How the radar echo coverage field is derived
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The radar echo coverage field is based upon data matched from the 
national radar mosaic to each RUC grid point.  Each radar mosaic 
grid point is assigned to the RUC grid point closest to it, thus 
making a certain number of radar mosaic grid points valid for each 
RUC point.  The values of radar echo coverage indicate the percentage 
of those valid grid points which contain reflectivity of VIP level 1 
(~18 dBZ) or greater.  At least 10 radar grid points must be valid 
for a percentage to be calculated.


How the radar echo coverage field is used
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The radar mosaic is used in different ways for different icing
scenarios, because the presence or lack of 18+ dBZ echoes can have 
different implications.  In situations where the surface, satellite
and RUC datasets have indicated that ice crystals are likely to
be present at a given RUC grid point, the observation of significant
echo within that grid area implies that ice crystals are quite abundant.

When ice crystals are quite abundant, supercooled liquid water within
the column will tend to be diminished, thus decreasing the icing threat.
Hence, in this case, the higher the percentage of radar grid points
with VIP level 1 or greater that are matched to the RUC grid point, the 
more the diagnosed icing potential will be decreased.  The exception to 
this is the case when only light snow is observed and less than 10% of 
the RUC grid point area is filled with VIP level 1 or greater.  

Field project experience has shown that when patches of light snow are 
present, patches of supercooled liquid water sometimes exist in adjacent, 
echo-free areas.  Thus, the icing potential may be slightly higher for 
the remaining 90%+ of the grid box.  We may have to tweak the knobs on 
this a bit in the future.

The occurrence of 18+ dBZ echo is used in the opposite fashion in 
locations where freezing drizzle, freezing rain and ice pellets are 
observed at the surface, or they are implied from the combination of 
surface, RUC and satellite data.  If the existence of precipitation-
sized supercooled water drops is expected, then the occurrence of echo 
on the radar implies that the drops are abundant and/or large in size.  
Both of these situations imply a potential significant threat to some 
aircraft, so the higher the percentage of radar grid points matched to 
the RUC grid point that have VIP level 1 or greater in this situation, 
the more the radar data will act to increase the diagnosed icing potential.

The scenarios presented here are the simplest ones to explain, but other 
situations do exist.  Overally, the same basic concepts are applied - 
1) if ice crytals are likely to be dominant, decrease icing when VIPGE1 
percentage increases, and 2) if supercooled liquid water is likely to be 
dominant, increase icing when VIPGE1 percentage increases.