How the SLD potential field is derived
--------------------------------------

The SLD potential field is designed to identify locations where 
precipitation-sized supercooled water drops are most likely to exist.  
Similar to the icing field, this potential is based on a combination 
of information from surface observations, satellite, radar and RUC 
model data.  Essentially, this portion of the algorithm looks for 
the following situations:

1) Freezing precipitation below a warm nose.  When a warm nose has been 
identified in the RUC model data, and liquid or freezing precipitation 
(ZL, ZR, IP, L or R) is occurring at the surface, SLD is likely to be 
present at any altitude below the warm nose where temperatures are 
subfreezing.

2) Freezing precipitation without a warm nose.  The mere occurrence of 
freezing drizzle, freezing rain, or ice pellets at the surface is an 
absolute indicator of the existence of SLD aloft.  Without the presence 
of a warm nose, the SLD must be forming via the collision-coalescence 
process.  It will typically extend from the surface to the top of the 
lowest cloud deck.

3) Non-freezing liquid precipitation (drizzle and rain) without a warm 
nose but with warm cloud top temperatures.  Both drizzle and rain
can form via a collision-coalescence process or via snow melting to reach 
the ground as liquid precipitation.  The trick is to know which of these 
processes is occurring.

Cloud top temperature data from satellite is used to help answer this 
question.  When cloud tops are rather warm (> -12C or so), the likelihood 
becomes relatively hight that the entire cloud deck is made up of liquid 
water.  On the flip side, when cloud top temperatures become relatively 
cold, the likelihood increases that the rain or drizzle is forming via 
snow melting as it falls through the freezing level.  Thus, SLD potential 
will only be indicated in this situation when warm cloud tops are present, 
and the potential increases as as the cloud top temperature increases.

4) Above the warm nose, when liquid or freezing precipitation is occurring 
at the surface and cloud top temperatures are warm.  Field program and case 
study data have revealed that SLD can exist above the warm nose in 
situations when cloud top temperatures are quite warm (> -15C or so).  
This is because the layer above the warm nose is one where overrunning 
is often occurring, which is conducive to the formation of SLD ONLY WHEN 
THE CLOUD IS PRIMARILY LIQUID IN PHASE.  The phase is very important, 
because gradual lift in a cloud that is primarily ice is not likely to 
yield SLD.  Even if the cloud is primarily liquid in phase, either SLD 
or cloud drop icing will form.  

*NOTE: The only reason we include scenarios 3 and 4 as possibilities for 
SLD is because the likelihood of SLD seems to be higher than normal, but 
it is not nearly as likely as in siuations 1 or 2.  With this in mind, 
the potential for SLD is identified, but tempered in these situations.

When one of these four situations is occurring, specific mathematical 
equations are applied to the pertinent meteorological data to indicate 
the likelihood of SLD. These equations are designed to be physically 
consistent with the known SLD formation scenarios.