ASSESSING IMPACTS OF RESIDUAL
GROUND CLUTTER. Residual ground clutter near
the radar may be recognized by its speckled appearance.
When it is imbedded in a meteorological signal over the
radar, the velocity dealiasing algorithm may introduce
errors in the velocity field due to radial and azimuthal
gate-to-gate shears greater than the Nyquist velocity.
This problem is most likely to occur in the Clear Air
mode using VCP (volume coverage pattern) 31, where
the Nyquist velocity is about 21 kts. If this problem
occurs in the Precipitation mode, the induced shears
may be picked up by the mesocyclone algorithm and
carried as a feature.
Ground Clutter Returns from Anomalous
Anomalous propagation of the radar beam is caused
by nonstandard atmospheric temperature or moisture
gradients (part B of the FMH-11). Super refraction,
which is frequently caused by temperature inversions,
bends the beam toward the earth and can cause the radar
to detect ground returns from distances far exceeding
the normal ground clutter area.
RECOGNITION OF GROUND CLUTTER
R E T U R N S
F R OM
A N O M A L O U S
PROPAGATION. With increasing antenna
elevation, these returns will usually disappear. A time
lapse of Reflectivity products may show apparent
motion or changing patterns. There can be a 20 dBZe
(a decibel of the equivalent radar reflectivity factor) or
more difference between adjacent returns in the absence
of precipitation, mean velocities maybe near zero, and
spectrum widths may be narrow. Ground returns from
anomalous propagation mixed with precipitation may
result in large spectrum width values and low velocities.
Erratic movement of ground returns from anomalous
propagation, in comparison with the motion of
precipitation echoes, may also be seen.
ASSESSING IMPACTS OF GROUND
CLUTTER RETURNS FROM ANOMALOUS
PROPAGATION. Ground return from anomalous
propagation mainly affects interpretation of reflectivity
echoes in the affected areas. It can cause erroneous
output and increase edit time of the radar coded message
in these areas. It may also affect algorithmic output; for
example, if reflectivity is greater than 30 dlil~, erroneous
identification of a storm may occur and precipitation
accumulation values may be degraded. Super refraction
of the radar beam frequently occurs behind the cold air
outflow regions of thunderstorms. In these instances,
the precipitation processing algorithms may
erroneously interpret the ground returns as precipitation
echoes and significantly overestimate the precipitation.
REMOVAL OF GROUND CLUTTER
R E T U R N S
F R O M
A N O M A L O U S
PROPAGATION. Anomalous propagation can be
removed, to a large extent, by application of the clutter
filter to the elevation angles affected. This is
accomplished by overriding the clutter map resident in
the RDA through editing of the Clutter Suppression
Regions menu at the Unit Control Position. Up to 15
clutter suppression regions can be edited in which three
levels of suppression can be defined for the reflectivity
and doppler channels. Unit Radar Committee guidance
on the use of the clutter map editor must be obtained.
If weather is not a factor, that is, when operating in
the Clear Air mode or when ground clutter or anomalous
propagation is in a precipitation-free sector in the
Precipitation mode, it is reasonable to apply the clutter
filter. If anomalous propagation is mixed with
precipitation, the filter should not be applied.
An occasional source of data contamination is
simultaneous reception of signals at comparable power
levels through both the main antenna pattern and its
sidelobes (part B of the FMH-11).
RECOGNITION OF SIDELOBES. Sidelobes
are found to the right, left, above, and below high
reflectivity areas. Potential interference from sidelobes
can be diagnosed by knowing how much difference in
power there is between the main beam and the sidelobe.
The location of potential sidelobe interference will be
specified by a particular number of degrees between the
axis of the main beam and the sidelobe. The velocity
field of sidelobes will display noisy or erratic values.
Spectrum widths will achieve extreme values and are
the best indicator of sidelobe interference.
ASSESSING IMPACTS OF SIDELOBES. The
presence of sidelobes may indicate erroneously high
storm tops or new storm growth where there is none.
Sidelobes can also impact velocities in a weak echo
region, where mesocyclones occur, by providing noisy
or erroneous values that mask true velocity patterns.
Algorithmic output may be affected.
Due to the sensitivity of the WSR-88D, anomalous
returns near sunrise or sunset usually appear for several
radials. These returns are generated because the sun