熱帶氣旋於臺灣附近形成之個案分析－以桔梗颱風（2013）為例

Abstract

桔梗颱風(2013)為少數在臺灣附近形成之個案；本研究之目的在分析桔梗初始渦旋之形成和發展成熱帶氣旋之過程，並探討康芮颱風(2013)殘餘之中層渦旋在桔梗初始渦旋形成過程所扮演的角色。 分析觀測資料與雙都卜勒反演風場之結果顯示，康芮颱風侵臺時受環境強垂直風切影響而有高低層環流分離情形；低層環流中心沿臺灣東部近海往北移動，而中高層環流中心則向西北移動，經過中央山脈後在臺灣海峽滯留並減弱。受康芮中高層殘餘渦旋影響，海峽地區之中高層渦度明顯大於低層者；在桔梗初始渦旋形成前，海峽上出現強且長延時之線狀對流系統，其中並似伴隨有數個渦度熱塔(vortical hot tower, VHT)。桔梗初始渦旋於海峽建立後，其中層環流東移過山，並與臺灣東北部的低層低壓垂直重整，發展為桔梗颱風。 本研究利用Weather Research and Forecasting Model （WRF）進行模擬與分析，探討桔梗初始渦旋於海峽之形成和發展過程。結果顯示在桔梗初始渦旋形成之前，臺灣海峽附近持續有間歇性的對流出現，並伴隨中低層位渦之形成；中層位渦在康芮殘留的中層環流中合併、增強。初始渦旋形成期間，一低層風切線於浙江福建沿海建立，其西南端並有一正位渦擾動形成，此擾動隨後往海峽中央移動；當其移至康芮殘留之中層環流下時，形成一旋轉性強的中低層位渦柱（中低層低壓系統）。當低層低壓東移至海峽中央時，其東側的西南風受臺灣地形影響形成低層噴流(barrier jet)。低壓南側的西風與西南風合流，一狹長輻合區由近系統中心往南延伸，並伴隨有具數個VHTs的線狀對流系統發展；此線狀對流亦同時增強輻合區的西風與西南風，其產生之位渦亦能有效的平流至系統中心，此系統因而於臺灣海峽上快速增強，形成桔梗颱風之初始渦旋。地形敏感度實驗顯示，若移除臺灣地形，上述線狀對流不會產生，桔梗之初始渦旋亦不會於海峽形成。

Tropical Storm Toraji (2013) is one of the few tropical cyclones (TCs) that formed near Taiwan. Purpose of this study is to analyze the formation and development process of the incipient vortex of Toraji. The influence of the mid-level remnant vortex of Kong-Rey (2013) on the formation of Toraji’s incipient vortex will also be discussed. As revealed by the Dual Doppler radial winds and the ECMWF high-resolution reanalysis (ERA-interim, at 0.125∘ resolution), a decoupling of the lower-level and upper-level centers of Kong-Rey occurred on 29 August 2013 due to the strong vertical wind shear. The low-level circulation center kept moving northward, while the mid-upper lever circulation center moved northwestward, crossing the central mountain range (CMR) of Taiwan to over the Taiwan Strait. An area with mid-high level high positive vorticity existed at the Taiwan Strait for 2 days (29-30 August). Before the formation of Toraji’s incipient vortex on 00 UTC, 31 August, a line convection with several vortical hot towers appeared at the Taiwan Strait. A barrier jet caused by the southwesterlies and Taiwan topography is also observed. The mid-level circulation of the incipient vortex then moved eastward, crossing the CMR. TC Toraji formed after the coupling of the mid-level circulation with the low-level low pressure located to the east of CMR. The formation and development process of Toraji’s incipient vortex was simulated and analyzed using WRF model. Results show that a low-level potential vorticity (PV) core formed at the southwest extremity of a shear line located along the east coast of China before the formation of the incipient vortex. The low-level PV core then moves eastward to over the Taiwan Strait and couples with the mid-level PV core (Kong-Rey’s remnant) and a surface low pressure system forms. A convergent zone with embedded line convection occurs from near the PV core extending southwestward, resulting in the formation of the incipient vortex. The convergent zone is triggered by the westerly winds located to the south of the low pressure system and the barrier jet caused by the Taiwan topography. Thefore, the remnant of Kong-Rey’s mid-level circulation might have created a favorable environment for the formation of Toraji’s incipient vortex. Results of terrain-sensitivity experiments show that no incipient vortex will form if the Taiwan topography is removed in the simulation.