Terminal Doppler Weather Radar

By Milwaukee Area Skywarn Association,
Last updated March 2009

The Terminal Doppler Weather Radar (TDWR) is an advanced technology weather radar deployed near 45 of the larger airports in the U.S. The radars were developed and deployed by the Federal Aviation Administration (FAA) beginning in 1994, as a response to several disastrous jetliner crashes in the 1970s and 1980s caused by strong thunderstorm winds. The crashes occurred because of wind shear--a sudden change in wind speed and direction. Wind shear is common in thunderstorms, due to a downward rush of air called a microburst or downburst.

The TDWRs can detect such dangerous wind shear conditions, and have been instrumental in enhancing aviation safety in the U.S. over the past 15 years. The TDWRs also measure the same quantities as our familiar network of 148 NEXRAD WSR-88D Doppler radars--precipitation intensity, winds, rainfall rate, echo tops, etc. However, the newer Terminal Doppler Weather Radars are higher resolution, and can "see" details in much finer detail close to the radar. This high-resolution data has generally not been available to the public until now. Thanks to a collaboration between the National Weather Service (NWS) and the FAA, the data for all 45 TDWRs will be made available in real time over the next few months via a free satellite broadcast (NOAAPORT). Six radar sites are already available (Figure 1), and the remaining radars will be added by June 2009. I'm pleased to announce that the Weather Underground is now making the TDWR data available to the public, and will be adding new sites as they become available. We're calling them "High-Def" stations on our NEXRAD radar page. The six TDWR sites available so far are:

Detroit, MI
Cleveland, OH
Fort Lauderdale, FL
Miami, FL
West Palm Beach, FL
Newark, NJ

Since thunderstorms are uncommon along the West Coast and Northwest U.S., there are no TDWRs
Figure 1
Summary of the TDWR products
The TDWR products are very similar to those available for the traditional WSR-88D NEXRAD sites. There is the standard radar reflectivity image, available at each of three different tilt angles of the radar, plus Doppler velocity of the winds in precipitation areas. There are 16 colors assigned to the short range reflectivity data (same as the WSR-88Ds), but 256 colors assigned to the long range reflectivity data and all of the velocity data. Thus, you will see up to 16 times as many colors in these displays versus the corresponding WSR-88D display, giving much higher detail of storm features. The TDWRs also have storm total precipitation available in the standard 16 colors like the WSR-88D has, or in 256 colors (the new "Digital Precipitation" product). Note, however, that the TDWR rainfall products generally underestimate precipitation, due to attenuation problems (see below). The TDWRs also have such derived products as echo height, vertically integrated liquid water, and VAD winds. These are computed using the same algorithms as the WSR-88Ds use, and thus have no improvement in resolution.

Improved horizontal resolution of TDWRs
The TDWR is designed to operate at short range, near the airport of interest, and has a limited area of high-resolution coverage--just 48 nm, compared to the 124 nm of the conventional WSR-88Ds. The WSR-88Ds use a 10 cm radar wavelength, but the TDWRs use a much shorter 5 cm wavelength. This shorter wavelength allow the TDWRs to see details as small as 150 meters along the beam, at the radar's regular range of 48 nm. This is nearly twice the resolution of the NEXRAD WSR-88D radars, which see details as small as 250 meters at their close range (out to 124 nm). At longer ranges (48 to 225 nm), the TDWRs have a resolution of 300 meters--more than three times better than the 1000 meter resolution WSR-88Ds have at their long range (124 to 248 nm). 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. At distances within 48 nm of the TDWR, these radars can pick out the detailed structure of tornadoes and other important weather features (Figure 2). Extra detail can also been seen at long-ranges, and the TDWRs should give us more detailed depictions of a hurricane's spiral bands as it approaches the coast.
Figure 2. View of a tornado taken by conventional WSR-88D NEXRAD radar (left) and the higher-resolution TDWR system (right). Using the conventional radar, it is difficult to see the hook-shape of the radar echo, while the TDWR clearly depicts the hook echo, as well as the Rear-Flank Downdraft (RFD) curling into the hook. Image credit: National Weather Service.
No change to time resolution
Like the old NEXRAD data, the new TDWR data will update once every six minutes. The NWS advertises that the TDWR data will be sent out within one minute of when it is measured. The TDWR does scan the atmosphere once per minute at the lowest elevation angle of the radar, but unfortunately, there are no plans to make this rapid scan data available via the free public NOAAPORT feed.

