不同的夏季降雨型態對於極端時雨量與大氣溫度對應關係的影響

Abstract

全球暖化使得溫度上升，大氣溫度變化可能對於極端降雨型態與特性有所影響，許多前人指出全球暖化可能會改變極端降雨的頻率、降雨強度以及延時長短，從熱力學的克勞修斯-克拉佩龍關係式，可以推測當氣溫在上升1度時，大氣的飽和水氣壓會上升大約7%，所以許多過去的研究認為，由於水氣增加，極端降雨強度也會以相似比例增加，比較不像平均降雨的變化會直接受到平均環流變化的影響。但是過去研究發現，當氣溫上升到較高的氣溫度時，雖然大氣中的水氣壓隨溫度上升的速度變化不大，但是極端降雨隨溫度上升速率變成克勞修斯-克拉佩龍關係的兩倍或更多。不過當氣溫或是露點溫度上升至25度至26度以上時，隨著氣溫或露點溫度升高，極端降雨強度反而下降。以臺灣測站資料進行類似分析，並針對夏季降雨型態做分類後，可以發現在上述大氣背景氣溫與極端降雨強度關係中，在高溫所得到的負斜率關係多半是由於午後雷陣雨以及其他未歸類的降雨型態為主，這兩種降雨型態往往伴隨的日均溫度較高，但降雨強度較其他降雨型態弱，導致負斜率的發生。從降雨事件分析，極端降雨強度在降雨延時變短時也會隨之減弱。而露點溫度–極端降雨強度關係中，負斜率的露點溫度主要是由颱風與其他未歸類的降雨型態為主，屬於午後降雨型態的極端降雨所伴隨的露點溫度反而較低。在結合降雨事件延時做分析後，目前我們可推斷，午後降雨型態在負斜率的露點溫度–極端降雨強度關係中，極端降雨強度隨著露點溫度反而增加，降雨事件的延時也增加，從物理的角度來看，午後降雨是往往是局部區域的對流性降雨，小尺度對流系統降雨的延時卻較長，代表不只是大氣的溫度高，而且水氣也較為充足，因此露點較高；然而在颱風降雨型態下，較強的極端降雨強度往往伴隨較低的露點溫度，是因為當颱風降雨較強時，往往是屬於較長降雨延時的事件，整體環境溫度隨著持續降雨而降低，使露點溫度較短延時的颱風外圍雨帶的露點溫度低，對於上述負斜率有較大的貢獻。

Past studies showed that both frequency and intensity of extreme rainfall increase when atmospheric temperature increases. From basic thermodynamic principle, the saturated atmospheric moisture content increases about 7% per °C. If there is no significant change in the dynamical forcing for extreme rainfall events, the extreme precipitation might increase with temperature in a similar fashion. However, there are studies indicating that, using the local observed data from surface stations, the extreme rainfall intensity can increase twice as large as compared to the thermodynamic theory. Further, the extreme rainfall actually decreases as the background surface air temperature or dew point temperature increases when the surface air temperature exceeds 25°C or the dew point temperature reaches 26 °C. Our study tries to use the rainfall type and event duration to explain the negative relationship between temperature increase and extreme rainfall intensity decrease.After classifying the summer rainfall types, we found that the negative temperature–rainfall intensity relationship is mainly due to diurnal convection and other unclassified convective rainfall types. The daily mean temperatures are higher in those two rainfall types due to the relatively high morning temperature before the start of a heavy rainfall event while their intensities are not as strong as compared to the extreme rainfall associated with typhoons or mesoscale convective systems during the Mei-Yu front. In addition, the extreme rainfall at higher temperature is typically with a shorter duration and weaker intensity. Both contribute to the negative relationship between temperature and extreme rainfall intensity. For the negative dew-point temperature and extreme rainfall intensity relationship, the weaker extreme rainfall at higher dew-point temperature is mainly due to rainfall associated with typhoons and other unclassified convective rainfall types. With additional rainfall event duration information, we found that both the rainfall intensity and duration increases with the dew point temperature for diurnal convective rainfall. Since diurnal convective rainfall is typically a local scale system triggered by instability, both higher temperature and more moisture, therefore higher dew-point temperature favor larger extreme afternoon rainfall. The lower background dew point temperatures associated with more extreme rainfall for typhoon events is typically due to the fact that the stronger typhoon rainfall has a longer rainfall duration. The background temperatures drop substantially for those long-lasting extreme rainfall events.