碩士 / 國立臺灣大學 / 大氣科學研究所 / 105 / 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.
| Identifer | oai:union.ndltd.org:TW/105NTU05022009 |
| Date | January 2017 |
| Creators | Xun-Ze Liu, 劉巽澤 |
| Contributors | Chung-Hsiung Sui, 隋中興 |
| Source Sets | National Digital Library of Theses and Dissertations in Taiwan |
| Language | zh-TW |
| Detected Language | English |
| Type | 學位論文 ; thesis |
| Format | 45 |
Page generated in 0.0112 seconds