Dynamics of boundary layer and lower‐level vortices east of the second‐step terrain over the Yangtze River Valley

Abstract

Boundary layer vortices (BLVs) forming east of the second‐step terrain over the middle Yangtze River Valley are often overlooked in operational forecasting, yet they may play an important role in extratropical cyclogenesis and associated severe heavy rainfall. Using diagnostics analysis of reanalysis data and convection‐permitting simulations, this study identifies a previously unrecognized mechanism linking BLVs to cyclogenesis. Nocturnal BLVs developed through terrain‐induced convergence between northerly and southwesterly flows, with pressure‐gradient–Coriolis adjustment driving rapid vortex intensification. BLVs subsequently promote low‐level vortex (LLV) development through vertical vorticity advection, while horizontal advection further accelerates LLV growth. The resulting BLV–LLV coupling forms a coherent vortex column that evolves into an extratropical cyclone and organizes mesoscale convection. Sensitivity experiments further show that surface roughness mainly modulates BLV development by boundary layer convergence and stretching, whereas the trumpet‐shaped terrain regulates the efficiency of vorticity redistribution and LLV intensification. These results identify BLVs as lower‐tropospheric precursors to cyclogenesis and provide new insight into vortex evolution over complex terrain.