未來長期氣候變遷對西北太平洋超級颱風個案降水潛在影響之模擬研究

Abstract

氣候變遷所衍生的極端天氣現象是人類在未來須面對的環境議題之一，近年來逐漸在國際間獲得重視，而對於未來的災害推估也是學界近期正努力鑽研的一大領域。對於西北太平洋區域而言，颱風的肆虐往往對陸地造成嚴重災損，除風力驚人，其挾帶的劇烈降水更具有破壞力，尤其薩菲爾·辛普森等級C4或C5的強颱更是不可言喻。為探討未來暖化下強烈颱風的降水變化，本研究以此為動機，自Thean (2021) 選取雲解析風暴模式CReSS模擬之梅姬(2010)、海燕(2013)、莫蘭蒂(2016)三個典型超級颱風個案及未來21世紀末RCP 4.5及RCP 8.5情境的模擬實驗結果，加以分析相關降水參數及環流結構變異，另使用水收支做更進一步的定性與定量診斷作為研究主軸。研究結果指出，三個案在未來情境模擬下，背景水氣增加與垂直次環流上升運動的加強導致颱風劇烈降水皆有顯著成長，在0~400公里半徑範圍內，經全模擬時間平均，梅姬颱風分別在RCP 4.5及RCP 8.5情境下增加6.04%、12.85%，海燕颱風分別增加12.85%、29.6%，莫蘭蒂分別增加4.63%、6.3%；若縮小至0~200公里半徑，梅姬颱風於兩環境下分別成長11%、19.99%，海燕颱風分別增加10.42%、19.43%，莫蘭蒂颱風則成長5.92%、19.58%。各類水象粒子的軸對稱垂直剖面分布也呼應到降水，水氣、降水粒子在內核有顯著增長，而冰相粒子因溶解層提高，生成高度會些許增加且有總含量減少的可能性。在水收支分析上，考慮不同個案的環流半徑與計算精確度，三個案使用不同半徑與時長進行計算，不過皆有一致的結果：在暖化情境下，密度輻合增強，密度平流加強，粒子絕對溼度時變率亦增加，而整體水收支以水氣的密度輻合為最大主導項。本研究另有分析中低層0~5.5公里之可降水量與水平輻合積分，儘管發現三個案於不同暖化情境下誘發降水的偏好歸因不同，一部分實驗的降水差異來自水平輻合的增大，另一部分則來自可降水量的增加，不過兩機制皆會對未來超級颱風環流降水增加的可預期性是不變的。

The extreme weather under climate change is one of the environmental problems that the humanity will face in the future. Related researches of estimating future disasters are still ongoing recently. In western North Pacific (WNP), typhoons often cause huge damage. In addition to the strong wind, its torrential precipitation is even more destructive, especially for category 4 and 5 super-typhoons. To explore the changes in precipitation caused by intense typhoons under future warming scenarios, this study selected three super-typhoon cases from Thean (2021): Megi (2010), Haiyan (2013), and Meranti (2016). This study uses cloud-resolving storm simulator (CReSS) to reproduce these cases and simulate their precipitation at the end of the 21st century under RCP 4.5 and RCP 8.5 warming scenarios combined with Coupled Model Intercomparison Project Phase 5 (CMIP5) data. Following analysis in this research includes the variations of relevant parameters, circulation structures, and water budget.The results indicate the increase of background moisture and the strengthening of upward motion in the typhoon circulation cause significant increase in precipitation in future scenarios. In radii of 0-400 km, the growth of Typhoon Megi precipitation under RCP 4.5 and RCP 8.5 is 6.04% and 12.85%, respectively; Typhoon Haiyan grows by 12.85% and 29.6%; Typhoon Meranti also increases by 4.63% and 6.3%. For the inner core of 0-200 km, rainfall of Megi grows by 11% and 19.99% under the two scenarios; Haiyan also grows by 10.42% and 19.43%, and precipitation of Meranti has 5.92% and 19.58% increases, respectively. The mean radial profiles of water species also reflect the rainfall distributions: the water vapor and precipitation particles significantly increase in the inner core, and due to the rising of melting layer, there is a possibility that the generation height of ice-phase particles will increase and the quantities will slightly decrease.As for the water budget, considering different circulation radius and computational accuracy, these three cases were calculated with different radii and durations, but all the results are consistent. In the warming future, total water transport would increase, including the enhancement of flux convergence (i.e. summation of density convergence and density advection), and absolute humidity change rate; meanwhile, the domination term of water budget is water vapor density convergence. In addition, by calculating precipitable water (PW) and integrated horizontal convergence (IHC) within the height of 0-5.5 km, this study shows the attribution inducing precipitation in different experiments is different: rainfall variation in some experiments come from the changes in IHC, and some of which are caused by PW. Nevertheless, the fact that the increase of future super typhoon rainfall by the interaction between these two mechanisms remains unchanged.