Abstract: Using the data from surface meteorological stations,observatory sounding and the NCEP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research) reanalysis,the characteristics of synoptic process and the occurrence conditions for freezing rain observed at 20 surface meteorological stations in Beijing from 1960 to 2013 were analyzed.The results show that the freezing rains mainly occur from November to April of the next year.The freezing rain phenomena normally appear in Daxing,Tongzhou and Changping districts.The conditions of abundant water vapor and uplifting at the low atmospheric level are favorable to the formation of freezing rain.The vertical structure of atmospheric stratification can be divided into two types,i.e.,the melting and non-melting layers.The occurrence frequency of freezing rain for both of them is equal.Based on the comparative analyses,it indicates that the warm advection between 700 hPa and 850 hPa plays a crucial role in the change of inversion temperature intensity.For the cases without melting layer,the height of cloud top is higher.The air temperature under the 700 hPa is from -10℃ to 0℃.The precipitation drops to the ground as supercooled water,and it happens to freeze and forms freezing rain.For the cases with melting layer,the wet layer is shallower,only existing under the 850 hPa.The warm moist air sliding on the cold air near the surface is one of the favorable conditions for the formation of this kind of freezing rain.

Key words: Freezing rain, Atmospheric stratification, Melting layer, Inversion temperature intensity