Terminal Doppler Weather Radar (TDWR) is a Doppler weather radar system with a three-dimensional "pencil beam" used primarily for the detection of hazardous wind shear conditions, precipitation, and winds aloft on and near major airports situated in climates with great exposure to thunderstorms in the United States. As of 2011, all were in-service with 45 operational radars, some covering multiple airports in major metropolitan locations, across the United States & Puerto Rico. Several similar weather radars have also been sold to other countries such as China (Hong Kong). Funded by the United States Federal Aviation Administration (FAA), TDWR technology was developed in the early 1990s at Lincoln Laboratory, part of the Massachusetts Institute of Technology, to assist air traffic controllers by providing real-time wind shear detection and high-resolution precipitation data. The primary advantage of TDWRs over previous weather radars is that it has a finer range resolution—meaning it can see smaller areas of the atmosphere. The reason for the resolution is that the TDWR has a narrower beam than traditional radar systems, and that it uses a set of algorithms to reduce ground clutter.
Characteristics
TDWR uses a carrier wave in the frequency band of 5600–5650 MHz (5 cm wavelength), with a narrow beam and angular resolution of 0.5 degrees, and has a peak power of 250 kW. In reflectivity, the resolution in distance is 150 metres (500 ft) within 135 kilometres (84 mi) of the radar and 300 metres (1,000 ft) from 135 kilometres (84 mi) to 460 kilometres (290 mi) to the radar. The reason for this difference is that since the width resolution is angular, at larger range the width of the beam becomes quite large and to obtain a better averaging of data in a resolution volume, one has to increase the number of range pulse bins. This cut off is arbitrarily set for the software at 135 kilometres (84 mi). In radial velocities, data are available up to 90 kilometres (56 mi) from the radar with the full angular resolution of 0.5 degrees and range resolution of 150 metres (490 ft). Because of the Pulse Repetition Frequency (PRF) used, there is aliasing and the maximum non-ambiguous velocity is 20 to 30 knots (23 to 35 mph; 37 to 56 km/h). TDWR can perform near-surface scans at a 0.1-0.3 degree angle of inclination from the Earth's surface every minute. It can also perform composite scans in which the radar observes at several different angles of inclination in order to obtain a fuller picture of the atmospheric conditions; each such composite scan requires 6 minutes.
Comparison with NEXRAD
Advantages A NEXRAD weather radar currently used by the National Weather Service (NWS) is a 10 cm wavelength (2700-3000 MHz) radar capable of a complete scan every 4.5 to 10 minutes, depending on the number of angles scanned, and depending on whether or not MESO-SAILS is active, which adds a supplemental low-level scan while completing a volume scan. Its resolution is 0.5 degrees in width and 250 metres (820 ft) in horizontal. The non-ambiguous radial velocity is 62 knots (71 mph; 115 km/h) up to 230 kilometres (140 mi) from the radar. The range resolution of the TDWR is nearly twice that of that classic NEXRAD scheme. This will give much better details on small features in precipitation patterns, particularly in thunderstorms, in reflectivity and radial velocity. However, this finer resolution is only available up to 135 kilometres (84 mi) from the radar; beyond that, the resolution is close to that of the NEXRAD. However, since August 2008, oversampling on NEXRAD has increased its resolution in lower elevations in reflectivity data to 0.25 km (0.16 mi) by 0.5 degree, and increased the range of Doppler velocity data to 300 km (190 mi). This lessens the advantages of TDWR for those elevations.
Shortcomings
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