A Possible Dynamic Mechanism for Rapid Production of the Extreme Hourly Rainfall in Zhengzhou City on 20 July 2021

In this study, the unprecedented extreme rainfall event during 19-20 July 2021, which caused devastating flooding in Zhengzhou City and its nearby areas, is examined based on observational data analysis and WRF model 40-h simulations on 1-km horizontal resolution. The results show that the model successfully reproduces (i) major synoptic-scale weather systems (i.e., the western Pacific subtropical high, the Tibetan high, two typhoons, and the Huang-Huai cyclone), (ii) convective initiation along the east to north edge of the Songshan Mountain, where orographic lifting is obvious, and (iii) subsequent formation of the convective storm producing the extreme rainfall in Zhengzhou. In particular, the model generates the maximum rainfall rate of 233 mm h(-1) and 40-h accumulated rainfall of 704 mm, corresponding well to the observed extreme values of 201.9 mm h(-1) and 818 mm, at nearly observed timing and location. Importantly, the model reproduces an intense quasi-stationary, well-organized meso-gamma-scale convective system, surrounded by an arc-shaped convergence zone, allowing the development of convective updrafts in a three-quarter circle around the convective system, in a way similar to "multidirectional pumping," attracting all associated precipitation overlaid and concentrated into the same trailing region to generate the extreme hourly rainfall over Zhengzhou. Our study emphasizes the significant contribution of the unique dynamic structure of the well-organized meso-gamma-scale convective system to the record-high hourly rainfall. A possible dynamic mechanism for short-time extreme rainfall production is proposed. That is, the arc-shaped convergence zone of the mesoscale convective system, acting like multidirectional lifting pumps, transports precipitation from different directions into the same region, and thus produces the extreme rainfall. The results gained herein may shed new light on better understanding and forecasting of short-time extreme rainfall.