NEXRAD
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NEXRAD or Nexrad (Next-Generation Radar) is a network of 158 high-resolution Doppler 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 time for tracking active weather. NEXRAD has an increased emphasis on automation, including the use of algorithms and automated volume scans.
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[edit] Deployment
After more than 30 years of research on operational Doppler 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. The last system was completed in northern Indiana in June of 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.
[edit] 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)
| VCP | Scan Time (min) | Elevation angles (°) | Usage | Special attributes |
|---|---|---|---|---|
| 11 | 5 | 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. |
| 12 | 4 | 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. |
| 121 | 5.5 | 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 repetitions to greatly enhance velocity data. |
| 21 | 6 | 0.50, 1.5, 2.4, 3.4, 4.3, 6.0, 9.9, 14.6, 19.5 | Shallow precipitation | Rarely used for convection due to sparse elevation data and long completion time. |
| 31 | 10 | 0.5, 1.5, 2.5, 3.5, 4.5 | Detecting subtle boundaries or wintry precipitation | Long-pulse |
| 32 | 10 | 0.5, 1.5, 2.5, 3.5, 4.5 | Slow rotation speed allows for increased sensitivity. Default clear-air mode, reduces wear on antenna. | Short-pulse |
[edit] Future Enhancements
Currently, a large hardware upgrade to the Radar Data Acquisition (RDA) units is underway. The original hardware and software at the radar sites had very limited expandability. To enable more efficient future enhancements to radar products, an "Open RDA" is being installed at all WSR-88D sites nationwide. In addition to large increases in computing power, the new software architecture allows for easier installation of upgrades, including the latest algorithms. This will pave the way for numerous planned improvements over the next decade.
The next major upgrade is likely to be 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, sleet, 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. [1]
Beyond dual-polarization, the advent of phased array radar will probably be the next giant leap 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 the end of the decade. 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.
[edit] See also
| US Weather Radars |
|---|
| Military weather radars: AN/APQ-13 | AN/APS-2F | AN/CPS-9 | AN/FPS-41 | AN/FPS-77 |
| Weather Surveillance Radars: WSR-1 and -1A | WSR-3 | WSR-4 | WSR-57 | WSR-74C and -74S | WSR100-S | WSR-88D (NEXRAD) |
| Research radars: ADRAD | ARMOR | Cimarron | CORAD | CSU-CHILL | CSU-Pawnee | DOW | KPOL | NOAA/ETL | NOAA Ron Brown's Doppler | NPOL | NSSL 10 cm Doppler | PAR at NSSL | SKYWATER | SMART-R | TOGA | UND |
[edit] External links
- 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://www.wunderground.com/radar/help.asp
- http://weatheratmyhouse.com
- NEXRAD Weather Links
