Analysis of the Environmental Field and Unstable Conditions on A Rainstorm Event in the Ili Valley of Xinjiang

Taking a torrential rainfall that occurred in the Ili Valley of Xinjiang on June 26, 2015 as an example, this study analyzes the circulation background and unstable conditions of the rainstorm process by using observation data and the high-resolution numerical simulation results of WRF. Some results are as follows: (1) Precipitation occurs under the background of synoptic circulation with the “two-ridges-and-one-trough” pattern over the middle and high latitudes in the middle troposphere and the “double highs” pattern of the South Asian High in the upper troposphere. Under the effect of the terrain of the Ili Valley (i.e., trumpet shaped topography with an opening to the west), the Central Asian vortex located in Kazakhstan causes westerly winds in the lower layer of the Ili Valley, whereas that located in the Tarim Basin causes easterly winds in the middle layer of the Ili Valley. The vertical shear of horizontal winds 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 Asian vortexes, the low-layer convergence line is formed and coupled with a divergence area caused by the upper jet, which enhances the upward motion. The low-layer westerly wind transports water vapor into the Ili Valley, and the water vapor accumulates in the valley. The enhancement of the upward motion lifts the water vapor in the Ili Valley. (2) The simulation results of WRF can reproduce the location, intensity, and evolution process of precipitation during this weather process and provide data with high spatial and temporal resolution for analyzing the evolution of the rainstorm process. The analysis of the simulation results shows that the divergence distribution, water vapor, the vertical shear of horizontal wind, and thermal stratification distribution over the precipitation area have important contributions to the generation of precipitation. The analysis of the vertical and horizontal components of moist potential vorticity reveals that the convective instability affected by thermal stratification influences the generation of precipitation, and the symmetric instability affected by the vertical shear of the 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 entire precipitation area is mainly caused by the vertical shear part of potential divergence, whereas the divergence part of the potential divergence can strengthen the convective instability in the leeward slope of the small terrain. These results show that the dynamic and thermodynamic factors are coupled with each other in the entire precipitation evolution process, thus affecting the precipitation intensity and area.