Abstract: Using ground, high altitude conventional meteorological observation data and NCEP/NCAR FNL 1°×1° 6 h analysis data, the temperature stratification characteristics under different precipitation phases and the mechanism of multiple phase transition was analyzed during the January 2018 precipitation type in Hunan. The results showed that the cold rain and snow freezing weather process was generated under the background of cold wave outbreak and large-scale circulation with active low trough in mid-latitude; The temperature characteristics of the middle-lower layers of different phases were obviously different. In freezing rain phase, the inversion layer appeared higher, during the freezing rain period, the frontal inversion thickness was thinner than 70 hPa, and the gradient was significant. There were obvious warm layers near and below 800 hPa. The warm layer thickness was thickest, and the temperature stratification had the structural characteristics of cold layer-warm layer-cold layer. When snow falled, the frontal inversion thickness was thicker than 100 hPa, the inversion gradient was weaker than that of freezing rain, and there was no warm layer or shallow warm layer. Temperature stratification often had the structural characteristics of ice crystal layer-cold layer （weak warm layer-cold layer）. NCEP/NCAR FNL temperature could reflect the actual temperature in a certain extent. There was a process mechanism of “over-cooling and warm rain” in frozen rain, and the precipitation mechanism of cold cloud was obvious in snowfall. The strong cold air of the ground and the active downward movement of the southern branch front were the main reasons for the rapid phase transition. The intensification of low-level convergence and the abnormal enhancement of the southwest jet of 700 hPa were the direct reasons for the rapid transformation from snow to freezing rain. The multi-layer convergence and divergence structure during freezing rain period was conducive to the formation of cloud water and rainwater in the middle and lower layers, and to the stable maintenance of the warm stratification in the middle layers.

Key words: phase, multiple conversions, temperature characteristics, precipitation mechanism, southern branch front