摘要:本文针对2021年7月20日河南省郑州市发生的“7·20”特大暴雨天气过程，主要基于FY-4A静止气象卫星成像仪和地基天气雷达遥感数据，利用光流法分别计算遥感数据图像的光流场（Optical Flow Field）。经与FNL数据水平风和地面风速观测对比表明，气象卫星和雷达光流场可以近似反映大气和云系的高空和低空的运动特征。在此基础上，分析了与暴雨天气过程有关的动力条件和水凝物输送特征。结果显示，在20日午后，存在从华南经河南延伸至华北“西南—东北”走向的水汽和云水输送带，其中对流活动非常明显，并一直延伸至河南中北部的既有云系中，为河南郑州地区特大暴雨的形成提供了有利的水汽和云水输送条件。20日午后至16时（北京时）最强降水发生前，郑州地区低空由辐散转为强烈的气旋状辐合，并且高空的反气旋涡度增强，表明郑州地区整个降水系统上升运动增强。在最强降水发生前，从郑州地区南侧输入的水凝物急剧增加。这些结果表明，郑州地区不仅存在大量水汽输入，同时还有大量水凝物随强对流云输送进入到大范围降水系统的上升运动区，可能极大地加速了水汽转化为云水进而形成降水的微物理过程转化速率，这可能是此次郑州特大暴雨快速增强的主要成因。本文提出的基于遥感数据光流场的分析方法在暴雨短临预报和预警中有显著的应用潜力。
关键词:FY-4A卫星云图/
天气雷达/
光流场/
“7·20”郑州特大暴雨/
水凝物输送
Abstract: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.
Key words:FY-4A weather satellite cloud image/
Weather radar/
Optical flow field/
“7·20” Zhengzhou superheavy rainfall/
Hydrometeor transportation
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