2019年利奇馬颱風之長生命期雙眼牆結構維持機制： 軸對稱觀點

Abstract

本論文從軸對稱的觀點探討2019年利奇馬颱風的長生命期雙眼牆結構。本論文使用網格大小細至1公里的高解析WRF模式模擬利奇馬颱風，探討可能的雙眼牆維持機制。藉由解Sawyer-Eliassen方程式，可診斷出對應潛熱加熱分布所反映的次環流(徑向風、垂直速度)結構；將位於內眼牆、外眼牆、兩眼牆間的moat區域的潛熱加熱分別代入Sawyer-Eliassen方程式，就可診斷出對應區域潛熱加熱所貢獻的moat區下沉氣流。藉由計算dynamic efficiency factor (DEF)，水氣被外眼牆限制的內眼牆處，潛熱加熱轉換至切向風的能量轉換效率也可被診斷。 Sawyer-Eliassen診斷結果顯示，雙眼牆形成後，moat區的下沉氣流主要由內眼牆及外眼牆的潛熱加熱貢獻，由moat區的潛熱冷卻為次要貢獻。DEF診斷結果顯示，內眼牆處的潛熱加熱轉換至切向風的轉換效率較外眼牆處大。雖然邊界層內切向風因摩擦力而減弱，內眼牆補償的切向風較外眼牆補償的切向風大，抵銷一部分摩擦減弱的切向風。補償的切向風可以在moat區海洋表層吸收更多水氣，增加內眼牆的水氣供應，增加潛熱轉換至動能的總能量。雖然利奇馬颱風內眼牆最後受外眼牆的水氣阻斷而減弱，內眼牆仍可因潛熱加熱的高DEF而維持數十小時。

This study examines the long-lived concentric eyewall structure of Typhoon Lekima (2019) from an axisymmetric perspective. Possible maintenance mechanisms for the concentric eyewalls are investigated using a high-resolution WRF simulation (nested down to 1-km horizontal grid size). The secondary-circulation responses to the latent heating in the inner eyewall, moat and outer eyewall are diagnosed by solving the Sawyer-Eliassen equation individually to examine the corresponding contribution to the moat downdraft. By calculating the dynamic efficiency factor (DEF), the conversion of latent heating to kinetic energy is evaluated in the moisture-restricted inner eyewall. The Sawyer-Eliassen diagnoses show that the moat downdraft was contributed mainly by latent heating in the inner and outer eyewall, with a secondary contribution from latent cooling in the moat after concentric eyewall formation. DEF diagnoses show that the conversion of latent heating to kinetic energy in the inner eyewall was more efficient than in the outer eyewall. Although tangential wind within the boundary layer was weakened by friction, the compensative tangential wind in the inner eyewall was larger than in the outer eyewall. The compensative tangential wind indirectly accumulated moisture from the sea surface in the moat, aiding the moisture supply to the inner eyewall and enhancing the amount of kinetic energy converted from latent heating. Although the inner eyewall of Typhoon Lekima eventually weakens due to the moisture cut off from the outer eyewall, the inner eyewall can still be maintained for tens of hours by the high DEF from latent heating.