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
NEXRAD sites within the Continental U.S.
NEXRAD sites in Alaska, Hawaii, U.S. territories, and military bases.
In the 1970s, the U.S. Department of Commerce, Department of Defense and Department of Transportation found the need to replace the existing national radar network, consisting of WSR-74 and WSR-57 radars not utilizing Doppler technology that were respectively developed in 1974 and 1957, 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 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 (JSPO) 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.[1][2]
When the proposal was presented to the Reagan administration, options were considered to build the radar systems: either allowing 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.[1][2]
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 forecasting was in Sterling, Virginia on June 12, 1992. The last system deployed as part 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; other radars also filled gaps in coverage 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.[3]
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 any rotation present within the storm at different scan angles. It provided improved resolution and sensitivity, allowing operators to see features such as cold fronts, thunderstormgust 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.[4]
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:
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 half that span without the need for hardware upgrades.[7][8]
Upcoming 2017 VCP changes
In October, 2015, the National Weather Service announced[9] that an upcoming software update would eliminate multiple precipitation mode VCPs, and replace them with a single VCP intended to combine the best features of the VCPs being eliminated. An additional clear air mode will also be made available.[1]
As of the upcoming Build 18 update, scheduled for deployment in the spring and summer of 2017, the following VCPs will be available:
Severe weather, including tornadoes, located closer to the radar (within 85 miles for storms travelling up to 55 MPH, but shorter distances for faster-moving precipitation)
Severe weather, including tornadoes, over 70 miles away from the radar, or widespread severe convection. Completion time for VCP 212 + 1 SAILS scan is similar to VCP 12 + 2 SAILS scans
General-purpose precipitation, including tropical systems capable of producing tornadoes. Most vertical resolution of any VCP
Yes (up to one per volume scan)
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
No
31
9.75
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
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
9
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 (up to one per volume scan)
Enhancements
Super resolution
Deployed from March to August 2008,[11] the Super Resolution upgrade permitted the capability of the radar to produce much higher resolution data. Under legacy resolution, the WSR-88D provides reflectivity data at 1 km (0.62 mi) by 1 degree to 460 kilometres (290 mi)* range, and velocity data at 0.25 km (0.16 mi) by 1 degree to a range of 230 km (140 mi). Super Resolution provides reflectivity data with a sample size of 0.25 km (0.16 mi) by 0.5 degree, and increase the range of Doppler velocity data to 300 km (190 mi). 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.[12]
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. Along with providing better detail of detected precipitation and other mesoscale features, 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.[13]
Dual polarization
Non-Polarimetric Radar
Polarimetric Radar
WSR-88D sites across the nation have been upgraded to polarimetric radar, which adds vertical polarization to the horizontalradar 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 showed 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.[14] The deployment of the dual polarization capability (Build 12) to NEXRAD sites began in 2010 and was completed by the summer of 2013. The radar at Vance Air Force Base in Enid, Oklahoma is the first operational WSR-88D to be modified to utilize dual polarization technology; the modified radar went into operation on March 3, 2011.[15]
Coverage gaps
NEXRAD coverage below 10,000 feet
WSR-88D has coverage gaps below 10,000 feet (or no coverage at all) in many parts of the continental United States, often for terrain or budgetary reasons, or remoteness of the area. Such notable gaps include most of Alaska; many portions of the Rocky Mountains; Pierre, South Dakota; portions of northern Texas; large portions of the Nebraska panhandle; and areas near the borders of the Oklahoma and Texas Panhandles. Notably, many of these gaps lie in tornado alley. At least one tornado has gone undetected by WSR-88D as a result of such a coverage gap – an EF1 tornado in Lovelady, Texas in April 2014. As a result of the coverage gap, initial reports of tornadic activity were treated with skepticism by the local National Weather Service forecast office.[16][17]
Coverage gaps can also be caused during radar outages, especially in areas with little to no overlapping coverage; an outage on July 16, 2013 resulted in an outage that lasted through early August[18]
It is not likely that additional WSR-88Ds will be deployed, as the production line was shut down in 1997, and the National Weather Service has an insufficient budget to restart production.[17]
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 National Severe Storms Laboratory predicts that a phased array system will eventually replace the current network of WSR-88D radar transmitters.[19]
Applications
Usage
NEXRAD data is 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 hydrological forecasting. Data is provided to the public in several different forms, the most basic form being 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, consisting 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 which in turn distribute the data to private organizations.[20]