Attenuation problems
The most serious drawback to using the TDWRs is the attenuation of the signal due to heavy precipitation falling near the radar. Since the TDWRs use the shorter 5 cm wavelength, which is closer to the size of a raindrop than the 10 cm wavelength used by the traditional WSR-88Ds, the TDWR beam is more easily absorbed and scattered away by precipitation. This attenuation means that the radar cannot "see" very far through heavy rain. It is often the case that a TDWR will completely miss seeing tornado signatures when there is heavy rain falling between the radar and the tornado. Hail causes even more trouble (Figure 3). Thus, it is best to use the TDWR in conjunction with the traditional WSR-88D radar to insure nothing is missed.
Figure 3. View of a squall line (left) taken using a TDWR (left column) and a WSR-88D system. A set of three images going from top to bottom show the squall line's reflectivity as it approaches the TDWR radar, moves over the TDWR, than moves away. Note that when the heavy rain of the squall line is over the TDWR, it can "see" very little of the squall line. On the right, we can see the effect a strong thunderstorm with hail has on a TDWR. The radar (located in the lower left corner of the image) cannot see much detail directly behind the heavy pink echoes that denote the core of the hail region, creating a "shadow". Image credit: National Weather Service

Range unfolding and aliasing problems
Another serious drawback to using the TDWRs is the high uncertainty of the returned radar signal reaching the receiver. Since the radar is geared towards examining the weather in high detail at short range, echoes that come back from features that lie at longer ranges suffer from what is called range folding and aliasing. For example, for a thunderstorm lying 48 nm from the radar, the radar won't be able to tell if the thunderstorm is at 48 nm, or some multiple of 48 nm, such as 96 or 192 nm. In regions where the software can't tell the distance, the reflectivity display will have black missing data regions extending radially towards the radar (Figure 4). Missing velocity data will be colored pink and labeled "RF" (Range Folded). In some cases, the range folded velocity data will be in the form of curved arcs that extend radially towards the radar.

Figure 4. Typical errors seen in the velocity data (left) and reflectivity data (right) when range folding and aliasing are occurring. Image credit: National Weather Service.

Ground clutter problems
Since the TDWRs are designed to alert airports of low-level wind shear problems, the radar beam is pointed very close to the ground and is very narrow. The lowest elevation angle for the TDWRs ranges from 0.1 to 0.3, depending upon how close the radar is to the airport of interest. In contrast, the lowest elevation angle of the WSR-88Ds is 0.5. As a result, the TDWRs are very prone to ground clutter from buildings, water towers, hills, etc. Many radars have permanent "shadows" extending radially outward due to nearby obstructions. The TDWR software is much more aggressive about removing ground clutter than the WSR-88D software is. This means that real precipitation echoes of interest will sometimes get removed.

For more information
For those of you who are storm buffs that will be regularly using the new TDWR data, I highly recommend that you download the three Terminal Doppler Weather Radar (TDWR) Build 3 Training modules. These three Flash files, totaling about 40 Mb, give one a detailed explanation of how TDWRs work, and their strengths and weaknesses. There is also a full product documentation guide available. I'll be adding the info in this blog entry into the radar help link available on each of our radar pages.

Jeff Masters  12/15/08 Blog
3 Part audio-visual training program showing  successful uses and weak signal shortcomings.
WSR 88D and TDWR level 2 & 3 plans and products.      19 pages
http://wdtb.noaa.gov/buildTraining/TDWR/TDWR_SPG_ICD_v43.pdf     42 pages  1.42 Mb
Full Product Documentation    111 pages   Micheal Istok
Alphabetical List of Airports with TDWR.........http://www.nws.noaa.gov/tg/tdwr-radar-list.pdf
Supplemental Data; other radars, Ridge map displays, transmitter specs.
http://ams.confex.com/ams/pdfpapers/145466.pdf      11 pages  Istok and others.