不同氣候情境對長江流域暖季東移雨帶的影響與評估

Abstract

觀測資料顯示，5月至7月在青藏高原東部產生的降雨，常有沿長江流域(29°N–34°N, 100°E–120°E) 向東邊傳播的現象。針對此現象，本論文主要探討的議題有二。其一為利用Weather Research and Forecasting (WRF) 區域模式，對2009年5月的長江流域東移雨帶進行三種不同氣候條件下的模擬，以求瞭解氣候變遷對長江流域東移雨帶可能造成的影響。其二為探討在計算資源充足的情況下，如何有效提高WRF模式模擬長江流域東移雨帶個案的能力。 針對議題一，本論文的研究結果發現： (1) 在現今的氣候條件下（現代情境），WRF對長江流域東移雨帶的模擬結果與觀測資料相似； (2) 在加入過去氣候變異的情況下（過去情境），WRF模擬的東移雨帶個數明顯減少，東移距離明顯變短，降雨強度明顯變弱； (3) 在加入未來氣候變異的情況下（未來情境），WRF模擬的東移雨帶個數、東移距離與現代情境下的模擬結果無明顯差異，但降雨強度明顯增強。分析平均環流場的差異顯示，在過去情境下，高層西風的減弱會造成東移雨帶個數減少與東移距離變短，而水氣傳輸的減弱是造成降雨強度變弱的主因。在未來情境下，因為高層西風無明顯變化，所以東移雨帶個數與東移距離與現代情境下的模擬結果無明顯差異，而水氣傳輸的增強(減弱)會造成東移雨帶降雨強度變強(變弱)。針對議題二，本論文選用不同解析度、不同嵌套方式、不同積雲參數法及雲物理方案，對WRF模擬長江流域東移雨帶的能力進行敏感度測試。結果證明提高水平解析度最能夠顯著改善模擬長江流域東移雨帶的結果。在相同解析度的情況下，雙向嵌套優於單向嵌套。而使用不同的積雲參數法或雲物理方案，則對模擬結果無顯著影響。Using WRF model driven by three different climate conditions (theoriginal, the past and the future) for May 2009 as an example, this study examines the effects of climate change on the eastward propagating rainfall events over the Yangtze River Valley. Results show that (1) under the original climate condition (i.e. Control Run), the characteristics of eastward propagating rainfall events simulated by the model are similar to the observations; (2) with the effects of past climate change (i.e. Past Run), the simulated number of propagating event is fewer; the propagating distance is shorter; and the rainfall intensity is weaker than the Control Run; (3) with the effects of future climate change (i.e. Future Run), only the rainfall intensity is stronger than the Control Run. Diagnoses on the circulation change suggest that (1) the change in upper-level westerly is responsible for the change in occurrence frequency and propagating distance of rainfall events, and (2) the change in moisture flux convergence is responsible for the change in rainfall intensity over the Yangtze River Valley. Furthermore, analyses also suggested that increasing the resolution of the model can improve model’s capability in simulating the rainfall in the Yangtze River Valley, but changing the schemes of physics processes has less impact.