2021年8月西南地区东部降水异常偏多的水汽条件分析

2021年8月西南地区东部降水量为1961年以来同期第2多，比历史最多的1998年仅差0.9 mm，极为异常。利用西南地区东部118个气象台站1961—2021年夏季8月逐日降水资料及ERA5逐月高度场、纬向风、经向风、比湿和水汽场等再分析资料，采用拉格朗日水汽轨迹模式和现代统计诊断方法，分析了2021年8月西南地区东部降水出现异常偏多的主要水汽输送条件和水汽来源、造成降水异常的大气环流特征等。结果表明：2021年8月西南地区东部水汽的净流入主要来自于对流层中、低层，以低层的水汽贡献最大；2021年8月水汽输送路径有一定的独特性，以低层来自于我国东部地区的水汽路径的轨迹数量最多，与以往水汽主要轨迹和贡献来自南部路径的特征有所不同。2021年8月西南地区东部降水异常偏多与大气环流异常紧密相关，主是体现在500 hPa高度场上乌拉尔山附近和鄂霍次克海阻高异常偏强、中纬度低值系统（东北低涡等）异常活跃、印度低压偏弱和和西太副高异常西伸且强度偏强；大气环流系统引导下有利的水汽输送造成了西南地区东部降水的异常偏多。印度洋海温的持续偏暖可能是维持2021年8月西太副高持续偏强、偏西的重要外强迫因子；而同期鄂霍次克海阻塞高压的异常偏强可能与西北太平洋海温的异常偏暖有关，并对西南地区降水偏多起到了积极的作用。

Analysis of Water Vapour Conditions for Excessive Precipitation in Chongqing in August 2021

In August 2021, extremely unusual precipitation in the eastern part of Southwest China reached the second highest next to 1998 since 1961. Based on daily precipitation and monthly ERA5 reanalysis data during 1961 2021, the main water vapour transport conditions and water vapour sources, along with the abnormal atmospheric circulation characteristics for anomalous more than normal precipitation in August 2021 over the eastern part of SW China, are analyzed. The results show that the net inflow of water vapour in the eastern part of southwest China in August 2021 is mainly from the mid lower troposphere, with the largest contribution from the lowest layer. In August 2021, the water vapour transport pathway is the most unique one, different from the main trajectory and contribution from the southern pathway. In August 2021, the abnormal precipitation in the eastern part of southwest China is closely related to the abnormal atmospheric circulation, mainly reflected in the 500 hPa with blocking system over the Ural Mountains and the sea of Okhotsk resistance, and the abnormally active mid latitude low system (i.e. Northeast cold vortex), the weak Indian low pressure and the western subtropical high abnormal westward extension and strong intensity. The favourable water vapour transport guided by the atmospheric circulation system results in more abnormal precipitation in the eastern part of Southwest China. Persistent warming of the Indian Ocean is likely an important external forcing factor maintaining the continuously intensified Pacific subtropical high in August 2021, while the abnormal strength of the Okhotsk high in the same period could be related to the abnormal warming of the Northwest Pacific SST. As a result, they play a positive role in the excessive precipitation in the Southwest China region.