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Terminal Doppler Weather Radar - Supplemental Product Generator (TDWR-SPG)

The Terminal Doppler Weather Radar (TDWR) network is a Doppler weather radar system operated by the Federal Aviation Administration (FAA), which is 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. NCEI archives the products produced by the NWS Supplemental Product Generator (SPG), called Level III, which are in the same data format as Next Generation Weather Radar (NEXRAD) Level III. NCEI does not archive the base data (called Level II).

History

TDWR was developed in the early 1990s at Lincoln Laboratory, which is part of the Massachusetts Institute of Technology. The TDWR system was funded by the U.S. Federal Aviation Administration (FAA) to assist air traffic controllers by providing real-time wind shear detection and high-resolution precipitation data.

In 2004, the NWS established the policy with the FAA to allow the operational use of TDWR data and began to develop the SPG. Initial deployment of the SPG began in 2005 and finished in 2008.

As of 2014, there were 45 operational TDWR-SPG radar systems in major metropolitan locations across the United States and Puerto Rico.

Data Access

Radar Data Access
Weather radar data at NCEI are stored on our tape archive system and are accessed by placing orders through the NCEI website. There are several ordering systems available, optimized for different situations, including small orders, multisite and multiday orders, and large bulk orders. Currently, TDWR-SPG data are only available from the "Select by Site" and "Select by File" ordering systems. Small orders are typically completed in less than 15 minutes.

There are occasional gaps and missing data for each site in the archive. These gaps are caused by a number of reasons including scheduled maintenance at the Radar sites, unplanned downtime due to severe weather, communications problems, or archival problems. The data access web pages include lists or visualizations of file availability.

Products

TDWR-SPG Products
This complete list of all available data products (called Level-III products) includes descriptions and possible uses. There are 26 TDWR-SPG Level-III products routinely available from NCEI, including precipitation estimates, storm relative velocity, and echo tops.

Comparison of the TDWR to the WSR-88D

The range resolution of the TDWR is finer than what is available in the Weather Surveillance Radar, 1988 Doppler (WSR-88D), or any other FAA radar that has weather channel capability. The TDWR utilizes a range gate resolution of 150 m for Doppler data. It has a resolution of 150 m for reflectivity data within 135 km and 300 m from beyond 135 km to 460 km. By contrast, the WSR-88D employed by the National Weather Service, FAA, and Department of Defense has a maximum range gate resolution of 250 m for Doppler and 1 km for surveillance data.

The angular (azimuth) resolution of the TDWR is nearly twice what is available in the WSR-88D. Each radial in the TDWR has a beam width of 0.55 degrees. The average beam width for the WSR-88D is 0.95 degrees. The following table shows a comparison of technical specifications between the TDWR and the WSR-88D.

	WSR-88D	TDWR
Wavelength	10 cm	5 cm
Volume Scan Time	4 minutes in VCP 12	1 min 0.2 degree, HAZ
Beam Width	1.25 degrees	0.5 degrees
Range Gate	0.13 nm in velocity 0.54 nm in reflectivity	0.067 nm
Max Unambiguous Velocity	Up to 62 kts	20-30 kts
Max Doppler Range	230 km	90 km

NOAA's Radar Operations Center has detailed information.

Technical Information

How Does the Radar Collect Data?

The FAA designed the TDWR to look for low-altitude phenomena such as wind shifts over the runways, wind shear along the immediate approach and departure corridors, and downbursts. These radars are located close to major airports with the scanning strategy optimized to sample the atmosphere over its associated airport. The TDWR employs a scanning strategy called monitor mode, which is similar to the WSR-88D clear-air mode. The TDWR remains in monitor mode until one of the two following conditions are recognized:

A region of 20 dBZ echoes located within 24.3 nautical miles (nm) from the associated airport with a nominal areal extent of 1.3 nm and an altitude of at least 1.3 nm above ground level or
The radar detects wind shear or a microburst.

This is a departure from the WSR-88D operations that can switch from clear-air mode to precipitation mode if the areal coverage of precipitation exceeds an adaptable parameter anywhere on the scope. The TDWR ties its decision on the area of influence directly to its associated airport.

Volume Coverage Pattern (VCP) 90
VCP 90 is the TDWR monitor mode (clear air) and consists of 17 scans in about six minutes. The first cut is always a low PRF, long-range scan (276 km). All remaining scans are short range (90 km). There is a small variation in the elevation angles of the lower scans, but all scans above 5 degrees are the same in all systems. Cuts two and three employ a split cut strategy. Cut two contains no dealiased Doppler data. Cut three (at the same elevation as cut 2) is the first cut to contain dealiased Doppler velocity data. The first three cuts are used to initialize wind field models and velocity unfolding algorithms.

Volume Coverage Pattern (VCP) 80
VCP 80 is the TDWR hazardous mode (precipitation mode). The first cut is always a low PRF, long-range scan (276 km). Cuts two and three are the short-range split cut scans used to collect information for unfolding and clutter migration algorithms. The first cut with dealiased Doppler data is cut three. There are two sub-volumes or aloft scans contained in each full volume as denoted by the use of red and orange. Every fourth scan, the TDWR provides a short-range elevation scan at the same elevation as cuts two and three.