The role of southwesterly flow in MCS formation during a heavy rain event in Taiwan on 12-13 June 2005

This paper presents a numerical study of a heavy rain event that occurred in southern Taiwan in June 2005. From 11-13 June 2005, a weak Mei-yu front moved southeastward from China to Taiwan, while mesoscale convective systems (MCSs) were developing and moving northward over the northern South China Sea (SCS). During the first day of the event the southwesterly flow intensified when a ridge associated with the Pacific high extended northwestward from the Philippines to the southern Taiwan Strait (TS). This pressure pattern produced a large northwestward pressure gradient force that created a southeasterly wind speed increase followed by intensification of the southwesterly flow through Coriolis acceleration. An low-level jet (LLJ) formed consequently and transported moisture and unstable air toward the southwestern coast of Taiwan. MCSs were triggered in the southwesterly flow because the potentially unstable air was lifted in a low-level convergence and shearing vorticity environment. They intensified, became organized, and moved northeastward overland, resulting in heavy rainfall in southern Taiwan. On the second day, low pressure formed near the southern TS because of the combined effect of a travelling short-wave trough and a pressure reduction resulting from the latent heat release by the evolving MCSs. This pressure change produced down-gradient acceleration in the northeastward direction, resulting in southwesterly flow strengthening. The local wind acceleration was smaller than that of the first day because the dominant pressure system was local scale, while that of the first day was synoptic scale.