The Current System
As a baseline it is appropriate to begin with a review of the existing system. The primary weather surveillance radar system operated by U.S. agencies is the WSR-88D (NEXRAD) system, consisting of about 150 nearly identical radars. Most of these radars were deployed over the United States and some overseas locations in the 1990s. Data from this system support activities of the National Weather Service (NWS), Federal Aviation Administration (FAA), and the Department of Defense (DoD); they are distributed to a wide variety of other users as well. Although there are some differences among the radars operated by the three agencies, the design is essentially uniform for all radars. A common design for all the radars helped ensure high reliability and performance while reducing maintenance complexity and life-cycle costs. The primary functions of the system are to provide measurements for monitoring and forecasting severe storms, developing flash flood warnings, and contributing to other hydrologic applications. In addition to these, the radar system has evolved into a critical tool that supports numerous other meteorological applications. Data from other radar systems can also augment the NEXRAD data set. These systems include the FAA’s Terminal Doppler Weather Radar (TDWR) and short- and long-range surveillance radars [Air Route Surveillance Radar systems (ARSR) and Airport Surveillance Radar (ASR)], atmospheric wind profilers (Rich, 1992), and radar systems operated by television stations and other private entities.
THE NEXRAD NETWORK
Appendix A summarizes the technical characteristics of the WSR-88D radar system. Crum and Alberty (1993) and Serafin and Wilson (2000) provide addi-
tional background on the system characteristics. Coverage over the eastern two-thirds of the country is essentially complete, though significant limitations exist in coverage near the surface. There are some gaps in western regions, and the combination of high-altitude sites and mountainous terrain presents difficult problems in several areas (Westrick et al., 1999).
Surveillance of the atmospheric volume surrounding a NEXRAD site is provided through one of several available volume coverage patterns (VCPs). The VCPs summarized in Table 2–1 are commonly used. The “clear air” patterns cover the lowest layers of the atmosphere in 10 minutes and provide such things as wind profiles and indications of sea breeze fronts or storm outflow boundaries that could trigger convective activity. The “precipitation” and “severe weather” patterns cover the full depth of storm activity in 5 to 6 minutes and provide more frequent updates on evolving storms.
Primary Data and Derived Products
The NEXRAD is a pulse-Doppler system that measures three primary characteristics of the radar echoes: equivalent radar reflectivity factor, commonly referred to as reflectivity and designated by Zc; Doppler (radial) velocity, designated by v or vr; and the width of the Doppler spectrum, designated by sv. These base data variables, derived in the radar data acquisition (RDA) unit, express the zeroth, first, and second moments, respectively, of the Doppler spectrum of the echoes. A value for each quantity is available for every “resolution cell” of the radar, as defined by its antenna beamwidth and the sampling rate along the beam axis (though the latter is constrained presently to no less than half the pulse duration).
These displays, together with products (summarized in Radar Operations Center, 2002) and results of the algorithms discussed below, are developed from
TABLE 2–1. WSR-88D Volume Coverage Patternsa
the base data in a radar product generator (RPG) unit. In addition, a series of computer algorithms operate upon the base data (some also incorporate auxiliary information such as temperature profiles), examining the echo patterns and their continuity in space and time in order to identify significant weather features such as mesoscylones, tornado vortex signatures, or the presence of hail. Outputs of these algorithms are displayed as icons superimposed on the basic radar displays or in auxiliary tables. The number and the variety of potential algorithms continue to increase as scientific knowledge about the relationship between echo characteristics and storm properties improves and the available computational resources increase.
Data Display, Dissemination, and Archiving
A “principal user processor (PUP)” associated with each NEXRAD installation, and numerous additional “remote PUPs,” provided the initial data display capability. The PUP was essentially a mainframe minicomputer, with a monitor, that operated programs to generate displays from a rather limited set of possibilities. As computer technology has advanced, open-systems architecture is being implemented to replace both the RPG and PUP units. Thus, the “open RPG” (ORPG) will generate and display the various products as well as relaying the relevant data on for display on other systems.
