A physically based, nondimensional parameter for discriminating between locations of freezing rain and ice pellets

A nondimensional parameter is presented that can be used to help distinguish between conditions favorable for the occurrence of freezing rain and ice pellets. The parameter was derived from the well-established condition that most incidents of freezing rain and ice pellets are associated with a layer of above-freezing air elevated above a layer of below-freezing air adjacent to the Earth's surface and the requirement that any cloud ice must completely melt for freezing rain, otherwise ice pellets would result. The parameter was obtained from the ratio of the time available for melting to the time required for complete melting. The parameter was tested on the mesoscale thermodynamic conditions that existed with the 1990 St. Valentine's Day ice storm that affected much of the Midwest and on a number of other episodes of freezing rain and ice pellets in the Midwest. Testing showed excellent spatial agreement between diagnosed and observed locations for freezing rain and ice pellets. An isonomogram is presented to allow the parameter to be easily used as a tool in determining winter precipitation type.