Abstract:The snowfall in Beijing on January 12, 2023 was the first snowfall appearing in early and mid-January in last 42 years, and the precipitation types underwent complex changes during this process. In this paper， we utilize various conventional and non-conventional observations such as cloud radar and micro rain radar (MMR) data as well as ERA5 reanalysis data to analyze this event, focusing on causes of complex phase distribution and variation. Our findings indicate the following: The low level warm and wet southerly jet stream provides abundant moisture for precipitation. However, a lack of uplifting of warm and wet air by cold east winds and weak upper trough results in relatively low precipitation amounts, meanwhile the northeast wind just above the surface did not play the role of cold cushion, which was not conducive to the maintenance of snow and snow in the whole city. The disparity in the height of the 0 ℃-layer between western and eastern Beijing, caused by low-level warm advection, explains the snowfall in the west and rain in the east. Additionally, the cooling effect caused by melting and evaporation process is the main reason for the rapid decrease of 0 ℃-layer height just lower than 500 m, which caused rain to snow in plain areas. Thickened > 0 ℃ warm layer and decreased snowflake size and density immediately above the melting layer caused snow to turn into rain reaching the ground. When it comes to forecasting precipitation types, deterministic models struggle to accurately depict the snow-melting process. However, short-range ensemble forecast results can compensate for the errors in deterministic models. Integrating cloud radar, micro rain radar, and microwave radiometer data ETC. enhances our ability to monitor and analyze snow formation within clouds and melting in the boundary layer, thus improving the accuracy of precipitation type now casting.