Although the NEXRAD system of radars is of major value as a stand-alone weather-observing network, additional value is obtained through the integration of NEXRAD data with other weather observations [e.g., other radar systems, wind profilers, satellites, the National Lightning Detection Network (NLDN), or surface measurements] and associated analyses. A number of systems have evolved to accomplish the integration of weather observations, many of which are intended to support commercial and general aviation. Among these are:
Advanced Weather Interactive Processing System
Corridor Integrated Weather System
Integrated Terminal Weather System
U.S. Air Force Open Principal User Processor
Algorithm Testing and Display System
Weather Support to Deicing Decision Making
These and similar systems are expected to mature dramatically and grow in use over the next two decades as the science of meteorology and the technology of information processing and dissemination continue to advance and merge with social needs for improved weather information and forecasting.
Dissemination of NEXRAD data within the NWS is now handled through the AWIPS system. Equivalent systems support the FAA and DoD users of NEXRAD data. Dissemination to outside users, formerly handled by vendors, is
now accomplished through the Base Data Distribution System (BDDS) with distribution over the Internet.
Archiving of the NEXRAD base data (the “Level II” data) has been accomplished by magnetic tape recording at the sites, with the tapes being shipped to the National Climatic Data Center (NCDC) (Crum et al., 1993). Experience has shown that only a little more than half of the national data set reaches the archive in retrievable form. Thus, the Collaborative Radar Acquisition Field Test (CRAFT) (Droegemeier et al., 2002) is under way to test the capability to transmit NEXRAD data over the Internet to NCDC and increase the fraction of retrievable data. A separate archive of a set of the derived products (the “Level III” data) provides basic data for such things as research, training, and legal inquiries.
USERS AND USES OF THE DATA AND PRODUCTS
The NEXRAD principal user agencies are the Department of Commerce (DOC), DoD, and Department of Transportation (DOT). The primary mission organizations within these agencies are the NWS,1 the Air Force Weather Agency (AFWA),2 the Naval Meteorological and Oceanography Command (NMOC) and the FAA.3 NWS is responsible for the detection of hazardous weather and for warning the public about these hazards in a timely, accurate, and effective way. The Service also provides essential weather information in support of the nation’s river and flood prediction program as well as in support of civilian aviation, agriculture, forestry and marine operations. The national information database and infrastructure formed by NWS data and products, can be used by other governmental agencies, the private sector, the public, and the global community. The AFWA provides worldwide meteorological and airspace environmental services to the Air Force, Army, and certain other DoD organizations. NMOC supports the Navy, Marine Corps and certain other DoD organizations. The primary missions of these DoD agencies are to provide timely information on severe weather for the protection of DoD personnel and property; to provide weather-related information in support of decision-making processes at specific locations; and to support military aviation. The FAA’s responsibility requires the FAA to
gather information on the location, intensity, and development of hazardous weather conditions as well as to provide this information to pilots and air traffic controllers and managers. The current mission of the principal user agencies—to protect life and property—is expected to remain the same in the future. It is the quality, delivery, and use of this service that will change over time and will need to be addressed in considering the radar system of the future.
The group of users has expanded dramatically to include, among others, a very large atmospheric sciences and hydrometeorological research community in universities and research laboratories throughout the world; other federal, state, and local governmental organizations and private sector providers; and distributors and users of weather and climate information gleaned from meteorological radar measurements and other associated products. The latter include data that are either taken or derived directly from radar measurements as well as information derived through intelligent integration of radar data with other measurements and analyses of weather events. The NEXRAD system has been, and continues to be, immensely valuable in providing weather observations to this vast and diverse array of data users.
Many other federal agencies now use and rely on weather radar data to help meet their operational and other responsibilities. Among these are the Federal Emergency Management Administration (FEMA), Environmental Protection Agency (EPA), Nuclear Regulatory Commission (NRC), Department of Energy (DOE), U.S. Geological Survey (USGS), Department of Interior (DOI), Department of Agriculture (USDA), Forest Service, Bureau of Land Management (BLM), National Oceanic and Atmospheric Administration (NOAA),4 National Park Service (NPS), Federal Highway Administration (FHWA), U.S. Coast Guard (USCG), and National Aeronautics and Space Administration (NASA).
