Analysis on environmental conditions and dual-polarization radar characteristics of the phase transformation of precipitation in a rain and snow event in Hunan

Based on the conventional meteorological observation data, the NCEP reanalysis data, the dual-polarization radar data, wind pro? filer radar data, and other datasets, the environmental conditions and dual-polarization radar characteristics of the phase transformation of precipitation in a rain and snow event in Hunan in late December 2021 is studied. The results are as follows: (1) The disappearing of dry lay? er, the thickening of wet layer, the whole-layer wet bulb temperature below 0 ℃, and the saturated stratification in the cold pad under the in? version layer before the snowfall favored the further cluster growth of falling snowflakes, resulting in the further enhancement of snowfall. (2) That the southwest jet exits at 700hPa and the strong water vapor convergences at 600-800 hPa, combined with the significant vertical up? draft at the middle and lower layers, provided sufficient moisture for the snowstorm. (3) The intermittent light rain (snow) in the evening of the 25th is closely related to the transit of short-wave trough frequently. In the morning of the 26th, the southwesterly wind turned to the westerly wind, which led to a short-term weakening of snowfall in Changsha. Furthermore, the warm-humid southwesterly flow strengthened over the cold pad, and the southeast wind in the cold pad turned to the northeast wind, which corresponded to the two peaks of snowfall in Changsha on the 26th. (4) There are certain differences in the correlation coefficient (CC), differential reflectance (Z_dr), and horizontal reflectance factor (Z_h) of different phases of precipitation. The dual-polarization products have a certain indication effect on the identification of rain and snow phases of this case. (5) The low-level"zero-velocity-level"is straight, and has the characteristics of wind-speed convergence, enhanced southwest jet core, and downward extension, etc., indicating that the dynamic climbing of warm-humid airflow over the cold pad would increase, and the snowfall could be maintained for a long time and would increase gradually. However, with the appearance of divergence of wind field, the snowfall gradually weakened. The findings of this study provide helpful criteria for the development, maintenance, and weak? ening of snow, which can be applied to the operational work of snowstorm nowcasting and forecasting.