How the precipitation fields are derived
----------------------------------------

The precipitation fields are based solely upon reported precipitation 
types from surface observations (METARs) across the United States and 
Canada.  Each RUC model grid point is assigned all of the precipitation 
types (freezing drizzle, freezing rain, ice pellets, snow, rain, 
drizzle) reported by any surface station within an allowable radius 
of influence.  The radius of influence was set to 125 km and 150 km to 
the east and west of -100 degrees longitude, respectively.  A larger 
radius of influence is used in the western portion of the grid due to 
the relative sparsity of surface observations there.  The seemingly 
large 125 and 150 km values are used to minimize gaps in the cloud 
base field.

The "precipitation" field shows all of the RUC grid points where 
precipitation was indicated within the given radius of influence.

The "freezing precipitation" field shows all of the RUC grid points 
where freezing precipitation (freezing drizzle, freezing rain, ice 
pellets) was indicated within the given radius of influence.

The "snow-only precipitation" field shows all of the RUC grid points 
where the only precipitation falling within the given radius influence 
was snow.


How the precipitation fields are used
-------------------------------------

The use of the precipitation fields is rather complicated.  When
a particular precipitation type is being reported at the surface,
it implies a certain range of meteorological scenarios in which
it could have formed.  A decision tree has been developed for each
precipitation type, and decisions within the tree are based
upon information available from the satellite, radar and model data.
A few cases are outlined below:

1) When snow is the only precipitation type observed (i.e. the snow-
only field is turned on at the grid point), then an abundant amount 
of ice crystals is expected to exist within the cloud deck.  This
information is typically used to decrease the potential for icing 
somewhat, especially when a good deal of echo is showing up in the 
radar field (implying that a very efficient ice process is present).

2) When freezing rain is present, then precipitation-sized supercooled
water drops MUST BE PRESENT from the surface to some altitude.  When
a warm nose exists in the column of RUC temperature data, it is 
assumed that these drops are forming via the classical melting process, 
and thus exist from the surface to the height of the warm nose (given 
that the temperatures within this layer are appropriate).  Such 
information is used to increase the likelihood of icing in that layer, 
especially when a good deal of echo is present in the radar field 
(implying the existence of very large drops and/or a high concentration 
of drops).

3) When only drizzle (not freezing drizzle) is present, this could 
imply several possibilities.  Among these possibilities are a) the 
drizzle is being formed by light snow falling through a melting layer 
to form harmless, warm (T > 0 C) drizzle drops, and b) the drizzle was
formed via the collision-coalescence process and may be the result of
freezing drizzle aloft which has fallen into a layer of warmer 
temperatures near the surface.  Scenario A is no big deal because there
are ice crystals above and warm drizzle below, neither of which poses
an icing threat.  Scenario B implies a potentially dangerous situation
aloft, since ZL can seriously affect aircraft performance.  To decide
which scenario is probably causing the drizzle at the surface, additional
data from the satellite is used to determine the likelihood that the
cloud is primarily made up of liquid, rather than light snow.  WITHOUT
THE ADDITIONAL INFORMATION, ONE CANNOT TELL WHETHER WARM DRIZZLE IMPLIES
A SIGNIFICANT HAZARD TO AIRCRAFT OR NO HAZARD AT ALL.

There are many more scenarios for ZL, ZR, IP, R and L.... far too 
many to describe here.  The key thing to note is that the surface 
precipitation type offers us additional clues about what's happening 
aloft.  By combining this information with what we can gather from 
the other data sources, we can often paint a fairly accurate picture 
of the icing potential in the column.

More information on the relationship between surface precipitation and
aircraft icing, as well as its usefulness can be found in the following
articles:

Bernstein, B.C., T.A. Omeron, F. McDonough and M.K. Politovich, 1997:  The 
	relationship between aircraft icing and synoptic scale weather 
	conditions. Wea. and Forecasting, 12, 742-762.

Bernstein, B.C., T.A. Omeron, M.K. Politovich and F. McDonough, 1998:  
	Surface weather features associated with freezing precipitation and 
	severe in-flight aircraft icing.  Atmospheric Research, 46, 57-74.

Bernstein, B.C., 1996:  A new technique for identifying locations where 
	supercooled large droplets are likely to exist:  The stovepipe 
	algorithm.  Preprints, 15th Conf. on Wea. Analysis and Forecasting, 
	Norfolk, VA, 19-23 August.  Amer. Meteor. Soc., Boston, 5-8.

Politovich, M.K. and B.C. Bernstein, 1995:  Production and depletion of
	supercooled liquid water in a Colorado winter storm.  J. Appl. 
	Meteor., 34, 2631-2648.