Mechanisms of MCS Initiation and Maintenance During Extreme Rainstorm Events in Semi-Arid Regions: A Case Study of Qingyang

Abstract

Extreme rainstorm events are becoming more frequent in semi-arid regions, yet the mechanisms behind their development and maintenance, especially from dynamical and thermal perspectives, which remain insufficiently quantified, limiting forecasting capabilities. This study employs the WRF model to investigate an extreme rainstorm case in a semi-arid valley region. Results show that the rainstorm was significantly influenced by the northward extension of the Western Pacific Subtropical High (WPSH) and a low pressure system. The onset of the Mesoscale Convective System (MCS) was facilitated by moist-unstable stratification, valley wind circulation, and strong terrain-induced vertical motion. During the mature stage of the MCS, a complex interplay of dynamic systems and unique trumpet-shaped valley topography sustained the extreme rainfall. The interaction between orographically modified valley winds and a persistent low-level jet (LLJ) formed a strong convergence zone, characterized by a LLJ-left shear line, intensified surface convergence, and enhanced northerly flows, which increased positive vorticity. Vorticity budget analysis revealed that vorticity advection and divergence effects were the dominant contributors to vorticity generation, emphasizing the importance of low-level convergence and spin transport in maintaining the MCS. In parallel, strong diabatic heating from moist air ascent created a positive feedback loop that further energized convection. The valley's topography acted as a key amplifier by concentrating moisture and enhancing vertical motion. These findings highlight the critical role of terrain-induced dynamics in sustaining extreme rainfall in semi-arid environments, offering valuable insights for improving heavy rain forecasts in vulnerable regions.