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

Hui Ma,Yanwei Jing,Guiping Wu,Xiaolei Ma

doi:10.3390/atmos12050556

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.

Highlights

Every year, floods cause serious economic losses and heavy casualties in different regions, which make them one of the most serious natural disasters worldwide [1]
Since the mean height of 750 hPa is about 2500 m above sea level, we defined the levels with a height of between 2000 m and 3000 m as the levels similar (LSM) to 750 hPa
In the summer of 2020, an abnormal Meiyu season appeared over the Yangtze River Valley (YRV), which caused the worst disasters of recent decades

Summary

Introduction

Floods cause serious economic losses and heavy casualties in different regions, which make them one of the most serious natural disasters worldwide [1]. For. China, on average, the direct economic loss due to floods is around 250 billion RMB each year [2]. The most severe floods of China mainly appeared in the Yangtze River Valley (YRV), due to the heavy precipitation during the Meiyu season [3], such as the famous 1931, 1954, and 1998 floods [4,5]. In summer 2020, the YRV experienced another abnormal Meiyu season [5,6], with its accumulated precipitation breaking the historical record of events since 1961. The 2020 flood caused more than 140 dead/missing and about $12 billion of economic losses [6], which made it the worst flood in recent decades. The Sichuan Basin (SCB) and its surroundings were one of the most severely affected areas during the 2020 Meiyu season, with some sections of the basin experiencing a once-in-a-hundred-year flood [5,6]

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