理想颱風對海洋影響之數值模擬

Abstract

海洋受到颱風的反應最主要是由於颱風移速、颱風強度以及海洋混合厚度的影響。本研究利用DieCAST（Dietrich/Center for Air-Sea Technology)海洋模式，首次有系統的比較DieCAST其海溫在強風速下的反應。 首先以DieCAST海洋模式為基礎，加上理想颱風風場阮肯渦漩（Rankine Vortex），設計颱風於緯度25度由西向東通過一固定理想海洋。比較的方法利用三種不同的初始條件：颱風移速、颱風強度和海洋初始混合層厚度，以海洋溫降的反應來探討DieCAST海洋模式在強風速下的合理性。實驗的結果顯示移速較慢及強度較強的颱風所引起的Ekman pumping較強，其所造成溫降也較強。移速較快的颱風混合較大，慣性震盪也比較強，溫降持續較久。初始混合層越厚的海洋對颱風的反應越不明顯。 值的注意的是在數值模式中不同的紊流參數化過程也是會造成溫降改變的因素之一，本研究使用兩種不同的紊流混合模式PP82 （Pacanowski and Philan der,1982）、PWP（Price-Weller-Pinkel,1986）來模擬並互相比較，實驗發現PWP紊流參數化在高風速下上層海洋混合過程較強，更能符合實際的海洋反應。 最後，我們仿照並設計了三個颱風的無因次參數:颱風移速（S）、Burger number（B）、Mach number（C）。S代表了慣性週期對颱風影響海洋時間的比值，其值決定了慣性震盪的強弱。B為混合層水流和斜溫層水流間的壓力強度指標，代表了脫離颱風影響後混合強弱的程度。C為海表面流速對颱風移速的比值，其代表湧升流的強弱。本實驗的結果顯示DieCAST模式對S和C值表現與一般海洋模式相當，S值越大則慣性震盪強，溫降持續時間較長，C值越大則湧升流越大，溫降也比較強。但不論在哪一種的紊流模式下B值的作用都不太明顯。

The oceanic response to the idealized typhoon is studied using a 4th-order -accurate basin-scale ocean model. We examined the ocean response systematically under several wind strengths by adding idealized typhoons based on Rankine vortex. We set up the idealized typhoons with three different initial conditions: translation speed of typhoons, intensity of typhoons, and the thickness of initial ocean mixed layer. Nu-merical experiments showed that slower moving and stronger intensity typhoons lead to large Ekman pumping, which causes vigorous sea surface cooling. The faster moving typhoons lead to stronger mixing process and inertial oscillation, which enhances longer temperature drop and oscillation. Our results also showed that thicker initial mixed layer (larger momentum) has weaker ocean response to temperature drop. We also find that different turbulence parameterizations (Pacanowski and Philan-der, 1982) and Price et al., 1986) also lead to different sea temperature cooling in the numerical model. Our results show stronger and more intensive surface mixing process in PWP occurs at high wind speed, which is closer to the reality. Finally, three non-dimensional parameters are used to quantify the ocean response: Storm speed (S), Burger number (B), and Mach number (C). The Storm speed, S, which is the ratio of the local inertial period to the hurricane residence time, is expected to be large when the response of upper-ocean currents will include strong inertial motions. The Burger number, B, which is the pressure coupling between the mixed-layer current and the thermocline current, is expected to be large when the pressure coupling and the relaxation stage dynamics would be most pronounced. The Mach number, C, which is the ratio of sea surface current speed to typhoon moving speed, is expected to be large when the upwelling is strong. Our results showed that both Storm speed and Mach number show strong impacts on the ocean response to typhoons while the Burger number play only a minor role.