Further investigation of a physically based, nondimensional parameter for discriminating between locations of freezing rain and ice pellets

The general applicability of an isonomogram developed by Czys and coauthors to diagnose the position of the geographic boundary between freezing preci pitation (freezing rain or freezing drizzle) and ice pellets (sleet or snow grains) was tested using a 25-yr sounding database consisting of 1051 soun dings, 581 where stations were reporting freezing drizzle, 391 reporting fr eezing rain, and 79 reporting ice pellets. Of the 1051 soundings, only 306 clearly had an environmental temperature and moisture profile corresponding to that assumed for the isonomogram. This profile consisted of a three-lay er atmosphere with 1) a cold cloud layer aloft that is a source of ice part icles. 2) a midlevel layer where the temperature exceeds 0 degreesC and ice particles melt. and 3) a surface layer where T < 0<degrees>C. The remainin g soundings did not conform to the profile either because 1) the freezing p recipitation was associated with the warm rain process or 2) the ice pellet s formed due to riming rather than melting and refreezing. For soundings co nforming to the profile, the isonomogram showed little diagnostic skill. Fr eezing rain or freezing drizzle occurred about 50% of the time that ice pel lets were expected. Ice pellets occurred in nearly a third of the cases whe re freezing precipitation was diagnosed. Possible reasons for the poor diag nostic skill of the method are suggested.