Weather data have become valuable to the operations of many State and other governmental agencies. These typically include organizations with the following designations: agriculture, environmental protection, conservation and natural resources, fish and game commissions, transportation, emergency management, and water resources.
Numerous academic programs within the university community work with NEXRAD weather data in their research; these include departments or programs that represent studies in the fields of meteorology, atmospheric sciences, climatology, physics, chemistry, air pollution, hydrology, earth sciences, geography, geology, transportation, civil engineering, electrical engineering, geophysics, signal processing, computer science, computer engineering, natural resources, agriculture, forestry, economics, transportation, aviation, environmental science,
and engineering. Many government agencies and other public and private sector organizations are also involved in many investigations in fields that utilize weather radar data as an essential resource for their investigations.
As access and understanding of the use of NEXRAD weather data have grown, so has the list of users of this information within the private sector. Examples include broadcasters, commercial aviation, agriculture, trucking, recreation providers, economic forecasters, the insurance industry, and energy companies.
SHORTCOMINGS OF THE SYSTEM
A variety of limitations impede the ability of the NEXRAD system to meet the needs of all the varied users. Some, such as the divergence of the radar beam with increasing range, are inherent to any radar system. Others, such as the inability to acquire data in small elevation steps during shallow winter precipitation episodes, can be overcome by rather straightforward hardware or software modifications (the latter will be facilitated by the greater flexibility afforded by forthcoming open-systems architecture). The ongoing program of research and development should provide at least partial solutions to some of the other problems. But the opportunity to introduce newer technologies in a subsequent generation of weather surveillance radar systems offers promise of even further improvements.
Serafin and Wilson (2000) provide a good summary of the recognized deficiencies of the NEXRAD system. Those that affect the primary variables directly include contamination by ground clutter, both that in the normal radar environment and that arising during anomalous propagation conditions, and the occasional impact of bird echoes on the Doppler velocity data. The problem of range-velocity folding, common to all pulse-Doppler radars, has proved to be quite serious in the NEXRAD system. Study of several techniques now underway should yield means of mitigating the range-velocity folding problem, and versions of those techniques may well be applicable to future systems.
Spatial coverage limitations are imposed in the first instance by the curvature of the earth. This limitation constrains the available coverage to minimum altitudes, which increase with distance from the radar site. The problem is exacerbated by obstacles in the radar environment, which constitute a radar horizon extending above 0 deg elevation angle. The NEXRAD scans are restricted to some maximum elevation angle (currently 20 degrees), mainly to provide an acceptable scan update rate (see below); the result is a “cone of silence” data gap above each radar site. The cone of silence and limited low-level coverage inhibit the value of NEXRAD data to aviation interests. A further constraint on the NEXRAD system, limiting the minimum elevation angle to no lower than 0.5 deg, adds to this difficulty. This problem is of special concern for radars at high-altitude sites in mountainous areas; such radars are often unable to sense signifi-
cant precipitation occurring entirely below their minimum usable elevation angle. Similar difficulties arise in areas subject to intense precipitation from shallow cloud systems, such as places in the lee of the Great Lakes affected by lake-effect snowstorms. Coverage over coastal and adjacent waters, a special concern for hurricane-prone regions, is limited. Such regional variations raise questions about the ability of a universal radar system configuration to meet the requirements for weather surveillance in all locations.
Weather surveillance needs should be evaluated by geographic region to determine if a common radar system design is appropriate for all regions.
Regions devoid of data pose a difficult problem for the NEXRAD algorithms. The primary causes of data voids are beam overshoot, cone of silence near the radar, beam blockage due to obstructions, gaps in vertical coverage, low signal strength, data masking due to data corruption, and planned and unplanned outages.
