渦旋交互作用與結構改變

Abstract

本研究利用區域淺水模式模擬，探討渦旋交互作用、颱風渦旋結構改變與起轉效應。我們以淺水模式延續前人之正壓雙渦旋實驗，雖然強渦旋周圍的羅士比變形半徑遠小於背景環境值，但是渦旋交互作用結果和正壓模式類似，亦即渦度動力（vorticity dynamics）和位渦動力（potential vorticity dynamics）在強颱風形成雙眼牆的結構之下並沒有很大不同，皆屬於慢速流形（slow manifold）動力，重力波造成的影響很有限。 我們以渦旋交互作用探討颱風的結構改變，包括颱風最大風速增強或暴風圈半徑變大，並分為兩個範疇：（1）強颱風的增強作用（intensification）；（2）弱渦旋的旋生（cyclogenesis）。依據颱風衛星觀測與RAINEX觀測Hurricane Rita的近中心渦度場，以初始值問題及強迫外力問題，設計理想實驗，探討中渦旋對強、弱中心渦旋造成的起轉效應。並由其特徵，定義出兩種不同的起轉類型，並依此分析實驗結果。第一種為中心渦旋之最大切向風速增強，第二種為最大切向風速不變或減弱，但最大風速半徑增加。結果顯示：（1）若中渦旋發生在中心渦旋的最大風速半徑內，將與中心渦旋合併並增強中心渦旋之最大切向風速，屬於第一種起轉類型；反之，若發生在中心渦旋的最大風速半徑外，則屬第二種起轉類型；（2）中心渦旋最大切向風速增加的幅度，與中渦旋強度成正比；（3）當中心渦旋之渦度結構愈鬆散，中渦旋愈能使中心渦旋產生起轉效應；若結構較緊密，則中心渦旋較難起轉。這些實驗結果皆能以渦旋之「快速帶狀化區域（rapid filamentation zone）」動力理論來解釋。此外，以背景亂流之實驗結果亦支持以上觀點。

This study utilizes a shallow water model to understand the vortex interaction, structural change and spinup effect of typhoons. Despite the Rossby Radius of Deformation around strong vortexes is much smaller than of the background, the results of experiments in the nondivergent barotropic model and in the shallow water model are similar. This indicates that there is little differences between vorticity dynamics and potential vorticity dynamics in the formation of the concentric eyewall structure of typhoons. The dynamics of this formation belongs to a slow manifold dynamics and the effect of the gravity waves is limited. We attempt to study the structural change of typhoons in vorticity interactions, including both (1) The intensification of maximum tangential wind and radius of maximum wind of typhoons and (2) The cyclogenesis of weak vortexes. We designed ideal experiments according to observations of RAINEX in order to study the spinup effect of the vortex contributed by mesovortex. The results of these experiments are classified according to its structural characteristic . Type I includes the cases which the maximum tangential wind of the core vortex increases and Type II includes the cases which the maximum tangential wind does not increases , but the radius of maximum wind increase. The Result shows that: (1) If the mesovortex arise within the radius of maximum wind, the structural change of core vortex belongs to Type I. Otherwise, it belongs to Type II, (2) The raise of the maximum tangential wind is proportion to the strength of the mesovortex, (3) If the vorticity structure of the core vortex is looser, the mesovortex will more profound impact on the core vortex. These results can be explained by the rapid filamentation zone dynamic theory of the vortex. Finally, we have also perform simulations with turbulent vorticity background. In general, the result of the turbulent background experiments supports the ideal experiments.