| Underlying terrain or coast is an important factor for the tropical cyclones (TC) intensity and structure change during its landfall process. Taiwan is located at the forefront of China southeast coast, which is frequently attacked by TCs, and has significant effects on TC movement, structure, intensity and rainfall due to its complex topography. However, the impact mechanism of Taiwan terrain is not understood until now. Statistical features of TCs mading landfall on Taiwan Island is studied firstly based on 60 years (1949-2008) TC historical data. Then, diagnostic analysis is made on the structure and intensity change of two TC cases, which crossed Taiwan Island or Taiwan Strait in north-south direction. Finally, a set of sensitivity experiments are employed to investigate the terrain impact of Taiwan Strait with a mesoscale numerical model WRF.Statistical results show that most of the TCs can maintain over Taiwan Island for about 6 h, and the minimum sea-level pressure near TC center increases about 6 hPa averagely. TBB composite analysis indicates that TC convection structure is in slightly elliptic shape with the long axis in northeast to southwest direction when it makes landfall on the east coast of Taiwan. It is found that the convection near TC center dissipates and the cloud system structure becomes loose when crossing the Island. Besides, TC scale decreases and the radius of maximum wind speed increases after entering Taiwan Strait from the Island.Typhoon Nari (2001) made landfall on the northern coast of Taiwan and maintained its center over the Island for about 36 h, then re-entered the sea from the south side of Taiwan. Study shows that the ascending air flow in Nari's western circulation is strengthened by the windward slope'of mountainous terrain along Taiwan Strait, however, the leeward slope sinking effect on eastern region to the Island inhibits convection development in typhoon eastern circulation, that is responsible for the formation of Nari semicircular asymmetric structure. Beside, the channel effect of Taiwan Strait benefits the strengthening and stretching of the lower layer jet stream in Nari periphery, which is not only favorable for water vapor convergence in the Strait, but also for the increasement of the angular momentum, thus maintains Nari intensity.Another case, Meranti intensified rapidly into a mini typhoon just before its landfall in Fujian when it moved northwardly and entered the Taiwan Strait. Study shows that southerly flow on Meranti's western periphery is diverted and accelerated due to Taiwan terrain, which is benefit to the water vapor inflows and vortex sustention. Meranti shrank its scale because of the narrow terrain when it entered Taiwan Strait. Accordingly, the typhoon air column was elongated in the vertical direction and tangential wind speed was increased, which enhanced Meranti's intensity. The study also indicates that the development and involved of convective clouds in TC's southern region is favorable to the strengthening of Meranti. Moreover, the upper lever cold vortex in the south side of the subtropical high also plays an active role in Meranti intensification.Sensitivity experiments on different terrain of Taiwan Strait indicate that the Taiwan terrain has significant effect on Nari structure. Without Taiwan terrain, Nari structure would be symmetric and the intensity would be stronger. Fujian's terrain only has a certain influence on the intensity and the structure of Nari. Without Fujian terrain, the channel effect would be diminished. Doubling the terrain height of both side of Taiwan Strait, the channel effect would be increased, that benefits formation of the asymmetric structure and maintenance of Nari.Finally, a conceptual model of Taiwan terrain and Taiwan Strait affecting TC structure and intensity is generalized based on the main results.
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