基于地基遥感垂直观测的宜昌地区两次雨雪过程相态差异及识别模型构建

Phase differences and development of a precipitation phase identification model for two rain-snow weather events in the Yichang area based on ground-based remote sensing vertical observations

摘要:

针对2024年2月宜昌地区的两次雨雪过程(“02.01”过程和“02.20”过程),利用微波辐射计与毫米波测云仪垂直观测资料,研究了降水相态变化特征与成因,分析了相态转换过程中温度层结、云顶温度以及云物理结构的演变特征,并基于云顶温度、暖层与冷垫厚度等关键参数,构建了降水相态识别模型。结果表明:1)两次过程雨、冰粒、雪等相态的转换与地面降温同步,毫米波测云仪反射率因子和径向速度与降水强度变化一致。2)两次过程云中产生充足冰相粒子的云顶温度阈值为-5 ℃,暖层厚度小于0.5 km和大于1.0 km是冰相粒子不融化和完全融化的阈值,冷垫厚度小于0.8 km和大于2.0 km是液态粒子不冻结和完全冻结的阈值。3)固态降水发生时,两次过程的温度层结差异明显:雨夹冰粒时,分别为“冷-暖-冷-暖”4层模式和“冷-暖-冷”3层模式;降雪时,分别为单“冷”结构和“冷-暖-冷”3层结构。

Abstract:

Low-temperature rain,snow,and freezing weather events frequently occur during seasonal transitions (autumn-winter and winter-spring) and can have substantial societal impacts.These weather events are characterized by rapid transitions in precipitation phase,posing significant challenges for cold-season precipitation forecasting,particularly in identifying phase changes.Traditional phase identification methods primarily rely on conventional observations and numerical model outputs,focusing on vertical temperature distributions and threshold-based diagnostics at specific atmospheric levels.However,such approaches are unable to capture the continuous and fine-scale evolution of the entire atmospheric structure.Recent studies have demonstrated that high-resolution vertical observations—such as cloud-top temperature,the 0 ℃ level height,and cloud microphysical structure—provide valuable information for understanding precipitation phase formation and improving prediction.Nevertheless,previous studies in Hubei province have mainly emphasized large-scale circulation,vertical structure,and moisture transport,with limited attention to the detailed characteristics of phase transitions.Therefore,investigating the evolution and mechanisms of precipitation phase changes is essential for improving forecasts of low-temperature rain-snow weather events.
Two rain-snow weather events occurred in the Yichang area of Hubei province in February 2024:the “02.01” weather event (February 1—6) and the “02.20” weather event (February 20—23).Based on vertical observations from a microwave radiometer and a millimeter-wave cloud radar,this study examines the evolution of precipitation phase and its underlying mechanisms during these two weather events.First,variations in temperature stratification,cloud-top temperature,and cloud microphysical structure during phase transitions are analyzed.Subsequently,a precipitation phase identification model is developed based on key parameters,including cloud-top temperature,warm layer thickness,and cold-layer (cold cushion) thickness.
The results indicate that:1) Transitions among rain,ice particles,and snow are closely associated with surface cooling,and the reflectivity and radial velocity observed by the millimeter-wave cloud radar are consistent with variations in precipitation intensity.2) Significant differences in temperature stratification exist between the two weather events.During mixed-phase precipitation (rain with ice particles),temperature strictures exhibit four-layer (“cold-warm-cold-warm”) and three-layer (“cold-warm-cold”) configurations,respectively.During snowfall,the structures are characterized by a single cold layer and a three-layer (“cold-warm-cold”) pattern,respectively.3) For both weather events,the threshold cloud-top temperature for sufficient ice-phase particle generation is approximately -5 ℃.Warm layer thicknesses of <0.5 km and >1.0 km correspond to conditions under which ice particles do not melt and completely melt,respectively,whereas cold-layer thicknesses of <0.8 km and >2.0 km correspond to conditions under which liquid particles do not freeze and completely freeze,respectively.
These findings elucidate the mechanisms governing precipitation phase transitions in rain-snow weather events.The proposed phase identification model provides a useful reference for forecasting winter precipitation phase changes and low-temperature precipitation weather events.It should be noted that the threshold values identified in this study may vary across regions and depend on the intensity of temperature advection.Additionally,precipitation-induced attenuation of millimeter-wave signals may reduce the accuracy of echo height detection.Therefore,future studies should integrate multiple vertical observation systems and numerical simulations to further investigate phase transition processes from the perspectives of moisture conditions,thermal structure,and microphysical processes such as melting and freezing.