Contrasting Multiscale 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. Despite being initiated under a remarkably similar synoptic-scale backdrop characterized by dual typhoons and an anomalous Western Pacific Subtropical High (WPSH), 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 critical divergence points. First, moisture transport pathways differed markedly: the 2021 Zhengzhou event drew moisture from both the northwestern Pacific and the South China Sea through a dual-channel setup, whereas the 2024 Xiayi event relied on a single dominant corridor 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), low-level jets (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 how subtle differences at finer scales can lead to vastly different extreme outcomes under seemingly identical large-scale conditions, offering crucial insights for improving regional forecasting.