Overshoot refers to the void caused by the elevation of the lowest beam above the surface, which results from a combination of the elevation angle of the lowest tilt and the curvature of the earth. The consequential data voids affect every product. There is an additional complication. In order to achieve fairly rapid volume scans, the usual practice is to use a scan strategy, which has fairly coarse vertical beam spacing. The result is coverage gaps in the vertical. The trade-off here is between accepting longer times for the volume scan, accepting larger vertical gaps (fewer tilts), or enlarging the cone of silence by limiting the elevation of the highest tilt. Some products are more tolerant of vertical gaps than others. Early termination of volume scans by NEXRAD operators seeking more rapid updates of low-level base data occasionally introduces additional voids in the high-level data.
Data voids resulting from overshoot, beam blockage, vertical gaps, and the cone of silence are determined by the geometry of the radar installation and by the scan strategy. With the absence of reflectors, the signal strength can become so weak that the wind velocity cannot be resolved. The result is an enlarged velocity data void. Some compensation is possible by changing the waveform or the scan strategy, as in the present clear air mode VCP, if the radar was designed with the flexibility to trade increased volume scan time for greater sensitivity. Consideration should be given to providing the flexibility to adaptively adjust the sensitivity of the next generation radar.
The update cycle of the NEXRAD system, or indeed of any mechanically scanning system, in conditions of rapidly evolving convective weather is a serious limitation. Measurement of the primary variables along any given beam direction requires some minimum dwell time, which is governed by the required precision
of the measurements. The dominant dwell-time constraint is imposed by the required precision in reflectivity (Smith, 1995), and in the case of NEXRAD, relaxing the 1 dB requirement could moderate this required precision. However, the necessity of covering the full volume of the atmosphere will always restrict the available update intervals.
A variety of other concerns about data quality exist. The false alarm rates for many of the current algorithms are higher than desirable. The limited spatial resolution at long ranges impedes the ability to identify small-scale weather features such as small tornadoes. Except for VAD winds, wind products have not been reliable enough for introduction into numerical weather prediction (NWP) models, largely because of artifacts in the data stream. The reliability of operation in remote and unattended locations can be a significant concern.
A final deficiency concerns the NEXRAD precipitation estimates (e.g., Smith et al., 1996; Anagnostou et al., 1998), which are important to a variety of applications. The spatial and temporal resolution of the data are not sufficient for many flash flood situations. There is a fundamental problem in converting the measured reflectivities to precipitation rates, in that no universal relationship exists, and means for establishing the relationship appropriate to a given situation are not at hand. Higher reflectivities in the “bright band” region contaminate many precipitation products. Moreover, the coverage limitations discussed above mean that reflectivity data over most of the surveillance area are only available for altitudes some distance above the ground. Methods for projecting the reflectivity data down to the surface are the subject of much research (e.g. Seo et al., 2000) but have yet to be applied in NEXRAD. Modifications to NEXRAD such as a polarimetric capability should help with the precipitation-rate problem, but the vertical-profile problem will remain ubiquitous.
The national coverage, improved accuracy, and rainfall estimation capabilities of the NEXRAD system have advanced the practice of hydrologic forecasting and water resource management. If dual-polarization capabilities are incorporated into the current system as planned, further improvements in precipitation measurements will occur, particularly in the quantification of high-intensity rainfall rates and the characterization of snowfall. A key to the success of any future radar system will be the preservation of capabilities such as dual polarization to provide improved data quality and high-accuracy rainfall measurements. Improved spatial sampling will be needed for representative near-surface coverage throughout the continental United States (CONUS) (possibly excluding regions in complex, highly mountainous terrain). The latter implies the need for an affordable means of dealing with the inherent inability of any widely spaced network to provide adequate near-surface surveillance over large portions of the country.
THE EVOLVING NEXRAD SYSTEM
Most of the field-testing of NEXRAD concepts and prototype systems prior to deployment took place in the central U.S. and dealt with warm-season convective weather. As the NEXRADs were deployed in other regions, further needs developed, and limitations that had not been elucidated in the earlier field-testing surfaced. The established mechanism of an Operational Support Facility (now the Radar Operations Center), advised by a Technical Advisory Committee, was able to deal with many of these concerns. Through this mechanism, algorithms have been revised, new ones have been added, and such capabilities as Level II data archiving have been implemented. But in hindsight, more comprehensive field testing covering a wider variety of regional and climatic conditions earlier in the system development process would have revealed some of the concerns soon enough to allow earlier and more effective action to mitigate their impacts.
