Estimating Precipitation from Radar

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Estimating Precipitation from Radar
Jon W. Zeitler
Science and Operations Officer National Weather
Service Austin/San Antonio Forecast Office
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Radar Beam Basics
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Energy Scattering
As pulse volumes within the radar beam encounter
targets, energy will be scattered in all
directions. A very small portion of the
intercepted energy will be backscattered toward
the radar. The degree or amount of backscatter is
determined by target size (radar cross section)
shape (round, oblate, flat, etc.) state
(liquid, frozen, mixed, dry, wet) concentration
(number of particles per unit volume) We are
concerned with two types of scattering, Rayleigh
and non-Rayleigh. Rayleigh scattering occurs
with targets whose diameter (D) is much smaller
(D lt /16) than the radar wavelength. The
WSR-88D's wavelength is approximately 10.7 cm, so
Rayleigh scattering occurs with targets whose
diameters are less than or equal to about 7 mm or
0.4 inch. Raindrops seldom exceed 7 mm so all
liquid drops are Rayleigh scatters. Potential
problem Nearly all hailstones are non-Rayleigh
scatterers due to their larger diameters.  
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Probert-Jones Radar Equation
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Simplified Radar Equation
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Equivalent Reflectivity (Ze)
Since we technically don't know the drop-size
distribution or physical makeup of all targets
within a sample volume, radar meteorologists
oftentimes refer to radar reflectivity as
equivalent reflectivity, Ze. The assumption is
that all backscattered energy is coming from
liquid targets whose diameters meet the Rayleigh
approximation. Obviously, this assumption is
invalid in those cases when large, water-coated
hailstones are present in a sample volume. Hence,
the term equivalent reflectivity instead of
actual reflectivity is more valid.
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Reflectivity (Z) vs. Decibels of Reflrectivity
(dBZ)
dBZ 10log10Z
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Beam-Filling
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Sending vs. Listening
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Sending vs. Listening
99.843 of the time the WSR-88D is listening for
signal returns.
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The Doppler Dilemna
A low PRF is desirable for target range and
power, while a high PRF is desirable for target
velocity. The inability to satisfy both needs
with a single PRF is known as the Doppler
Dilemma. The Doppler Dilemma is addressed by the
WSR-88D with algorithms.
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Range Folding

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Subrefraction dry adiabatic, moisture increases
with height. In addition to underestimated echo
heights, this phenomenon tends to reduces ground
clutter in the lowest elevation cuts.
Superrefraction temperature inversion. In
addition to overestimated echo heights, increases
ground clutter in the lowest elevation cuts and
is the cause of what we normally refer to as
anomalous propagation or AP echoes.
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The Earth is Round!
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Storms Too Close!
Each pulse has a volume with dimensions of 500
meters ( 1500 meters) in length by 1 wide in
short pulse (long pulse) mode. This means that
two targets along a radial must be at least 250
(750) meters apart for the radar to be able to
distinguish and display them as two separate
targets (i.e., more than H/2 range separation
distance).
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Storms or Bats?
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Strategies to Fix Problems
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Drop Size Distribution
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Drop Size Distribution
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Rainfall Rate
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Rainfall Rate
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Rainfall Rate
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