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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. SMITH1

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 final 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 meteorological research The wartime Rad Lab investigators recognized the possibility that radar systems could employ the Doppler effect to measure target velocities. This offered a potential for remote measurement of wind speeds. The Weather Bureau followed up on this potential beginning in fall 1956 and continued through 1960. This early effort involved conducting tests on an experimental, 3cm, continuous-wave (CW) Doppler weather radar system at Wichita Falls, Texas, and Wichita, Kansas. Although plagued by noisy magnetrons and attenuation by rain, these early systems were capable of detecting 205mph (;30 km h21) winds near a tornado vortex (Holmes and Smith 1958; Rockney 1960; Smith and Holmes 1961). The inability of a CW radar system to determine the range to the target was a serious impediment to *Current affiliation: Science Applications International Corporation, O’Fallon, Illinois. 1 Current affiliation: Institute of Atmospheric Sciences, South Dakota School of Mines and Technology, Rapid City, South Dakota. # Current affiliation: Amherst Systems, Inc., Warner Robins, Georgia. Corresponding author address: Dr. Roger C. Whiton, SAIC, 619 W. Hwy 50, O’Fallon, IL 62269. E-mail: [email protected]

q 1998 American Meteorological Society

operational application of this capability. Thus, operational use of Doppler weather radar had to await the development of pulse-Doppler technology (that provided the range capability) for the extraction of moments such as mean radial velocity and spectrum width from pulse Doppler spectra and techniques for interpreting the velocity patterns observable with a single radar. Rogers (1990) reviews the history of early efforts to apply Doppler techniques in radar meteorology. In 1961, the Air Force Cambridge Research Laboratories (AFCRL) put into operation a 5-cm pulsed Doppler radar called Porcupine that was adapted for meteorological measurements. A signal and data processor called the plan shear indicator (PSI) (Armstrong and Donaldson 1969) was developed in connection with the Porcupine Doppler to enable the Doppler data to be displayed in real time. The PSI employed a coherent memory filter (CMF) to make coarse, real-time Doppler spectral analyses over the entire range of the radar (Chimera 1960; Atlas 1963; Groginsky 1965, 1966). Using the PSI, the first mesocyclone detected by Doppler radar was recorded on 9 August 1968 (Donaldson et al. 1969). Donaldson (1970) investigated Doppler radar’s ability to resolve vortices of different sizes and showed that the mesocyclone, which Fujita and others had linked with tornadoes, could be identified at more distant ranges than the smaller tornado vortex signature could.

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Based on this, Donaldson (1970) developed a set of minimum values of shear, persistence, and vertical extent as requisites for identification of a mesocyclone signature. Kraus (1973) identified the Brookline, Massachusetts tornado of 1972 based on its vortex signature. Investigations by the Weather Bureau to determine desirable characteristics of a Doppler weather radar and its utility in meteorology were continued in the 1960s at the National Severe Storms Laboratory (NSSL) (e.g., Lhermitte and Kessler 1964). The initial experiments used an X-band system with a comb filter for spectral analysis of the echoes; this system used the same trailer and some of the hardware employed in the Smith and Holmes (1961) experiments (E. Kessler 1997, personal communication). In 1971, NSSL put into operation their first S-band Doppler weather radar designed specifically for severe storm studies (Sirmans and Doviak 1973). NSSL added a similar system at Cimarron Field, 42 km northwest of NSSL, in 1973, with full dual-Doppler operations beginning in 1974. Using these radars, Burgess, Brown, and others frequently observed mesocyclone signatures and produced some of the first realtime Doppler displays. The first reported tornado vortex signature was associated with the Union City tornado (Burgess et al. 1975). Keystone research in the structure of mesocyclones (Burgess 1976) and tornado vortices (Brown and Lemon 1976), combined with an abundance of severe storms in Oklahoma, helped NSSL set the stage for the Joint Doppler Operational Project (JDOP) by providing the fundamental knowledge of storm dynamics, based on dual-Doppler information, that was necessary before single-Doppler schemes could be tested in JDOP. In signal processing, important mathematical underpinnings had been provided by Cooley and Tukey (1965), developers of the fast Fourier transform, and by the real-time Doppler velocity processing schemes of Rummler (1968a,b,c) and Miller and Rochwarger (1972). In 1974, based on Rummler’s scheme, Groginsky (1972), Lhermitte (1972), and Novick and Glover (1975) placed into operation the first multichannel pulse pair processor (PPP) at the AFCRL. Work at NSSL (e.g., Sirmans and Bumgarner 1975) led to improvements in the PPP technique; using different pulse repetition intervals (Sirmans et al. 1976; Doviak et al. 1978) provided a means of resolving some of the range-Doppler ambiguities troublesome even with S-band radars. The improved PPP technique was implemented on the NSSL radars in 1975 by Sirmans and at the National Center for Atmospheric Research (NCAR) by Gray et al. (1975). After the PPP made possible real-time Doppler velocity calculations, color monitors, then becoming commercially available, permitted real-time color displays of the radar reflectivity factor, mean radial velocity, and spectrum width (Gray et al. 1975; Jagodnik et al. 1975). A single Doppler radar measures only one component of the wind velocity, namely, the component radial to

