OVER SEA
Scatterometry
Satellites
Research Programs

OVER LAND
Doppler Radar
U.S. Radar Coverage
Research Programs

ON SITE
Mini- SODAR
Research Programs

WIND POWER
Growth in Wind Energy
Finding Strong Winds
Wind Atlases
Site Studies

Web site created by
Paul Kelly
Graduate Student
Geo Information Science
Department of Geography
Salem State College
Salem, Massachusetts, USA.

E-mail comments and suggestions to pmkelly@prodigy.net.

Last updated 4/24/2002.

OVER LAND

Doppler Radar
[NOAA has a good introduction to Doppler weather radar.]
In the United States, the backbone of daily meteorological forecasts is the National Weather Service’s network of NexRAD Doppler weather radars.  Canada has a similar radar network.  We see Doppler radar images daily on television weather reports, showing the location and movement of clouds and storm systems.  These common images derive from the radar backscatter of water droplets and ice crystals in the atmosphere.  The movement of these particles, towards or away from the radar, can be calculated by Doppler processing.  As the storm system moves closer, the radar returns have a higher frequency than the transmitted signal.  As the storm moves away, the radar returns have a lower frequency.

NexRAD radars can operate in two modes: clear air mode and precipitation mode.  Clear air mode is slower and more sensitive, picking up the movement of dust, light snow, and other suspended particles.  It takes about ten minutes to scan from 0.5° to 4.5° above the horizon.  Precipitation mode is faster, taking five to six minutes to scan from 0.5° to 19.5°.  The higher elevation angle allows better understanding of the vertical wind profile in storm conditions.

The movement of wind toward or away from the radar—called radial velocity—does  not yet give us wind speed and direction.  Determining wind movement in two or three dimensions is an ongoing challenge.  One approach is to compare radial velocities from two (or more) separately located radars scanning overlapping areas.  Radars are expensive, however, and the network is not dense enough to allow this solution generally.  Another approach is radar interferometry, in which multiple receivers are located separately, all receiving backscatter from the same transmitter.  Since receivers are much less expensive than transmitters, this approach may prove more feasible economically.  Finally, work continues on algorithms that can extract two or three-dimensional wind fields from radial velocities as detected by a single Doppler radar.

U.S. Geographic Coverage
An individual NexRAD weather radar scans the atmosphere within a 230 km radius.  The map below shows the installed network of WSR-88D radars.

Examination of precipitation mosaics created from this network show fairly comprehensive coverage, with some gaps in the west created by both the distance between radars and obstruction by mountain ranges.

A detailed discussion of the operation and output of WSR-88D radars (in a lab for a Texas A&M meteorology course) includes a discussion of velocity products.  It is pointed out that reliable velocity data, especially in clear air, extend over only 80 nautical miles, about two thirds the radius of standard precipitation products.

It is not yet clear to how much Doppler radar data is being used in the development of newer wind atlases.

Research Programs
The extraction of wind data from Doppler radar is simplest in a uniform wind field, where the air mass in the region of the radar is all moving in the same direction.  Research continues on algorithms to analyze the data.

The University of Essex’s Doppler Wind Retrieval Project focuses on two general areas: the possibility of running convential and Doppler measurements at the same time (despite different sampling requirements) and presentation of Doppler data, to experts and non-experts.  They are particularly concerned with VADs (velocity azimuth diagrams) and fitting wind models to them.   Images of VADs and PPIs (plan position indicators) may be viewed on their site.

University of Nebraska-Lincoln’s Environmental Remote Sensing Laboratory (ERSL) has several atmospheric Doppler radar projects.  These investigate methods of wind profiling throughout the lower 3 km of the atmosphere, spatial interferometry (which employs an array of receivers), and imaging techniques for clear air turbulence, including animations of vertical and horizontal slices of clear air turbulence.

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