Analysis of Circulation Characteristics and Precipitation Phase Difference during an Extreme Freezing Rain and Snow Weather Event in Southwest China

Extremely catastrophic compound weather events are a common occurrence within the backdrop of global warming; the frequent snowstorms or frozen-rain events in southern China are examples of typical compound extreme weather events. Thus, this study examined the characteristics of the circulation and the phases of heavy snow and freezing rain during a typical compound weather event in Southwest China, and it described the two main phases. It was revealed that the freezing rain event occurred in the frontal strong baroclinic environment in the Guizhou Province, whereas the snowfall occurred in the cold zone to the north of the Sichuan Province front. Furthermore, a notable difference was observed in the vertical circulation patterns, wherein the ascent motions occur over snowfall regions, ranging from lower to higher altitudes, whereas a temperature inversion layer was detected in the lower atmosphere over the freezing rain region, resulting in a cold–warm–cold temperature configuration. Thus, the vertical motions exhibit a two-layer circulation mode, wherein the powerful ascent motions at low levels are suppressed by the descent motions at medium levels. The ascending motions are stronger over the snowfall area, and the cloud-top temperature is colder. This is accompanied by the development of convection currents, which leads to a phase difference from the freezing rain. The generalized moist potential vorticity (GMPV) theory can thoroughly show the circulation features and phase transition of water vapor, and it has been employed to diagnose the characteristics of GMPV distribution during the occurrence and development of heavy snow and freezing rain. It has been discovered that the anomalous distribution of the baroclinic term can more accurately reflect the atmospheric baroclinicity close to the quasistationary front and the location or evolution of heavy snow and freezing rain. The results obtained by calculating the disturbed pressure equation terms demonstrate that the balanced diversity between the downward perturbance pressure gradient force and the buoyancy is the primary reason for the difference in the vertical circulation characteristics between snowfall and freezing rain. This study can provide references for weather analysis and operational forecasts on the perspective of circulation features of this type of weather and provide references for power operation guarantees for power generation enterprises.