6
Findings and Recommendations

As a consequence of the development of the science of radar meteorology and the advent of the NEXRAD system, weather radar has become the primary means of detecting, describing, tracking, and nowcasting or short-term forecasting precipitation-laden and, to a more limited extent, clear air weather over the contiguous United States. It has value both as a stand-alone system and as a major component of the entire atmospheric observing system. The NEXRAD system contributes to the preservation of life and property (e.g., through significantly improved tornado, severe storm, flash flood, and hurricane landfall warnings); to public safety; to the advancement of meteorology, hydrology and climatology; to aviation; and to the conduct of numerous weather-dependent activities. Even though the system’s current configuration provides outstanding public service, innovations and technological advances will add significant value to the evolving NEXRAD system and ultimately to the future weather radar system. This future system will most likely include a variety of different sensor types that will ensure continuation of present capabilities, address current deficiencies, and take advantage of opportunities afforded by new knowledge.

The more important uses of weather radar data in the future will be within the context of an integrated observing system. Radar data will be combined with data from satellites, surface networks, commercial aircraft, and other sources to produce weather depictions that are more effective than those in place today. Radar data will also be used increasingly in advanced data assimilation schemes with numerical weather and associated hydrologic prediction models. These applications will require real-time data of very high quality, with error statistics that are well quantified.

The committee was neither constituted nor charged to design the future



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Weather Radar Technology: Beyond Nexrad 6 Findings and Recommendations As a consequence of the development of the science of radar meteorology and the advent of the NEXRAD system, weather radar has become the primary means of detecting, describing, tracking, and nowcasting or short-term forecasting precipitation-laden and, to a more limited extent, clear air weather over the contiguous United States. It has value both as a stand-alone system and as a major component of the entire atmospheric observing system. The NEXRAD system contributes to the preservation of life and property (e.g., through significantly improved tornado, severe storm, flash flood, and hurricane landfall warnings); to public safety; to the advancement of meteorology, hydrology and climatology; to aviation; and to the conduct of numerous weather-dependent activities. Even though the system’s current configuration provides outstanding public service, innovations and technological advances will add significant value to the evolving NEXRAD system and ultimately to the future weather radar system. This future system will most likely include a variety of different sensor types that will ensure continuation of present capabilities, address current deficiencies, and take advantage of opportunities afforded by new knowledge. The more important uses of weather radar data in the future will be within the context of an integrated observing system. Radar data will be combined with data from satellites, surface networks, commercial aircraft, and other sources to produce weather depictions that are more effective than those in place today. Radar data will also be used increasingly in advanced data assimilation schemes with numerical weather and associated hydrologic prediction models. These applications will require real-time data of very high quality, with error statistics that are well quantified. The committee was neither constituted nor charged to design the future

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Weather Radar Technology: Beyond Nexrad generation weather surveillance radar system. The findings and recommendations summarized herein deal with technologies having the potential to mitigate some of the limitations of the evolving NEXRAD system. For many of the approaches, and certainly for those categorized as “far-term” or “visionary,” technical feasibility remains to be established. In most cases benefit-cost evaluations will have to precede any move toward implementation in the design of the future system. The committee encourages the agencies that commissioned this study to follow through with the investigations necessary to establish the technical feasibility of the “far-term” and “visionary” technologies and to conduct benefit-cost analyses of the feasible ones. The success of the findings and recommendations summarized herein will depend critically on the development of a parallel end-state user process (i.e., extending beyond the hydrologist or meteorologist) that defines needs in user contexts. The committee believes that without this user check and balance system, the best plans for the future could produce scientific and technical value, but could be of limited user value. Likewise, the concept of computer augmentation and decision support is fundamental to this end process. The complexities of an integrated observing system, and the four-dimensional sorting thereof for specific user needs, may require sophisticated routines that provide for decision support systems. GROUP I: RADAR TECHNOLOGIES The findings and recommendations of this committee are summarized here in two groups. The first group concerns those technical approaches considered most promising for the development of future weather radar systems, and is presented in order of our estimate of their maturity (starting with the most mature) of the various approaches. Finding The current NEXRAD (WSR-88D) system, a highly capable weather surveillance radar, has proved to be of great value to many sectors of our society, and its applications have extended beyond the traditional goal of protecting life and property. The Radar Operation Center and the NEXRAD Product Improvement Program mechanisms have provided an evolutionary process for improvements to the system. Recommendation—Near-term (Chapter 2) The Radar Operation Center and the NEXRAD Product Improvement Pro gram mechanisms should be extended to permit continual improvement to the NEXRAD system. Provisions should be made to carry features found to be beneficial, such as polarization diversity, over to the succeeding generation of systems.

