| During the early summer rainy season over southern China and Taiwan, heavy rainfall is frequently accompanied by a strong southwesterly flow in the lower troposphere. Previous studies on the physical mechanisms responsible for the intensification of the southwesterly flow during this season have been inconclusive. In this research, I investigate the dynamics of the intensification of the southwesterly flow during TAMEX.;Before the seasonal transition, the spin-up of the lee cyclone in the lower troposphere is caused by the arrival of the upper-level trough, the low-level cold air advection behind a midlatitude trough, and the positive feedback from latent heating. The lee trough provides the initial low-level vorticity. The dynamically induced vertical motion associated with the upper-level trough and the low-level cold air advection results in the initial deepening of the low-level lee vortex. The southwesterly flow develops in response to the increased pressure gradients related to the development and movement of a lee trough and transports the warm, moist air from the south. The condensation heating within the warm, moist flow feeds back to the large-scale flow and leads to the further deepening of the lee cyclone and the intensification of the southwesterly flow. The secondary circulation across the front system is characterized by a thermally direct circulation with rising motion in the southwesterlies and sinking motion in the postfrontal northeasterlies and by a weak thermally indirect circulation to the south.;After the seasonal transition in mid-June, the major baroclinic zone shifts northward, and the upper-level south Asian anticyclone moves over the Tibetan Plateau preventing baroclinic disturbances from moving into southern China. An orographically induced lee trough forms southeast of the Tibetan Plateau. As a lee vortex develops within the lee trough, a strong southwesterly flow is observed over the southern China coast. The spin-up of the lee vortex is mainly attributed to latent heating. The secondary circulation across the southwesterly flow is characterized by a rising motion northwest of the southwesterly flow and a broad sinking motion to the south. |
