The Formation Mechanisms for a Short-Term Extreme Rainfall Event. Part I: Mesovortex Pairs and Typhoon Impacts

Abstract Short-term extreme rainfall events can cause severe damage, with formation mechanisms remaining highly uncertain. In this study, the formation mechanisms for the record-breaking hourly rainfall of 201.9 mm in Zhengzhou, China, on 20 July 2021 are investigated based on observations and high-resolution numerical experiments with large-eddy simulation (LES) and data assimilations. This study has two parts: Part I (the present paper) highlights the formation of mesovortex pairs (MVPs) and typhoon impacts under the local topography, as well as roles in the extreme rainfall event, while Part II will investigate cloud microphysical processes in the MVPs. The results indicate that the quasi-steady wind belt induced by the southeasterly outer-region circulations of Typhoons “In-fa” and “Cempaka” persistently transport strong warm moist air toward Zhengzhou. Similar to the “Venturi effect,” when the strong wind belt encounters the local trumpet-shaped topography, it is further strengthened and results in the formation of cyclonic and anticyclonic mesovortices (MVs) in the mesoscale convective system (MCS). The MVs tend to move out of the valley and finally evolve as the strong MVPs. The tilting updrafts of the MVPs pointing toward the same position induce additive effects on rainfall by transporting cloud/rainwater, resulting in the short-term extreme rainfall event in Zhengzhou City, which is just located on the central axis of the wind belt. The intensity of the MVPs is strongly dependent on the location of the central axis of the wind belt. Therefore, the formation of MVPs through typhoon impacts under the unique local trumpet-shaped topography plays a critical role in the extreme rainfall event. Significance Statement Short-term extreme rainfall events are one of the destructive meteorological disasters. Under a warming climate, short-term extreme rainfall events have been exhibiting an increasing trend. However, the formation mechanisms for extreme rainfall events remain unclear and pose significant challenges to current weather forecasting. It is essential to improve our understanding and forecasting ability of short-term extreme rainfall events. This study aims to clarify the formation mechanisms for an extreme rainfall event with a record-breaking hourly rainfall of 201.9 mm that occurred in Zhengzhou, China, on 20 July 2021. To reproduce the process, various state-of-the-art modeling techniques are used in this study. It is found that the formation of the short-term extreme rainfall is closely associated with some small vortices with a horizontal scale of several to tens of kilometers [mesovortex pairs (MVPs)] induced by the outer-region circulations of Typhoons “In-fa” and “Cempaka” under a local trumpet-shaped terrain condition. All updrafts in the MVPs tend to tilt toward the same position and induce additive effects on rainfall in the position, resulting in short-term extreme rainfall. These findings highlight the importance of typhoon circulation, local topography, and small-scale MVPs in the formation of extreme rainfall events and have potential applications in weather forecasting models.