颱風環流維持機制之數值模擬

颱風的能量主要來自於海洋之潛熱及可感熱通量，但深對流中所釋放的潛熱是否可有效加強颱風之環流和暖心（或加熱效率），則受颱風內部動力特性所影響。本文利用CSU-RAMS，模擬似颱風渦旋在不同初始背景大氣穩定度下，受加熱作用的反應狀況；分析重點在於颱風透過那些大氣物理過程以有效提高加熱效率，並探討伴隨颱風眼形成之物理機制。結果顯示，在前24小時的積分中，初始渦旋的強度持續減弱，但底層近渦旋中心之高慣性穩定度使摩擦內流層的空氣塊無法直接進入渦旋中心，而於接近最大切向風速半徑之中低層造成輻合，並逐漸形成眼牆（約積分33小時）；眼牆之上升運動使中心附近之中、高對流層出現水平風場的輻合，進而導致中心之下沈運動而形成颱風眼。當模擬之渦旋形成颱風眼後，大氣之地轉調整過程乃是由風場向質量場調整，渦旋之加熱效率因而提高，並促使暖心及環流增強（或形成CISK所述之正反饋）。然而，颱風眼中下沈增溫所產生之浮力效應則逐漸抑制下沈運動，而限制正反饋過程的持續。結果同時顯示，初使大氣的對流不穩定度極易受海氣間的交互作用而改變，故大氣之不穩定度不是影響颱風能量運作的最重要因子（CISK則過於重視大氣之潛在不穩定度）。然而，颱風眼牆內沿空氣塊軌跡之溫壓變化並不滿足卡諾熱機，且並非所有從海洋獲得之能量皆可用於加強系統之暖心；因此，大氣內部動力特性的改變在颱風環流增強上可能亦扮演相當關鍵的角色。綜合而言，本文的模擬結果似指出，CISK理論所提出的動力過程，可相當程度應用於解釋颱風環流之增強過程。

關鍵字

颱風；颱風強度；颱風眼；海氣交互作用

並列摘要

The latent and sensible heat fluxes from ocean are the most important energy source of typhoon. However, whether the latent heat released in deep convections can be used efficiently to increase the circulation and the warm core of typhoon (or the heating efficiency) is affected by the internal dynamics of typhoon. This study attempted to simulate the response of a typhoon-like vortex to heating under different background stability conditions, using CSU-RAMS. Focus of this analysis was placed on the atmospheric physical processes, associated with a typhoon, that caused the increase of the heating efficiency. The mechanisms leading to the formation of the typhoon eye were also discussed.Results showed that the intensity of the initial vortex kept decreasing in the first 24-hour integration. The frictionally driven low-level inflow could not penetrate through to the vortex center due to the high inertia stability near the center. This resulted in a mid-low level convergence around the radius of maximum tangential wind and the gradual forming of the eye wall (at about 33 hours of integration). In consequence, a mid-upper level convergence occurred at vortex center. Air was forced to subside and to form the eye. After the formation of the eye, the geostrophic adjustment process appeared that the wind field was adjust to the mass field. The heating efficiency of the vortex increased and both the warm core and the circulation of the vortex were enhanced (a positive feedback as CISK described). The bouyance effect caused by the subsidence warming, however, tended to suppress further development of subsidence. The positive feedback mechanism thus was limited.Results also showed that the initial background atmospheric stability could be modified easily by the effect of the air-sea interaction. Therefore, the stability is not the most important parameter in affecting typhoon energetics (CISK tended to over-emphasize the importance of the potential instability). However, the temperature and pressure changes along the air parcel trajectories inside typhoon eye-wall region were not exactly the same as those described by the Carnot heat engine. In addition, not all the energy acquired from the ocean (or the released latent heat) could be efficiently used to enhance the warm core of typhoon. Thus, the changes in typhoon internal dynamics seemed to play a key role in typhoon intensification. In summary, results of this study seem to indicate that the dynamic processes as proposed by CISK theory can be used, to a certain degree, to explain the internal dynamics during typhoon intensification process.

並列關鍵字

Typhoon ； Typhoon intensity ； Typhoon eye ； CISK ； Air-sea interaction

國際替代計量

颱風環流維持機制之數值模擬

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