This paper presents an observational and numerical study of the southwesterly flow and heavy rainfall associated with Typhoon Mindulle (2004). When Mindulle made landfall on the east coast of Taiwan on 1 July 2004, a secondary low formed over the Taiwan Strait. The low then dissipated after Mindulle moved out over the ocean north of Taiwan. Subsequently, the accompanying strong southwesterly flow brought extremely heavy rainfall over southern and central Taiwan. The modeling study of the 15-km and 5-km grids shows that the southwesterly flow, which resulted from a downgradient acceleration toward the low system over the Taiwan Strait, transported the convectively unstable air northeastward over the northern South China Sea. When low-level air convergence provided enough lifting, strong mesoscale convective systems (MCS) were triggered in this region. This contributed to the formation of a series of mesolows and mesohighs which prevented air from accelerating farther northeastward. Only after these MCSs dissipated and the Pacific high extended westward was the moist unstable air able to accelerate into the southern Taiwan Strait, where it converged with the westerly flow of the typhoon circulation, producing convective rainbands southwest of Taiwan. When moving over land, these MCSs were enhanced, resulting in heavy rainfall. It is concluded that although the typhoon circulation was critical on the rainband development, the importance of the southwesterly flow in bringing the moist unstable air to converge with the typhoon circulation can not be left out. The southwesterly flow was first induced by typhoon's low pressure system at early stage, and was enhanced as a result of the westward extension of the Pacific high at later stage. The moisture budget analysis further shows that the development of the MCSs was mainly contributed by the horizontal moisture transport in the southwesterly flow. Evaporation locally from the ocean played only a minor role.
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