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Introduction
Radar data and derived products are available at no cost through the National Climatic Data Center. The system from which these data are collected is the Weather Surveillance Radar-1988 Doppler network (WSR-88D Radar) which began operation at a few sites in 1991.
What is NEXRAD?
The Next Generation Weather Radar system (NEXRAD)
comprises 159 Weather Surveillance Radar-1988 Doppler
(WSR-88D) sites throughout the United States and
select overseas locations. This system is a joint effort
of the United States Departments of Commerce (DOC),
Defense (DOD), and Transportation (DOT). The controlling
agencies are the National Weather Service (NWS),
Air Force Weather Agency (AFWA) and Federal Aviation
Administration (FAA), respectively. Level II data are the three
meteorological base data quantities: reflectivity,
mean radial velocity, and spectrum width. From these
quantities, computer processing generates numerous
meteorological analysis products known as Level III
data. Level II data are recorded at all NWS and
several select CONUS DOD WSR-88D sites. Level III products
are recorded at 155 of the 159 sites. The data are
sent to the National Climatic Data Center (NCDC)
for archiving and dissemination.
See the NEXRAD Inventory
or the NEXRAD Product List
for additional information.
Detailed information can be found at the NEXRAD Radar Operations Center http://www.roc.noaa.gov/
How does the radar collect data?
Three functional components make up the WSR-88D: Radar Data
Acquisition (RDA), Radar Product Generator (RPG), and the
Open Principal User Processor (OPUP). To adequately sample the
atmosphere, the WSR-88D employs nine scanning strategies or
volume coverage patterns (VCPs). A VCP is a series of 360 degree
sweeps of the antenna at pre-determined elevation angles completed
in a specified period of time. Other scan strategies maybe implemented
in the future.
Volume Coverage Pattern 121: This VCP implements the multi-pulse repetition frequency detection algorithm (MPDA) which helps mitigate range/velocity aliasing (the Doppler Dilemma). This VCP has the same elevation angles (9) as VCP 21, but completes 20 azimuthal scans in 5 minutes.
Volume Coverage Pattern 12: This VCP has the same number of elevation angles as VCP 11. However, denser vertical
sampling at lower elevation angles provides better vertical definition of storms, improves the detection capability of radars
impacted by terrain blockage for better rainfall and snowfall estimates, results in more storms being identified, and provides
quicker updates. This VCP has 14 elevation angles and completes 17 azimuthal scans in about 4.5 minutes.
Volume Coverage Patterns 11 and 21: These VCPs are used in the Precipitation Mode to better sample the vertical structure of convective weather echoes and to provide better temporal resolution. VCP 11 provides better vertical sampling of weather echoes than VCP 21 and is usually preferred in situations where convective precipitation is within 60 nmi of the antenna. The VCP 11 has 14 elevation angles and completes 16 azimuthal scans in 5 minutes while VCP 21 has 9 elevation angles and completes 11 azimuthal scans in 6 minutes.
Volume Coverage Patterns 31 and 32: Both of these VCPs are used in the Clear-Air Mode to optimize the sensitivity of the WSR-88D. The VCP 31 (long pulse) provides a better signal-to-noise ratio permitting lower reflectivity returns to be detected, while VCP 32 (short pulse) provides a higher unambiguous velocity. Both VCPs have 5 elevation angles, 7 azimuthal scans, and take 10 minutes to complete.
Volume Coverage Patterns 211, 212, and 221: These VCPs have the same elevation angles as VCPs 11, 12, and 21
respectively. These VCPs implement the Sachidananda-Zrnic Algorithm (SZ-2) which is applied on the "split cuts" (generally
elevation angles below 1.5° except for VCP 31 which has a split cut at 2.5°). See Chapter 5, Part C, of the
Federal
Meteorological Handbook No.11 (FMH-11) for additional information on split cuts. The SZ-2 Algorithm reduces range ambiguity
for Doppler data.
For more information on radar data visit the NEXRAD Radar Operations Center.
Where can I get details on NEXRAD radar?
Details on the NEXRAD RADAR system are available in Federal Meteorological Handbook No. 11 (FMH-11). The handbook consists of four
volumes and covers RADAR concepts, theory, products and analysis. The handbooks are available for download from the
Office of the
Federal Coordinator for Meteorology (OFCM)
at http://www.ofcm.gov/homepage/text/pubs.htm
The four FMH-11 volumes are:
A - System Concepts, Responsibilities, and Procedures.
B - Doppler Radar Theory and Meteorology.
C - Products and Algorithms.
D - Unit Description and Operational Analysis.
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A considerable amount of free information on the NEXRAD Radar System can be obtained through the NCDC web pages:
http://www.ncdc.noaa.gov/oa/radar/jnx/
http://www.ncdc.noaa.gov/nexradinv/index.jsp
http://www.ncdc.noaa.gov/oa/radar/radarproducts.html
http://www.ncdc.noaa.gov/oa/climate/climateresourcesother.html
What is the cost of NEXRAD data?
Digital Radar Data may be downloaded at no cost from NCDC. ( Download Radar Data )
Hardcopy Radar Maps may also be ordered for a small fee. ( Order Radar Hardcopy )
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