Variation Mechanism of Low-Level Jets during an Extreme Rainstorm

From July 19 to July 21, 2021, extreme rainstorms occurred in Henan Province, China, accompanied by many low-level jet (LLJ) activities. In this study, the horizontal motion, kinetic energy, and geopotential height gradient equations in the isobaric coordinate system are employed along with ERA5 (Fifth Generation of ECMWF Reanalyses) data to examine the crucial physical factors that influence the evolution of LLJs. Results reveal that the LLJ center initially formed at 925 hPa near Zhengzhou and subsequently developed and intensified to the northwest vertically, and its core location was elevated to 850hPa, and then to 700 hPa, respectively. The horizontal gradient of the geopotential height had a remarkable influence on the local change in zonal and meridian winds of the LLJ. The mountainous territory to the west of Zhengzhou had a low value of geopotential height system, while the plain region to the northeast of Zhengzhou had a geopotential height system with a high value. Both regions produced a strong zonal gradient force of geopotential height near Zhengzhou, which, together with the zonal east wind, propelled the growth of the kinetic energy of the LLJ. The anomalous distribution of geopotential height was mostly ascribed to surface geopotential height and vertical integration of near-surface virtual temperature. Furthermore, calculations of the equation of the zonal gradient of geopotential height disclosed that the vertical advection of potential temperature caused by the zonal gradient of vertical velocity and that of diabatic heating were the principal forcing terms for the local variation of the zonal gradient of geopotential height. The relatively low-temperature region in the mountainous region and the relatively high temperature region in the plain region had substantial influences on the development and evolution of LLJs.