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Terminal Doppler Weather Radar Information
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Background
Comparison of the TDWR to the WSR-88D
Frequently Asked Questions
Additional Information
Background
The Terminal Doppler Weather Radar (TDWR) is a high quality, dedicated meteorological surveillance radar deployed near many of the larger airports in the U.S. The Federal Aviation Administration (FAA) installed TDWRs near the largest airports which were vulnerable to wind shear conditions (thunderstorms, frontal passages, etc.). The TCLT TDWR is located 9.25 miles northeast of the Charlotte-Douglas International Airport, or 35.34N and 80.89W.
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 meters (m) for Doppler data. It has a
resolution of 150 m for reflectivity data within 135 kilometers (km) and 300 m from beyond 135
km to 460 km. By contrast, the WSR-88D employed by the National Weather Service (NWS),
Federal Aviation Administration (FAA) and Department of Defense (DoD) 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. Table 1 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 |
.13 nm in velocity
.54 nm in reflectivity |
.067 nm |
| Max Unambiguous Velocity |
Up to 62 kts |
20-30 kts |
| Max Doppler Range |
230 km |
90 km |
One of the greatest advantages of the TCLT TDWR is it's location under the KGSP WSR-88D coverage area. The TCLT radar is located 81 miles northeast of the KGSP radar. Due to the curvature of the Earth's surface, the lowest standard atmosphere view from the KGSP radar over center city Charlotte is 7245 feet above ground level. In comparison, the lowest scan from the TCLT radar is only 315 feet. Both the finer resolution and closer proximity of the TDWR to the Carolina piedmont makes significant improvements to the doppler radar data used by warning meteorologist. Figure 1 illustrates the difference between the KGSP WSR-88D and the TCLT TDWR during the Mother's Day 2006 supercell over northern Cabarrus County.
 
Figure 1.
Radar reflectivity at 0.5 degree scan from the KGSP WSR-88D at 1844 UTC (left). Radar reflectivity at 1.0 degree scan from the TCLT TDWR at 1842 UTC. Click on image to enlarge. |
Frequently Asked Questions
1. Why does the radar detect high reflectivity over Avery, Mitchell, and Yancey Counties on the long range scan?
2. Why are the velocity and reflectivity images missing northwest of the radar?
3. What do the lines on the map represent?
4. How often does the radar generate a new image?
1. Why does the radar detect high reflectivity over Avery, Mitchell, and Yancey Counties on the long range scan?
The long range scan of the TDWR uses the 0.5 degree slice. Due to the low angle, the radar beam intersects the high terrain and energy is returned to the radar. Since the beam is nearly blocked by the mountains, the returns are usually assigned a high dBz, see Figure 2 below.
Figure 2. TDWR radar returns from the eastern slopes of the Appalachians.
All regional radars encounter this problem and use several clutter suppression techniques to remove non-weather returns. However, the TDWR does not use clutter suppression in order to remain sensitive enough to detect areas of wind shear, low level boundaries, and other features important to aviation interests.
2. Why are the velocity and reflectivity images missing northwest of the radar?
Unfortunately, a water tower located northwest of the radar antenna blocks a portion of the radar transmission. The beam blockage results in missing to very poor quality data beyond the water tower, see Figure 3.
Figure 3. Beam blockage resulting in poor data along the 340 degree radial.
3. What do the lines on the map represent?
The TDWR background map outlines counties and lakes in white lines. In addition, the TDWR background map also uses a azimuth/range overlay. The range rings on the long range map are spaced 25 nm apart and 15 nm apart on the short range map. The primary focus of the TDWR is aviation, therefore, other landmarks (cities and interstates) are not plotted on the map.
4. How often does the radar generate a new image?
Like the WSR-88D, the TDWR scan strategy is separated into either a clear air or precipitation mode. The TDWR clear air mode is referred to as "monitor mode" and the precipitation mode is called "hazardous weather mode". Update times will vary depending on the mode the radar is using. The lowest short-range scan of the "monitor mode" will update every 5.4 minutes. However, the lowest scan of the "hazardous weather mode" will generate every minute. A long-range scan is required every 5 minutes. The radar mode is indicated in the lower left-hand corner of the radar image, see Figure 4.
Figure 4. The radar mode is listed as "Mon" for monitor mode and "Haz" for hazardous weather mode.
Additional Information
Doppler University
The Mothers Day 2006 severe weather outbreak across the Piedmont of the Carolinas
Tornadoes strike Liberty and Moore, South Carolina, and near Gastonia, North Carolina, on 5 January 2007
Observations of a non-supercell tornadic thunderstorm from a Terminal Doppler Weather Radar |
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