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1 Introduction Modernization and associated restructuring of the National Weather Service (NWS) will usher in a new era for severe weather and flood warning and forecast services in the United States. Important advances in the science of meteorology, coupled with major new technological capabilities for observing and analyzing the atmosphere, will provide unprecedented weather service improvements over the next decade. The existing systems for making weather observations and processing and communicating data and information about the weather are obsolete and costly to maintain. The modernized NWS will obtain substantially more weather data through the use of new technology, which will include automated surface observations, Doppler radars, satellites, and tiara assimilation on supercomputers. Advanced information processing technology will be used to integrate the various data fields and provide them to the forecaster in useful forms, as well as to assist in generating and issuing improved forecasts, warnings, and other products. In the transition from He old system to the new, Congress and others have expressed concerns about the adequacy of the new radar network to detect significant weather events over vulnerable areas of the contiguous United States such that there will be no degradation of the service currently provided by the NWS.i It is important to note that degradation in any one component of the new system, including NEXRAD coverage, does not necessarily imply a net degradation in the overall system. Con- gress asked the National Research Council (NRC) and its National Weather Service Modernization Committee (NWSMC) to address these concerns. This report is the result of a 6-month study conducted by a pane! of radar engineers and meteorologists qualified by training and experience. The background leading to the study, the charge to the committee/panel, and the study process followed by the pane} are described in the section entitled "Background: Modernization of the National Weather Service." To determine the technical adequacy of the new Doppler radar with respect to the "no degradation of service" requirement;, the pane! defined and mapped the radar-detection capabilities in Chapter 2. This analysis provided the basis for a comparison of the old and new systems, which is discussed in Chapter 3. The panel's comparison was made with respect to weather services, that is, warnings that involve a forecaster and his/her expertise, supported by all of the available information. This information includes bow radar observations and a wide variety of other observational information Mat is provided by the new technology and by other elements of the composite national weather system. ~ Use of "contiguous United States" in this report will denote the 48 contiguous states (i.e., not including Alaska or Hawaii). Degradation of services related to radar-detection services is not a factor in Alaska and Hawaii since NEXRADs will provide new coverage capabilities in both states. (There was no NWS radar coverage prior to NEXRAD.) 4

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Introduction 5 Guidelines for assessing potential degradation of service at specific sites are discussed in Chapter 4. The panel's conclusions and recommendations are listed in Chapter 5. General criteria for evaluating a weather radar system and the technical aspects of ra- dars system configuration, spatial coverage, resolution, and sensitivity are detailed in Appendix A. While a site-by-site evaluation was excluded from the panel's task, these criteria, guidelines, and data will allow others to conduct such evaluations. References, a list of acronyms, and a glossary are provided at the end of the report. BACKGROUND: MODERNIZATION OF THE NATIONAL WEATHER SERVICE In the late 1980s, the NWS embarked on a modernization and restructuring of its operations and the associated field office structure. When completed, this modernization will provide a much-needed updating of the technologies used to observe weather features (especially often-destructive phenomena, such as severe thunderstorms, tornadoes, wind shear, flash floods, and hurricanes) and will include new information systems that will enable Me NWS to provide more accurate and timely weather forecasts and warnings to the public. The new observing systems include the Weather Surveillance Radar-1988 Doppler (WSR-~8D) Next Generation Weather Radar (NEXRAD), the Automated Surface Observing System (ASOS), and the next generation of Geostationary Operational Environmental Satellites (GOES). In the new system, an Advanced Weather Interactive Processing System (AWIPS) will process the large volume of information, provide the forecaster with information in useful forms, and provide an interactive communications link among the offices. The associated restructuring of the NWS is driven by these new technologies. The current field office structure includes 52 Weather Service Forecast Offices (WSFOs), whose responsibilities are organized on a geographical basis; about 200 smaller offices, including Weather Service Offices (WSOs) and Weather Service Meteorological Observatories, which manually take surface observations and, in some instances, weather radar observations (and, in Me case of WSOs, issue local-area forecasts and warnings); and 13 River Forecast Centers (RFCs) (NWS, 1985, 7 and 21~. The new, modernized structure of the NWS will include ~ IB Weather Forecast Offices (WFOs), ~12 of which will be located in the contiguous United States, at sites determined primarily by the locations of the NEXRADs, and Me 13 RFCs (NRC, 1993, 12~. Even though there will be a substantial reduction in Me total number of field offices, the new organizational structure is intended to provide equivalent or ~ Improver . services. The modernization process is now well underway with the ongoing deployment of the new NEXRAD and ASOS observational technologies and the successful launch of a GOES-8 satellite in April 1994. Development of AWIPS is in progress. The NEXRAD One cornerstone of the modernization program is Me NEXRAD, more accurately described as the WSR-~8D. In addition to conventional reflectivity observations, this advanced radar uses the