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Weather Radar Technology: Beyond Nexrad Finding Other radar systems provide various kinds of weather surveillance. Examples include the Terminal Doppler Weather Radar (TDWR) and Airport Surveillance Radars (ASR) operated by the Federal Aviation Administration (FAA), radars operated by commercial broadcasters and other private entities, clear air profilers, and airborne weather avoidance radars carried on commercial and private aircraft. Recommendation—Near-term (Chapter 4) The potential value and technology to incorporate data from complementary radar systems to provide a more comprehensive description of the atmosphere should be investigated Finding Mobile radars can provide highly detailed views of weather events. Such observations not only have scientific interest, but also could be valuable in support of emergency services in cases such as fires, contaminant releases, and nuclear, chemical, or biological attacks. Recommendation—Near-term (Chapter 4) The potential of operational mobile radar systems to contribute to the nation’s weather surveillance system for emergency response and for improved short- term forecasts should be evaluated. Finding Adaptive waveform selection and volume scan patterns are important for optimizing radar performance in different weather situations. Beam and waveform management will be determined by the prevailing atmospheric phenomena and threats. Recommendation—Far-term (Chapter 3) Adaptive waveform selection, which may even be applied to present systems, and agile beam scanning strategies, which require an electronically scanned phased array system, should be explored to optimize performance in diverse weather. Finding Radar systems with phased array antennas and advanced waveforms can support a broad spectrum of applications with observation times sufficiently short to deal with rapidly evolving weather events such as tornadoes or downburst winds.

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Weather Radar Technology: Beyond Nexrad Recommendation—Far-term (Chapter 3) The technical characteristics, design, and costs of phased array radar systems that would provide the needed rapid scanning, while preserving important capabilities such as polarization diversity, should be established. Finding A closely spaced network of short-range radar systems would provide near-surface coverage over a much wider area than the current NEXRAD system. This network would expand geographic coverage of low-level winds, precipitation near the surface, and weather phenomena in mountainous regions. Recommendation—Far-term (Chapter 4) The potential for a network of short-range radar systems to provide enhanced near-surface coverage and supplement (or perhaps replace) a NEXRAD-like network of primary radar installations should be evaluated thoroughly. Finding The satellite-borne Tropical Rainfall Measurement Mission (TRMM) radar has demonstrated the ability to measure precipitation over regions not reached by land-based radars. The TRMM data are useful for climate monitoring and extended-range weather and climate forecasting. Future satellite technology is likely to allow on-orbit operation of weather radar systems with larger antenna apertures and higher power outputs than are currently used in space. Satellite constellations operating as distributed array antennas would provide high-resolution global coverage, thereby supplementing ground-based networks. Recommendation—Visionary (Chapter 4) The capabilities of future space-based radar systems to supplement ground- based systems should be determined. Finding Both piloted and Unmanned Aerospace Vehicles (UAV) are being developed for a variety of remote sensing and other applications. As the capabilities of these airborne platforms increase it may become possible to place weather radar systems on station at a variety of altitudes, for extended duration. These systems could support weather forecasting, weather warnings and other emergency response applications.

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Weather Radar Technology: Beyond Nexrad Recommendation—Visionary (Chapter 4) The capabilities of Unmanned Aerospace Vehicles and piloted aircraft to carry weather radar payloads should be monitored for their potential to provide weather surveillance over the continental United States and over the oceans. GROUP II: PROCEDURES This second group of findings and recommendations concerns matters important to the design, development, and implementation of future generations of weather surveillance radar systems, independent of the specific technologies that may be adopted. No order is intended for the following recommendations. Finding Weather forecasting and warning applications are increasingly relying on integrated observations from a variety of systems that are asynchronous in time and are nonuniformly spaced geographically. Weather radar is a key instrument that provides rapid update and volumetric coverage. Recommendation (Chapter 1) The next generation of radars should be designed as part of an integrated observing system aimed at improving forecasts and warnings on relevant time and space scales. Finding Weather surveillance needs vary from region to region and from season to season, depending on factors such as the depth of precipitating cloud systems and local topography. Recommendation (Chapter 2) Weather surveillance needs should be evaluated by geographic region to determine if a common radar system design is appropriate for all regions. Finding Early field-testing of NEXRAD concepts and systems in a limited range of geographic and climatological situations did not evaluate and elucidate the full range of operational demands on the system.

