新疆西部一次大暴雨形成机理的数值模拟初步分析

2016年7月31日至8月1日新疆西部发生了一次罕见的大暴雨过程,利用常规观测资料、FY-2G卫星TBB（Black-Body Temperature）资料和NCEP/NCAR（1°×1°）再分析资料,在天气尺度环流背景和中尺度系统分析的基础上,利用WRF（Weather Research and Forecasting）模式对此次大暴雨过程进行了高分辨率数值模拟,利用模拟资料对大暴雨的形成进行了分析。结果表明:此次暴雨发生在稳定维持的"两脊一槽"环流形势下,巴尔喀什湖低槽、高空偏西急流、低空偏东急流和近地面辐合线是造成此次大暴雨过程的主要天气系统。中尺度云团沿近地面的辐合线在天山迎风坡附近不断生成,云团生成后,在向东北方向移动过程中,经过伊犁地区上空时,受天山地形抬升影响不断发展增强,造成伊犁地区出现持续性较强降水。天山迎风坡附近持续较长时间的辐合线是造成此次新疆西部大暴雨的直接中尺度系统,其生成与低层风场辐合、低空急流和地形均有关系。低层辐合引发的垂直运动在地形迎风坡附近加强,风场辐合及地形抬升共同导致强垂直运动发展并维持,类似于"列车效应",不断生成的尺度更小的对流系统沿着辐合线持续移过新疆西部的伊犁地区,是该次暴雨持续的重要原因。

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.