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6 Assessment of NEXRAD Coverage "Doppler effect") to measure motion of clear air and atmospheric phenomena within storms, up to a maximum distance of 230 km from the radar. This allows much more accurate detection of circulations associated with tornadoes and other significant weather. NEXRADs will replace a network of 128 older radars that were concentrated east of the Rocky Mountains, primarily in the central and eastern United States. A comparison of radar characteristics is provided in Appendix A. The 138 NEXRADs to be deployed in the contiguous United States will provide national coverage. The NWS will operate 116 of the NEXRADs; the Department of Defense (r)oD) will operate 22. Through AWIPS, forecasters in the new field offices will work interactively win all of the avail- able data. The deployment of these radars nationwide is spread across a 6-year period (1990-1996~. As of April 1995, 116 NEXRADs had been installed. CHARGE TO THE COMMITTEE The charge transmitted by the Secretary of Commerce to the NRC (Appendix B) calls for the NWSMC to assess the adequacy of NEXRAD coverage from a scientific and technical standpoint, in terms of the no degradation of service requirement of the Weather Service Modernization Act, P.L. 102-567. The Act, section 702 (4), states that degradation of service means any decrease in or failure to maintain the quality and type of weather services provided by the National Weather Service to the public in a service area, including but not limited to a reduction in existing weather radar coverage at an elevation of 10,000 feet. This request from the Secretary of Commerce was amended by an October 5, 1994, agreement between National Oceanic and Atmospheric Administration (NOAA) and Congressman Cramer of Alabama. In accordance with this agreement (see sections 4 and 5, "Study Guidelines," in Appendix B) the pane! agreed to receive public comments and "to conduct an independent scientific assessment of Me proposed NEXRAD radar coverage and consolidation of field offices in terms of 'no degradation of services' and to establish criteria for identifying service areas where the decommis- sioning of existing radars could degrade service." The panel did not explicitly address nontechnical issues, such as economic, safety, and population risk factors, that were beyond the scope of this study. The Statement of Task of the NRC's NEXRAD Panel of the NWSMC is in Appendix C. ~ The Doppler effect is the change in frequency of electromagnetic radiation (light, radio waves, radar pulses, etc.) or sound waves as the point of emission or reflection approaches or recedes from an observer. An example is the change in pitch of an emergency vehicle siren as it approaches and passes an observer. 2 "Consolidation of field offices" is, among other things, tied to the decommissioning of existing radars and associated transfer of service responsibilities. Thus, the only scientific assessment that is applicable pertains to the technical aspects of radar-detection coverage and the associated forecast and warning services for the service area of the radar.

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Introduction 7 STUDY PROCESS Pane! members interviewed many people who were familiar with NEXRAD and solicited opin- ions from meteorologists and others on NEXRAD's performance. The interviewees reflected a high level of satisfaction with the capabilities of the NEXRAD and an enthusiasm for the operation and nutnuts of the radar This suggests that there is a high level of confidence in the performance of the system and the quality of its associated services. Making a balanced comparison of weather radar systems to ascertain a possible "degradation of service" requires the use of objective criteria upon which to base the comparison. In a broad sense, the important criterion is the system's ability to recognize and respond to significant weather features of concern. These criteria and the technical- aspects of a radar that relate to its detection capability are discussed in Appendix A. The pane! reviewed the history of the NEXRAD siting plans and was briefed by NOAA personnel and others on technical aspects of the program. At the panel's request, NOAA asked SRI International to provide spatial coverage, resolution, and sensitivity charts for elevations 4,000 ft. 6,000 ft. 10,000 ft. and 40,000 It above site level for the entire contiguous United States. It soon became apparent to the pane! that the detection of specific types of weather phenomena needed to be quantified and compared (see Chapter 2) in order to assess NEXRAD and pre-NEXRAD detection coverage realistically. The pane! derived detection ranges for several significant weather phenomena. Site-specific coverage characteristics for five weather phenomena were used to generate coverage charts for the pre- NEXRAD and NEXRAD networks. At the panel's request, NOAA asked SR} to prepare "difference charts," which the pane! analyzed to determine areas where the decommissioning of old radars could result in a degradation of radar-detection coverage (see Chapter 2~. Where sufficient data were available (see Chapter 3), the pane! reviewed warning performance data and compared pre-NEXRAD and NEXRAD operations at weather offices in `different areas of the country. The pane! also reviewed these data in the context of the overall composite system considerations of the pre-NEXRAD and NEXRAD networks. Finally, the pane! developed criteria (see Chapter 4) to use as guidelines in assessing possible degradation of service in areas where the decommissioning of existing radars, and changes in related services provided by the associated weather offices, might be in question.