Mesoscale factors contributing to the extreme rainstorm on 20 July 2021 in Zhengzhou, China, as revealed by rapid update 4DVar analysis

The rainstorm produced 201.9 mm of rainfall in 1 h, breaking the record of mainland China for 1-h rainfall accumulation in the past 73 years. Using 2-km continuously cycled analyses with 6-min updates that were produced by assimilating observations from radar and dense surface networks with a four-dimensional variational (4DVar) data assimilation system, we illustrate that the modification of environmental easterlies by three mesoscale disturbances played a critical role in the development of the rainstorm.

Among the three sys-tems, a mesobeta-scale low pressure system (mesolow) that developed from an inverted trough southwest of Zhengzhou was key to the formation and intensification of the rainstorm. We show that the rainstorm formed via sequential merging of three convective cells, which initiated along the convergence bands in the mesolow. Further, we present evidence to sug-gest that the mesolow and two terrain-influenced flows near the Taihang Mountains north of Zhengzhou, including a bar-rier jet and a downslope flow, contributed to the local intensification of the rainstorm and the intense 1-h rainfall.