2021年盛夏西藏持续性强降雨过程的水汽输送特征分析

Analysis of Water Vapor Transport Characteristics during Persistent Heavy Rainfall in Xizang in the Midsummer of 2021

利用NCEP和ERA5再分析资料,首先分析了2021年7月8—11日持续性强降雨天气过程的大尺度环流背景和水汽输送特征,然后引入NOAA轨迹模式HYSPLIT,定量分析强降水的水汽输送特征。结果表明:(1)此持续性强降雨主要是由500 hPa高原切变线和副热带高压位置变动所致,稳定的高低空影响系统有利于形成持续性降水,南部的暖湿气流与北部干冷气流在强降雨区持续辐合。10日后低空700 hPa上的偏南气流引导孟加拉湾水汽北上至高原东南部,受特殊地形增幅作用形成了对流性降雨,也是持续性强降水在西藏维持的重要原因。(2)经过水汽轨迹后向追踪和聚类发现,西藏强降水的主要水汽来源是阿拉伯海和孟加拉湾;其次是高原附近或本地受特殊地形形成的水汽;最后一部分水汽则伴随着北部冷空气南下至高原。(3)根据分析各层的水汽通量发现700 hPa主要水汽源地是非洲索马里附近和西太平洋;600 hPa主要水汽源地是印度南部的洋面和西太平洋;500 hPa主要水汽源地是印度附近的洋面和西太平洋。水汽通量从低层(700 hPa和600 hPa)上高原时有明显的减弱,在中层(500 hPa)上有明显的水汽垂直输送,并向东部和南部输送,也为此次持续性强降水提供了充足的水汽。

Based on the NCEP and ERA5 reanalysis data,the large-scale circulation background and water vapor transport characteristics of the persistent heavy rainfall weather process from July 8 to 11,2021 were first analyzed,and then the NOAA trajectory model HYSPLIT was introduced to quantitatively analyze the water vapor transport characteristics of heavy precipitation.The results indicate that(1)This persistent heavy rainfall is mainly caused by changes of the 500 hPa plateau shear line and the position of the subtropical high pressure.A stable high and low alti-tude influence system is conducive to the formation of continuous precipitation,and the warm and humid airflow in the south and the dry and cold airflow in the north continue to converge in the heavy rainfall area.After 10 days,the southerly airflow over 700 hPa at a low altitude guided the water vapor from the Bay of Bengal northward to the southeastern part of the plateau,which formed convective rainfall due to the special topographic amplification,which was also an important reason for the persistence of heavy precipitation in Xizang.(2)The backtracking and clustering of water vapor trajectories showed that the main source of water vapor leading to heavy precipitation in Xi-zang were formed from the Arabian Sea and the Bay of Bengal,then a part formed by the special topography near the plateau or locally,and a last part of the water vapor traveled south to the plateau with cold air from the north.(3)Ac-cording to the analysis of water vapor flux in each layer,it was found that the main sources of water vapor at 700hPa were near Somalia in Africa and the western Pacific Ocean;the main sources of water vapor at 600 hPa were the ocean surface of southern Indian and the western Pacific Ocean;and the main sources of water vapor at 500 hPa were the ocean near India and the western Pacific Ocean.The water vapor flux weakened significantly from the lower layer(700 hPa and 600 hPa)to the upper plateau,and there was obvious vertical water vapor in the middle layer(500 hPa)transported to the east and south,which also provided sufficient water vapor for the persistent heavy precipitation.