The role and characteristics of low-level jet during a persistent rainstorm in Guangxi

In 2022, Guangxi experienced the strongest Dragon Boat Precipitation since the founding of the People's Republic of China. The persistent rainstorm process from 17 June to 22 June 2022 had large accumulations and overlapping falling areas, leading to floods, torrential floods, landslides, and other disasters. The impacts of the low-level jet and its characteristics during this process were analyzed based on multi-source observation data and ERA5 reanalysis data. The results show that: (1) The low-level jet enhanced significantly during the night, and the convective system developed on the north side of the jet core. During the daytime, the low-level jet weakened and the convective system gradually disappeared, resulting in the heavy precipitation being mainly concentrated during the night. (2) During the night, the positive vorticity zone on the left of the 850 hPa low-level jet coincided with the exit zone of the 925 hPa boundary-level jet in the northeast of Guangxi, which combined with the mountain terrain barrier, and caused deep low-level convergence. The low-level convergence was conducive to the enhancement of the upward movement in northeast Guangxi and favored the continuous development of the convective system. At the same time, the convective instability in the lower atmosphere also increased rapidly, which provided a favorable unstable stratification environment for heavy precipitation. (3) The variation of the low-level jet can be well explained by the inertial oscillation mechanism. During the day, the surface heating in the central and southern regions of Guangxi led to the gradual enhancement of turbulent friction, resulting in jet deceleration with the characteristics of sub-geostrophic. During the night, the turbulent friction was weakened, and the jet accelerated, which gradually presented the characteristics of super-geostrophic. The Coriolis force acting on ageostrophic wind was the main contributor to the momentum of the jet, while friction dissipation and vertical transport were the momentum sinks.