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Weather Radar Technology: Beyond Nexrad Recommendation (Chapter 2) The development program for the next generation weather surveillance radar system should incorporate adequate provision for beta testing in the field in locations with diverse climatological and geographic situations. Finding Despite the great utility and value of NEXRAD, data-quality issues, addressed in part through the NEXRAD Product Improvement Program (NPI), remain a significant impediment to some important applications. Recommendation (Chapter 3) The quality of real-time data should receive prominent consideration in the design and development of a next generation weather surveillance radar system. Realtime data quality assessment should be automated and used in deriving error statistics, and alerting users to system performance degradation. Finding Present and prospective users of the electromagnetic spectrum are competing for the current weather radar spectrum allocation. There is particular concern that the use of S-band may be lost for weather radar applications. S-band minimizes attenuation effects and ambiguities in Doppler wind measurements. The loss of S-band would compromise measurements of heavy rain and hail, warnings of flash floods and tornadoes, and the monitoring of hurricanes near landfall. The cost of rectifying these impacts in the current NEXRAD system would be substantial. Transmitters, receivers, and software would all require replacement and/ or significant modifications. It is also possible that antennas would need to be replaced. The current state of science is such that certain capabilities would be lost. Recommendation (Chapter 3) Policy makers and members of the operational community should actively participate in the arena of frequency allocation negotiation. The impact, including the economic and societal costs, of restrictions on operating frequency, bandwidth, and power should be assessed for current and future weather radar systems. Finding Weather radar data are being increasingly used in climatological studies as well as for a wide variety of other research. Weather radars provide continuous high-resolution monitoring in space and time.

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Weather Radar Technology: Beyond Nexrad Recommendation (Chapter 5) To support the use of radar data in the climate observing system and other research areas, standards for calibration and continuity of observations should be established and implemented. Finding Weather radar provides observations, on small spatial and time scales, that are essential for monitoring precipitation and diagnosing certain weather events as well as for supporting nowcasting systems, hydrologic models, and numerical weather prediction models. Effective assimilation of radar data in these models requires accurate error statistics. Recommendation (Chapter 5) The value of radar data, as part of an integrated observing system, in diagnostic applications nowcasting systems, and hydrologic and numerical weather prediction models should be considered in the design of the next generation weather radar system. The characteristics of radar observations and associated error statistics must be quantified in ways that are compatible with user community needs. Finding Broad dissemination of weather radar data in real time facilitates the application of those data to diagnostic and forecasting operations. Archiving of radar base data, as well as product data, facilitates research activities, retrospective studies, and climatological investigations. Recommendation (Chapter 5) Plans for next generation weather radar systems should include provisions for realtime dissemination of data to support forecast, nowcast, and warning operations and data assimilation for numerical weather prediction, and certain research applications. Routine reliable data archiving for all radars in the system for research, climatological studies, and retrospective system evaluation must be an integral part of the system. Convenient, affordable access to the data archives is essential. Finding A long-term objective of the radar and other weather observation systems mentioned in this report is the establishment of an integrated observational system, whereby most or all of these observations (e.g., ground-based and space-borne radar; hyperspectral, visual, and IR satellite data; and directly and remotely sensed

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Weather Radar Technology: Beyond Nexrad data from manned and unmanned aircraft, commercial aircraft, surface-based remote sensors, and radiosondes), would be assimilated on to a fixed four-dimensional grid to provide the most complete diagnosis of weather impacts possible. Additionally, numerical weather prediction (NWP) models and nowcasting techniques would readily provide forecasts for times ranging from a few minutes to many hours. A broad array of products will be used to support decisions that will improve safety to humans, improve operational efficiency, and make homeland defense efforts more effective. Recommendation (Chapter 5) Tactical Decision Aids and means for collaborative decision-making capabilities should be developed for both meteorological and nonmeteorological users of the system, with attention to the demands on the integrated observing system.