未來氣候變遷對西北太平洋強烈颱風可能影響之模擬研究

Abstract

氣候變遷影響著環境的趨勢，使我們開始致力於減少溫室氣體。因此，有些研究嘗試透過模式建立各種情境，以探索未來氣候變遷對環境的影響。由於未來氣候變遷中，逐漸升高的溫度、水氣有利於颱風形成，所以許多科學家進行了颱風的實驗，並驗證了它們在未來氣候變遷的環境中，確實有強度增強、雨量增加、暴潮高度上升的趨勢。這篇論文使用建模的方式，針對 21 世紀西北太平洋最強烈的，而且擁有至少 900 hPa 最低中心海平面氣壓的颱風進行模擬研究。2010 年的強烈颱風梅姬、2013 年的強烈颱風海燕、2014 年的強烈颱風黃蜂、2015 年的強烈颱風蘇迪勒、2016 年的強烈颱風莫蘭蒂、2018 年的強烈颱風玉兔，是本論文進行實驗的 6 個颱風，以討論未來氣候變遷對它們強度的影響為何。論文的實驗設計，是將這些颱風移植到 2 種 21 世紀末的未來環境，它們是將 4.5 W m⁻² (RCP4.5) 及 8.5 W m⁻² (RCP8.5) 溫室氣體排放濃度投射到 21 世紀末的 2 個預估情境。這裡使用的代表濃度路徑，是由 38 個第 5 階段耦合模式對比計畫氣候模式預估的資料。資料裡有稍微增多的重力位高度、大氣溫度、水氣、海表面溫度，以及些許的反氣旋風場，這些差異值將被加入未來環境的初始場中。以這些未來環境重力位高度、風場的初始場為例，它們是不利於颱風發展的負面因素。雖然這些初始場的差異很小，但它們的差異值在模式裡一段時間後變大了。2.5 km 解析度雲解析風暴模式的模擬結果顯示，颱風在未來的環境將有更弱的強度。起初，未來環境較高的溫度降低了水氣的飽和度。隨著颱風的發展，在後面階段模擬裡，溫度梯度逐漸變小。因此，環境的穩定度提升了。在針對特定變數進行的敏感度實驗裡，梅姬對水氣有較高的敏感度，但海燕對水氣有較低的敏感度。在軸對稱的分析裡，未來氣候變遷環境中的颱風內核發展半徑正在擴大，因此它們強度減弱了。儘管海燕是所有研究個案中擁有最大的切向風、垂直速度、水氣、相對濕度，但它們很早就被耗盡了，導致海燕的強度急劇的減弱了。另外，現代環境中梅姬較大的剪力速率、未來環境中海燕較大的方向風切使它們強度減弱了。The circumstances of climate change underway have brought our concern to reduce emission of greenhouse gases. For that reason, there are different attempts on modelling studies to answer the possible impacts of future climate change. Since rising temperature and increasing water vapour in climate change are favourable for typhoons, a number of scientists have verified the outcome of stronger intensity, intensified rainfall, and rising storm surge height from typhoons in future climate change. In this thesis, modelling study is conducted for the strongest Western North Pacific typhoons in 21ˢᵗ century, which consist at least 900 hPa minimum central pressure. Typhoon Megi in 2010, typhoon Haiyan in 2013, typhoon Vongfong in 2014, typhoon Soudelor in 2015, typhoon Meranti in 2016, and typhoon Yutu in 2018 are the 6 study cases selected for discussions on possible impacts of future climate change on their intensity. They are shifted to 2 environments at the end of 21ˢᵗ century, which overshoot 4.5 W m⁻² (RCP4.5) and 8.5 W m⁻² (RCP8.5) greenhouse gases emission to the end of 21ˢᵗ century. The Representative Concentration Pathway used is published by 38 Coupled Model Intercomparison Project phase 5 models. There are slightly increased geopotential height, atmospheric temperature, water vapour, sea surface temperature, and a slight anticyclonic wind field added into initial fields of the future environments. It is known that the initial geopotential height and wind field are negative factors. Although the changes are small, they are enlarged after some time. Results in 2.5 km resolution simulations with Cloud Resolving Storm Simulator have shown them weaker in future. Initially, the increasing temperature has lowered saturation of water vapour. Across simulations, temperature gradients are decreasing. So the environment is stabilised. From sensitivity experiments on certain variables, Megi is more sensitive to water vapour, but Haiyan is less sensitive to it. Axisymmetric analyses have shown the radius of inner convection for typhoons is expanding in future, which has caused them weaker. Even though Haiyan has the largest increment in tangential wind, vertical velocity, water vapour, and relative humidity, but they are exhausted immediately in early stage of deepening, eventually it is drastically weakened. In addition, the larger directional shears in present day Megi and the larger speed shear in future Haiyan have weakened their intensity.