Analysis of Dynamic Conditions and Hydrometeor Transport of Zhengzhou Superheavy Rainfall Event on 20 July 2021 Based on Optical Flow Field of Remote Sensing Data

In this paper, FY-4A stationary satellite imager data and ground-based weather radar data are used to produce an optical flow field of the “7·20” superheavy rainfall event in Zhengzhou City, Henan Province, on 20 July 2021. Compared with the FNL (Final Operational Global Analysis) horizontal wind speed and observed ground data, the flow fields can approximately reflect the motional characteristics of cloud systems and the upper and lower levels of the atmosphere. On this basis, an analysis of the dynamic conditions and hydrometeor transport related to the “7·20” superheavy rainfall event is presented. The results show that there was a “southwest to northeast” transport zone of water vapor and cloud on the afternoon of 20 July, extending from southern China to northern China through Henan Province. There was active convection in this transport zone, extending to the existing cloud system over the northern–central parts of Henan, providing favorable transport conditions for a superheavy rainfall event. In the Zhengzhou region, the anticyclonic vorticity of the upper troposphere increased as divergence in the lower troposphere converted to strong cyclonic convergence on the afternoon of 20 July, prior to the onset of the heaviest precipitation, indicating that the updraft in the precipitation system in the Zhengzhou region was intensifying. The hydrometeor input at the south boundary of the Zhengzhou region increased ahead of the stage of heaviest precipitation. These results indicate that in the large-scale precipitation system, not only was there abundant water vapor, but also hydrometeors in the severe convective cloud were transported to the updraft area. This process may greatly accelerate the microphysical process of water vapor transformation into cloud water droplets, ultimately, precipitation, which may be an important cause of the rapid enhancement of superheavy rainfall in turn. The analysis method used, based on an optical flow field of remote sensing data as proposed in this study, has significant application potential in improving early warning systems for impending superheavy rainstorms.