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.