Observational and Numerical Studies of the Intensification of the Southwesterly Flow in the Lower Troposphere during the Taiwan Area Mesoscale Experiment 1987

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.