NEXRAD

NEXRAD or Nexrad 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 within the United States Department of Commerce, the Federal Aviation Administration within the Department of Transportation, and the U.S. Air Force within the Department of Defense. Its technical name is WSR-88D.
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.

Deployment

In the 1970s, the U.S. Departments of Commerce, Defense, and Transportation, agreed that to better serve their operational needs, the existing national radar network needed to be replaced. The radar network consisted of WSR-57 developed in 1957, and WSR-74 developed in 1974. Neither system employed Doppler technology, which provides wind speed and direction information.
The Joint Doppler Operational Project was formed in 1976 at the National Severe Storms Laboratory to study the usefulness of using Doppler weather radar to identify severe and tornadic thunderstorms. Tests over the next three years, conducted by the National Weather Service and the Air Weather Service agency of the U.S. Air Force, 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 was formed to move forward with the development and deployment of the proposed NEXRAD radar network. That year, the NSSL completed a formal report on developing the NEXRAD system.
When the proposal was presented to the Reagan administration, two options were considered to build the radar systems: allow corporate bids to build the systems based on the schematics of the previously developed prototype radar or seek contractors to build their own systems using predetermined specifications. The JSPO group opted 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. However, it took four years to allow the prospective contractors to develop their proprietary models. Unisys was selected as the contractor, and was awarded a full-scale production contract in January 1990.
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 daily forecasting was in Sterling, Virginia on June 12, 1992. The last system deployed as part of the installation program was installed in North Webster, Indiana on August 30, 1997. In 2011, the new Langley Hill NEXRAD was added at Langley Hill, Washington to better cover the Pacific Coast of that area; other radars also filled gaps in coverage at Evansville, Indiana and Ft. Smith, Arkansas, following the initial installations. The site locations were strategically chosen to provide 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 quick access by maintenance technicians.
The NEXRAD radars incorporated a number of improvements over the radar systems that were previously in use. The new system provided Doppler velocity, improving tornado prediction ability by detecting rotation present within the storm at different scan angles. It provided improved resolution and sensitivity, enabling 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 examine the vertical structure of storms and could act as wind profilers by providing detailed wind information for several kilometers above the radar site. The radars also had a much increased range allowing detection of weather events at much greater distances from the radar site.
WSR-88D development, maintenance, and training are coordinated by the NEXRAD Radar Operations Center located on the grounds of the University of Oklahoma Westheimer Airport in Norman, Oklahoma.
The University of Louisiana at Monroe in Monroe, Louisiana operates a "WSR-88D clone" radar that is used by local National Weather Service offices in Shreveport, Little Rock and Jackson to fill gaps in NEXRAD coverage in northeastern Louisiana, southeastern Arkansas and western Mississippi. However, the radar's status as being part of the NEXRAD network is disputed.

Radar properties

A standard WSR-88D operates in the S band, at a frequency of around 2800 MHz, with a typical gain around 53 dB using a center-fed parabolic antenna. The pulse repetition frequency varies from 318 to 1300 Hz with a maximum power output of 700 kW at Klystron output, although dependent on the volume coverage pattern selected by the operator. All NEXRADs have a dish diameter of 9.1 m and an aperture diameter of 8.5 m. Using the predetermined VCPs, NEXRADs have a traditional elevation minimum and maximum ranging from 0.1 to 19.5 degrees, although the non-operational minimum and maximum spans from −1 to +45 degrees. Unlike its predecessor, the WSR-74, the antenna can not be manually steered by the operator. WSR-88D Level I data is the recorded output of the digital receiver. Spatial resolution varies with data type and scan angle – level III data has a resolution of 1 km x 1 degree in azimuth, while super-res level II,, has a resolution of 250m by 0.5 degrees in azimuth below 2.4 degrees in elevation.

Scan strategies

The NEXRAD radar system continually refreshes its three-dimensional database via one of several predetermined scan patterns. These patterns have differing PRFs to fit the respective use, but all have a constant resolution. Since the system samples the atmosphere in three dimensions, there are many variables that can be changed, depending on the desired output. With all traditional VCPs, the antenna scans at a maximum of 19.5 degrees in elevation, and a minimum of.5, with some coastal sites scanning as low as.2 or lower. Due to the incomplete elevation coverage, a phenomenon known as "The Cone of Silence" is present with all NEXRAD radars. The term describes the lack of coverage directly above the radar sites.
There are currently seven Volume Coverage Patterns available to NWS meteorologists, with an eighth in the process of replacing one of the existing seven. 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, 32, and 35
Shallow Precipitation: VCP 35, 112, and 215
Non-Tropical Convection: VCP 12, 212, and 215
Tropical System Convection: VCP 212, 215, 112, and 121

VCP	Scan time	Elevation scans	Elevation angles	Usage	SAILS available?
12	4.2	14	0.5, 0.9, 1.3, 1.8, 2.4, 3.1, 4, 5.1, 6.4, 8, 10, 12.5, 15.6, 19.5	Severe weather, including tornadoes, located closer to the radar	Yes
212	4.5	14	0.5, 0.9, 1.3, 1.8, 2.4, 3.1, 4, 5.1, 6.4, 8, 10, 12.5, 15.6, 19.5	Severe weather, including tornadoes, over 70 miles away from the radar, or widespread severe convection. Best VCP for MRLE use. Completion time for VCP 212 + 1 SAILS scan is similar to VCP 12 + 2 SAILS scans	Yes
112	5.5	14	0.5, 0.9, 1.3, 1.8, 2.4, 3.1, 4, 5.1, 6.4, 8, 10, 12.5, 15.6, 19.5	Variant of VCP 212 designed for tropical systems and strong, non-severe wind shear events. Uses a combination of MPDA and SZ-2 to form a contiguous velocity display. MRLE use is not possible with this VCP	Yes
215	6	15	0.5, 0.9, 1.3, 1,8, 2.4, 3.1, 4, 5.1, 6.4, 8, 10, 12, 14, 16.7, 19.5	General-purpose precipitation, including tropical systems capable of producing tornadoes. Most vertical resolution of any VCP	Yes
121	6	9	0.5, 1.5, 2.4, 3.4, 4.3, 6, 9.9, 14.6, 19.5	Legacy VCP, originally designed for tropical systems. Has significant gaps in vertical resolution above 6°. Scan strategy ensures 20 rotations in six minutes, heavily wearing antenna mechanical components. Similar completion time to VCP 215. To be replaced by VCP 112	No
31	10	5	0.5, 1.5, 2.4, 3.4, 4.3	Long-pulse clear air mode designed for maximum sensitivity. Excellent for detecting light snow or subtle boundaries. Prone to detecting ground clutter. May be prone to detecting virga	No
32	10	5	0.5, 1.5, 2.4, 3.4, 4.3	Short-pulse clear air mode designed for clear air or isolated light rain and/or wintry precipitation. Ideal to use when no precipitation is in the radar range, to reduce wear on antenna mechanical components	No
35	7	7	0.5, 0.9, 1.3, 1,8, 2.4, 3.1, 4, 5.1, 6.4	Short-pulse clear air VCP designed for scattered to widespread light to moderate precipitation from non-convective cloudforms, especially nimbostratus. Not recommended for convection, except for pop-up thundershowers produced by Cumulus congestus clouds located 30 miles or more away from the radar	Yes

The specific VCP currently in use at each NEXRAD site is available.