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the radar. Use of a second Doppler radar with a different viewing aspect permits determination of the full wind vector (with some restrictions). Dual-Doppler radar observations were conducted by Peace and Brown (1968) as early as 1967, by Lhermitte (1970) and Lhermitte and Miller (1970) in 1969, by the National Hail Research Experiment (Knight and Squires 1982) in the 1970s, and by others. Scan procedures and methods for deriving two- and three-dimensional velocity structures, for regions containing precipitation echoes, were developed. It was evident, however, that for operational purposes improved means for displaying and interpreting the velocity patterns observed by a single Doppler radar would be necessary. The fortuitous advent of affordable microprocessors for digital signal processing capable of operating in a real-time environment and the later availability of high-resolution, graphical, color displays, greatly facilitated these Doppler radar developments. Fundamental research using dual-Doppler data produced the understanding needed to interpret the single-Doppler velocity patterns. The early interest focused on the storm-scale velocity patterns, which would provide clues to storm severity. However, the color plan position indicator (PPI) velocity displays also revealed intriguing patterns even in widespread stratiform precipitation (Kraus and Donaldson 1976). They were identified as reflections of the variation of the wind velocity with height; sometimes the patterns contained sharp gradients indicating the presence of frontal boundaries (Wilson et al. 1980). Such information is useful in weather analysis and forecasting. Wood and Brown (1986) provide a convenient summary of the characteristics of these velocity patterns and their interpretation. The increased sensitivity of weather radar systems, due in part to the Doppler processing performed on the signal, meant that some of the velocity patterns could even be observed in clear air, adding to the forecasting value of the data. The velocity azimuth display (VAD) technique earlier developed by Lhermitte and Atlas (1961) had been improved upon by Browning and Wexler (1968). That fundamental underpinning would later be extended by Rabin and Zrnic´ (1980) to provide wind information in the clear air using data from a single Doppler radar. Rabin and Zrnic´’s work would later serve as the fundamental basis of one of the Weather Surveillance Radar-88 Doppler’s (WSR-88D’s) most important operating modes, the clear-air or VAD-wind mode. Based in part on the early echo isolation algorithms developed in the late 1960s, Captain D. Forsyth, then an Air Force officer assigned to the Air Force Geophysics Laboratory (AFGL), was asked by K. Glover to develop an echo-tracking algorithm. Working with his co-investigator Captain C. Bjerkaas, another Air Force officer assigned to the laboratory, Forsyth produced the tracking algorithm that was used with some success in JDOP (see section 3). From 1980 to 1982,

