NEXRAD or Nexrad (Next-Generation Radar) is a network of 159 high-resolution S-band Doppler weather radars operated by the National Weather Service, an agency of the National Oceanic and Atmospheric Administration (NOAA) within the United States Department of Commerce. Its technical name is WSR-88D, which stands for Weather Surveillance Radar, 1988, Doppler. NEXRAD detects precipitation and atmospheric movement or wind. It returns data which when processed can be displayed in a mosaic map which shows patterns of precipitation and its movement. The radar system operates in two basic modes, selectable by the operator – a slow-scanning clear-air mode for analyzing air movements when there is little or no activity in the area, and a precipitation mode, with a faster scan for tracking active weather. NEXRAD has an increased emphasis on automation, including the use of algorithms and automated volume scans.
In the 1970s, the US Department of Commerce, Department of Defense, and the Transportation Department found the need to replace the existing national radar network, consisting of non-Doppler WSR-74 and WSR-57 radars developed in 1974 and 1957, respectively, to better serve their operational needs. The Joint Doppler Operational Project (JDOP) was formed in 1976 at the National Severe Storms Laboratory to study the usefulness of using Doppler radar to identify severe and tornadic thunderstorms. Tests over the next three years, conducted by the National Weather Service and the US Air Force Weather Service, found that Doppler radar provided much improved early detection of severe thunderstorms. A working group that included the JDOP published a paper providing the concepts for the development and operation of a national weather radar network. In 1979, the NEXRAD JSOP was formed to move forward with the development and deployment of the proposed NEXRAD radar network. The JSOP group needed to select a contractor to develop and produce the radars that would be used for the national network. Radar systems developed by Raytheon and Unisys were tested during the 1980s. Unisys was selected as the contractor, and was awarded a full-scale production contract in January 1990.
Installation of a prototype was completed in the Fall of 1990 in Norman, Oklahoma. The first installation of a WSR-88D for operational use in everyday forecasts was in Sterling, Virginia on June 12, 1992. The last system of this installation campaign was installed in North Webster, Indiana on August 30, 1997. In 2011 a single new radar was added at Langley Hill, WA to better cover the Pacific coast in that area. The site locations were strategically chosen to provide the most overlapping coverage between radars in case one failed during a severe weather event. Where possible, they were co-located with NWS Weather Forecast Offices to permit quicker access to maintenance technicians.
The NEXRAD radars incorporated a number of improvements over the radar systems previously in use. The new system provided Doppler velocity, improving tornado prediction ability. It provided improved resolution and sensitivity, allowing operators to see features such as cold fronts, thunderstorm gust fronts, and mesoscale features of thunderstorms that had never been visible on radar. The NEXRAD radars also provided volumetric scans of the atmosphere allowing operators to interrogate the vertical structure of storms and provide detailed wind profiles above the radar site. The radars also had a much increased range allowing detection of weather features at much greater distances from the radar site.
Unlike its predecessors, the WSR-88D antenna is not directly controllable by the user. Instead, the radar system continually refreshes its three-dimensional database via one of several predetermined scan patterns. Since the system samples the atmosphere in three dimensions, there are many variables that can be changed, depending on the desired output. There are currently nine Volume Coverage Patterns (VCP) available to NWS meteorologists. Each VCP is a predefined set of instructions that control antenna rotation speed, elevation angle, transmitter pulse repetition frequency and pulse width. The radar operator chooses from the VCPs based on the type of weather occurring:
- Clear Air or Light Precipitation: VCP 31 and 32
- Shallow Precipitation: VCP 21
- Convection: VCP 11, 12, 121, 211, 212, and 221 
|VCP||Scan Time (min)||Elevation scans||Elevation angles (°)||Usage||Special attributes|
|11||5||14||0.5, 1.5, 2.4, 3.4, 4.3, 5.3, 6.2, 7.5, 8.7, 10, 12, 14, 16.7, 19.5||Convection, especially when close to the radar||Has the best overall volume coverage.|
|211||Convection, especially when close to the radar||Improves range-obscured velocity data over VCP 11|
|12||4.5||14||0.5, 0.9, 1.3, 1.8, 2.4, 3.1, 4.0, 5.1, 6.4, 8.0, 10.0, 12.5, 15.6, 19.5||Convection, especially activity at longer ranges||Focuses on lower elevations to better sample the lower levels of storms.|
|212||Widespread severe convective events||Improves range-obscured velocity data over VCP 12|
|121||6||9||0.5, 1.5, 2.4, 3.4, 4.3, 6.0, 9.9, 14.6, 19.5||Large number of rotating storms, tropical systems, or when better velocity data is needed.||Scans lower cuts multiple times with varying pulse repetition rates to greatly enhance velocity data.|
|21||5||Shallow precipitation||Rarely used for convection due to sparse elevation data and long completion time.|
|221||Widespread precipitation with embedded convection. (i.e., tropical systems)||Improves range-obscured velocity data over VCP 121|
|31||10||5||0.5, 1.5, 2.5, 3.5, 4.5||Detecting subtle boundaries or wintry precipitation||Long-pulse|
|32||Slow rotation speed allows for increased sensitivity. Default clear-air mode, reduces wear on antenna mechanical components.||Short-pulse|
Deployed from March to August 2008, the Super Resolution upgrade is the capability of the radar to produce much higher resolution data. Under legacy resolution, the WSR-88D provides reflectivity data at 1 km by 1 degree to 460 km range, and velocity data at 0.25 km by 1 degree to a range of 230 km. Super Resolution provides reflectivity data with a sample size of 0.25 km by 0.5 degree, and increase the range of Doppler velocity data to 300 km. Initially the increased resolution is only available in the lower scan elevations. Super resolution makes a compromise of slightly decreased noise reduction for a large gain in resolution.
