莫拉克颱風雨帶內對流胞擾動氣壓分布之探討

Abstract

本論文研究颱風環境中對流胞與低層噴流的交互作用，莫拉克颱風(2009) 的觀測中顯示，東西走向的雨帶座落於臺灣海峽颱風環流與西南季風的氣流交會所產生的輻合帶中，雨帶由多個向東移動的對流胞組成，並伴隨30 m/s的低層西風噴流。成熟的對流胞西側常有新的對流發展，對流胞東移的速度一般而言比起低層噴流的速度更慢。我們使用空間解析度1公里的雲解析風暴(CReSS)模式模擬、分析莫拉克颱風雨帶內的對流胞，我們的研究假設為，當低層噴流伴隨的垂直風切與對流胞中垂直速度具有的水平差異交互作用時，可以在對流胞的上風處產生顯著的垂直氣壓梯度力，莫拉克颱風雨帶內對流胞西側的垂直氣壓梯度力有可能強化原本的對流胞。 我們使用數值模式資料分別求出浮力擾動氣壓與動力擾動氣壓的大小，並且可進一步的將動力擾動氣壓分離為線性與非線性項的貢獻，線性項包含低層噴流形成的垂直風切與在對流中的上衝流產生的水平差異貢獻，非線性項則來自對流胞本身。透過氣壓的診斷結果，線性的作用對動力擾動氣壓的貢獻最為重要，低層噴流與對流的交互作用，將分別在對流胞西側的低層與高層形成相對擾動高壓與低壓，以及正的垂直擾動氣壓梯度力。成熟的對流胞中除了浮力以外，動力垂直擾動氣壓梯度力也在對流胞的西側貢獻正的垂直加速度。 我們的分析強調颱風環境中低層噴流的重要性，當低層噴流與現有的對流胞交互作用時，對流胞的西側可能會在上風處產生顯著的垂直動力擾動氣壓梯度力，此作用力可能會在颱風雨帶內強化原本的對流胞。

This thesis studies the convective cell interaction with the low-level jet in the typhoon (TY) environment. Observations of TY Morakot (2009) indicate that the east-west oriented rainband in the Taiwan Strait is in the convergent zone caused by the interaction of the TY circulation and the southwesterly monsoon flow. The rainband is with low-level westerly jet of 30 m/s and composed of many eastward moving convective cells. The new convection often developed to the west of the mature cells and the eastward speed of the convective cell in general is slower than the low-level jet speed. We use 1-km resolution Cloud Resolving Storm Simulator (CReSS) model to simulate and to analyze the convective cells in rainbands associated with TY Morakot. Our hypothesis that the vertical wind shear associated with the low level jet, when interacted with the horizontal vertical wind variation in the presence of the convection, can generate significant vertical pressure gradient force on the upwind side of the convective cell. The vertical pressure gradient force to the west of the convective cells may strengthen the convective cell in the TY Morakot rainband. With the numerical model data, we solve for the buoyancy pressure perturbation and dynamic pressure perturbation. The dynamic pressure perturbation can be further separated into the linear and non-linear contributions. The linear term mainly involves the product of the vertical wind shear from the low-level jet and the horizontal variation of the convective updraft and the nonlinear terms are from convective cells. Our pressure diagnosis suggests that the linear contribution is the most important in the dynamic pressure perturbation. The interaction of the low-level jet with convection will produce relative high (low) dynamic pressure perturbation in the lower (upper) level and positive vertical perturbation pressure gradient to the west of the convective cells. Together with the buoyancy, the dynamics vertical pressure perturbation gradient force also contributes to the positive vertical acceleration to the west of the mature convective cells. Our analysis highlights the importance of the low-level jet in TY environment when interact with existing convective cells may produce significant dynamic vertical pressure perturbation gradient to the upwind side. The dynamic vertical pressure gradient force may strengthen the convective cells in the TY rainband.