Mesoscale Factors Contributing to the Extreme Rainstorm on 20 July 2021 in Zhengzhou, China, as Revealed by Rapid Update 4DVar Analysis

Abstract

The purpose of this study is to diagnose mesoscale factors responsible for the formation and development of an extreme rainstorm that occurred on 20 July 2021 in Zhengzhou, China. The rainstorm produced 201.9 mm rainfall in one hour, 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 systems, a meso-beta-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 suggest that the mesolow and two terrain-influenced flows near the Taihang mountains north of Zhengzhou, including a barrier jet and a downslope flow, contributed to the local intensification of the rainstorm and the intense 1-h rainfall. The three mesoscale features co-existed near Zhengzhou in the several hours before the extreme one-hour rainfall and enhanced local wind convergence and moisture transport synergistically. Our analysis also indicated that the strong midlevel south/southwesterly winds from the mesolow along with the gravity-current-modified low-level northeasterly barrier jet enhanced the vertical wind shear, which provided favorable local environment supporting the severe rainstorm.

ZDR Backwards Arc: Evidence of Multi‐Directional Size Sorting in the Storm Producing 201.9 mm Hourly Rainfall

“…(2022) using Weather Research & Forecasting (WRF)‐ARW model simulations reproduced the major synopticscale weather systems and proposed that the arc‐shaped updrafts transporting rain water into the same trailing region were the main dynamical driver for the 201.9 mm hourly rainfall. The presence of an arc‐shaped convergence zone was also found in another independent simulation study (Wei et al., 2022) and in analyses with real‐time radar data assimilation (Sun, Li, et al., 2023). In spite of the consensus on the presence of arc‐shaped updrafts, the modeling studies above employed one‐moment schemes in which the variations of raindrop size distributions, which were characterized by rather high temporal‐spatial variability in this event (Cui et al., 2024) and directly linked to surface rainfall accumulations (Huang et al., 2020; H. Li et al., 2023), was overlooked, suggesting a need to examine the interactions between the storm‐scale kinematics and microphysics.…”

“…(2022) employed the similar rational to quantify the size sorting signatures in convections. These approaches assume the presence of a single inflow, and there is a lack of study addressing the case of multiple directions of size sorting in presence of multi‐directional inflows (Sun, Li, et al., 2023; Wei et al., 2022; Yin et al., 2022). Here, we present a seminal approach to quantify the size sorting directions based on coupled Z DR ‐ K DP separation signatures.…”

“…This event took place under favorable synoptic conditions (Sun, Fu, et al., 2023; J. Xu et al., 2022; S. Zhang et al., 2022), including the weak Huang‐Huai cyclone, the western Pacific subtropical high, the deep Tibetan high, and Typhoons In‐Fa and Cempaka (Yin et al., 2022). The meso‐ β ‐scale low pressure system developed from southwest of Zhengzhou was conducive to the intensification of the rainstorm (Sun, Li, et al., 2023). Although operational models showed some skills in heavy rainfall prediction, the extremity of rainfall intensity was severely underpredicted (Q. Zhang et al., 2023; Zhu et al., 2022).…”

Spatial Inhomogeneity of Synoptic‐Induced Precipitation in a Region of Steep Topographic Relief: A Case Study

“…Based on the convection-permitting simulation, Hua et al (2020) found that the orographic gravity waves generated by the small-scale Taiyue Mountain cause adiabatic cooling and saturation of the lower troposphere, which is conducive to the subsequent convection initiation. Sun et al (2023) illustrated that the development of barrier airflow and downslope airflow in the vicinity of the Taihang Mountains is key to the intensification of rainstorm. Observations from the Mesoscale Alpine Programme project revealed that the presence of isolated mountains and valleys, with horizontal scales much smaller than the main Alpine ridge, significantly affects the precipitation patterns over the southern Alpine slope (Medina & Houze, 2003;Rotunno & Ferretti, 2003;Smith et al, 2003).…”

“…Sun et al. (2023) illustrated that the development of barrier airflow and downslope airflow in the vicinity of the Taihang Mountains is key to the intensification of rainstorm. Observations from the Mesoscale Alpine Programme project revealed that the presence of isolated mountains and valleys, with horizontal scales much smaller than the main Alpine ridge, significantly affects the precipitation patterns over the southern Alpine slope (Medina & Houze, 2003; Rotunno & Ferretti, 2003; Smith et al., 2003).…”