Analysis on Precipitation Efficiency of the “21.7” Henan Extremely Heavy Rainfall Event

A record-breaking heavy rainfall event that occurred in Zhengzhou, Henan province during 19–21 July 2021 is simulated using the Weather Research and Forecasting Model, and the large-scale precipitation efficiency (LSPE) and cloud-microphysical precipitation efficiency (CMPE) of the rainfall are analyzed based on the model results. Then, the key physical factors that influenced LSPE and CMPE, and the possible mechanisms for the extreme rainfall over Zhengzhou are explored. Results show that water vapor flux convergence was the key factor that influenced LSPE. Water vapor was transported by the southeasterly winds between Typhoon In-Fa (2021) and the subtropical high, and the southerly flow of Typhoon Cempaka (2021), and converged in Zhengzhou due to the blocking by the Taihang and Funiu Mountains in western Henan province. Strong moisture convergence centers were formed on the windward slope of the mountains, which led to high LSPE in Zhengzhou. From the perspective of CMPE, the net consumption of water vapor by microphysical processes was the key factor that influenced CMPE. Quantitative budget analysis suggests that water vapor was mainly converted to cloud water and ice-phase particles and then transformed to raindrops through melting of graupel and accretion of cloud water by rainwater during the heavy precipitation stage. The dry intrusion in the middle and upper levels over Zhengzhou made the high potential vorticity descend from the upper troposphere and enhanced the convective instability. Moreover, the intrusion of cold and dry air resulted in the supersaturation and condensation of water vapor, which contributed to the heavy rainfall in Zhengzhou.