The relative importance of warm rain and melting processes in freezing precipitation events

The importance of warm rain and melting processes in freezing precipitation events is investigated by analyzing 972 rawinsonde soundings taken during freezing precipitation. The soundings cover regions of the United States ea st of the Rocky Mountain states for the period 1970-94. The warm rain proce ss was found to be unambiguously responsible for freezing precipitation in 47% of the soundings. In these soundings, the clouds had temperatures entir ely below freezing, or had top temperatures that were above freezing. Anoth er 28% of the soundings had cloud top temperatures between 0 degrees and -1 0 degrees C. Clouds with top temperatures >-10 degrees C also can support a n active warm rain process. Considered together, the warm rain process was potentially important in about 75% of the freezing precipitation soundings. This estimate is significantly higher than the estimate of 30% in a previo us study by Huffman and Norman. The temperature, moisture, and wind profile s of the soundings, their geographic distribution, and the common occurrenc e of freezing drizzle at the sounding sites suggest that most of these even ts were associated with shallow cloud decks forming over arctic cold air ma sses. The "classic" freezing rain sounding, with a deep moist layer and a midleve l warm (>0 degrees C) layer, was observed in only 25% of the sample. In the se soundings, the depth of the cloud layer implies that melting processes w ere important to precipitation production. From the geographic distribution , the common occurrence of freezing rain, and the sounding profile, these c ases appear to be associated primarily with cold air damming and overrunnin g along the U.S. East Coast, and with warm-frontal over-running in the midw estern United States.