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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.)
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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.
Representative terms from entire chapter:
national weather
