淺對流及深對流過程對於熱帶降水的影響

Abstract

本研究利用垂直積分水氣收支及濕靜能收支探討淺對流過程和深對流過程對於熱帶降水的影響。其中濕靜能收支分析中，利用粗濕穩定度(Gross moist stability, M)分析淺深對流環流各自在熱帶大氣中所扮演的角色。M過去主要使用於探討深對流過程，因此本研究目的之一即求得淺對流過程的M。 將垂直運動之垂直結構分成淺對流模及深對流模，其中淺對流模為所有邊界層輻合所產生的過程；深對流模則為自由大氣因具有不穩定度受到激發所產生的深對流過程。透過兩對流模的垂直水氣平流及垂直濕靜能平流探討各自在收支中的情形，其中利用多變量線性迴歸求得各對流模之M、粗乾穩定度及粗濕分層度。 水氣收支分析中，深對流模的垂直水氣平流在SST高的地方有較大貢獻；淺對流模的垂直水氣平流在對流地區有很大的貢獻，尤其SST梯度大的地方，因此淺對流模在降水的水氣供應上扮演了重要的角色。從濕靜能收支觀點，M的正負號決定了對流的角色，淺對流模之M (M1)為負值，即著其環流輸入濕靜能到大氣柱中，增加大氣的不穩定度；深對流模之M (M2)為正值，表示其環流將大氣中的濕靜能輸出，將大氣調整回穩定狀態。因此淺對流模所扮演的角色和淨熱通量相似，皆可視為自由大氣的熱力強迫項。M主要受到低層水氣和對流高度影響：淺對流模之M (M1)主要和低層水氣呈負相關，與對流高度的關係較小；而深對流模之M (M2)因低層水氣和對流高度兩個效應的抵消，和邊界條件無明顯關係。 透過本研究從水氣及濕靜能觀點能更進一步了解淺對流過程及深對流過程在熱帶大氣中所扮演的角色，並且本研究也檢視了兩對流過程之M的特徵。

Column-integrated moisture and moist static energy (MSE) budgets are used to examine the roles of shallow and deep convective processes in the tropical precipitation. In this study, the gross moist stability (M), usually used for deep convection, is also derived for shallow convection. The roles of shallow and deep convective processes in MSE budget are analyzed by the sign of M. Vertical structure of vertical velocity is decomposed into a shallow-mode, responsible for all boundary layer convergence, and a deep-mode, driven by atmospheric instability. Vertical moisture and MSE advections are used to examine the effects of the shallow- and deep-mode in the tropical hydrological cycle. Also, a multiple linear regression is used to derive M, gross dry stratification (Ms), and gross moisture stratification (Mq) of each mode. The moisture budget shows that the vertical moisture advection of the deep-mode contributes more moisture to precipitation over regions where SSTs are high, while the vertical moisture advection of the shallow-mode supplies moisture over convective regions, with the most over regions where SST gradients are strong. Therefore, shallow-mode plays an important role to supply moisture for precipitation. In the MSE budget, the sign of M determines its role in tropical circulation. M of the shallow-mode (M1) is usually negative, indicating that the circulation associated with the shallow-mode tends to import MSE and destabilizes the atmosphere. On the other hand, M of the deep-mode (M2) is positive, indicating that the circulation associated with the deep-mode tends to export MSE and stabilize the atmosphere. Hence, the shallow-mode, similar with a net-flux term, can be treated as a forcing to free atmosphere. M is mainly controlled by low-level moisture and cloud-top effects. M of the shallow-mode (M1) is negatively related to low-level moisture, while M of the deep-mode (M2) does not have a clear relationship with lower boundary condition, due to a strong cancellation between low-level moisture and cloud-top effects. In this study, the roles of shallow and deep convective processes in the tropics are better understood through both moisture and MSE views. Moreover, the characteristics of M associated with two convective processes are also examined.