A Preliminary Analysis of the Formation Mechanism for a Heavy Rainstorm in Western Xinjiang by Numerical Simulation

Synoptic circulation pattern and mesoscale system associated with the torrential rain occurring in western Xinjiang from 31 July to 1 June 2016 are analyzed based on conventional observational data, TBB (Black-Body Temperature) data of FY-2G satellite and the NCEP/NCAR reanalysis data at 1.0°×1.0° spatial resolution. By using the WRF (Weather Research and Forecasting) model, the torrential rain is simulated with high resolution, and the simulation results are used to make a preliminary diagnosis of this rainfall formation. The results are as follows. A stable circulation pattern of "two ridges one trough" was the background of the rainstorm. The Balkh Lake trough, the upper level westerly jet, the lower level easterly jet and the surface convergence line were the main weather systems conductive to the heavy rain. Mesoscale clouds continually formed in the windward slope of Tianshan, and moved to the northeast after their formation. Mesoscale convective clouds continually developed as the clouds passed over the Ili area, leading to strong continuous precipitation there. The mesoscale convergence line caused by continuous wind field convergences at the low-level near the windward slope of Tianshan was the system that directly caused this heavy rainstorm in western Xinjiang, which was related to low-level wind convergence, low-level jet and terrain effect. The vertical motion caused by the low-level convergence was further enhanced by the orographic lifting, and both the wind field convergence and terrain elevation contributed to the strong updraft development and maintenance. Similar to the "train effect", mesoscale systems that constantly moved across western Xinjiang along the convergence line also contributed to the persistence of the heavy rain.