云南一次极端强降水过程成因和地形影响分析

基于多种站点观测资料和ERA5再分析资料，对2020年8月15~19日云南一次影响全省的强降水过程的持续性和预报偏差原因进行探究。结果表明:青藏高压的持续东移，是此次降水过程得以维持的关键因素。500hPa影响降水的关键天气系统是由滇缅高压和西太平洋副热带高压形成的两高辐合系统，其演变为北槽南涡，最终发展为青藏高压和西太平洋副热带高压形成的两高辐合系统。此过程中，中高纬度中高层冷平流促使短波槽发展，中层入侵云南的冷空气加强了其上空的层结不稳定性，低层冷空气则增强了对暖湿空气的抬升。在有利天气形势下，云南西南部哀牢山对该地区降水有明显的增幅作用，尤其是迎风坡，海拔高度和降水的正相关性较好，但地形对降水的增幅作用并不一直随海拔高度的增加而增大。ECMWF数值模式没有预报出影响云南降水的两高辐合系统的西移，导致云南中部至西南部降水量预报明显偏小。 

Analysis on Cause and Topographical Influence of an Extremely Heavy Precipitation Event over Yunnan Province

Based on multiple observations and ERA5 reanalysis data, the causes of persistence and forecast deviation of a heavy rainfall process affecting Yunnan Province from 15 to 19 August 2020 are explored. The results show that the continuous eastward movement of Tibetan Plateau High plays a critical role in the long duration of the event. The dominant synoptic system for the precipitation on 500hPa is the convergence area between the Yunnan-Burma High and the West Pacific Subtropical High, which evolved into the south vortex of the northern trough and eventually developed into the two-high convergence system formed by the Tibetan Plateau High and the Western Pacific Subtropical High. In this process, the high-level cold advection at middle and high latitudes promotes the development of the shortwave trough. The cold air invading Yunnan at the middle level strengthens the stratification instability over it, and the cold air at the low level enhances the uplift of warm and humid air. Under favorable weather conditions, Ailao Mountain in southwestern Yunnan has a significant increase in precipitation in the region. Especially on the windward slope, the positive correlation between altitude and precipitation is good, but the effect of terrain on the increase of precipitation does not always increase with the increase of altitude. The ECMWF numerical model did not predict the westward shift of the two-high convergence system affecting precipitation in Yunnan, resulting in a significantly smaller precipitation forecast from central to southwestern Yunnan.