Key thermal dynamic process of extreme rainstorm caused by residual vortex of Typhoon Haikui (2311) in Guangxi

Affected by the residual vortex of Typhoon Haikui (2311), an extreme rainstorm occurred in the southeast coast of Guangxi from September 10 to 11, 2023, with large rainfall intensity, overlapping falling areas and obvious night rain characteristics, and the rainfall in many places broke the observation records since the establishment of the station. Based on the multi-source observations and ERA5 reanalysis data, the cause of the extreme rainstorm and the possible mechanism of residual vortex maintenance were analyzed. The results are as follows: Under the background of the weakening of the continental high and the stable maintenance of the western Pacific subtropical high, the "Haikui" residual vortex between the two is weakly guided by the steering flow, remaining stationary over Guangxi, which cooperated with the strong southwest monsoon flow and contributed to the extreme rainstorm. There is obvious strong precipitation potential near the rainstorm center, and the precipitable water is abnormally large before the heaviest precipitation. The boundary layer jet on the southeast side of the residual vortex is the key influencing system for extreme rainstorm, which plays a cross-scale pivotal role in connecting the residual vortex with local extreme rainstorm. Its development at night is related to the local kinetic energy increase driven by work done by the meridional pressure gradient force in the southeast side of the residual vortex.The negative meridional potential height horizontal gradient on the southeast side of the residual vortex combined with southerly wind increases the local kinetic energy, resulting in enhanced development of boundary layer jet at night. The development of boundary layer jet is conducive to the formation of strong vertical helicity through low-level convergence uplift combined with positive vorticity circulation of residual vortex, on the one hand, and the frontogenesis forcing of convergence and deformation at low level provides dynamic conditions for the development of heavy precipitation. On the other hand, is conducive to the rainstorm center maintains high temperature and high humidity at low level, and thicker wet layer depth. The deep warm advection heating layer and the middle troposphere latent heating provide favorable thermal conditions for the maintenance of residual vortex circulation. At night, the perturbation downslope wind on the eastern slope of the Yunnan-Guizhou Plateau and the perturbation onshore wind in the Beibu Gulf cooperate with each other to point to the center of the residual vortex, enhance the convergence effect of the wind field, and provide dynamic conditions for the development and maintenance of the cyclonic vorticity of the residual vortex.