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Forsyth continued developing the algorithm after moving to NSSL. The improved algorithm was used in the Boston area Next-Generation Weather Radar (NEXRAD) demonstration in 1983–84. The algorithm was later incorporated into the WSR-88D as the storm series of segment identification, centroid location, tracking, position forecasting, and structure analysis routines (Forsyth et al. 1981). Other algorithms for mesocyclone, tornado vortex, and hail detection were developed later and fielded with NEXRAD. Operational radar meteorologists, except those at a few uniquely equipped radars, did not have access to this advanced technology; however, they could see that the results being achieved in the research community might have the potential for a great payoff if they could be afforded on operational weather radar systems. 2. Limitations of operational weather radars and identification of the need for follow-on, improved weather radar systems Beginning in 1971, Air Weather Service (AWS) recognized a need for a follow-on weather radar, a successor to the aging FPS-77. Limitations in the FPS-77’s design—particularly its limited quantitative capabilities and lack of associated data processing and communications—even then drove AWS to consider alternatives for a successor system. Because at that time it took 10 yr to build the requirements documentation, fund, produce, and field a major sensing system that would be maintainable by the Department of Defense (DOD), AWS stated a requirement in the 1970s for such processing as a part of what was then called the Weather Radar of the 1980s program. At that time, there was no consensus (and no budget) for a Doppler weather radar; Doppler systems were considered expensive research tools whose operational benefits had not yet been quantified. Early requirements statements for the follow-on system did not call explicitly for a Doppler capability but did not exclude it either (Barad et al. 1973). Research in the 1970s pointed strongly toward the feasibility of an operational Doppler weather radar system that might be expensive, although affordable. In 1976, Dr. D. Atlas, a leader in radar meteorological research and president of the American Meteorological Society, produced testimony on severe storm identification techniques before the U.S. House of Representatives Subcommittee on Environment and the Atmosphere, recommending that the nation’s next operational weather radar be a Doppler system. Atlas was joined by Dr. E. Kessler of NSSL and other leading meteorologists in endorsing a field experiment to test Doppler radar’s potential for operational use. The nation’s operational weather agencies, at the time, had their own, separate programs for follow-on radar systems. In 1976, the National Weather Service (NWS) and NSSL announced their intention to design, acquire, and field a Doppler weather radar to replace the aging

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WSR-57 without need for participation by the Department of Transportation or DOD (Johannessen and Kessler 1976). AWS in 1976 began considering the operational use of air traffic control radars as an alternative to acquiring a follow-on weather radar. A feasibility study showed that the air traffic control alternative was not very capable, but the alternative was inexpensive and therefore attractive. Captain T. Linn at AWS headquarters believed strongly that a single, national Doppler weather radar system was affordable and certainly a better alternative than using air traffic control radars. In frustration over not being able to advance what he believed to be the best alternative, Linn finally wrote his congressman recommending a single national system be 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 development effort. As to whether the target system would be a Doppler radar, it was recommended that a study be made on the technical characteristics and benefits of an operational Doppler weather radar system. 3. Joint Doppler Operational Project (JDOP) The two principal laboratories involved—the Department of Commerce’s NSSL and the air force’s AFGL—agreed to combine their capabilities under NSSL’s leadership to make the project a success. This agreement was the genesis of the Joint Doppler Operational Project. As part of the preparation before JDOP, a working group included several members from AWS headquarters, including Captain R. Bonesteele, one of the present authors (ACH) and D. Sirmans of NSSL. One of the purposes of that working group was to develop some of the requirements for quantitative measurements using JDOP radars and displays. The group’s work focused on alignment capability, calibration procedures, and basic testing requirements. During 1977, the Federal Committee for Meteorological Services and Supporting Research established an interagency working group for the Next-Generation Weather Radar. R. Bonesteele laid the planning groundwork for the JDOP effort but moved to the Pentagon in 1978, where he established research and development funding lines for the air force portion of what would later be called NEXRAD. Other members of the working group are listed in the JDOP Final Report (JDOP Staff 1979). JDOP itself began in 1977 and ended in 1979, with major participation from both NSSL and AFGL. Federal Aviation Administration (FAA) participation in JDOP began in late 1978. The project was as much an operational effort as a research effort, and its objectives were operationally focused. Operational weather organizations such as AWS and NWS as well as the research laboratories were well represented in JDOP. One of the present authors (KEW) served as overall project coordinator, as NSSL had been selected as the location of

