Abstract:

From November 17 to 20, 2020, there was a large-scale freezing rain and snow weather process in Jilin province, and there was a long-term and large-scale freezing rain during the transition of rain and snow, causing a rare degree of disaster in history.In this paper, the conventional ground observation data, radiosonde second data, microwave radiometer data, and ERA5 reanalysis data were used to analyze the cause mechanism of the freezing rain.The results show that the process is influenced by the deepening of the eastward movement of the upper trough, the southward invasion of the Mongolian High, and the northward surface cyclone.The low-level jet brings abundant water vapor to the precipitation in Jilin province, and the specific humidity is generally 3~7 g·kg^-1.At the same time, under the influence of a warm ridge at 850 hPa, the 0℃ isotherm maintains and moves slowly in the central part of Jilin province for a long time, and the cold and warm air masses confront each other for a long time, forming a stable inversion layer in the middle and lower troposphere, which provides favorable temperature stratification conditions for freezing rain in Changchun city and Jilin city.Through the analysis of the temperature and humidity stratification during freezing rain in Changchun and Jilin stations, it is found that the freezing rain occurs in the cold-warm-cold temperature stratification from the bottom up of the vertical temperature field.The height of the cold pad is higher than 950 hPa, the thickness is 0.7~1.0 km, the temperature at the lower 1000 hPa is -3~0℃, the temperature at 850 hPa is 1~3℃, and the thickness of the warm layer is at 950~750 hPa, about 1.5 km.The maximum temperature in the center of the warm layer is greater than 3℃.The content of solid water in the middle and upper altitudes (800~350 hPa) should not be less than 0.2 g·kg^-1, and the content of liquid water in the middle and lower altitudes (900~700 hPa) should not be less than 0.2 g·kg^-1.The greater the updraft, the greater the water content of solid and liquid, and the greater the precipitation.The wind speed at 10 m observed by the automatic meteorological station drops to 0 m·s^-1 or there are no measured results.The time of freezing rain can be roughly estimated, and the results of manual observation can be effectively corrected or supplemented.

Key words: Inversion layer, Melting layer, Supercooled water