一次新疆伊犁河谷特大暴雨过程的环境场及不稳定条件分析

伊犁河谷是新疆地区暴雨多发且暴雨强度最强的地区。本文以该地区的一次特大暴雨过程为例，利用观测资料以及WRF高分辨率数值模拟结果对该次暴雨过程的环流背景及不稳定条件进行了分析。结果表明:（1）此次降水过程发生在对流层高层南亚高压“双体型”，中层中高纬度“两脊一槽”以及两个中亚低涡发展移动的环流形势下。在伊犁河谷特殊的向西开口的喇叭口地形作用下，中心位于哈萨克斯坦的中亚低涡导致伊犁河谷低层为偏西风，中心位于塔里木盆地的中亚低涡使得伊犁河谷中层为偏东风，导致伊犁河谷内中低层水平风的垂直切变增强；伊犁河谷内，地形及哈萨克斯坦中亚低涡环流的共同作用形成了低空辐合线，辐合线附近形成的辐合区正好与高空急流辐散区垂直叠加，引发河谷内的上升运动增强。低层西风将水汽输送进河谷，并在河谷内迎风坡附近堆积，上升运动增强后导致河谷内堆积的水汽得以抬升。（2）WRF模拟结果分析显示，散度分布、垂直风切变、水汽及热力层结分布等对降水产生均有重要贡献。通过对湿位涡垂直及水平分量的分析得出热力层结影响的对流不稳定对前期降水的产生有影响，同时，垂直风切变影响的对称不稳定对降水增强维持有重要作用。位势散度分析进一步指示出整个降水区低层的对流不稳定主要是由于位势散度的垂直切变部分造成，而位势散度的散度部分能加强河谷内小地形背风坡处的对流不稳定，说明整个降水演变过程中，动热力因子的相互作用共同影响了降水强度和落区。

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.