989 and 1993, because it contained a fast scan sequence for better temporal resolution over no instruction on the WSR-88D; the decision had been the limited region along and near air®eld approach and made to have WSR-88D radar meteorology instruction departure paths. The TDWR's scan strategy and algo- presented at the OSF's well-equipped and professionally rithms are particularly suitable for automated detection staffed training facility in Norman, Oklahoma. Initial and reporting of microbursts, wind shear, and gust WSR-88D operator training for both NWS and DOD fronts, which constitute serious hazards to aircraft on was conducted at the OSF, beginning in 1990, as a 20- approach or during takeoff. Initial speci®cations for the day course, later shortened to 18 days. At the height of TDWR had called for an S-band system. FAA's fre- the program, 720 students per year moved through the quency-control of®ce, aware of increased use of the 2.7± Norman course, attendance at which was a criterion for 2.9-GHz frequency band by the WSR-88D and many commissioning of NWS WSR-88Ds. Mobile training other systems, decided it should not ®eld another radar teams traveled to hub DOD WSR-88D sites to admin- in that band. FAA was aware that, with deployment of ister initial training. Beginning in 1993, the DOD radar the WSR-88D, many C-band weather radars such as the course, consisting of 24 training days, began being FPS-77 and WSR-74C would be decommissioned, taught at Keesler Air Force Base (AFB), Mississippi. opening up space in the C-band for potential use by the The National Weather Service Training Center provided TDWR. all WSR-88D maintenance training in a seven-week The FAA reasoned that the TDWR could be designed course. as a C-band system because Doppler data were required The quality of self-study material on radar meteorology only out to a range of 89 km, and range coverage could improved steadily. In the mid-80s, part B of FMH-7 was be traded for the shorter wavelength. The so-called revised and improved. In 1990, a summary of the use Doppler dilemma is that the product of the maximum of Doppler radar data to identify severe thunderstorms unambiguous range and maximum unambiguous veloc- was already available (Burgess and Lemon 1990). In ity is a constant, that constant being the speed of light the early 1990s, FMH-11, Doppler Radar Meteorolog- times the wavelength divided by 8. For a maximum ical Observations, was written and distributed to the unambiguous range of 89 km, the Doppler dilemma lim- ®eld for operational use and self-study. With deploy- its the C-band maximum unambiguous velocity to Ϯ24 ment of the WSR-88D, the National Center for Atmo- msϪ1. TDWR speci®cations, on the other hand, called

Unauthenticated | Downloaded 09/27/21 09:54 AM UTC 250 WEATHER AND FORECASTING VOLUME 13 for a maximum unambiguous velocity of Ϯ40 m sϪ1. practical problems. It is easier than it has been for a To meet that requirement out to a distance of 89 km, quarter-century to apply research results and interact Raytheon used a velocity dealiasing algorithm; to sup- productively with those conducting the research. press contamination by echoes from beyond the maxi- Naturally, the ®eld does not stand still. Techniques mum unambiguous range, they had to use a pulse rep- such as differential re¯ectivity, the more general polar- etition frequency (PRF) agility scheme, where the pro- ization diversity, bistatic weather radars, and multiple- cessor selects the PRF that works best in removing un- Doppler analysis have been developed that are not ca- wanted data from second and succeeding trips pabilities of today's operational weather radar systems. (Michelson et al. 1990). The interest in nearby weather Nevertheless, we are probably a lot closer to realizing echoes uncontaminated by ground clutter drove the de- further advances that have an operational payoff now sign toward a narrow beamwidth with exceptionally that the nation has ®elded operational Doppler systems high sidelobe suppression and additional features (in- and has a commitment to keeping them fully capable cluding the shorter wavelength) for ground clutter sup- and in step with research ®ndings. So the best is yet pression. In addition to providing Doppler mean radial ahead, as long as we heed D. Atlas's warning to train velocity and spectrum width data out to a range of 89 the nation's next generation of observing-oriented me- km, the TDWR also collects radar re¯ectivity factor data teorologists well. out to a range of 460 km using an interleaved scan sequence. Acknowledgments. The authors thank R. Donaldson The FAA's program manager for the TDWR, D. Turn- Jr., consultant to Hughes STX Corporation, for his thor- bull, is largely responsible for the program's success. ough review of an early version of this paper. We also Dr. J. Evans, MIT Lincoln Laboratory, led a multiagency appreciate the assistance proved by R. Bonesteele, C. team effort to extend relevant WSR-88D algorithms to Bjerkaas, T. Crum, R. Elvander, D. Forsyth, R. Kandler, operate in the TDWR environment and to provide new R. Saf¯e, T. Sieland, C. Tidwell, and J. Wieler for pro- algorithms to meet unique FAA requirements for reli- viding valuable information. The contribution of one of able, automated detection of microbursts and wind shear the authors (PLS) was supported in part by National events. Improved clutter suppression, a microburst de- Science Foundation Grant ATM-9509810. tection algorithm, and a gust front algorithm resulted from the initial work on behalf of the TDWR. After deployment of the TDWR, a machine-intelligent gust APPENDIX front algorithm was developed and is being deployed List of Acronyms and Abbreviations in 1998. Evans' team included S. Campbell, R. Delanoy, M. Eilts, D. Klingle-Wilson, M. Merritt, S. Olson, and AFB Air Force Base S. Troxel. Evans and Bernella (1994) provide an early AFCRL Air Force Cambridge Research Laborato- summary of the work. ries AFGL Air Force Geophysics Laboratory AWS Air Weather Service 8. Conclusions CMF Coherent memory ®lter Part of the fascination early radar meteorologists had COMET Cooperative Program for Operational Me- with their new ®eld must have been due to this remote teorology, Education, and Training sensor's ability to see cloud and precipitation processes CW Continuous wave at work in a direct way that would have been impossible DOD Department of Defense had they been restricted to conventional surface and FAA Federal Aviation Administration upper-air observations, even at the mesoscale. Surely FMH Federal Meteorological Handbook tomorrow's radar meteorologists will have no less a fas- IOTF Interim Operational Test Facility cination as they observe, measure, understand, and pre- JDOP Joint Doppler Operational Project dict the phenomena that seem puzzling and dif®cult to JOR Joint Operational Requirements understand today. Tempting as it might be to romanticize JSPO Joint System Program Of®ce over radar meteorology's past, it is likely that the best LLWAS Low-Level Wind Shear Advisory System years of the ®eld lie ahead. At no time in the past, except NCAR National Center for Atmospheric Research perhaps in the mid-1950s, were the instrumentation and NEXRAD Next-Generation Weather Radar techniques of the operational radar meteorologist closer NSSL National Severe Storms Laboratory to those of the research meteorologist than they are now. NTR NEXRAD technical requirements The tools in the hands of the practicing radar meteor- NWS National Weather Service ologist are far better now than they have ever been. The OSF Operational Support Facility prospects for advances due to applied research or tech- PPI Plan position indicator nique development on forecasting and warning prob- PPP Pulse pair processor lems are exceptionally promising as the nation builds PRF Pulse repetition frequency an archive of weather radar data that can be applied to PSI Plan shear indicator

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