Figure 2-25.—WSR-88D system configuration.
REFLECTIVITY.—Recall, some degree of
energy is likely to be returned to the antenna as a result
of backscattering, and that size, shape, and
composition all contribute to a target’s ability to
backscatter this energy. Stronger targets have higher
levels of reflectivity and return more energy. Thus,
stronger targets have higher reflectivity values; that is,
higher dBZ levels. For example, large raindrops
backscatter many times more power than drizzle and
will produce reflectivity levels 5 to 10 dBZ higher.
Because of its characteristics, hail tends to have
exceptionally high reflectivity values. And, since the
WSR-88D displays information in the form of echo
intensity levels, a severe thunderstorm with hail will
appear much more intense than an area of light rain or
drizzle.
VELOCITY.—Velocity is used to determine
wind speed, direction, and target motion (perhaps the
movement of a thunderstorm cell). The WSR-88D can
detect small areas of convergence, divergence, and
even circulation. This has proven valuable in alerting
whole communities to the onset of strong surface
winds and tornadic activity.
SPECTRUM WIDTH.—Spectrum width data is
another method of determining atmospheric motion.
Spectrum width examines the degree of velocity
variance ofparticles (scatterers) in the atmosphere that
results from dispersion and mixing. Within a single
sample volume of the atmosphere, many particles of
varying size, shape, and composition move in different
directions and speeds. Because of their differences,
each particle moves about independently of the others.
Within this single sample volume, the WSR-88D does
not sense each individual motion, but rather a
combination of all velocities for the overall sample.
This combination of different velocities is what
spectrum width measures.
Spectrum width data is quite useful in determining
atmospheric stability, and is a good indicator of
environmental turbulence. By itself, it offers very little
guidance. Combined with other products, the user can
determine radar performance and reliability of the data
being displayed. The concept of evaluating spectrum
width data is still relatively new and operational uses
are still being developed.
RDA Subcomponents
The RDA consists of all the hardware and software
needed to transmit and receive radar energy. It also
preprocesses raw data by filtering inconsistencies. The
transmitter, receiver, and signal processor are housed
together in a small building near the antenna, as
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