近年侵台颱風的個案中有幾個都是具有明顯對流不對稱的例子，即敏督利颱風 (2004)、凡那比颱風(2010)、南瑪都颱風(2011)等。這些颱風除了內核區具有很強的降 水外，由於另一側較廣泛對流區存在的關係，也會造成較多的累積降水。此外，由於 台灣或菲律賓地形的關係，對這些對流不對稱的影響也是值得研究的。因此本研究計 畫將探討颱風對流不對稱的成因；另外，也會特別檢視地形對颱風對流不對稱所造成 的影響。 目前研究規劃是針對南瑪都颱風在顯著明顯垂直風切下，並與受呂宋地形影響下 所造成的對流不對稱分析。初步成果相當獨特，有別於過去用簡單模式所提出由於颱 風高層暖心往下風切處傾斜，為滿足熱力風平衡而在下風切處產生較強的垂直運動之 動力機制。本研究將提出另一種較基礎且符合動力直觀的颱風不對稱理論，即環境垂 直風切造成颱風次環流的不對稱，進而導致對流不對稱的機制。此外，本研究也發現 地形摩擦作用會稍微增強颱風次環流的不對稱，進而產生更顯著的對流不對稱。 透過 WRF (Weather Research and Forecasting Model)數值模擬結果顯示，儘管南瑪 都颱風在登陸台灣的位置與時間有些許的偏差，整體的路徑模擬與觀測相當接近，即 可以模擬出颱風同時通過呂宋島與台灣的情形。模擬的強度演變也與美軍聯合颱風警 報中心(Joint Typhoon Warning Center, JTWC)最佳路徑的分析一致。至於環境垂直風切 所產生的非對稱對流部分，模式同樣可以模擬出颱風在環境垂直風切影響下所造成的 下風切左側強對流的特徵。進一步分析顯示風切上游及右側回波相對較弱，最大回波 區在風切下游的眼牆附近產生，並順著眼牆平移到風切的左側。 由地形敏感度實驗顯示各組的路徑皆與觀測有相同的走向，差異在於地形存在 時，颱風會往地形靠近，無地形時颱風路徑較為平滑沒有偏折且遠離原先陸地範圍。 強度演變顯示了有地形時強度減弱，無地形時強度較強，或是脫離地形後重新加強的 情況。至於地形敏感度實驗對非對稱對流特徵的影響上，結果顯示地形的有無並不會 影響非對稱對流的出現，顯示環境垂直風切仍是造成南瑪都非對稱對流的主因。然而， 地形的存在，由於摩擦作用的關係，則會擴大下風切處的颱風次環流，即增強低層內 流、中層垂直運動、上層的外流的區域，進而導致對流不對稱更明顯。 In recent years, there are a few typhoons invading Taiwan with obvious convection asymmetry, i.e. Typhoon Mindulle (2004), Typhoon Fanapi (2010), and Typhoon Nanmadol (2011). These typhoons not only induce strong rainfall in the core areas, but also cause more cumulative rainfall over the more active convection side. In addition, the impact of Taiwan and the Philippines topography to these convective asymmetry is also worthy to investigate. This proposal will investigate the mechanisms for forming the convective asymmetry of storm and its adjustment when the storm passes the terrains. Research is currently planning for Typhoon Nanmadol which is obviously embedded within strong vertical wind shear, and is influenced by the Luzon Island. Preliminary results are quite unique and different from previous mechanisms raised form simple model. This study will propose an independent schematic diagram based on the secondary circulation of TC to easily and fundamentally describe why the convective asymmetry forms and its adjustment when TC passes the topography. Regarding to the asymmetric convection simulation, the active down-shear-left convection relative to storm center is well captured by the model. This asymmetric convection is consistent with the observed radar images. Further analyses indicate that the convection is originally enhanced over the down-shear side and then advected to the left of the vertical wind shear by the cyclonic circulation of the storm. Based on results from several sensitivity experiments with or without the terrain of individual or both islands, it is found that the asymmetric convection is not obviously influenced by the terrain of two islands. This result indicates that the environmental vertical wind shear plays a dominate role in forming the asymmetric convective pattern of Nanmadol. However, the existence of terrain of Luzon could slightly broadens the lower inflow, vertical motion, and upper outflow area at the down-shear side, then tends to induce more asymmetric convection of the storm. These findings about the convective asymmetry of tropical cyclone when it was embedded within the vertical wind shear or approached the topography is quiet interesting in Typhoon Nanmadol (2011). We have proposed an independent schematic diagram based on the secondary circulation of TC to easily describe why the convective asymmetry forms and its adjustment when TC passes the topography.
