A Study on the Mechanisms Accounting for the Generation of a Southwest Vortex That Caused a Series of Severe Disasters during the 2020 Abnormal Meiyu Season

Abstract

The abnormal 2020 Meiyu season caused the worst disasters over the Yangtze River Valley in recent decades. Of these, the Sichuan Basin (SCB) and its surrounding regions were one of the most severely affected areas. Disastrous weather frequently occurs in these regions, with a large proportion of it closely related to the southwest vortices (SWVs). In order to further the understanding of SWV generation, this study investigated the formation mechanisms of a quasi-stationary SWV (by using two sets of vorticity budgets), which caused torrential rainfall (resulting in flash floods in Sichuan and Chongqing), lightning activities (causing tripping incidents of transmission lines in Sichuan) and strong winds (leading to shutting down of wind turbines in Hubei). Results showed that the SWV was generated in a favorable background environment, during which an upper-tropospheric divergence and a middle-tropospheric warm advection appeared over the SCB. Trajectory analyses and vorticity budget showed that the air particles that came from the lower troposphere of the regions south of the Tibetan Plateau dominated the SWV formation. These air particles experienced notable ascending during which an increase in their cyclonic vorticity occurred mainly due to convergence-related stretching, whereas, tilting mainly decelerated this increase. The air particles sourced from the areas within the key region of the SWV and areas northeast of the key region were the second dominant factor for the vortex formation. Overall, for the air particles that formed the SWV, their most rapid changes of vorticity and divergence appeared in the period 24 h before SWV formation, implying that this was the critical period for the SWV generation.

Extreme Mei-yu in 2020: Characteristics, causes, predictability and perspectives

“…Moreover, the Sichuan Basin in the upper reaches of the YRV also experienced severe disasters during the 2020 Mei-yu season. Ma et al (2021) found that air particles from the low-level troposphere south of TP reached the interior of the Sichuan Basin, leading to divergence in the upper troposphere and warm advection throughout the mid-troposphere. This process was conducive to generating more southwest vortices with torrential rainfall in the Sichuan Basin and its downstream areas.…”