全球暖化的效應使地球表面溫度以及低層對流層溫度上升，同時也改變了全球的水 文循環。暖化的大氣可以容納更多水氣，根據 Clausius-Clapeyron relation可以得知，全 球平均地表溫度每上升攝氏一度，在相對溼度為常數的條件下，水氣將會增加 7%。藉 由分析衛星資料中的水氣，也得到與 Clausius-Clapeyron relation類似的變化。許多研究 使用氣候模式預估暖化下大氣水氣量，結果也都相當一致的約為 7.5% K-1，但是模式中 降雨的增加率則為 1~3% K-1。相關的降雨衛星資料分析預測全球平均降雨量變化應該要 與水氣的變化一致，因此是什麼機制造成模式中降雨率的變化趨勢低於蒸發率是目前正 在研究的課題之一。 除了機制的探討外，全球平均降雨量變化趨勢仍然有很大的不確定性存在。以能量 平衡的觀點來看，全球平均降雨量應該與大氣長波輻射量成正比，也就是說當大氣長波 輻射增加時會冷卻大氣，這時需要更多降雨來提供熱量（潛熱）來維持大氣的能量平衡， 反之亦然。在模式中有數個會影響大氣長波輻射的因子，其中雲量（特別是高雲）的模 擬不確定性最大，也是最有可能造成模式中降雨偏差的因子之一，因此本研究將使用 UCLA QTCM為工具探討在暖化情境下，雲量的變異度對降雨率的模擬誤差有多大的影 響，同時進一步探討相關的大環境場可能出現的改變，例如：大尺度環流的變化。 The influences of global warming include the warming of surface and lower troposphere and also the changes of hydrological cycle. A warmer atmosphere may contain more water vapor. According to Clausius-Clapeyron equation and assuming the relative humidity is constant, the rate of water vapor increasing is 7% per Kelvin. From the analysis of satellite data, scientists have researched the same result as expected from the Clausius-Clapeyron equation. Studies analyze water vapor increasing rate in warming scenario using global climate models (GCMs) show that the increasing rates of all models are consistent as 7.5 % per Kelvin. However, the increasing rate of precipitation of all models is 1—3 % per Kelvin. The result from satellite analysis predicts that the change of precipitation rate should have the same rate as the evaporation. Thus, through what mechanisms that all GCMs simulate much lower precipitation rate change is one of the most active research topics. Except the mechanism, there is also large uncertainty on the change of precipitation rate. Base on the constraint of global energy conservation, the global mean precipitation rate should be proportional to longwave cooling from the atmosphere and vise versa. In a GCM, there are a few factors that may influence radiative cooling and one of the most uncertain factors is the cloud amount. In this research, we propose to use UCLA QTCM to investigate the impact of cloud amount on the change of precipitation rate. This research may extend to include the rate change of large-scale circulation in the future.