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JDOP. R. Bonesteele served as principal agency representative for the air force, while Major T. Sieland had the same responsibility for AWS. Coinvestigators from AWS included Captains J. Bonewitz (JDOP during 1977), D. Forsyth, M. Mader, and M. Snapp. The AFGL Weather Radar Branch was represented by K. Glover. His research team included C. Bjerkaas and R. Donaldson. NSSL’s principal investigator was D. Burgess, with warning coordination assistance provided by D. Devore, a forecaster from the Oklahoma City forecast office. With the progress made in Doppler radar signal and data processing, AFGL was able to develop real-time signal analysis, data processing, and display software (including but not limited to echo tracking) that was the air force’s most significant contribution of weather radar technology to JDOP. JDOP bridged the gap between research and operations, as it considered operational details such as the type of displays most usable by operational meteorologists. Many of JDOP’s results pointed strongly to design specifics that a successful operational Doppler system would require. For example, during JDOP’s use of AFGL’s C-band Porcupine Doppler to observe the Wichita Falls storm of April 1979, microwave attenuation caused by precipitation from an intervening storm caused the Wichita Falls storm to disappear completely from the displays of the C-band radar, whereas the Sband NSSL radar was able to observe it with no problem (Allen et al. 1981). That finding later became useful in successfully combating the challenge that the S-band WSR-88D was too expensive and should be replaced by a more economical C-band Doppler system. JDOP concluded that Doppler radar is superior to conventional radar and storm spotters; it increases warning lead time for tornadoes from about 2 min to 20 min, reduces false alarm ratios for tornadoes and severe thunderstorms, and improves probability of detection for severe thunderstorms. JDOP further concluded that a Doppler radar with a suitably narrow beamwidth can identify potentially severe thunderstorms by detecting the mesocyclone signature at long ranges (230–350 km), whereas Doppler’s capability to separate tornadic from nontornadic storms is limited to closer ranges (less than 230 km). JDOP specifically recommended that the nextgeneration weather radar have a Doppler capability (JDOP Staff 1979). From 1960 through the end of JDOP, the pioneers, contributors, and supporters, and JDOP participants shown in Table 1 were instrumental in bringing the nation’s operational Doppler weather radar system into operation (D. Forsyth 1996, personal communication). If you know of anyone whose name was inadvertently left off this list, please send the name and achievements to D. Forsyth at NSSL. 4. Joint System Project Office (JSPO) and Interim Operational Test Facility (IOTF) JDOP strongly demonstrated the usefulness of an operational Doppler weather radar and indicated the nature

TABLE 1. Pioneers, contributors and supporters, and JDOP participants (D. Forsyth 1996, personal communication). Pioneers Graham Armstrong Dave Atlas Dan Barczys Louis Battan Ed Brandes Jim Brantley Rodger Brown Keith Browning Don Burgess Tony Chimera Ralph Donaldson Dick Doviak Ken Glover Grant Gray Herb Groginsky Dave Holmes Ed Kessler Mike Kraus Les Lemon Roger Lhermitte Gene Mueller Bob Peace Rod Rogers Dale Sirmans Robert Smith John Theiss Ray Wexler Dusan Zrnic´

Contributors and supporters Glen Anderson Rick Anthes Ken Banis Stanley Barnes Harold Baynton Al Bishop Bill Bumgarner Rit Carbone John Carter Al Chmela Ken Crawford Rosemary Dyer Cal Easterbrook Jim Evans Ted Fujita Speed Geotis Ken Hardy Larry Hennington Ted Linn Jay Miller Allan Pearson Bob Saffle Mike Schmidt Bob Serafin Buzz Shinn Bill Smith Paul Smith Gene Walker Roger Whiton Jim Wilson

