The dominant synoptic patterns under which frequent extreme hourly rainfall events occurred over Southwest China during the warm seasons of 1981–2020

Abstract Four dominant synoptic patterns associated with the frequent areas of extreme hourly rainfall days (EHRDs) and events (EHREs) over Southwest China during the warm seasons of 1981–2020 are identified by applying the simulated annealing and diversified randomization algorithm after evaluating 15 objective classification algorithms. Results show that the four dominant patterns account for 30.8%, 24.5%, 23.2%, and 21.5% of the total EHRDs. Three of these patterns exhibit higher daily rainfall rates and EHREs in Yunnan, the Sichuan Basin, and the eastern part of Southwest China, which correspond well to the presence of stronger moisture flux convergence, and the fourth one has similar rainfall characteristics only in the western Sichuan Basin, where steep mountains are distributed. Results also show that the south Asian high, the western North Pacific subtropical high, some low‐level circulations (e.g., jet streams, vortices, and shear lines), northerly cold air intrusion, and local topography play different roles in influencing the intensity and spatial distribution of daily rainfall and frequent EHREs over Southwest China. In particular, longer duration, higher daily rainfall rates, and more EHREs are found to correlate with more intense moisture flux convergence in the northeast of Southwest China (where the moist flows from the Bay of Bengal and the South China Sea meet) and more favorable low‐level circulations with significant mesoscale instability. It is concluded that dominant synoptic patterns and moisture flux convergence associated with frequent EHRE areas at the 40 years time‐scale can be used to help improve our understanding and prediction of regional EHREs.