雙眼牆結構生成與尺度大小變異：正壓動力探討

Abstract

本研究探討在正壓動力下，雙眼牆渦度結構生成的尺度大小變化。觀測上颱風核心渦度結構可以是環狀結構，而渦度環為不穩定結構，在發生正壓不穩定後渦度會有側向上的混合，在最大風速半徑外渦度緩慢遞減，產生類似裙帶的渦度結構，且渦旋中心壓力加強。因此本文延續雙渦旋交互作用的實驗，以渦度環做為核心渦旋，研究發生自身重組的核心渦旋如何影響雙渦旋模擬的最終型態與其大小，並藉以研究最終型態與強度變化之間的關係。 在兩渦旋強度比與無因次距離不大的雙渦旋實驗中，粗或滿的核心渦度環會形成雙眼牆渦度結構，且中心壓力下降較少；較偏細且空的核心渦度環不會形成雙眼牆渦度結構，其中心壓力下降較多。而其壓力下降量值又受單一渦度環模擬所下降的量值主導，因此雙渦旋實驗是否發生強度快速增強（壓力下降量值大），核心渦旋結構扮演關鍵角色。在雙渦旋實驗的最終型態中，除了中心壓力較初始時增強，也伴隨最大風速半徑內縮，內縮後由於高風速區增加，為維持模式中能量保守，最大風速將會減小，因此導致外圍的旋轉差異減弱，使帶狀化時間增長。實驗中細且空的核心渦度環在最終型態有較大的風場內縮範圍，也因此細且空的核心渦度環雖會導致系統強度快速增強，但無法形成雙眼牆結構。以核心渦旋單獨模擬的觀點來看，偏細且空的渦度環發生渦度重組後，在中心外側會有渦度裙帶的結構出現，所以不利於與較近的周遭渦旋形成雙眼牆渦度結構，這再次顯示渦度環結構對雙渦旋實驗的最終形態有決定性影響。然而細且空的核心渦度環在兩渦旋強度比較大與無因次距離較大時，最終型態和阮肯渦旋並無不同。因此核心渦度環所生成的雙眼牆渦度結構尺度偏大，造成雙眼牆渦度結構的尺度變異。 若將細且空的核心渦度環加上渦度裙帶（α-渦度環），和未加上渦度裙帶的渦度環相比，需要較遠的無因次距離才能在雙渦旋實驗中形成雙眼牆渦度結構。而細且空的α-渦度環和粗或滿的α-渦度環相比，細且空的核心結構依然會發生混合較為劇烈的情形。

This study discusses the size variability in the formation of concentric eyewall under barotropic dynamics. In observation, the vorticity structure of the core of tropical cyclone can be regarded as an annular ring. The ring is an unstable structure because of barotropic instability. Radial vorticity mixing process occurs while barotropic instability happens and the vorticity outside the radius of maximum wind forms a skirt-like structure then. It has been already known that mixing process also caused central pressure fall in former research. Therefore, We extend the experiment of binary vortex interaction and investigate how the core vortex under active vorticity mixing influence the end state and size of binary vortex interaction by using vorticity ring as core vortex. Moreover, we can understand the relation between the end state of binary vortex interaction and intensity change. In the binary vortex interaction experiments with small vorticity ratios and dimensionless gaps, thick or filled enough rings tend to turn into concentric eyewall vorticity structure and their pressure fall slightly. Thin and hollow rings tend to become monopole or tripole structure while their central pressure fall dramatically. The decreasing value of pressure is dominated by the process of single vorticity ring mixing in thin and hollow ring, so the core vorticity structure is a key factor for rapid intensification. In the end states of binary vortex interaction, the central pressure decrease accompany with the contraction of the radius of maximum wind. The contraction provides more area of high velocity. For the maintenance of kinetic energy conservation, the maximum velocity needs to decrease. This makes the differential rotation weaker and lengthens the filamentation time. The thin and hollow core has the most raising region of high velocity, so it’s difficult to become concentric eyewall vorticity structure -- even it’s pressure intensify rapidly. In the point of view of single core vortex simulation, thin and hollow rings generate vorticity skirt after vorticity rearrangement. It’s hard to format concentric eyewall vorticity structure when the core vortex is skirted. This emphasizes the importance of core vorticity structure again. However, the results of thin and hollow cores with large vorticity ratios or non-dimensional gaps are similar to the results of Rankine. Therefore, the core vorticity rings cause larger size of concentric eyewall structures. Comparing to the ring with vorticity skirt (α – Ring), the skirted one need a further gap between neighboring vortex to form concentric eyewall vorticity structure. It’s also easier to mix into monopole or tripole in thin and hollow α – Ring than in thick or filled α – Ring.