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Weather Radarthe WSR-88D

Information taken from the Federal Meteorological Handbook No. 11

Part B – Doppler Radar Theory and Meteorology June 1990

Part C – WSR-88D Products and Algorithms Feb 1991

Chris Allen (callen@eecs.ku.edu)

Course website URL people.eecs.ku.edu/~callen/823/EECS823.htm

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OutlineIntroduction

Basic characteristics

Scanning patterns

Propagation considerations

Post processing

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IntroductionNEXRAD or Nexrad (Next-Generation Radar) is a network of 158 fine-resolution Doppler weather radars operated by the National Weather Service, an agency of the National Oceanic and Atmospheric Administration (NOAA) within the United States Department of Commerce.

NEXRAD is used to warn the people of the United States about dangerous weather and its location.

Its technical name is WSR-88D, which stands for Weather Surveillance Radar, 1988, Doppler.

After more than 30 years of research on operational Doppler weather radar systems, the National Weather Service began to deploy the WSR-88D in 1988 and the last system was installed in 1997.

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NEXRAD Radar (WSR-88D)

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WSR-88D radar characteristics

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WSR-88D radar characteristics

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Clear Air Scanning Pattern

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Precip/Severe Weather Scanning Pattern

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Propagation considerationsBeam refraction due to atmospheric effects

The classical method of accounting for refraction in radar-height computations is to replace the actual Earth radius, R, by an equivalent Earth with radius k·R and to replace the actual atmosphere by a homogeneous atmosphere in which electromagnetic waves travel in straight lines.Normally the vertical gradient of the refractive index is negative and, if it is assumed constant, the value of k is 4/3.

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Propagation considerationsAtypical atmospheric conditions can result in anomalous propagation. Temperature inversion or instances where the atmospheric water content is sharply lower close to the surface can modify the propagation characteristics to create a “duct” whereby radio waves are bent substantially more than in the standard atmosphere.

B shows the most common duct-producing situation.

Situations C and D are less common.

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Propagation considerationsAtmospheric signal attenuationClear air two-wayattenuation due to atmospheric gases

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Propagation considerationsRainfall attenuationSignal attenuation due to rainfall is accounted for in WSR-88D data products.

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Post processingThe data undergo various conditioning prior to meteorological analysis.

Unit conversion is applied to relate received signal power to reflectivity and velocity is extracted from Doppler analysis of the signal.

Point target suppression is accomplished by an analysis that monitors the width and reflectivity gradient of the target and suppresses the return when these correspond to those of a point target.

Data thresholding consists of suppressing data points whose SNR is below 3 dB.

Range unfolding and velocity dealiasing are also performed to remove these artifacts.

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Weather phenomenon characteristicsPrior knowledge of the typical temporal and spatial characteristics of various weather events are used in mode configuration and data interpretation.

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Relating backscatter to precipitation type

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Relating backscatter to precipitation rate

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Range unfoldingBy operating in multiple modes, range and velocity ambiguities can be resolved.

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Range-velocity ambiguity tradeoffs

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Data characteristics

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Example weather radar: NEXRAD

Rain off the coast of Brownsville, Texas

ParametersS-band (2.7 to 3 GHz)PTX = 750 kW

Antennaparabolic reflector diameter: 8.5 mbeamwidth: 16.6 mrad (0.95°)

NEXRAD Radar (WSR-88D)