梅雨鋒面對流雨帶之個案研究（二）：動量收支計算及對流尺度的擾動壓力場

Case Study of TAMEX IOP13 Frontal Rainband Part II: Momentum Budget Analysis

洪景山(Jing-Shan Hong) ； 周仲島(Ben Jong-Dao Jou)

《大氣科學》 25卷1期 (1997/03) Pp. 51-78

摘要

本研究針對TAMEX IOP13之鋒面雨帶個案，利用雙都卜勒雷達合成的三維風場及反求之擾動壓力場進行雨帶中對流胞的動量收支計算，主要分為三個部份。第一部份是討論對流尺度動量傳送和對流胞演化之間的相關。其結果指出氣壓梯度力在雨帶的演化過程中扮演重要的角色，並且是導致對流順風切傾斜的主要原因。其二為探討對流系統的整體動量傳送行為。計算結果顯示平行雨帶方向的動量分量為順梯度垂直傳送，而垂直雨帶方向的動量分量則為逆梯度垂直傳送，其中雨帶順風切傾斜的結構正是維持水平動量逆梯度垂直傳送所必須具備的必要條件。逆梯度垂直傳送動量雖然違反亂流混和的原則，但也意味著組織性的對流行為在此可以反抗環境所具有傾向於被勻滑的趨勢。此外，垂直通量輻合項和氣壓梯度力項大小相近，方向相反，因此如果在參數化積雲對流的動量傳送過程中只考慮水平動量垂直傳送的效果，則勢將足以產生顯著的偏差。在第三部份中，本文從動量傳遞的觀點出發，指出所謂的“對流尺度上沖流和環境垂直風切的交互作用”基本上就是流場反應對流尺度上沖流動量垂直傳送的結果：當流場透過動量垂直傳送而產生水平加速時，同時在下游處因質量輻合而產生相對高壓區，上游處則產生相對低壓區，這樣的壓力擾動基本上可以想像成是流場為反應外力刺激所具有的慣性，而此一慣性取決於平行作用力方向尺度和垂直作用力方向尺度之間的相對大小；當平行作用力方向的尺度較大時則流場的反制作用較弱，反之則較大，因此淨作用力會變得較小，而傾向於維持流場原先的狀態。此一概念也可以推展至浮力產生的動量傳送和其伴隨的垂直擾動壓力場。本研究透過動量收支計算，不僅使我們對個案中雨帶系統的動量傳送行為有更深刻的了解，同時也藉此更進一步探討不同尺度系統之間的交互作用及對流尺度中氣壓梯度力形成的可能原因，這些知識的建立不僅只在拓展我們對本個案了解的深度，並且在模式模擬及驗證上提供一個量化比對的空間，這對於後續的研究工作是很有幫助的。

關鍵字

氣壓梯度力 ； 順風切傾斜 ； 逆(順)梯度垂直傳送 ； 參數化 ； 慣性

並列摘要

This paper focuses on the momentum transport of a severe frontal rainband during the TAMEX IOP13 by using the synthesized wind field and retrived perturbation pressure field from dual-Doppler radar. There are mainly three parts in this paper. First, the relation of the momentum transport and the evolution of the frontal rainband in convective scale are studied. The calculations show that the pressure gradient force plays important roles in the evolution of the convective system. The interaction of the convective updraft and the environmental vertical wind shear generates the rear-to-front pressure gradient force at mid-level and result in the tilting of the convective system toward the downshear side. The downdraft occurred at the warm sector which is at the downshear side constructs a front-to-rear pressure gradient force at the lower boundary. The pressure gradient force then enhance the low level convergence at the leading edge of the surface front. It is the major mechanism which the convective cell could maintain its long life.Second, the impact of the bulk effect of the convective momentum transport to the large scale circulation is investigated. The calculations show that the convection transports the along-line component of momentum down the mean gradient and the cross-line component upgradient. The vertical tilting of the convective system to the downshear side is a necessary condition which transports the cross-line momentum upgradient. Besides, the pressure gradient force term is comparable to the vertical momentum flux convergence term. It implies that there might be significant errors if we only consider the effect of the vertical momentum transport in cumulus parameterization.Finally, we try to interpret what the interaction of the convective updraft and the vertical shear is by the momentum transport thinking. The horizontal acceleration induced by the vertical momentum flux convergence term will generate a high pressure at downstream side and low pressure at upstream side. The pressure dipole, the same as the consequence induced by the interaction of the convective updraft and the vertical shear, can be considered as the inertia of the flow field to resist and tend to offset the external forcing which is due to the momentum vertical transport. The inertia doesn't depend on the fluid but on the aspect ratio of the external forcing. The reaction, the pressure dipole, of the flow field will be larger when the horizontal scale of the forcing is large than its vertical scale. In the case, the net force will be smaller and tend to keep the initial status of the flow field. From the momentum transport thinking, we can combine the field inertia, the midlevel pressure gradient force, and the vertical momentum transport in the same sense and it is especially important while parameterizing the cumulus momentum transport.

並列關鍵字

pressure gradient force ； downshear tilt ； upgradient (downgradient) vertical transport ； parameterize ； inertia