Multi-scale topographic control of southwest vortex formation in Tibetan Plateau region in an idealized simulation

The southwest vortex (SWV) is a lee vortex occurring on the leeside of the Tibetan Plateau in southwestern China, which is strongly affected by the different scale topography of the Tibetan Plateau, Hengduan Cordillera, and Sichuan Basin. The roles of these topographic features in SWV formation were investigated by conducting simulations with dry dynamics in an idealized background flow. Two shallow topographically induced vorticity streams are found to be the main contributors to SWV formation. The first vorticity stream extends out from the southeastern Hengduan Cordillera and the second from the east side of the Tibetan Plateau conjoint with the Hengduan Cordillera and Sichuan Basin. The stretching, tilting, and friction play different roles in vertical vorticity generation along the two vorticity streams, in which the stretching of the planetary vorticity dominates other vertical vorticity sources at the upper level of the first vorticity stream. The SWV forms due to the combined effects of the topographic features. The Hengduan Cordillera turns the southwesterly airflow around the Tibetan Plateau to induce the first vorticity stream, and the Sichuan Basin enhances the second one, which is associated with the stretching and tilting of airflow from the top of the Tibetan Plateau. Moreover, the Sichuan Basin provides a natural site favorable for the merging of the two vorticity streams and then promoting SWV formation. The sensitivity experiments show that the location and scale of the SWV are controlled mainly by the Tibetan Plateau and Hengduan Cordillera, and the Sichuan Basin plays a secondary role.