Abstract: Based on the surface and radiosonde observation data, Doppler weather radar data of Shanxi Province from 2014 to 2017 and reanalysis data of NCEP/NCAR, the rain-to-snow transformation processes were classified and analyzed in winter half year in Shanxi Province, and the effect of surface air temperature on transformation of precipitation phases was discussed, then the precursor information of precipitation phase transition was extracted. The relationship between snowfall and snow depth increment for snowfall processes was statistically analyzed, and the prediction indicators of snow depth were refined. Finally, two rain-to-snow processes from 23 to 24 November 2015 and 21 to 22 November 2016 were analyzed comparatively in order to reveal physical mechanism of precipitation phase transition. The results are as follows: (1) The rain-to-snow transformation process was the most in November in Shanxi Province, and followed by February. As an indicator of precipitation phase change, the surface air temperature was related to climatic and weather (such as the intensity and moving path of cold air) characteristics, geographic location, etc. (2) The ratio of snow depth increment and snowfall was about 0.68 cm·mm-1 in winter half year in Shanxi, and it increased with the decrease of temperature, so it had obvious temporal and spatial differences with the change of month, latitude and altitude. (3) During the change of rain to snow, the proportion of snow-ice layer thickness to total cloud layer increased with the cooling in lower layer of troposphere and the falling path of solid condensate in cloud shortened, which reduced the melting probability of solid condensate, and resulting in the change of phases.

Key words: multi-phase precipitation, surface air temperature, snow depth, phase transition mechanism

摘要： 利用2014—2017年山西省地面和高空气象观测资料、NCEP/NCAR FNL再分析资料、山西及周边地区多普勒天气雷达资料，对山西冬半年雨转雪过程进行归类与分析，探讨地面气温在降水相态转换中的作用，提取降水相态转换的前兆信息。针对降雪过程，统计分析降雪量和积雪深度增量的关系，总结提炼积雪深度预报指标。最后，选取气候特征相似的两次雨转雪过程进行对比分析，揭示降水相态转换的物理机制。结果表明：（1）山西省11月发生雨转雪的站次最多，其次为2月。地面气温作为降水相态变化的重要指标，其与气候和天气（如冷空气强度和路径）特征、地理位置等有关。（2）山西冬半年积雪深度增量与降雪量比值约0.68 cm·mm-1，且比值随着气温降低而增大，因此存在明显的时空差异。（3）在雨转雪的不同时段，随着对流层低层降温，冰雪层厚度在总云层的比例有所增加，且云中固态凝结物下落路径缩短，使得固态凝结物在下落过程中融化概率减小，造成相态变化。

关键词: 多相态降水, 地面气温, 积雪深度, 相态转换机制