Weather Radar the 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

Outline Introduction Basic characteristics Scanning patterns Propagation considerations Post processing

Introduction NEXRAD 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.

NEXRAD Radar (WSR-88D)

WSR-88D radar characteristics

WSR-88D radar characteristics

Clear Air Scanning Pattern

Precip/Severe Weather Scanning Pattern

Propagation considerations Beam 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.

Propagation considerations Atypical 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.

Propagation considerations Atmospheric signal attenuation Clear air two-way attenuation due to atmospheric gases

Propagation considerations Rainfall attenuation Signal attenuation due to rainfall is accounted for in WSR-88D data products.

Post processing The 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.

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

Relating backscatter to precipitation type

Relating backscatter to precipitation rate

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

Range-velocity ambiguity tradeoffs

Data characteristics

Example weather radar: NEXRAD NEXRAD Radar (WSR-88D) Parameters S-band (2.7 to 3 GHz) PTX = 750 kW Antenna parabolic reflector diameter: 8.5 m beamwidth: 16.6 mrad (0.95°) Rain off the coast of Brownsville, Texas