NEXRAD

NEXRAD Radar at the WSR-88D Radar Operations Center

NEXRAD or Nexrad (Next-Generation Radar) is a network of 159 high-resolution 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.

Deployment

Source: NOAA

After more than 30 years of research on operational Doppler weather radar systems, the National Weather Service (NWS) began to deploy the WSR-88D in 1988. It replaced WSR-74 and even WSR-57 units from 1974 and 1957 respectively. The first installation was completed in the Fall of 1990 in Norman, Oklahoma, however, the first installation of a WSR-88D for use in everyday forecasts was in Sterling, Virginia on June 12, 1992. The last system was installed in North Webster, Indiana on August 30, 1997. 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.^[1]

Initial development of the NEXRAD system started in 1982 at the National Severe Storms Laboratory in Norman, Oklahoma. Members of the early NEXRAD team included Timothy O'Bannon, a meteorologist and hydrologist who, as an amateur ornithologist also recognized the radar's additional capabilities for tracking bird migration patterns.^[2]

Lead contractor was Sperry Corporation and Concurrent Computer supplied the high speed minicomputer processing units.

Scan strategies

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 six Volume Coverage Patterns (VCP) available to NWS meteorologists. Each VCP is a predefined set of instructions given to the antenna that control the rotation speed, transmit/receive mode, and elevation angles. They use a specific numbering scheme:

• Clear Air: VCP 31 and 32 (two digits beginning with 3)
• Shallow Precipitation: VCP 21 (two digits beginning with 2)
• Convection: VCP 11 and 12 (two digits beginning with 1)
• Multiple Pulse Frequency Dealiasing: VCP 121 (three digits beginning with a 1, followed by the 2 digit number of VCP with similar elevation angles)

Future enhancements

Super resolution

In the process of implementation from March to June 2008, is the capability of the RDA to produce super resolution data. The WSR-88D provides reflectivity data at 1 km by 1 degree to 460 km range, and Doppler data at 0.25 km by 1 degree to a range of 230 km. Super Resolution will provide data with a sample size of 0.25 km by 0.5 degree, and increase the range of Doppler data to 300 km from the current 230 km. Initially the increased resolution will only be available in the lower scan elevations. Super resolution makes a compromise of slightly decreased noise reduction for a large gain in resolution.^[3]

To improve severe weather warning lead times, potential tornadic storms need to be identified as soon as possible. The improvement in beam width resolution increases the range at which small tornado parent circulation patterns (down to 4 km diameter) can be detected. Super-resolution also provides additional detail to aid in severe storm analysis. Extending the range of Doppler data and providing Doppler data earlier in the process of a volume scan provides velocity data more quickly than current scan techniques.^[4]As of August 1, 2008, 134 out of 172 nexrad sites provide super resolution.

Polarimetric radar

The next major upgrade is 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.^[5]The deployment of the dual polarization capability (Build 12) to NEXRAD sites will begin in 2010 and last until 2012.

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 2010. Such a system would more likely be installed separate from the existing WSR-88D network, perhaps only in areas like the Great Plains where tornadoes are more common.^[6]

Applications

One practical application under experiment is using the mosaic map to suggest alternate flight paths for airliners to avoid turbulence.^[7]

Bibliography

• David Atlas, 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.

In Popular Culture

In the movie Twister, references are made about NEXRAD, in the form of mobile computing systems used by storm chasers containing "NEXRAD realtime."

List of NEXRAD sites

See also

• LLWSAS
• Canadian weather radar network

References

External links

• http://www.weather.gov/
• http://weather.noaa.gov/radar/national.html
• http://www.srh.noaa.gov/srh/sod/radar/radinfo/radinfo.html
• http://www.srh.noaa.gov/srh/sod/radar/radinfo/about.html
• http://www.nssl.noaa.gov/researchitems/radar.shtml
• http://mi3.ncdc.noaa.gov/mi3report/MISC/nexrad-stations.TXT
• http://www.wunderground.com/radar/help.asp
• NEXRAD Weather Links
• Social & Economic Benefits of NEXRAD from "NOAA Socioeconomics" website initiative