1.中国科学院大气物理研究所;2.中国科学院大学;3.中国气象局乌鲁木齐沙漠气象研究所;4.Department of Meteorology, COMSATS Institute of Information Technology, Islamabad, Pakistan
国家自然科学基金项目（41661144024），国家重点研发计划项目（2018YFC1507104），中国气象局乌鲁木齐沙漠气象研究所专项资金项目（IDM2019007），巴基斯坦自然科学基金项目(PSF/NSFC- Earth/C-COMSATS-lsb (07)
1.Institute of Atmospheric Physics, Chinese Academy of Sciences;2.University of Chinese Academy of Sciences;3.Institute of Desert Meteorology, China Meteorological Administration
Taking a torrential rainfall occurring at Ili Valley of Xinjiang on June 26, 2015 as an example, the circulation background and unstable conditions of this rainstorm process were analyzed by using observation data and high-resolution numerical simulation results of WRF. Some results are drawn as following: (1) The precipitation occurs under the background of synoptic circulation with the “two-ridge and one-trough” pattern over the middle and high latitudes in middle troposphere and “double highs” pattern of the South Asia high in upper troposphere. Under the effect of the particular terrain of the Ili Valley, which is westward opening trumpet-shaped, the Central Asian vortex located in Kazakhstan causes westerly wind in the low layer of the Ili Valley, and the Central Asian vortex located in the Tarim basin causes easterly wind in the middle layer of the Ili Valley. The vertical shear of horizontal wind in the Ili Valley is enhanced by the interaction of two Central Asian vortexes. In the Ili Valley, affected by the topography and the Central Asia vortexes, the low-layer convergence line is formed and coupled with divergence area caused by the upper jet, which enhances the upward motion. The low-layer westerly wind transports water vapor into Ili Valley and the water vapor accumulates in the valley. The enhancement of the upward motion causes the water vapor to be lift in the Ili Valley. (2) The simulation results of WRF can basically reproduce the location, intensity and evolution process of the precipitation during this weather process, and provide data with high spatial and temporal resolution for analyzing the evolution of rainstorm process. The analysis of the simulation results shows that the divergence distribution, water vapor, vertical shear of horizontal wind and thermal stratification distribution over the precipitation area have important contributions to the generation of precipitation. Through the analysis of vertical and horizontal components of moist potential vorticity, it is concluded that the convective instability affected by the thermal stratification influences the generation of precipitation, and the symmetric instability affected by the vertical shear of horizontal wind influences the enhancement and maintenance of precipitation. The analysis of potential divergence further indicates that the convective instability in the lower layer of the whole precipitation area is mainly caused by the vertical shear part of potential divergence, while the divergence part of the potential divergence can strengthen the convective instability in the leeward slope of the small terrain. It indicates that the dynamic and thermodynamic factors are coupled with each other in the whole precipitation evolution process, which affects the precipitation intensity and area.