The Next Generation Weather Radar (NEXRAD) system currently comprises 160 sites throughout the United States and select overseas locations. The NCEI archive includes the base data, called Level-II, and the derived products, called Level-III. Level-II data include the original three meteorological base data quantities: reflectivity, mean radial velocity, and spectrum width, as well as the dual-polarization base data of differential reflectivity, correlation coefficient, and differential phase.
From the Level-II data, computer processing generates numerous meteorological analysis products known as Level-III data. All the National Weather Service (NWS) and several select continental U.S. Department of Defense NEXRAD sites record Level-II data. The majority of the sites record Level-III products. When NCEI receives the data, it is archived and disseminated.
Using World War II technology, the NWS operated the Weather Surveillance Radars–1957 (WSR-57) network. This was followed by Weather Surveillance Radars–1974 (WSR-74) and then Weather Surveillance Radars–1988 Doppler (WSR-88D) or NEXRAD. The NEXRAD system is a joint effort of the U.S. Departments of Commerce, Defense, and Transportation. The controlling agencies are the NWS, Air Force Weather Agency, and Federal Aviation Administration (FAA), respectively. In 1988, the NEXRAD agencies established the WSR-88D NOAA Radar Operations Center (ROC) in Norman, Oklahoma.
The ROC employees come from the NWS, Air Force, Navy, FAA, and support contractors. The ROC provides centralized meteorological, software, maintenance, and engineering support for all WSR-88D systems. WSR-88D systems are modified and enhanced during their operational life to meet changing requirements, technology advances, and improved understanding of the application of these systems to real-time weather operations. The ROC also operates WSR-88D test systems for the development of hardware and software upgrades to enhance maintenance, operation, and provide new functionality.
All NEXRAD Level-II data are available through NCEI. Data collection and recording are in the unit of files, which typically contain four, five, six, or ten minutes of base data depending on the volume coverage pattern. There are between 50 and 100 Level-III products routinely available from NCEI, including precipitation estimates, hail estimates, storm relative velocity, and echo tops.
Radar Data Access
Weather radar data at NCEI are stored on our tape archive system and are accessible by placing orders through the NCEI website. There are several ordering systems available, optimized for different situations, including small orders, multisite and multiday orders, and large bulk orders. Small orders typically complete in less than 15 minutes.
There are occasional gaps and missing data for each site in the archive. These gaps are caused by a number of reasons including scheduled maintenance at the Radar sites, unplanned downtime due to severe weather, communications problems, or archival problems. The data access web pages include lists or visualizations of file availability.
NEXRAD Radar Products
Access a complete list of all available data products (called Level-III products) with descriptions and possible uses. There are between 50 and 100 Level-III products routinely available from NCEI, including precipitation estimates, hail estimates, storm relative velocity, and echo tops.
How Does the Radar Collect Data?
Three functional components make up the WSR-88D: Radar Data Acquisition, Radar Product Generator, and the Open Principal User Processor. 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 predetermined elevation angles completed in a specified period of time. Other scan strategies are currently under development for the future.
Volume Coverage Patterns 11 and 21
The Precipitation Mode uses these VCPs 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 five minutes, while VCP 21 has nine elevation angles and completes 11 azimuthal scans in six 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 the identification of more storms, and provides quicker updates. This VCP has 14 elevation angles and completes 17 azimuthal scans in about 4.5 minutes.
Volume Coverage Patterns 31 and 32
The Clear-Air Mode to optimize the sensitivity of the WSR-88D uses both of these VCPs. 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 five elevation angles, seven azimuthal scans, and take 10 minutes to complete.
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 (nine) as VCP 21, but completes 20 azimuthal scans in five minutes.
Volume Coverage Patterns 211, 212, and 221
These VCPs have the same elevation angles as VCPs 11, 12, and 21 respectively. The "split cuts" (generally elevation angles below 1.5° except for VCP 31, which has a split cut at 2.5°) apply these VCPs, which implement the Sachidananda-Zrnic Algorithm. See chapter five, Part C, of the Federal Meteorological Handbook No.11 for additional information on split cuts. The Sachidananda-Zrnic Algorithm reduces range ambiguity for Doppler data.
Where Can I Get Details on NEXRAD?
Details on the NEXRAD system are available in Federal Meteorological Handbook No. 11. There are additional handbooks (publications) available for download from the Office of the Federal Coordinator for Meteorology.
Four volumes are available:
The official Interface Control Documents (ICDs) contain detailed information on the binary data formats and product specifications.