| 摘要: | IPCC報告指出，在全球暖化氣候變遷影響下，極端事件可能會有增加的趨勢。Trenberth et al.(1999,2003)指出全球暖化可能會改變降雨的頻率、強度和延時；假設低層對流層的相對濕度保持一常數，根據Clausius–Clapeyron equation熱力關係式，計算出全球的水氣隨溫度的增加大約為7%K -1。另一方面，全球強降水的強度和頻率也有逐漸增加的趨勢，而且趨於兩極化，即雨季越濕、乾季越乾，但這些變化會隨著地區的改變而有所不同。
本篇研究運用Liu et al.(2009)提出的年際變異法(Interannual Difference Method,IDM)，取得降雨強度與全球溫度距平（包含聖嬰等年際變化）關係上的定量結果：在1961-2015年間，台灣地區雖然總降雨量沒有明顯變化，但台灣的強降雨隨全球暖化有顯著增加，其中最強的前10%的降雨隨全球溫度增加約78%K -1，而十個強度的降雨隨溫度的變化量，隨降雨強度越強，有遞增的趨勢；因同一時期，中、小雨相對的減少，幾乎抵銷了強降雨增加的雨量，導致整體的總降雨量並沒有太大改變。另外，分析不同季節的降雨特性，發現降水強度最強的前10%的降水皆是增加，強度最弱的後10%的降水則都是減少，其降雨強度變化的趨勢與年變化趨勢大致一致；儘管增減幅度不一，甚至在台灣的雨季-梅雨季時的總降雨量是減少的，說明了台灣地區降雨隨氣候暖化的變化趨勢主要是受到「強降雨越強、弱降雨越弱」的機制驅動。
由過去研究顯示，造成劇烈降雨的氣象因子多且複雜。本篇為驗證台灣降雨的改變是因全球暖化，進而水氣增加等原因所導致，我們也運用再分析資料計算了台灣附近地區低層的相當位溫、垂直速度、水氣通量、可降水量等變化，結果呈現受到暖化影響，溫度、相當位溫及可降水量增加，同時在低層的水氣通量也因盛行風向而增多，加上有利的上升運動，特別是降雨強度前30%的降水顯著比過去增加。根據跨政府氣候變遷委員會第五次氣候變遷評估報告 (2013)，未來降雨趨勢乾濕對比將越來越明顯。
;The IPCC report indicates that extreme events may increase under the climate change. Trenberth et al.(1999,2003) pointed out that global warming may change the frequency, intensity and timing of rainfall. Assume that the lower part of the troposphere has a constant amount of relative humidity, by using Clausius-Clapeyron equation , it could be calculated that the water vapor in the whole world would increase approximately 7%k-1. On the other hand, significant increase of heavy precipitation and decrease of light and moderate precipitation have been found in many areas of the world, that is, wet-get-wetter and dry-get-drier mechanism (WWDD) (Zhou et al. 2011; Chou et al. 2013). But these changes will vary with the region.
In this study, we use the Interannual Difference Method (IDM) proposed by Liu et al.(2009) to evaluate quantitative changes of rainfall intensity due to global temperature changes (including El Nin ̃o). During the period from 1961 to 2015, the heavy precipitation in Taiwan showed an increase due to global warming. The top 10% heaviest precipitation increased by about 78% K-1,but there were significant decreases in the light and moderate precipitation in the same period,so that the trends of Taiwan annual total precipitation did not change significantly. In addition,we also analyzed the trend of Taiwan’s precipitation in different seasons. We found that the amounts of top 10% heaviest precipitation have all increased ,but the amounts of bottom 10% lightest precipitation have all decreased. Even though the change in each season’s annual total precipitation in response to one degree Kelvin increase in the global mean temperature are quite different. Therefore we conclude that the seasonal precipitation in Taiwan is controlled by the heavy-precipitation-get-heavier and light-precipitation-get-lighter mechanism (HHLL) (Liu et al.,2016),rather than the wet-get-wetter and dry-get-drier mechanism (WWDD).
The meteorological factors affecting the strong rainfall are very complicated. We have analyzed changes in equivalent temperature, moisture flux and precipitable water in the western North Pacific between the first five years of data (1961-1965) and the last five years (2011-2015) to provide further support for the theory of global warming as the cause of the increases in extremely heavy precipitation. |
