Terminal Doppler Weather Radar
Encyclopedia
Terminal Doppler Weather Radar (TDWR) is a doppler weather radar system used primarily for the detection of hazardous wind shear
conditions on and near major airports in the United States. As of 2011, there were 48 active radars, across the United States & Puerto Rico. Several more radars have also been sold to other countries such as China (Hong Kong
). Funded by the United States Federal Aviation Administration
, TDWR was developed in the early 1990s at Lincoln Laboratory
, part of the Massachusetts Institute of Technology
, to assist air traffic controller
s by providing real-time wind shear detection and high-resolution precipitation data.
The primary advantage of TDWR over previous 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.
between 5600-5650 MHz (5 cm wavelength
) carrier wave, with an angular resolution
beam of 0.55 degrees. In reflectivity, the resolution in distance is 150 meters within 135 kilometers of the radar and 300 meters from 135 km to 460 km to the radar. The reason for this difference is that the width resolution being 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 km.
In radial velocities, data are available up to 90 km from the radar with the full angular resolution of 0.5 degrees and range resolution of 150 meters. Because of the Pulse Repetition Frequency
used, there is aliasing
and the maximum non-ambiguous velocity is 20 to 30 knots.
TWDR 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.
weather radar currently used by the National Weather Service
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. Its resolution is 1.25 degrees in width and 250 meters in range. The non-ambiguous radial velocity is 62 knots up to 230 km from the radar.
The range resolution of the TDWR is nearly twice the one of the NEXRAD. 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 km from the radar; beyond that, the resolution is close to that of the NEXRAD.
, the range of the TDWR is further limited. Finally, hail in a thunderstorm scanned by a TDWR can entirely block the signal as its size is larger than the wavelength. So a total attenuation behind a storm should raise the possibility of hail in the observer's mind.
A second problem is the smaller non-ambiguous radial velocity or Nyquist velocity
. In the case of the TDWR, this means the velocity of precipitations moving at a speed beyond 30 knots away or toward the radar will be analyzed incorrectly because of aliasing
. Algorithm
s to correct for this do not always yield the proper results. NEXRAD has a threshold that is twice as high (62 knots) and thus less processing and interpretation are needed. Because of this, the resolution of radar reflectivity for small scale features such as mesocyclone
s might be better in TDWR, but the velocity resolution may be worse.
Thus, it is best to use the TDWR in conjunction with a traditional NEXRAD nearby to ensure that nothing is missed.
(NSSL) has a program of development and improvement of radar products extracted from data obtained from TDWR and NEXRAD radars. The Severe Weather Warning Applications and Technology Transfer (SWAT) group is sponsored by the National Weather Service
and the FAA. It is working in 2009 on better filtering of non-weather echoes, better dealiasing algorithms of velocities, techniques to extract horizontal the wind field from one or multiple radars. NSSL has been providing TDWR data to NWS office since the late 1990s.
Wind shear
Wind shear, sometimes referred to as windshear or wind gradient, is a difference in wind speed and direction over a relatively short distance in the atmosphere...
conditions on and near major airports in the United States. As of 2011, there were 48 active radars, across the United States & Puerto Rico. Several more radars have also been sold to other countries such as China (Hong Kong
Hong Kong
Hong Kong is one of two Special Administrative Regions of the People's Republic of China , the other being Macau. A city-state situated on China's south coast and enclosed by the Pearl River Delta and South China Sea, it is renowned for its expansive skyline and deep natural harbour...
). Funded by the United States Federal Aviation Administration
Federal Aviation Administration
The Federal Aviation Administration is the national aviation authority of the United States. An agency of the United States Department of Transportation, it has authority to regulate and oversee all aspects of civil aviation in the U.S...
, TDWR was developed in the early 1990s at Lincoln Laboratory
Lincoln Laboratory
MIT Lincoln Laboratory, located in Lexington, Massachusetts, is a United States Department of Defense research and development center chartered to apply advanced technology to problems of national security. Research and development activities focus on long-term technology development as well as...
, part of the Massachusetts Institute of Technology
Massachusetts Institute of Technology
The Massachusetts Institute of Technology is a private research university located in Cambridge, Massachusetts. MIT has five schools and one college, containing a total of 32 academic departments, with a strong emphasis on scientific and technological education and research.Founded in 1861 in...
, to assist air traffic controller
Air traffic controller
Air traffic controllers are the people who expedite and maintain a safe and orderly flow of air traffic in the global air traffic control system. The position of the air traffic controller is one that requires highly specialized skills...
s by providing real-time wind shear detection and high-resolution precipitation data.
The primary advantage of TDWR over previous 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 the frequencyFrequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
between 5600-5650 MHz (5 cm wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
) carrier wave, with an angular resolution
Angular resolution
Angular resolution, or spatial resolution, describes the ability of any image-forming device such as an optical or radio telescope, a microscope, a camera, or an eye, to distinguish small details of an object...
beam of 0.55 degrees. In reflectivity, the resolution in distance is 150 meters within 135 kilometers of the radar and 300 meters from 135 km to 460 km to the radar. The reason for this difference is that the width resolution being 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
Bin (computational geometry)
In computational geometry, the bin data structure allows efficient region queries, i.e., if there are some axis-aligned rectangles on a 2D plane, answer the question Given a query rectangle, return all rectangles intersecting it. kd-tree is another data structure that can answer this question...
