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NEXRAD Radar at the WSR-88D Radar Operations Center

NEXRAD or Nexrad (Next-Generation Radar) is a network of 160 high-resolution S-band Doppler weather radars operated by the National Weather Service (NWS), an agency of the National Oceanic and Atmospheric Administration (NOAA) within the United States Department of Commerce, the Federal Aviation Administration (FAA) within the Department of Transportation, and the U.S. Air Force within the Department of Defense. 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.


Continental US sites
Alaska, Hawaii, territories, and military base sites

In the 1970s, the U.S. Department of Commerce, Department of Defense, and the Department of Transportation 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 (NSSL) 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 U.S. Air Force weather agency, 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 Joint System Program Office (JSPO) was formed to move forward with the development and deployment of the proposed NEXRAD radar network. The JSPO 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.[1]

Installation of an operational 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, Washington to better cover the Pacific Coast of that area and other radars also filled gaps at Evansville, Indiana and Ft. Smith, Arkansas following the initial installations.[citation needed] 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 (WFOs) to permit quicker access to maintenance technicians.[2]

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 to even storm scale 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 additionally can act as wind profilers in providing detailed wind information for several kilometres 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.[3]

WSR-88D development, maintenance, and training are coordinated by the NEXRAD Radar Operations Center (ROC) located at the National Weather Center (NWC) in Norman, Oklahoma.[4]

Scan strategies

Unlike its predecessor, 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 [5]
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

Significant advancements in scanning techniques were instituted in the spring of 2014. These dynamic or adaptive scanning techniques include AVSET (Automated Volume Scan Evaluation and Termination) and SAILS (Supplemental Adaptive Intra-Volume Low-Level Scan). Both increase the temporal resolution of scans: (1) AVSET by automatically detecting when precipitation returns drop below a set threshold (typically 20 dBZ) at a certain height and then scanning at no higher tilts and (2) SAILS by rescanning the lowest tilt halfway through a volume scan. Volume scans previously required over four minutes to complete and these techniques cut that time by approximately one half without the need for hardware upgrades.[6][7]


Super resolution

Deployed from March to August 2008,[8] 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 Script error: No such module "convert". by 1 degree to Script error: No such module "convert". range, and velocity data at Script error: No such module "convert". by 1 degree to a range of Script error: No such module "convert".. Super Resolution provides reflectivity data with a sample size of Script error: No such module "convert". by 0.5 degree, and increase the range of Doppler velocity data to Script error: No such module "convert".. 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.[9]

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.[10]

Dual polarization

File:Non-Polarimetric Radar.gif
Non-Polarimetric Radar
File:Polarimetric Radar.gif
Polarimetric Radar

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.[11] The deployment of the dual polarization capability (Build 12) to NEXRAD sites began in 2010 and was completed 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.[12]

Future enhancements

Phased array

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.[13]



NEXRAD data are used in multiple ways. It is used by National Weather Service meteorologists and (under provisions of U.S. law) 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. [14]

Operational locations

See also



External links

Theory of Doppler Weather Radar
Real time data