JDOP participants Ron Alberty Bob Allen Carl Bjerkaas Ray Bonesteele Dave Bonewitz Norm Chaney Chuck Clark Ray Crooks Bob Davies-Jones Don Devore J.T. Dooley Bob Elvander Doug Forsyth David George Izzy Goldman Doug Greene Jack Hinkelman Karl Johannessen William Klein Al Koscielny Jean Lee Mike Mader Mary McCoy Dick Mitchem Pio Petrocchi Peter Ray Chuck Safford Ken Shreeve Tom Sieland Mike Snapp Merritt Techter John Weaver Ken Wilk Allen Zahrai Dave Zittel

of the system that should be built. The interagency working group wrote, in July 1979, a NEXRAD Concept Report. In the same year, J. Bonewitz and Lieutenant Colonel C. Tidwell, then working out of the Office of the Federal Coordinator for Meteorological Services and Supporting Research, performed the cross-cut (multiagency) studies that obtained Office of Management and Budget support for NEXRAD. Triagency funding lines were then established. The stage was set for formation, in January 1980, of a Joint System Program Office (JSPO). Assigned as the initial Air Force members of a JSPO that then consisted of only eight people were C. Tidwell, DOD deputy program manager, and J. Bonewitz, radar meteorologist. The program manager was A. Hansen, NWS. S. Williamson later served as deputy program manager, NWS. J. Sowar was deputy program manager, FAA. The JSPO’s engineer was F. Blake, NWS. Differing agency requirements and program management methods, fiscal pressures, and some distrust among the agencies made the first year difficult. A. Durham, the second JSPO program manager, was able to cut through some of the problems and move the program in a multiagency direction. The leadership of Dr. E.

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Friday Jr., then director of NWS, and N. Blake, then FAA’s deputy associate administrator for engineering and development, was responsible for keeping the WSR88D program going during the troublesome first years. Despite problems and disagreements, the JSPO authored a joint operational requirements (JOR) document and a NEXRAD technical requirements (NTR) document in 1981. A research and development (R&D) plan was written and a technical advisory committee established to guide the R&D program. The JSPO Interim Operational Test Facility (IOTF) was established in Norman, Oklahoma in 1981 to continue to make improvements and test concepts for NEXRAD, under the guidance of one of the authors (KEW) assisted by D. Zittel. In 1982, the DOD selected D. Forsyth as deputy director (DOD) of the IOTF, soon followed by IOTF members T. O’Bannon and M. Istok. After that the operational NEXRAD system was competitively acquired and fielded as the successful WSR88D. Many of the concepts implemented in the WSR88D were originated by the IOTF. During development of the WSR-88D, an extensive set of operating manuals was published as Federal Meteorological Handbook-11 (FMH-11). Acquisition and fielding included training courses for the meteorologists who would be users of the system and the electronics technicians who would maintain it. 5. WSR-88D The WSR-88D system, manufactured by Sperry and successor firms, is described in Crum and Alberty (1993). It is an S-band system with Doppler capability. Other important improvements over the WSR-57, WSR74C, and S, FPS-77, and FPQ-21 systems it replaces include the following: R Matched-filter receiver design, a narrow, 18 beamwidth, and signal processing features that act together to enhance sensitivity (the radar can detect radar reflectivity factors as low as 216 dBZ at a range of 50 km in the clear-air mode and 28 dBZ at the same range in the storm mode), improve spatial resolution, and enhance the radar’s utility as a clear-air sensor; R Digital signal processing and extensive online data processing to provide products (such as maps of echo tops or vertically integrated liquid) in addition to the base data; R Computer algorithms to identify significant echo features (e.g., mesocyclones) and track echoes, still using the nondynamical, centroid-tracking algorithm; R Quantitative calibration and measurement capabilities, and R Color displays with many user-selectable features. A major difference is that the operator no longer manipulates the radar manually. Instead, the radar continually updates the three-dimensional database and frees the operator for full-time data interpretation performed

