Analysis on the phase transformation of precipitation in a rain and snow event in spring

The rain and snow event in cold wave in Nanjing at the end of March 2020 is analyzed using conventional observation data and new detection data such as microwave radiometer, wind profile radar and laser precipitation spectrometer. The results show that: (1) The weather systems that caused this cold wave are Ural high pressure ridge, upper cold vortex, ground strong cold high pressure and ground cold front. The water vapor provided by the southwest airflow in front of the southern trough and the continuous decrease of temperature in the middle and low levels caused by the cold air below 850 hPa are the key factors to produce changes in precipitation phase. (2) The microwave radiometer data showed that the lower level temperature drops first, followed by the ground temperature. There is a warm layer near the ground. The height of 0 ℃ layer changed obviously during the precipitation phase transformation, and it reached minimum in snowfall period. The water vaper during this event is mainly distributed at 3 to 4 km. The liquid water and the vaper density reached maximum in sleet period and reached minimum in snowfall period. (3) The vertical wind from wind profile radar reflected that when the cold air moves southward, the cold lower layer became thicker, the warm and humid air raised and the temperature decreased, and that leaded to the precipitation phase transformation. There was no heavy snow because the southwest jet was not established stably. (4) The laser precipitation spectrometer observed that the snowfall particle's diameter is bigger and falling velocity is slower, while the rainfall particle's diameter is smaller and falling velocity is faster. It can be used to monitor and forecast changes in precipitation phase. From the period of rainfall, sleet and snowfall, the patterns experienced a process from double spectrum to undulation and multimodal spectrum. Furthermore, both spectrum width and number concentration are significantly in increasing trend.