. This cut off is arbitrarily set for the software at 135 km.
In radial velocities, data are available up to 90 km from the radar with the full angular resolution of 0.5 degrees and range resolution of 150 meters. Because of the Pulse Repetition Frequency
Pulse repetition frequency
Pulse repetition frequency or Pulse repetition rate is the number of pulses per time unit . It is a measure or specification mostly used within various technical disciplines Pulse repetition frequency (PRF) or Pulse repetition rate (PRR) is the number of pulses per time unit (e.g. Seconds). It...
used, there is aliasing
Aliasing
In signal processing and related disciplines, aliasing refers to an effect that causes different signals to become indistinguishable when sampled...
and the maximum non-ambiguous velocity is 20 to 30 knots.
TWDR 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 NEXRADNEXRAD
NEXRAD or Nexrad is a network of 159 high-resolution Doppler weather radars operated by the National Weather Service, an agency of the National Oceanic and Atmospheric Administration within the United States Department of Commerce...
weather radar currently used by the National Weather Service
National Weather Service
The National Weather Service , once known as the Weather Bureau, is one of the six scientific agencies that make up the National Oceanic and Atmospheric Administration of the United States government...
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. Its resolution is 1.25 degrees in width and 250 meters in range. The non-ambiguous radial velocity is 62 knots up to 230 km from the radar.
The range resolution of the TDWR is nearly twice the one of the NEXRAD. 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 km from the radar; beyond that, the resolution is close to that of the NEXRAD.
Shortcomings
The shorter 5 cm wavelength, which is closer to the size of a raindrop than the 10 cm wavelength, is partially absorbed by precipitation. This is a serious drawback to using TDWR, as the signal can be strongly attenuated in heavy precipitation. This attenuation means that the radar cannot "see" very far through heavy rain and could miss severe weather such as strong thunderstorms which may contain the signature of a tornado, when there is heavy rain falling between the radar and that storm. When heavy rain is falling on the radomeRadome
A radome is a structural, weatherproof enclosure that protects a microwave or radar antenna. The radome is constructed of material that minimally attenuates the electromagnetic signal transmitted or received by the antenna. In other words, the radome is transparent to radar or radio waves...
, the range of the TDWR is further limited. Finally, hail in a thunderstorm scanned by a TDWR can entirely block the signal as its size is larger than the wavelength. So a total attenuation behind a storm should raise the possibility of hail in the observer's mind.
A second problem is the smaller non-ambiguous radial velocity or Nyquist velocity
Nyquist frequency
The Nyquist frequency, named after the Swedish-American engineer Harry Nyquist or the Nyquist–Shannon sampling theorem, is half the sampling frequency of a discrete signal processing system...
. In the case of the TDWR, this means the velocity of precipitations moving at a speed beyond 30 knots away or toward the radar will be analyzed incorrectly because of aliasing
Aliasing
In signal processing and related disciplines, aliasing refers to an effect that causes different signals to become indistinguishable when sampled...
. Algorithm
Algorithm
In mathematics and computer science, an algorithm is an effective method expressed as a finite list of well-defined instructions for calculating a function. Algorithms are used for calculation, data processing, and automated reasoning...
s to correct for this do not always yield the proper results. NEXRAD has a threshold that is twice as high (62 knots) and thus less processing and interpretation are needed. Because of this, the resolution of radar reflectivity for small scale features such as mesocyclone
Mesocyclone
A mesocyclone is a vortex of air, approximately 2 to 10 miles in diameter , within a convective storm....
s might be better in TDWR, but the velocity resolution may be worse.
Thus, it is best to use the TDWR in conjunction with a traditional NEXRAD nearby to ensure that nothing is missed.
Data processing improvements
The National Severe Storms LaboratoryNational Severe Storms Laboratory
The National Severe Storms Laboratory is a National Oceanic and Atmospheric Administration weather research laboratory located at the National Weather Center in Norman, Oklahoma....
(NSSL) has a program of development and improvement of radar products extracted from data obtained from TDWR and NEXRAD radars. The Severe Weather Warning Applications and Technology Transfer (SWAT) group is sponsored by the National Weather Service
National Weather Service
The National Weather Service , once known as the Weather Bureau, is one of the six scientific agencies that make up the National Oceanic and Atmospheric Administration of the United States government...
and the FAA. It is working in 2009 on better filtering of non-weather echoes, better dealiasing algorithms of velocities, techniques to extract horizontal the wind field from one or multiple radars. NSSL has been providing TDWR data to NWS office since the late 1990s.
External links
- NEXRAD and TWDR high-resolution data available on Weather UndergroundWeather Underground (weather service)Weather Underground is a commercial weather service that provides real-time weather information via the Internet. Weather Underground provides weather reports for most major cities across the world on its Web site, as well as local weather reports for newspapers and Web sites...
. - High Resolution Nexrad and TDWR Radar Imagery for the Southeast on APRSFL.net.