Abstract:

Based on observation data and ERA5 reanalysis data, an analysis and diagnosis of thes "6·13" heavy rainfall process in Guangxi in 2022 were conducted, with a focus on the role of diurnal variation and its cause of low-level jet. The results show that the heavy rainfall process is jointly influenced by the upper level trough, the low-level shear line and the southwest low-level jet. The low-level jet weakens during the day and significantly strengthens at night, with convective systems continuously developing at the top of the jet and the south of the shear line, resulting in heavy rainfall mainly concentrated at night. The extreme heavy rainfall appears during the merging stage of mesoscale convective cloud clusters, located in the large TBB (Top Blackbody Brightness temperature) gradient region. The convergence in the left front of the low-level jet at night and the obstruction of mountain topography together enhance the low-level convergence and promote the development of upward movement. The intensification of low-level jet at night causes the intense convergence of warm and cold air in the northeastern region of Guangxi, resulting in the enhancement of low-level horizontal frontogenesis forcing, which is conducive to the maintenance and intensification of precipitation. Under the continuous warm and humid transport of the low-level jet, the unstable structure in the lower atmosphere is significantly enhanced, which provides a favorable environment for the development and maintenance of convective systems and heavy rainfall. In addition, the low-level jet promotes the water vapor convergence and uplift in the northeastern region of Guangxi at night, which corresponds well with the heavy rainfall area. The diagnosis of momentum balance shows that inertial oscillation is the main mechanism causing the diurnal variation of low-level jet. The Coriolis force at night dominated the momentum change of the geostrophic wind, which rotates clockwise to form the southwest wind and superimposes with the background wind field to strengthen the low-level jet. The daytime turbulent friction is enhanced and the residual and advection terms jointly dissipate local momentum.

Key words: Low-level jet, Frontogenesis force, Momentum balance, Inertial oscillation