當颱風凡那比在2010年9月19日01UTC於花蓮登陸後，被台灣中央山脈的複雜地形破壞並減弱。凡那比移至山脈西側時眼牆被重建，並在台灣西南部造成豪雨災情。在眼牆重建時期，移動式X波段雙偏極化都卜勒雷達(TEAM-R)觀測到南風增強由底層開始向中層延伸。本研究使用高解析度的WRF模式(巢狀網格最內層為1-km網格間距)，討論凡那比颱風自9月18日00 UTC至9月20日00 UTC期間過山後眼牆的重建機制。透過渦度收支分析的結果顯示：水平渦度平流在中高層都有氣旋式渦度貢獻。凡那比中心通過地形之前，高層正渦度過山使得渦管拉伸，造成山脈西側底層有氣旋式渦度累積並且透過垂直渦度平流由低層(1 km)向中層(3 km)增加。地形和邊界層的垂直風切是在颱風中心過山前造成抽拉項和傾斜項的主因。當颱風中心完整通過中央山脈後，南側的主要雨帶遇到地形向北轉並產生顯著的曲率。雨帶中二次水平風速極大值(SHWM)的噴流結構形成，配合下坡風和對流的空間分佈，造成低層水平渦管傾斜加強底層的正渦度。同時氣旋式渦度由低層(1 km)往中層(4 km)傳送，直到高低層渦旋中心結合並完成眼牆重建過程。同時進行地形敏感度實驗，以檢驗眼牆重建過程中渦度平流、拉伸和傾斜的不同角色。總結來說，凡那比颱風的眼牆重建應為由下而上(bottom-up)的物理過程。

In this study, numerical simulations of Typhoon Fanapi (2010) interacting with Taiwan terrain are conducted using the Weather Research and Forecasting model (WRF; version 3.3.1) on a triply-nested grid (with the finest grid size of 1 km and 55 vertical levels). Typhoon Fanapi made landfall on eastern Taiwan on 0040 UTC 19 September and left Taiwan on 1200 UTC 19 September 2010, producing heavy rainfall and severe floods over southwestern Taiwan. Kinematic and microphysical characteristics within typhoon eyewall and inner rainbands were observed by the operational Doppler radar over Chigu and a research mobile dual-polarmetric radar (TEAM-R) over Kaohsiung.
When Fanapi approached eastern Taiwan from the ocean, the low-level eyewall gradually weakened and broke down due to strong friction by steep terrain over the Central Mountain Range (CMR), and a secondary low was formed on the lee side (western Taiwan) by adiabatic subsidence. Above the CMR, the Fanapi vortex continued its westward track with a slightly southward deflection. Control simulation showed that as the Fanapi vortex passed through the CMR to the western foothill, positive vorticity was gradually built up within the lee-side secondary low and transported upward from the surface. Then the Fanapi eyewall was reconstructed with a complete vortex ring, as revealed from radar observations.
A series of reduced-terrain experiments are conducted to verify the eyewall reconstruction mechanism. In the absence of Taiwan terrain, Fanapi would continue its westward movement without any deflection and the eyewall remained intact. With only half of Taiwan terrain, the southward track deflection was reduced and the degree of eyewall breakdown over the CMR and eyewall reconstruction over the lee side of the CMR was less obvious. A southerly jet was formed along the western foothill of CMR as the secondary horizontal wind maximum (SHWM) within the principal rainband encountered the CMR. The southerly jet resulted from the strong vertical shear of horizontal wind shear within the SHWM or the tilting of horizontal vortex tube within the SHWM by the downslope winds above the CMR and convective updrafts within the rainband. Through the vorticity-budget analyses and terrain sensitivity experiments, it is found that the bottom-up processes is active to reorganize the eyewall when Typhoon Fanapi was over the southwestern plain of the CMR.

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