| 摘要: | 在過去數十年以來，台灣極端降水變得更為頻繁且更加劇烈，午後熱對流為造成極端降水的原因之一，亦是台灣重要的夏季降水來源。在氣候變遷下，根據前人研究指出，未來強降雨會更強，使得相關研究顯得更為重要，因此了解未來午後熱對流的變化成為一個值得探討的議題。
本篇使用WRF模式進行動力降尺度以研究未來午後熱對流之變化，透過動力降尺度能得到高解析度的氣候推估資料，並且適合用於模擬劇烈天氣狀況。將年份區分為2011年至2015年、2046年至2050年、2071年至2075年以及2096年至2100年四段討論。資料來源分別使用NCEP分析場和CMIP5中的GFDL在RCP 8.5情境下的模擬結果做為WRF的初始場。
首先利用2011年至2015年資料進行驗證，結果顯示GFDL透過WRF進行動力降尺度有能力模擬午後熱對流之特徵。接著模擬未來情境，台北地區顯示平均每次午後熱對流的降水量在未來會隨年份明顯增加，台北部份地區從原本降雨量為20至30mm增加到30至40mm。造成降水強度增強的原因主要包含：(1)大環境的西南風增強，(2)淡水河口輸送進入之海風增強，(3)台北地區日間輻合量增加，(4)850hPa混和比增加以及(5)垂直方向地面至500hPa相當位溫增加，且上述變化隨時間愈接近21世紀末增加愈為劇烈且明顯。另外，若將降雨強度前10%的個案取出並與所有午後熱對流平均狀況討論，可以發現上述氣象因子的變化會更為明顯。將降雨由小至大排列，發現小雨的降雨量無明顯變化，但中位數約增加25%，最強之個案降雨量增加約2倍，因此可以推測未來極端降雨會愈趨劇烈。
本研究利用WRF進行動力降尺度發現未來午後熱對流會增強，不論是動力、熱力或水氣條件都扮演重要角色。研究結果顯示得知未來午後熱對流的情況與現在之異同，並了解未來極端降雨狀況的變化，以及其變化之原因。
;In the past few decades, extremely severe rainfall events have become more frequent and intense. Most of them are due to the afternoon thunderstorms in summer. Afternoon thunderstorm causes floods and landslides, which not only hurts the sociality of humans, but also the biosphere. Hence, the most important topic is to understand the detail of rainfall changes in the future.
In this study, WRF model will be used to simulate the long-term projection of precipitation. First, we divide the 21st Century into four parts, which are 2011-2015, 2046-2050, 2071-2075 and 2096-2100. By using CMIP5 under RCP8.5 scenario, a standard of changes of the weather conditions, we utilize GFDL model’s results as WRF input. The results show that the intensity of the afternoon thunderstorm in Taipei will increase. There are five main reasons that cause changes in afternoon thunderstorm: (1) Stronger southwesterly, (2) stronger sea breeze from Tamsui River, (3) stronger convergence in the daytime in Taipei basin, (4) higher mixing ratio at 850hPa, (5) higher equivalent potential temperature from surface to 500hPa. The difference between the present and the future will become larger as time approaches the end of the 21st century.
Furthermore, the strongest 10% afternoon thunderstorm cases have been selected. After rearranging all the cases sorted by rainfall amount, we find out that the median will increase by 25% and the rainfall of the most extreme case will double. The extreme afternoon thunderstorm rainfall will become more intense in the future.
To sum up, this study mainly focuses on the difference between thunderstorm days in the future and in the present. The change in dynamic conditions, thermal conditions, and water vapor can lead to the intensity change of the afternoon thunderstorms over Taipei basin in the future. |
