Numerical Simulation and Mechanism Analysis of an Extreme Precipitation in Ili Valley, Xinjiang

The Ili Valley is an area with frequent heavy rain in Xinjiang. In this paper, a heavy rainstorm process in this area on June 26, 2015 is taken as an example. The observational data and WRF high-resolution numerical simulation results are used to analyze the synoptic background and the process of the precipitation. The results show that: (1) The Central Asian low vortex and the upper-level jet provides a favorable circulation background for this heavy rain. Northerly winds and westerly winds forms a low-level convergence line in Ili Valley. (2) In addition to the convergence of low-level airflow, the uplifting effect of the terrain on the westerly winds also intensifies the low-level ascending motion. At the same time, the uplifting effect of the terrain on the northerly winds causes the middle-level ascending motion. After the low-level ascending motion is connected with that of the middle level, precipitation begins to occur. The convection further develops, superimposed with the upwards phase of upper-level wave, and the precipitation increases strongly. (3) Through spectral analysis methods, the characteristics of the upper-level wave are obtained, and the wave is an inertial gravity wave. It is further obtained from the mesoscale three-dimensional Eliassen-Palm (EP) flux that during the period of heavy precipitation, the energy of the upper-level inertial gravity wave is transported down to the low level of the precipitation area. (4) Convective instability plays an important role in the enhancement of the precipitation in the Ili Valley. The analysis of potential divergence further indicates that the convective instability in the 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 terrain. It indicates that the dynamic and thermodynamic factors are coupled with each other, which affects the precipitation location, intensity and evolution.