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at a computer-display subsystem called the principal user processor (PUP). The user has limited interaction with the radar and more substantial interaction with the database, displays, and algorithm products. Many capabilities have been gained (e.g., continuous coverage of the full surveillance volume, time-lapse sequences of the radar echo patterns, and machine-assisted interpretation of the data) but some have been lost. As an example, although a low-resolution, synthetic range-height indicator (RHI) display can be constructed using volume scan data, the operator can no longer perform vertical sector scans using the radar to obtain a high-resolution RHI. WSR-88D products are summarized in Klazura and Imy (1993). Archival datasets produced by the WSR88D system are described in Crum et al. (1993). The first operational WSR-88D system (Twin Lakes, Oklahoma, near Norman) was installed in fall 1990. The last of 158 operational WSR-88Ds was installed in northern Indiana in late June 1997. In the context of the WSR-88D, the concept of a ‘‘radar’’ can be considered to include both the radar data acquisition and the radar product generation units; the PUP, used for display, is considered separately. WSR88Ds not readily accessible or otherwise difficult to maintain are installed in a redundant configuration with an operational backup ready to continue to supply critical radar weather data if the primary unit fails. For that reason, the total number of radar systems exceeds the total number of radar installations. NWS has now accepted 117 operational systems (three more will be accepted in the future), FAA 12, and DOD 26; six radar systems across all the agencies have been accepted for use in maintenance and training. The total across all agencies for all purposes is 161 WSR-88D radar systems, including the redundant or backup units. Equally important are the PUPs, the unit at which the operational meteorologist works. NWS has accepted 193 PUPs, FAA 26, and DOD 188, for a total of 407, including those used for maintenance and training. All agencies with substantial weather radar programs are now using the same radar as their principal radar system, so modifications to the radar or improvements in operational techniques for its use, such as the interpretation algorithms, can be used advantageously by all the agencies, with associated cost savings. Some using agencies (i.e., NWS) intend to replace their PUPs with forecaster workstations of the Automated Weather Information Processing System; the flexibility of the WSR-88D’s modular design is such that this can be done easily without replacing the basic radar components. A continuing triagency NEXRAD product improvement program is in place to provide ongoing upgrades to the system’s hardware and software (Saffle and Johnson 1997). The maintenance concept for the WSR-88D involves organizational maintenance provided by the using agency and depot-level maintenance provided by the NWS

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Kansas City Logistics Facility. The WSR-88D contains facilities for both automatic and manual calibration; when automatic calibration adjustments exceed certain thresholds, manual maintenance is performed. The NEXRAD Operational Support Facility (OSF) provides operational and technical support for the WSR-88D and can perform radar calibration visits or maintenance training visits when requested to do so. It has not been considered necessary to schedule routine calibration visits to WSR-88D sites. The benefits of the national network of these radars are thus made available at all the operational weather stations and weather centers that need the information to perform their jobs effectively. In fielding the WSR-88D, it was realized that some requirements cannot be met or are mutually incompatible. When the shortfall requirements are so serious they cannot be met by modifying the fielded system, they can form the basis for acquisition of a separate system that specifically meets the unfulfilled needs. Such was the case with the FAA requirement for rapidly updated surveillance of the weather close to airports. Those requirements, which could not be met by the volumescanning WSR-88D, formed the basis for the Terminal Doppler Weather Radar (TDWR) (see section 7). 6. Training An important aspect of the WSR-88D program has been an expanded training program to ensure more effective use of the system’s capabilities. The air force short course in radar meteorology was suspended between 1989 and 1993, because it contained no instruction on the WSR-88D; the decision had been made to have WSR-88D radar meteorology instruction presented at the OSF’s well-equipped and professionally staffed training facility in Norman, Oklahoma. Initial WSR-88D operator training for both NWS and DOD was conducted at the OSF, beginning in 1990, as a 20day course, later shortened to 18 days. At the height of the program, 720 students per year moved through the Norman course, attendance at which was a criterion for commissioning of NWS WSR-88Ds. Mobile training teams traveled to hub DOD WSR-88D sites to administer initial training. Beginning in 1993, the DOD radar course, consisting of 24 training days, began being taught at Keesler Air Force Base (AFB), Mississippi. The National Weather Service Training Center provided all WSR-88D maintenance training in a seven-week course. The quality of self-study material on radar meteorology improved steadily. In the mid-80s, part B of FMH-7 was revised and improved. In 1990, a summary of the use of Doppler radar data to identify severe thunderstorms was already available (Burgess and Lemon 1990). In the early 1990s, FMH-11, Doppler Radar Meteorological Observations, was written and distributed to the field for operational use and self-study. With deployment of the WSR-88D, the National Center for Atmo-

