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 Propagation 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. Sidelobes 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. Solar Effects 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 12-7