Transcript Surface Scattering

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 ([email protected])
Course website URL
people.eecs.ku.edu/~callen/823/EECS823.htm
1
Outline
Introduction
Basic characteristics
Scanning patterns
Propagation considerations
Post processing
2
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.
3
NEXRAD Radar (WSR-88D)
4
WSR-88D radar characteristics
5
WSR-88D radar characteristics
6
Clear Air Scanning Pattern
7
Precip/Severe Weather Scanning Pattern
8
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.
9
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.
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Propagation considerations
Atmospheric signal attenuation
Clear air two-way
attenuation due
to atmospheric gases
11
Propagation considerations
Rainfall attenuation
Signal attenuation
due to rainfall is
accounted for in
WSR-88D data
products.
12
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.
13
Weather phenomenon characteristics
Prior knowledge of the typical temporal and spatial
characteristics of various weather events are used in mode
configuration and data interpretation.
14
Relating backscatter to precipitation type
15
Relating backscatter to precipitation rate
16
Range unfolding
By operating in multiple modes, range and velocity
ambiguities can be resolved.
17
Range-velocity ambiguity tradeoffs
18
Data characteristics
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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 20