Based on the conventional meteorological observation data, NCEP 0.25°×0.25° analysis data and the high-resolution data from the microwave radiometers and the wind profile radars, the precipitation phase of rain and snow process in the Sha-Ying river basin occurred on 21 February 2017 was analyzed using diagnostic and statistical methods. The results showed that the transition of precipitation phase was caused by continuous cooling in the lower layers, which was induced by northerly cold air at cold high bottom below 925 hPa, in the background of the rain and snow weather caused by interaction of the upper air trough and easterly cold air. In the earlier stage of this process, there was a strong warm advection below 700 hPa and a shallow cold advection below 900 hPa. The temperature structure of the whole layer was cold-warm-cold-warm, which led to the ice crystals melting into raindrops while falling. In the medium and latter stages, the strongly development of cold advection led to temperature rapidly decreasing, so the entire temperature layer turned to cold, and the phase state turned to snow. Although there was a warm layer in the local area, the phase was also snow as the warm layer was shallow and the lower cold layer was deep. The height of the 0℃ layer decreased significantly during the rain-snow transition. The 0℃ layer was above the LCL in the rainfall, and fell below the LCL in the snowfall. The bright band echo at 0℃ layer showed a significant change in the precipitation phase transition, whose height gradually decreased. The temperature and humidity profiles, the cloud base height and liquid water from the microwave radiometer all showed significant changes in the rain-snow transition. The liquid water showed rapid increase in the rain-snow transition. The wind profile qualitatively reflected the cold air moving southwards continuously, and the cold lower layer became thicker, led to the increase of precipitation intensity and the change of phase state. The wind profile velocity quantitatively reflected the difference between rainfall and snowfall process. The speed range was 1.5~7.0 m·s-1 in the rainfall, while the speed range in the snowfall was 0.25~1.5 m·s-1. The falling speed decreased obviously during the rain-snow transition, which could be used to monitor and forecast the transitions of precipitation phase.
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