Mechanism Analysis of the Synergistic Development of a Squall Line and a Convective Southwest Vortex

Using multi-source observational and reanalysis data, this study employs diagnostic analysis, radar data assimilation, and vorticity budget analysis to investigate the multi-scale mechanisms of the synergistic development of a squall line and a Convective Southwest Vortex (CSWV) over the Sichuan Basin from the night of 15 April to the daytime of 16 April 2024. The key findings are as follows: 1) Under the large-scale circulation pattern characterized by a “northern ridge and southern trough” over the Tibetan Plateau, a topographic quasi-closed southwest vortex accompanied by a warm shear line formed at 700 hPa over the southern Sichuan Basin. The northward migration of the transverse shear line at 500 hPa into the warm sector served as the primary synoptic background for this event. 2) At 20:00 BJT on 15 April, the areas with convective available potential energy exceeding 2000 J/kg, lifted index below -6 ℃, and vertical wind shear between 700 and 850 hPa greater than 10 m/s were found to be well collocated with the initial genesis location and main activity region of the squall line in the central-eastern Sichuan Basin. 3) The CSWV at 700 hPa evolved through three stages: genesis from shear flow, synergistic development with the squall line, and weakening/eastward movement. This synergy was driven by a feedback loop: merged cold pools from the squall line strengthened a surface convergence zone, intensifying the squall line, while vertical vorticity transport within the squall line fed the CSWV. In turn, the organized rotational flow of the CSWV structured the squall line"s convection. 4) Vorticity budget analysis identified the stretching term as the primary positive contributor to CSWV vorticity growth. The deep CSWV resulted from coupled dynamic forcing (low-level convergence and upper-level divergence) and the thermodynamic forcing (latent heat release). Based on these results, a conceptual model of the squall-line–CSWV synergy is proposed. Compared with previous squall-line events in the Sichuan Basin, the unique feature of this event lies in the configuration of northward-moving transverse shear lines at both 500 hPa and 700 hPa, along with a mutually reinforcing synergistic mechanism between the squall line and the CSWV, which together resulted in a longer duration and a broader impact area.