納莉（2001）颱風之位渦收支

The Potential Vorticity Budget of Typhoon Nari (2001)

林柏旭(Po-Hsu Lin) ； 楊明仁(Ming-Jen Yang)

《大氣科學》 40卷1期 (2012/03) Pp. 1-27

摘要

熱帶氣旋是個具有強烈旋轉氣流且伴隨旺盛對流雲系的暖心低壓系統，而由於位渦（Potential Vorticity）此物理量同時結合動力及熱力因子，且在絕熱無摩擦的條件下具有保守的特性，故用位渦來分析熱帶氣旋變化可以凸顯出非絕熱作用對於熱帶氣旋的貢獻。近年來有不少與熱帶氣旋位渦相關的研究，但採用位渦收支對實際颱風個案進行診斷分析的研究並不多。因此本研究利用位渦進行收支分析，可以同時瞭解颱風於登陸過程中之動力及熱力過程的演變，進一步瞭解臺灣的陡峭地形會對颱風之位渦收支造成何種影響。本研究參考Pedlosky（1987）及Schubert et al.（2001），根據非靜力可壓縮之位渦收支方程式，使用Yang et al.（2008）以MM5數值模式模擬納莉（2001）颱風之高時間、空間解析度的控制組實驗模式輸出資料，進行位渦診斷分析，以探討納莉颱風從海上到登陸臺灣過程中之位渦收支變化情形。同時藉由（1）移除臺灣地形、及（2）將台灣島嶼替換為海洋的敏感度實驗，討論臺灣地形及洋面水汽通量對納莉颱風位渦收支之影響。由控制組實驗模擬結果得知，納莉颱風在尚未登陸前其位渦分佈呈現成熟熱帶氣旋於洋面上的典型分佈型態；颱風登陸瞬間由於地形的作用使其對流增強及眼牆收縮。透過位渦收支分析，水平平流會將位渦順著颱風渦旋環流傳遞，並隨著徑向入流將位渦帶入颱風內核區域，垂直平流則將於低層經由可感熱通量及潛熱釋放作用生成的位渦向上傳遞。平流作用僅扮演著於三度空間中使位渦重新分佈的角色，在登陸前潛熱作用項在低層大氣中是位渦的主要貢獻項，而在中高層則是位渦的主要消耗項，登陸後由於垂直加熱分佈改變，使得該項的分佈也隨之改變，不過在低層仍是很顯著的主要貢獻項。摩擦項之影響僅在低層大氣較為顯著，但由於此項包含亂流混合以及邊界層摩擦力作用，故並不全然都會造成位渦的負貢獻。在地形敏感度的實驗結果得知，臺灣地形的存在除了會使摩擦項增大，對位渦造成負貢獻之外，也會加強低層的亂流混合，從而導致位渦的局部增加。由純海洋的敏感度實驗得知，海表面可感熱及潛熱通量是維持颱風內核位渦環存在的重要要素之一。於敏感度實驗中當納莉颱風離開臺灣再次進入海洋時，不對稱的潛熱作用會使颱風重新形成一多邊形的位渦環，並可能使颱風路徑出現小幅度的擺線震盪。

關鍵字

位渦收支 ； 納莉颱風 ； 數值模擬 ； 台灣地形

並列摘要

Tropical cyclone (TC) is a warm-core low pressure system with strong tangential wind and active convective clouds. Potential Vorticity (PV) is a quantity that combines both dynamic and thermodynamic information, and it is conserved under the adiabatic and frictionless condition. In recent years, there have been considerable studies between TCs and the PV. However, only a few studies used the PV budget to analyze a real TC case. In this study, we can improve our understanding of the dynamical and thermodynamical evolution during the TC's landfalling stage from the PV budget analysis.A compressible nonhydrostatic PV budget equation, based on Pedlosky (1987) and Schubert et al. (2001), is used here to gain insights into the PV budget evolution of a typhoon from its oceanic stage to landfall stage. The budget is conducted using high spatial resolution (2-km horizontal grid size) MM5 outputs (CT experiment) from Yang et al. (2008). Two terrain-sensitivity tests, by removing Taiwan topogrpahy (NT) and substituting the Taiwan island with ocean (OC), are conducted to examine the effect of Taiwan's topography and the ocean surface fluxes on PV budget of Typhoon Nari.In the CT experiment, its PV distribution exhibited the typical feature in a mature oceanic TC. At the time of landfall, its eyewall was contracted and convection was intensified by the topography. From the budget perspective, PV was redistributed cyclonically with TC circulation by horizontal advection and entered the inner core region with the radial inflow. The low-level PV generated by sensible heat flux and latent heat release will be transported upward through vertical advection. Latent heating term accounted for major PV generation in lower levels during the oceanic and early landfall stage. It also acts as a major PV sink term at mid-upper levels. The friction term included both effects of eddy mixing and surface friction; hence, it did not just act as a PV sink term.In the terrain-sensitivity experiments, if the Taiwan topography was removed (NT), the friction term reduced PV over Taiwan in lower levels, opposed to that for the full-terrain run. As a result, the existence of Taiwan topography could enhance the vertical eddy mixing. Both the Taiwan topography and surface friction are prone to trigger convection, releasing more latent heat and leading to the increase of PV. Turning off the ocean fluxes (NT vs. OC) will cause the dissipation of the PV ring. The asymmetric latent-heating effect in the NT run (occurred on the land-sea interface) not only contributed to the formation of this new PV ring, but also make this new PV ring become polygonal. This PV phenomenon may be a reason to cause the typhoon to move in a trochoidal manner afterward.

並列關鍵字

Potential vorticity budget ； Typhoon Nari ； MM5