雲解析模式受海溫驅動的輻射對流平衡反應

Abstract

本研究針對暖化下熱帶環流中的對流與降雨的變異趨勢進行探討。不同於過去以觀測資料和全球模式的結果所做相關的研究我們以三維渦度方程雲解析模式VVM做為工具、提供雲解析模式下的理想化實驗架構來進行研究。透過海溫梯度及準二維的模式設置驅動接近輻射對流平衡的氣候狀態，模擬出類似熱帶哈德里胞、沃克環流的模式內輻射對流平衡態的大尺度環流，我們將本模式架構下的控制組跟三度海溫暖化實驗的平均態之變異作為討論氣候暖化趨勢的研究依據。 在模式架構內的大尺度環流中，高海溫區（亦即暖池區）為主要降雨區和環流上升區；低海溫區（亦即冷池區）則為乾區和環流下沈區。在降雨方面，強降雨為模式降雨量的主要貢獻來源；弱降雨和中度降雨則為頻率發生最高的降雨型態。藉由分析穩定層及水物的垂直分布可以看到有類似觀測資料中的熱帶對流三模型態；而海溫暖化下，上層暖化增強比下層顯著，整體環流有加深、對流結構有上移的趨勢，同時主要降雨區的環流上升速度有增強的趨勢，而在冷池區的整體結構分布亦隨暖化氣柱增厚有上移、下沈氣流增強的趨勢，然而近地層的淺對流隨逆溫層穩定度增強、變淺則有變淺的趨勢。在冷池區邊界層，水氣收支為地表蒸發及水氣通量的水平輻合項、垂直輻合項三項互相調節的結果；在暖化下，水氣通量水平輻散增強，平均狀態而言伴隨環流有下沈增強的趨勢和邊界層由冷池區帶往主要降雨區的水氣通量水平輻散增強，由以上我們推測有一邊界層淺層環流的增強，使得冷池區水氣總含量有變少的趨勢，同時對流、雲雨更不容易發展、淺對流發展高度變低。 暖化的變異下，主要降雨區的降雨量和降雨頻率皆有增加的趨勢，而冷池區降雨量和降雨頻率有減少的趨勢；為方便討論，我們將降雨強度分成四級：不降雨的乾區（亦即降雨強度為零）、降雨強度後50%的弱降雨、中間40%中度降雨以及3.19mm/hr以上的前10％強降雨事件，發現在整體範圍和主要降雨區中的強降雨事件之暖化趨勢為降雨頻率增多、降雨量和降雨強度增大；弱降雨事件的暖化趨勢為降雨頻率減少、降雨強度增強，而弱降雨事件在整體範圍和主要降雨區有不同的降雨量暖化趨勢，分別為整體範圍內降雨量減小、主要降雨區內增大。 然而此雲模式理想化實驗架構有其界線使其解釋度有限，其中一部份主要來自於實驗架構中的週期性震盪及模式輻射強度，上述使得輻射對流平衡有一定殘差，定量上較敏感，因此只能討論定性上的特性。然而本研究提供了一模擬熱帶環流下探討對流與降雨變異趨勢的實驗架構，並得以定性上的變異結果可作為相關議題的進一步佐證與瞭解的根據。

This research focuses on the trend of the variation of convection and rainfall in tropical circulation under global warming. Different from the previous related studies based on observation data and outcome of the global model, our study uses 3-Dimension Vorticity Equation Cloud Resolving Model (3D VVM) as the tool to provide an idealized experiment framework under CRM. Through the prescribed Sea Surface Temperature (SST) gradient and quasi-2D model setting, we drove a near radiative-convective equilibrium to simulate a climate regime with a large-scale circulation under the radiative-convective equilibrium similar to tropical Hadley-Cell, Walker Circulation. Our discussion on climate warming trend is based on the anomaly between the mean state of the control and 3-degree SST uniform warming experiments under model framework. In the large-scale circulation of model framework, high-SST region is the main-rain region (MR) and ascending region while low-SST region (Cold Pool, CP) is the dry-region and descending region. The heavy rain is the main contribution to the total model rain amount. The light rain and mediate rain are the most frequent rain types. Through analyzing the vertical profile of the stable layer and hydrometeors distribution, we find out it is a structure similar to the tropical convection tri-modal distribution in observation. Under SST warming, upper layer gets warmer than other layers. The total circulation becomes deeper and there is an up-shifting trend of the convection structure. At the same time, the upward velocity of MR region circulation branch becomes larger; on the other hand, there is also an upshift of the total structure in CP region and the downward velocity of CP region circulation branch becomes larger. However, as the low-level inversion becomes stronger and shallower, the shallow convection in low-level layer becomes shallower. In CP region, surface evaporation, the horizontal convergence of vapor flux, and the vertical convergence of vapor flux mutually regulate the water budget of CP boundary layer. Under warming, horizontal divergence of vapor flux becomes stronger. In terms of mean-state, from the results in our model, we find out that the downward motion of the circulation becomes more intense. , and that the horizontal divergence of the vapor flux from CP to MR region in boundary layer gets stronger, which, we suppose, are related to an enhanced boundary level shallow circulation. The shallow circulation results in a decrease on the total water vapor content, a tougher developing of convection and cloud rain, and a lower-height development of shallow convection in the CP region. Under the warming trend, rainfall amount and rainfall frequency increase in MR region but decrease in CP region. For the convenience of discussion on rainfall intensity, we classify all the rainfall events into four categories: dry region (DRY) with no rain, light rain (LIG) with bottom 50% event, mediate rain (MED) with intermediate 40% event, and heavy rain (HEV) with top 10% event stronger than 3.19 mm/hr. We find out that under the warming, in total domain region and MR region, in HEV’s case, the rainfall frequency of HEV increases, and rainfall amount and rain intensity get larger; on the other hand, in LIG’s case, the rainfall frequency of LIG decreases, and rain intensity gets larger. And interestingly, the rain amount of LIG in total domain region gets smaller while it gets larger in MR region. However, the framework of the idealized CRM experiment has its limitation in terms of its ability of explanation. Part of the limitation comes from the periodic oscillation of the experiment framework and model radiation intensity, which gives rise to the residual of model radiative-convective equilibrium, and thus it merely allows for discussion on quantitative properties since quantitative properties are too sensitive to the residual. Nonetheless, this research provides an experiment framework to discuss convection and rainfall variation trend under simulated tropical circulation, and further understanding as well as verification of related issues can be achieved based on the qualitative variation results obtained in this study.