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.
The WSR-88D system configuration is not static, but rather continues to evolve through an ongoing NEXRAD Product Improvement (NPI) program. Stated objectives of this program (Saffle et al., 2001) are to:
ensure the capability to implement advances in science and technology to improve forecasts, watches, and warnings,
minimize system maintenance costs, and
support relatively easy upgrades in technology so that a large-scale WSR-88D replacement program may be indefinitely postponed.
The NPI Program works to develop and introduce system improvements in an orderly and seamless manner. To the extent that this program succeeds, the need for a replacement radar system will recede further into the future. Moreover, the NEXRAD system a decade or two hence will be substantially improved over that of today.
The current NPI Program emphasizes two major thrusts. One is to replace the data acquisition and processing systems in the original WSR-88D with “open-system” hardware and software. This development will increase the overall capability of the NEXRAD system for data processing, display, dissemination, and archiving; will facilitate implementation of new algorithms for processing the radar data; and will reduce costs for system operation and maintenance. Field deployment of the ORPG component, which executes the NEXRAD algorithms
and produces the image products, should be completed in FY2002. Introduction of the second component, the open RDA (ORDA) unit, is scheduled to follow in about three years. The ORPG improvements provide a capability for (1) data quality improvements, such as AP clutter detection/suppression and identification of nonprecipitation echoes, (2) new polarimetric-based products such as improved precipitation estimation and hydrometeor particle identification, and (3) new products that may be directly assimilated into operational numerical models. The ORDA improvements will include (1) availability of a modern Doppler spectral processing platform including digital receivers for improved data fidelity, (2) a provision for range-velocity ambiguity mitigation techniques using phase coding and dual PRT waveforms, (3) capability for polarimetric sensing and processing to more accurately measure hydrometeor properties, such as drop size distributions and precipitation phase, and (4) custom VCPs to allow site-specific volume scans adapted to local weather needs. These improvements are expected to be operational in the next 5–10 years. The third major component of the WSR-88D, the PUP display unit, is being converted to open architecture in different ways by the three major NEXRAD user agencies.
The second major thrust of the NPI Program is directed toward introduction of a polarimetric capability for the WSR-88D. Such a capability could provide improved precipitation measurements as well as new capabilities for identifying hydrometeor types (e.g., recognizing the presence of hail or discriminating between rain and snow regions) and enhanced ability to screen out artifact echoes such as those caused by ground clutter or birds. The Joint Polarization Experiments (JPOLE) project planned for 2003 (Schuur et al., 2001) will evaluate these capabilities on a WSR-88D specially modified to provide a prototype polarimetric capability. Results of this project will influence the decision concerning full implementation of a polarimetric capability for the WSR-88D.
The NPI Program is not limited to modifications of the WSR-88D itself. An enhanced software environment, termed “Common Operations and Development Environment” (CODE), is being provided to facilitate use of the open-architecture system capabilities and linkage of the NEXRAD data to other agency weather data systems such as AWIPS. Plans and procedures are being developed to incorporate data from appropriate FAA radar systems, including the TDWR, ASR-9/11, and ARSR-4, as a means of expanding the available coverage and enhancing the backup capabilities in case of a WSR-88D outage.
The NPI Program provides a means for introducing continuing improvements in science and technology into the NEXRAD system on an ongoing basis. This evolutionary approach works to keep the system up to date, and can postpone the need for a replacement system until obsolescence issues such as mechanical wear and tear enter the picture. That can allow time for more complete evaluation of the various new technologies discussed elsewhere in this report. The NPI Program will also provide operating experience with new technical features, such as range-velocity ambiguity mitigation techniques or polarimetry,
to help determine those features that should be carried over into a successor system. Sustaining the NPI Program will thus offer benefits not only to the NEXRAD system, but also to the follow-on system. Indeed, plans for the future system could well include a similar program for continual evolution.
The Radar Operations 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.