Comparative Study of the Characteristics and Causes of Two Low-Vortex Heavy Rain Events in North China

In this work, the characteristics and causes of two low-vortex heavy rain events in North China on July 11–13, 2021 (Event 1) and July 19–21, 2016 (Event 2) were analyzed using multiple observations and ERA-5 reanalysis. Both events took place under similar background circulations: the eastward movement and strengthening of the South Asian high, the westward and northward movements of the subtropical high, the eastward and northward movements and development of the mid-latitude vortex, and the stable maintenance of the downstream high-pressure dam. However, Event 1 was inferior to Event 2 in terms of accumulated rainfall, rainfall intensity, influence area, duration, and extremity; furthermore, their low-level jets (LLJ) were also different. In Event 1, southwest wind dominated the LLJ and was mainly located at the south of the low vortex, while in Event 2, the southwest jet was stronger than Event 1, and the easterly jet to the north side of the vortex was more significant than Event 1. Moreover, this easterly jet generated heavier precipitation under the influence of topography at the eastern foot of the Taihang Mountains. In addition, the two events’ vortex intensity, structure, and path were different. In Event 1, the vortex was shallow, and the positive vorticity was mainly located at the middle and lower troposphere. It moved faster in the later stages of Event 1 northward along the Taihang Mountains and eventually dissipated in North Hebei. However, in Event 2, the vortex was deeper and rarely moved in Southwest Hebei until it dissipated. The stage characteristics of these two events were also different. In the first stage, the low-level convergence of Event 1 mainly happened in South Shanxi and North Henan, which is near the vortex center, while the convergence of Event 2 mainly occurred at the junction of the easterly jet and terrain on the north side of the vortex in West Hebei. In the second stage, both events presented low-vortex spiral-shaped convective rainbands that looked like spiral rainbands in the typhoon. These spiral rainbands mainly appeared to the east of the low vortex in Event 1, whereas to the north of the low vortex in Event 2. This may be due to the different distribution of the curl of horizontal vorticity, differential vertical vorticity advection, warm advection, and diabatic heating. In the third stage, the vortex of Event 1 moved to the northern part of North China, and the strong convergence of the horizontal winds under an unstable environment and topographic lifting near the vortex center produced heavy rainfall. Meanwhile, the vortex of Event 2 continued to be located in West Hebei. Although the warm shear convergence on the northeast side of the vortex was not as strong as that in Event 1, the convectively unstable stratification was deeper, leading to a larger vertical upward movement and rainfall. This work would provide a reference for understanding the spatial and temporal differences between the two heavy rain events and their possible causes.