Dynamic heat and water vapor transport effects of a midsummer rainstorm event in the Sichuan Basin under the impact of flow over and flow around

The Sichuan Basin is surrounded by complex topography, resulting in significant over flows and around flows. However, research on the roles of these flows in term of dynamic, thermodynamic processes and water transport remains insufficient. Based on the ERA5 reanalysis data and GPM satellite precipitation products, this study presents a diagnostic analysis of the roles of topographically forced over flows and around flows in heat and water vapor transport during a heavy rainstorm in the Sichuan Basin from July 12 to 13, 2023, using a set of decomposition equations for the over and around flows together with equations of water vapor flux divergence, vorticity and helicity. The results are as follows. (1) The over flow advection term and around flow divergence term synergistically drive positive vorticity growth during the initial stage of the rainstorm; In the mature stage, surge of the tilting term dominates vorticity enhancement while the suppressive effect of the around flow advection term is pronounced. During the dissipation stage, the persistently strongly suppressive divergence term of the over flow and advection term of the around flow jointly cause rapid decay of positive vorticity. (2) Throughout the process, the divergence terms of the around flow and over flow exhibit opposing contributions to vorticity, with the former consistently promoting vorticity development (64.6%) and the latter continuously inhibiting it (44.6%). (3) The moisture and thermal helicity of the over flow drive upward transport of water vapor and heat from the middle and lower troposphere; the moisture and thermal helicity of the around flow enhance the moisture transport from the lower to middle troposphere, while they continuously intensify mid-level potential instability by pumping high-temperature air from lower troposphere. Ultimately, rainstorms are generated through the release of unstable energy. This quantitative analysis of the over and around flows provides a theoretical reference for forecasting mountain-triggered rainstorms in the Sichuan Basin.