2020年长江中下游地区梅汛期强降水特征及其与对流层上层斜压Rossby波的关系

2020年梅汛期（6～7月）长江中下游地区发生了严峻的汛情。2020年梅雨期长度和强度均远超历史平均水平。本文利用逐日NCEP/NCAR再分析资料和全球降水量网格数据集，研究了本次梅汛期降水特征及其与对流层上层斜压波动活动的联系。结果表明:本次梅汛期，长江中下游地区的总降水量和降水异常大值区位于安徽南部，共有7次连续的降水过程发生。长江中下游地区在对流层中低层辐合、高层辐散，且该地区上空有强的异常上升运动，有利于异常强降水的发生发展。同时，水汽自孟加拉湾和中国南海地区输送至长江中下游地区，为强降水的发生提供了充足水汽。利用小波分析研究该地区的逐日降水标准化时间序列时，发现其存在2～4天和6～14天的显著周期。高频（2～14天）扰动所显示的Rossby波动在对流层上层表现出向下游频散的特征，波动源于贝加尔湖附近。波扰动能量和通量所显示的波动向下游的传播过程与波包的传播过程较为一致，分别源于地中海和贝加尔湖附近的波扰能向东或向东南频散至长江中下游地区，有利于该地区扰动加强并进而有利于强降水的发生和维持。以上结果加深了人们对2020年超长“暴力梅”成因的认识并可为有效预测类似事件提供线索。

Heavy Precipitation in the Middle and Lower Reaches of the Yangtze River during the 2020 Meiyu Period: Features and Relationship with Baroclinic Rossby Wave in the Upper Troposphere

Abnormal heavy precipitation in the middle and lower reaches of the Yangtze River during the 2020 Meiyu period (June–July) resulted in enormous loss of lives and property. Moreover, the length and intensity of precipitation during this period far exceed the historical average. Using daily National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis data and the Climate Prediction Center global daily precipitation data, this study investigated the features of heavy precipitation and their relationship with baroclinic Rossby wave in the upper troposphere. The results showed that the total precipitation and precipitation anomalies in the middle and lower reaches of the Yangtze River were located in the southern part of Anhui Province, and that there were seven consecutive precipitation processes. The middle and lower reaches of the Yangtze River featured a convergence, while the upper troposphere featured a dispersion, and a strong anomalous upward motion occurred over the region, which favored the development of anomalous heavy precipitation. Moreover, water vapor was transported from the Bay of Bengal and the South China Sea to the middle and lower reaches of the Yangtze River, which provided sufficient water vapor for heavy precipitation. Wavelet-based analysis of the standardized time series of daily precipitation in this region revealed significant cycles of 2–4 days and 6–14 days. The Rossby fluctuations shown by high-frequency (2–14 days) perturbations exhibited a downstream dispersion in the upper troposphere, with fluctuations originating near Lake Baikal. The propagation process of the fluctuations downstream, shown by the wave disturbance energy and flux, was more consistent with that of the wave packet. The wave disturbances originating near the Mediterranean Sea and Lake Baikal could disperse eastward or southeastward to the middle and lower reaches of the Yangtze River. The energy transmitted to the middle and lower reaches of the river favored the intensification of the disturbance in this region and thus the occurrence and continuation of heavy precipitation. The results of this study further clarify the causes of the 2020 super-long “violent Meiyu” and may help scientists effectively predict similar events.