| Summary: | 碩士 === 國立臺灣大學 === 大氣科學研究所 === 107 === Outside the eyewall, tropical cyclone rainbands (TCRs) are the most striking and threatening region, causing not only heavy precipitation but intense winds. When Typhoon Nesat (2017) and Haitang (2017) were located near Taiwan, outer TCRs oriented approximately normal to the coast were initiated off the southwestern coast of Taiwan and resulted in the severe flood over the coastal and inland regions. Thus, it is crucial to understand the formative mechanism of the studied TCRs. This study used ground-based dual-Doppler and surface observations and NCEP-CFS reanalysis data to document the structural feature of the outer TCRs. The multiple-Doppler-derived wind fields showed that the formation of the TCRs was related to the low-level convergence between the southwesterly flow and the northwesterly flow. According to the budget analysis of radial momentum equations, the southwesterly flow was more subgradient due to the interaction of Typhoon circulations associated with Nesat and Haitang. In contrast, the northwesterly flow that was dominated by the outer circulation of Typhoon Nesat was close to the gradient wind balance. In addition, the structure of the outer TCRs in the nearshore region is different from that seen in the offshore region, which was caused by more pronounced low-level convergence produced as the coastally blocked flow (south-southwesterly flow) that occurred under low Froude number regime encountered the northwesterly flow.
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