The improvement in azimuthal resolution increases the range at which tornadic mesoscale rotations can be detected. This allows for faster lead time on warnings and extends the useful range of the radar. The increased resolution (in both azimuth and range) increases the detail of such rotations, giving a more accurate representation of the storm. Super Resolution also provides additional detail to aid in other severe storm analysis. Super Resolution extends the range of velocity data and provides it faster than before, also allowing for faster lead time on potential tornado detection and subsequent warnings.
WSR-88D sites across the nation are currently being upgraded to polarimetric radar, which adds vertical polarization to the current horizontal radar waves, in order to more accurately discern what is reflecting the signal. This so-called dual polarization allows the radar to distinguish between rain, hail and snow, something the horizontally polarized radars cannot accurately do. Early trials have shown that rain, ice pellets, snow, hail, birds, insects, and ground clutter all have different signatures with dual-polarization, which could mark a significant improvement in forecasting winter storms and severe thunderstorms. The deployment of the dual polarization capability (Build 12) to NEXRAD sites began in 2010 and will be complete by the summer of 2013. The Vance AFB radar is the first operational WSR-88D to be modified to Dual Polarization. The modified radar went operational on 3 March 2011.
Beyond dual-polarization, the advent of phased array radar will probably be the next major improvement in severe weather detection. Its ability to rapidly scan large areas would give an enormous advantage to radar meteorologists. Any large-scale installation by the NWS is unlikely to occur before 2020. The NOAA Severe Storms Laboratory predicts that a phased array system will eventually replace the current network of WSR-88D radar transmitters. 
NEXRAD data are used in multiple ways. It is used by National Weather Service meteorologists and is freely available to users outside of the NWS, including researchers, media, and private citizens. The primary goal of NEXRAD data is to aid NWS meteorologists in operational forecasting. The data allows them to accurately track precipitation and anticipate its development and track. More importantly, it allows the meteorologists to track and anticipate severe weather and tornadoes. Combined with ground reports, tornado and severe thunderstorm warnings can be issued to alert the public about dangerous storms. NEXRAD data also provides information about rainfall rate and aids in hydrology forecasting. Data is provided to the public in several different forms. The most basic form is graphics published to the NWS website. Data is also available in two similar, but different, raw formats. Available directly from the NWS is Level III data. Level III data consists of reduced resolution, low-bandwidth, base products as well as many derived, post-processed products. Level II data consists of only the base products, but at their original resolution. Because of the higher bandwidth costs, Level II data is not available directly from the NWS. The NWS distributes this data freely to several top-tier universities who in turn distribute the data to private organizations. 
|State||City or Place Name||ICAO Location Identifier||Coordinates|
|NY||New York City||KOKX|
|OK||Norman (Testbed)||No Level III Data|
|ND||Grand Forks (Mayville)||KMVX|
|UT||Salt Lake City||KMTX|
|CA||San Joaquin Valley||KHNX|
|CA||Santa Ana Mountains||KSOX|
|NA||Lajes Field, Azores||LPLA|
|NA||Kunsan Air Base, South Korea||RKJK|
|NA||Camp Humphreys, South Korea||RKSG|
|NA||Kadena Air Base, Japan||RODN|
- Crum, Timothy D., Alberty, Ron L."The WSR-88D and the WSR-88D Operational Support Facility." Bulletin of the American Meteorological Society. 74.9 (1993).
- WSR-88D Radar, Tornado Warnings and Tornado Casualties.
- NEXRAD Product Improvement – Current Status of WSR-88D Open Radar Data Acquisition (ORDA) Program and Plans For The Future
- Polarimetric Radar Page
- Technical Implementation Notice 10-22 Amended, 7 March 2011, Radar Operations Center, NOAA.
- Weather Research: Weather Radar
- Unidata Internet Data Distribution (IDD)
- NEXRAD sites and coordinates by the National Climatic Data Center
- Atlas, David, Radar in Meteorology: Battan Memorial and 40th Anniversary Radar Meteorology Conference, published by the American Meteorological Society, Boston, 1990, 806 pages, ISBN 0-933876-86-6, AMS Code RADMET.
|Wikimedia Commons has media related to: WSR-88D NEXRAD|
- Theory of Doppler Weather Radar
- Frequently Asked Questions by NOAA
- Radar Frequently Asked Questions (FAQ) by Weather Underground
- Social & Economic Benefits of NEXRAD from "NOAA Socioeconomics" website initiative
- Real time data