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spheric Research, NWS, and AWS established the Cooperative Program for Operational Meteorology, Education, and Training (COMET), which includes several modules on radar meteorology and Doppler weather radar applications. 7. TDWR The TDWR emerged from shortfalls of the NEXRAD program and the Joint Aviation Weather Studies conducted by NCAR. In 1985, the FAA asked the Martin Marietta Air Traffic Control Division to investigate alternatives for detection of wind shear affecting aircraft on approach to airports. Two alternatives were investigated: the Low-Level Wind Shear Advisory System (LLWAS) and the TDWR. The FAA had at first decided on the former because of its lower life cycle cost. The 1985 crash of Delta Flight 191 at Dallas–Fort Worth airport caused the FAA to reopen the decision and to request Martin Marietta expand on the advantages of the TDWR and provide a refined cost estimate. Using the new analysis, the FAA was able to convince a receptive Congress to fund both the TDWR and the LLWAS (C. Tidwell 1997, personal communication). Under a contract awarded in November 1988 to the Raytheon Manufacturing Company, FAA is acquiring 47 TDWRs, including two to be used for training and testing. The TDWR is now being installed, principally at large commercial airports that are vulnerable to lowlevel wind shear and microbursts. The TDWR has a narrow beamwidth for improved spatial resolution and a fast scan sequence for better temporal resolution over the limited region along and near airfield approach and departure paths. The TDWR’s scan strategy and algorithms are particularly suitable for automated detection and reporting of microbursts, wind shear, and gust fronts, which constitute serious hazards to aircraft on approach or during takeoff. Initial specifications for the TDWR had called for an S-band system. FAA’s frequency-control office, aware of increased use of the 2.7– 2.9-GHz frequency band by the WSR-88D and many other systems, decided it should not field another radar in that band. FAA was aware that, with deployment of the WSR-88D, many C-band weather radars such as the FPS-77 and WSR-74C would be decommissioned, opening up space in the C-band for potential use by the TDWR. The FAA reasoned that the TDWR could be designed as a C-band system because Doppler data were required only out to a range of 89 km, and range coverage could be traded for the shorter wavelength. The so-called Doppler dilemma is that the product of the maximum unambiguous range and maximum unambiguous velocity is a constant, that constant being the speed of light times the wavelength divided by 8. For a maximum unambiguous range of 89 km, the Doppler dilemma limits the C-band maximum unambiguous velocity to 624 m s21 . TDWR specifications, on the other hand, called

