How the warm nose height is derived and used
--------------------------------------------

A "warm nose" is simply a layer of air with temperatures > 0C
sandwiched between two layers of air with temperatures < 0C.
We look for this feature because it indicates the possibility
of a "classical" freezing rain process (Zerr, 1997).  

To find this structure in a given column, we loop through the
RUC model temperature data from the top of the grid (near the
tropopause) to the bottom of the grid (near the surface).  
Temperatures are typically subfreezing in the upper altitudes,
so as we descend through the grid, we search for the first
temperature which is above freezing.  If we find one, we consider
this to be the "melting level."  Once a melting level is found,
we continue to descend through the column in search of temperatures
which drop below freezing again.  If this is found, then we have
a classical freezing rain structure, and the height of the melting 
level is assigned as the "height of the warm nose."

If a warm nose is present in combination with liquid or freezing 
precipitation (ZL, ZR, IP, L or R) at the surface, then it is 
likely that ZL or ZR exists within the layer of sub-freezing 
temperatures below the melting level.  We use this information
to boost the likelihood of icing and SLD below the warm nose.

REFERENCES
----------

Zerr, R.J., 1997: Freezing rain: an observational and theoretical
	study.  Jour. Appl. Meteor., 36, 1647-1662.