Analysis of the characteristics and environmental conditions of an extreme rainfall event on Qinghai Plateau

An extreme rainfall event occurred in Qinghai on August 13, 2022, causing severe floods and landslides. The extreme rainfall also indirectly triggered the deadly flash flood in Datong County on August 18. This study analyzed this rainfall event in terms of its rainfall characteristics, synoptic systems, environmental conditions, and topographic influences, using high-resolution multi-source observational data, the ERA5 reanalysis data, the HYSPLIT4 trajectory model, standardized anomaly method, and percentile method. The results show that the extreme rainfall occurred in the eastern valley regions of Qinghai and the windward slope of the Laji Mountain, featuring high cumulative rainfall, intense hourly rainfall, long duration, exceptional extremity, and severe disaster-causing impacts. The anomalous westward and northward positioning of the Western Pacific subtropical high and the unusually strong South Asian high provided the synoptic-scale circulation backgrounds conducive to the occurrence of extreme rainfall. Pseudo-equivalent potential temperature (θ_se) was anomalously high by more than 3~4 standard deviation, accompanied by a deep convective instability structure and strong convective available potential energy, providing thermal and instability conditions for the development of mesoscale convective systems and extreme rainfall. The low-level meso-γ-scale shear line, wind convergence, and frontal zone led to an anomalously strong convergence on 700 hPa (3~4 standard deviation), providing a strong moisture convergence center (5 standard deviation above normal) in the rainfall region. The low-level strong convergence generated intense upward motion, triggering the release of convective instability energy, lifting and condensation of moisture, which induced extreme rainfall. The eastward airflow was later forced to ascend the Laji Mountain barrier, resulting in the continuous generation, development, merging, and slow movement of mesoscale convective cloud clusters, which contributed to the long duration and large cumulative rainfall of this extreme event.