Causes of a spring extreme rain-snow event in Henan province of Central China under warm and dry weather conditions

Extreme rain and snow events during the transitional season are always difficult to forecast. In this paper, a spring extreme rain-snow event that occurred on 16 March 2023 in Henan provionce is studied by using conventional surface observations, radiosonde observations, radar and ERA5 reanalysis data, with a focus on the causes of the extreme precipitation and complex phase transition of rain and snow. Major conclusions are as follows: This process is an extreme rain and snow event caused by an eastward moving upper trough combined with a northward moving low vortex shear under a warm-dry background. It was characterized by a sharp transition, high snowfall intensity, a prolonged period of heavy snowfall, and complex precipitation phase transitions, as the surface temperature remained near 0℃ throughout the snowfall. During the process, the abnormally strong extreme dynamic forcing, which was resulted from the thermodynamic forcing accompanied by abnormally strong low-level jet in the lower troposphere and the conditional symmetric instability, was an important factor that affected the high hourly precipitation rate; meanwhile, the slow movement of the low vortex and the small angle between the trend and direction of the echo system were key factors for the long-term precipitation. Temperature at the early stage of the process was abnormally high, and the cooling process occurred on 15 March, making the transition from rain to snow possible. At the beginning of the process, there was an obvious dry layer in the lower troposphere. During the process, the cooling effect caused by phase transition processes such as precipitation evaporation was an important cooling mechanism for the occurrence of the rain-to-snow weather, and the existence of the dry layer and the continuous development of precipitation were the key factors that determined whether the temperature could drop to around 0℃. After the atmosphere became saturated, the cooling mechanism caused by ice crystal melting also played a very important role in the maintenance of the snowfall phase.