颱風與環境交互作用下之長生命期雨帶

Abstract

本研究針對西北太平洋地區熱帶氣旋與季風環境交互作用所導致之兩類長生命期雨帶，進行系統性分析。第一類雨帶為由熱帶氣旋外圍雨帶發展而來、且伴隨大範圍冷雲頂之線狀對流系統，稱之外圍中尺度對流系統（OMCS），如莫拉克颱風（2009）所伴隨者；第二類雨帶則由熱帶氣旋主雨帶發展而來，具較長之生命期與大範圍冷雲頂，稱之加強型雨帶（ERB）。分析紅外線衛星雲圖與微波頻道衛星資料顯示，在1999年至2009年間，西北太平洋之熱帶氣旋分別有22% 和21% 伴隨有OMCS和ERB形成；而OMCS和ERB之總個數分別為109和90。分析發生於熱帶氣旋南側之85個OMCS與80個ERB顯示，OMCS常在距中心200至700公里處發展，且向外移動；而ERB則常在半徑100至300公里處發展，且以氣旋式繞中心移動。結果亦顯示，當ERB形成後，熱帶氣旋之暴風半徑常顯著增大，但增強速率維持不變；而OMCS形成後，熱帶氣旋之暴風半徑維持原有之變化趨勢，但增強速率減慢。值得注意的是，暴風半徑較大之颱風(15 m s-1暴風半徑大於3.6°)，有70%在其增強至颱風強度(65 kt)前，皆伴隨有一個以上的ERB發生。 針對伴隨有OMCS之風神颱風(2008)，利用Weather Research and Forecasting Model (WRF)進行模擬與分析，結果顯示WRF可合理模擬風神颱風所伴隨OMCS之形成與發展過程，唯因模擬之颱風外圍環流較大，而導致系統往南偏移約100公里。分析模擬結果顯示，風神颱風低層環流受呂宋島地形影響，颱風中心西側邊界層中之南北走向水氣帶向外圍區偏移；此南北向水氣帶因而能與西南氣流合流(而非旋入內核區)，並導致OMCS之形成。颱風西側之水氣帶形成則因西南季風所夾帶水氣以氣旋式軌跡繞過颱風中心，並受颱風與垂直風切交互作用影響所導致；換言之，颱風渦漩受垂直風切影響而傾斜，導致下風切方向之邊界層因不對稱渦度產生摩擦輻合；而摩擦造成之水平水氣通量輻合使水氣帶形成。在OMCS維持方面，受低層西南氣流與中層颱風北風環流之影響，此OMCS向南移動，並具有後方(北方)內流與前方(南方)層狀降雨區。南北向水氣帶中、低層具高對流可用位能之空氣，不斷隨後方內流進入系統，並在伴隨層狀降雨區之冷池(∆θ < -3 K)北方邊界、連續形成新對流胞，有利OMCS生命期之延長。分析1999年至2009年85個OMCS形成時之環境特徵顯示，51%之OMCS在颱風西側南北向水氣帶與西南氣流合流處形成，與風神颱風之OMCS形成過程相似。

This study analyzes two types of long-lasting rainbands associated with tropical cyclones (TCs) in the western North Pacific. These rainbands are separated from the eyewall convection and form due to the interactions of TCs and the monsoonal environments. The first type is the outer mesoscale convective system (OMCS), which is a linear convective system with a large cold cloud shield that develops from the TC distant rainband (e.g. OMCS embedded in Typhoon Morakot 2009). The second type is the enhanced rainband (ERB) that develops from TC principal rainband and is accompanied with active and long-lasting convections. A total of 109 OMCSs and 90 ERBs that occurred in the western North Pacific during 1999-2009 are identified using infrared and passive microwave images. About 22% (21%) of all TCs have at least one OMCS (ERB) during their life cycle. Eighty five OMCSs and 80 ERBs that developed in the southern part of TC are further analyzed. Results show that the south-type OMCSs developed at 200-700 km radii from the TC center and moved predominantly outward. The south-type ERBs developed at 100-300 km radii from the TC center and moved predominantly cyclonically. The TC intensification rate decreased but the rate of TC size change did not change when after an OMCS was present. However, the TC size increased significantly after an ERB was present. Seventy percent of very large typhoons (radius of 15 ms-1 wind > 3.6°) had an ERB during the period when they intensified from tropical storms to typhoons. The Weather Research and Forecasting (WRF) model was used to simulate the development and maintenance of an OMCS that occurred to the southwest of Typhoon Fengshen (2008). Results show that the WRF does a reasonably-well simulation except that the OMCS is shifted southward by 100 km because the simulated TC outer circulation is larger. The low-level TC circulation of Frngshen is deflected by the Luzon terrain causing a westward shift of an elongated north-south moisture band which then converges with the low level monsoon southwesterly flows. As a result, the OMCS develops at the outer region of Fengshen instead of being spiraled into the inner-core region. The formation of this moisture band is attributed to i) the moisture from the southwest monsoon that is transported cyclonically around the TC center, and ii) the tilting of the TC due to the strong northeasterly vertical wind shear (VWS). The TC-VWS interactions result in down-shear frictional convergence associated with asymmetric vorticity in the boundary layer and the formation of the moisture band. The OMCS develops when this moisture band interacts with the low-level southwesterly flow. Results also show that a characteristic structure of rear-fed inflow with leading stratiform in the cross-line direction (toward the south) is established when the OMCS becomes mature and moves southward. Such a structure contributes to the long duration of the OMCS because the high-CAPE air in the moisture band keeps feeding into the system and new cells form continuously at the trailing (north) edge of a cold pool (Δθ < - 3K) associated with the large stratiform precipitation. The synoptic conditions of all 85 south-type OMCS formations are examined. Results show that 51% of these OMCSs that formed at the intersection of an elongated moisture band in the TC northerly circulation and the southwest monsoon flow have similar feature with that of the OMCS embedded in Fengshen.