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for a maximum unambiguous velocity of 640 m s21 . To meet that requirement out to a distance of 89 km, Raytheon used a velocity dealiasing algorithm; to suppress contamination by echoes from beyond the maximum unambiguous range, they had to use a pulse repetition frequency (PRF) agility scheme, where the processor selects the PRF that works best in removing unwanted data from second and succeeding trips (Michelson et al. 1990). The interest in nearby weather echoes uncontaminated by ground clutter drove the design toward a narrow beamwidth with exceptionally high sidelobe suppression and additional features (including the shorter wavelength) for ground clutter suppression. In addition to providing Doppler mean radial velocity and spectrum width data out to a range of 89 km, the TDWR also collects radar reflectivity factor data out to a range of 460 km using an interleaved scan sequence. The FAA’s program manager for the TDWR, D. Turnbull, is largely responsible for the program’s success. Dr. J. Evans, MIT Lincoln Laboratory, led a multiagency team effort to extend relevant WSR-88D algorithms to operate in the TDWR environment and to provide new algorithms to meet unique FAA requirements for reliable, automated detection of microbursts and wind shear events. Improved clutter suppression, a microburst detection 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 front algorithm was developed and is being deployed in 1998. Evans’ team included S. Campbell, R. Delanoy, M. Eilts, D. Klingle-Wilson, M. Merritt, S. Olson, and S. Troxel. Evans and Bernella (1994) provide an early summary of the work. 8. Conclusions Part of the fascination early radar meteorologists had with their new field must have been due to this remote sensor’s ability to see cloud and precipitation processes at work in a direct way that would have been impossible had they been restricted to conventional surface and upper-air observations, even at the mesoscale. Surely tomorrow’s radar meteorologists will have no less a fascination as they observe, measure, understand, and predict the phenomena that seem puzzling and difficult to understand today. Tempting as it might be to romanticize over radar meteorology’s past, it is likely that the best years of the field lie ahead. At no time in the past, except perhaps in the mid-1950s, were the instrumentation and techniques of the operational radar meteorologist closer to those of the research meteorologist than they are now. The tools in the hands of the practicing radar meteorologist are far better now than they have ever been. The prospects for advances due to applied research or technique development on forecasting and warning problems are exceptionally promising as the nation builds an archive of weather radar data that can be applied to

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practical problems. It is easier than it has been for a quarter-century to apply research results and interact productively with those conducting the research. Naturally, the field does not stand still. Techniques such as differential reflectivity, the more general polarization diversity, bistatic weather radars, and multipleDoppler analysis have been developed that are not capabilities of today’s operational weather radar systems. Nevertheless, we are probably a lot closer to realizing further advances that have an operational payoff now that the nation has fielded operational Doppler systems and has a commitment to keeping them fully capable and in step with research findings. So the best is yet ahead, as long as we heed D. Atlas’s warning to train the nation’s next generation of observing-oriented meteorologists well. Acknowledgments. The authors thank R. Donaldson Jr., consultant to Hughes STX Corporation, for his thorough review of an early version of this paper. We also appreciate the assistance proved by R. Bonesteele, C. Bjerkaas, T. Crum, R. Elvander, D. Forsyth, R. Kandler, R. Saffle, T. Sieland, C. Tidwell, and J. Wieler for providing valuable information. The contribution of one of the authors (PLS) was supported in part by National Science Foundation Grant ATM-9509810. APPENDIX

List of Acronyms and Abbreviations AFB AFCRL

Air Force Base Air Force Cambridge Research Laboratories AFGL Air Force Geophysics Laboratory AWS Air Weather Service CMF Coherent memory filter COMET Cooperative Program for Operational Meteorology, Education, and Training CW Continuous wave DOD Department of Defense FAA Federal Aviation Administration FMH Federal Meteorological Handbook IOTF Interim Operational Test Facility JDOP Joint Doppler Operational Project JOR Joint Operational Requirements JSPO Joint System Program Office LLWAS Low-Level Wind Shear Advisory System NCAR National Center for Atmospheric Research NEXRAD Next-Generation Weather Radar NSSL National Severe Storms Laboratory NTR NEXRAD technical requirements NWS National Weather Service OSF Operational Support Facility PPI Plan position indicator PPP Pulse pair processor PRF Pulse repetition frequency PSI Plan shear indicator

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WHITON ET AL.

Principal user processor Radiation Laboratory Research and development Range-height indicator Terminal Doppler Weather Radar Velocity azimuth display REFERENCES

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VOLUME 13

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