The formation and evolution mechanism of the boundary layer vortex east of thesecond-step terrain along the middle reaches of the Yangtze River

Mesoscale vortices in the boundary layer are characterized by short lifespans, small spatial scales, and difficulty in prediction, leading to their frequent oversight in operational forecasting. This oversight often results in lower accuracy for precipitation forecasting associated with these vortices. From April 2 to April 3 2023, a squall line event triggered by vortices extending from the lower troposphere to the boundary layer occurred across eastern Hubei to western Anhui. This event developed ahead of a shallow mid-tropospheric trough, while the lower levels were influenced by southwest flow. High-resolution numerical simulations successfully reproduced the evolution of the vortex and the organizational development of the squall line. Dynamic diagnosis revealed that the nocturnal boundary layer vortex (925 hPa) was initiated by the intensification of the nocturnal jet and the blocking effect of terrain. Subsequently, through vertical advection of horizontal vorticity from boundary layer to lower level, the vortex at the lower troposphere (850 hPa) developed and intensified. Later, under the combined influence of horizontal divergence and horizontal advection, the vortex rapidly strengthened, creating favorable convergence conditions for the squall line's development due to the northerly flow west of the vortex and the southwest flow south of it.