| The surface observation and sounding data of winter cold in 2008-2013, combining with optical disdrometer data collected during the freezing weather observation from Jun.3th to Feb.15th in 2014, were used to analyze the annual mean days of freezing rain of North China in January, the frequency of occurrence of warm layer, the classification of freezing rain with warm layer, and different formation mechanisms in different meteorological stations. In the paper, the statistics of cloud base height, cloud top height, cloud top temperature, cloud thickness and variation characteristics of atmospheric structure were also studied, meanwhile, combining with the microphysical characteristics of Weining site, the formation mechanism of freezing rain and snow was figure out. The main results showed that:(1)The formation mechanism of freezing rain varies with different sites. It has close relationship with altitudes, Supercooled Warm Rain Process tends to occur in high altitudes, while Melting Ice Process tend to occur in low altitudes;(2)There are huge errors when using the traditional way to classify formation mechanism by the occurrence of warm layer, and the error percentage can be over 67%. So this traditional and widely accepted Melting ice Process may not be the formation mechanism of most freezing rain in China.(3)The cloud base height of snow is slightly higher than or roughly equal with that of freezing rain, however, its cloud top height is much higher than that of freezing rain. The cloud thickness of freezing rain and snow increases with the increase of altitudes; the cloud thickness of snow is greater than that of freezing rain. The thickness of cloud is an important factor of what kind of precipitation that supercooled water will form.(4) It is better to use relative warm layer ratio to distinguish freezing rain and snow than to use warm layer ratio. This indicates that choosing -5 ℃ to discuss atmospheric structure is better than choosing 0 ℃ for distinguishing freezing rain and snow.(5) Warm layer only occurs in the beginning of freezing rain, therefore the melting effect of warm layer is not relative to the transformation of precipitation types. But it may has relationship with the intensity of snow because the warm layer intensity in the beginning of freezing rain may affect cloud top height, which in turn affects precipitation types and precipitation intensity. |
