On the Near-0.DEG.C. Frequency Maximum in Surface Air Temperature under Precipitation: A Statistical Evidence for the Melting Effect.

Abstract

By using hourly data at 808 automated stations for 19 years, the frequency distribution of surface air temperature was analyzed, with attention to the near-0°C frequency maximum resulting from the melting of snow particles. Temperature in precipitation cases has a distinct frequency maximum just above 0°C (0.3°C on the average), whereas no peak exists in no precipitation cases. The peak height, defined by the maximum frequency deviation from neighboring temperature ranges, is 122% for the average over all the precipitation cases. The peak height is dependent on precipitation intensity and strongly on wind speed, and reaches several hundred or a thousand percent for calm cases with heavy precipitation (4-6 mmh-1 and ≧ 7 mmh-1). There are some regional differences in peak height, with relatively high values in plains facing the Pacific coast, especially in the inland part of the Kanto plain. Finally, the depth of the isothermal layer due to melting of snow particles was estimated by applying some simple assumptions to the present results. It was shown that the average case corresponds to an isothermal layer of a few hundred meters depth, in rough agreement with some vertical sounding data. ©2001, Meteorological Society of Japan.