Contrasting Large-Scale and Mesoscale Mechanisms of Extreme Rainfall Events in Henan Province: The July 2021 and September 2024 Cases

Two catastrophic rainfall events struck Henan Province, China, on 20 July 2021 and 18 September 2024, producing record-breaking 24-hour precipitation totals of 624.1 mm in Zhengzhou and 272.6 mm in Xiayi, respectively. Both events were initiated under a remarkably similar low-level synoptic-scale backdrop characterized by dual remote typhoons, a southeasterly low-level jet (LLJ), and a westward-extending low-level ridge of the Western Pacific Subtropical High (WPSH). Despite these similarities, the two events diverged dramatically in their evolution, intensity, and spatial distribution. This study investigates the multiscale processes responsible for this divergence using surface observations and ERA5 reanalysis data. We identify two divergence points. First, moisture transport pathways differed markedly: the 2021 Zhengzhou event drew moisture from both the northwestern Pacific and the South China Sea (SCS) through a dual-pathway setup, whereas the 2024 Xiayi event relied on a single dominant pathway originating from the northwestern Pacific, steered by the landfalling Typhoon Bebinca. Second, the triggering and organization of the mesoscale convective systems (MCSs) were governed by distinct mechanisms—the synergistic coupling of a Huang-Huai vortex (HHV), LLJs, and orographic lifting along the Taihang Mountains in 2021 versus cold-air-induced frontogenesis during Bebinca's extratropical transition in 2024. These findings highlight the importance of the coupling between large-scale circulation patterns and mesoscale convective processes in shaping extreme rainfall characteristics, offering crucial insights for improving regional forecasting.