A Numerical Study of Cloud Structure and Precipitation Mechanism of Low-Trough Low-Vortex Weather Process over the Liupan Mountain Area

The Liupan Mountain area is a water-conservation forest base in northwestern China; however, drought and lack of rain restrict the agricultural and economic development of this region. Through the weather research and forecasting model, a precipitation process that occurred in the Liupan Mountain area in southern Ningxia on August 21, 2018, was simulated as the basis of further research on enhancing the technology of artificial precipitation in this area. Based on observational data, a favorable circulation situation was analyzed, and the microphysical structure and precipitation formation mechanism in the precipitation cloud system were investigated. Results show that the weather system of this precipitation process developed in the dynamic field of high-altitude trough associated with low vortex, and the low vortex featured a slower movement compared with the high-altitude trough because of the blocking effect of the Liupan Mountain terrain. The vertical structure of cloud showed a remarkable “seeding-feeding” stratified structure; however, the vertical microstructure differed in different parts of the cloud system. This resulted in differences in the contributions of cold and warm-cloud processes to precipitation. In addition, the precipitation on the east windward side of the Liupan Mountain area was stronger than that on the west. Rain water was mainly produced by the melting of graupel and collection of cloud water. The accretion of supercooled rain was the main process of graupel growth. The layer of cloud water on the windward slope was deep, having high water content. This promoted the process of rain accretion by graupel in the supercooled layer and provided abundant cloud water for the coalescence growth process of raindrops, which enhanced both cold cloud and warm-cloud precipitation processes. The terrain had an impact on clouds development and precipitation formation; when the terrain height was reduced, the cloud water decreased, and consequently, the warm-cloud processes weakened, which also affected the graupel growth process.