| 摘要: | 在過去研究中，對於全球暖化下熱帶降水之變化及物理機制的探討有幾種不同的理論，其中最廣為人知的理論有Chou and Neelin (2004)和Held and Soden (2006) 所提出之 “wet-get-wetter” 及 Xie et al. (2010) 所提出之 “warmer-get-wetter” 理論。本篇研究使用高解析度大氣模式High Resolution Atmospheric Model (HiRAM)探討在RCP8.5暖化情境下，熱帶區域降水之變化，並進一步去了解影響降水變化之物理機制。
研究結果顯示，在全球暖化下，赤道東太平洋地區為最敏感區域，降水增加最為顯著，而降水減少則圍繞上述區域呈現弓狀的分布。整個熱帶海洋上，只有略微超過1/2的面積符合 “wet-get-wetter” 理論。利用水氣收支分析，我們得到熱帶降水變化主要與對流增強與減弱之動力效應項-⟨ω^′ ∂_p q ̅ ⟩有關，而熱力效應項-⟨ω ̅∂_p q^′ ⟩則扮演加強或抵消降水增加的作用。在符合 “wet-get-wetter” 理論的WW (wet-get-wetter)及DD (dry-get-drier)區域，其降水變化為熱力作用與動力效應合作所致；而在違背 “wet-get-wetter” 理論的WD (wet-get-drier)及DW (dry-get-wetter)區域，其降水變化主要為動力效應所主導，熱力效應則抵銷一部分動力效應。
另外，比較赤道東太平洋氣候敏感區域之降水變化機制差異，使用濕穩定度指標Normalized Gross Moist Stability (NGMS)及對流垂直變化情形作分析，結果顯示：在氣候上升運動區有深對流加深的情形(more top-heavy convection)，並伴隨著濕穩定度指標上升之現象(NGMS變化為正值)；而在氣候下沉運動區有淺對流增強的趨勢(more bottom-heavy convection)，並伴隨濕穩定度指標降低之現象(NGMS變化為負值)。濕穩定度指標變化之差異可解釋為什麼在全球暖化下，氣候上較為穩定的赤道東太平洋對流下沉區降水變化為何能與氣候上較為不穩定的對流上升區有相當幅度降水變化的原因。
最後，熱帶降水變化的分布與海表面溫度(SST)變化及雲輻射效應(CRE)變化相似，因此我們嘗試利用Temperature-Cloud-Radiation正回饋機制去解釋赤道東太平洋地區之強降水變化：當SST增暖幅度最大區域，空氣中水汽含量增加也最多；空氣中水氣含量增加最多區域，成雲效率也最高；成雲效率最高區域，CRE作用也最為顯著；CRE作用最顯著區域造成向下長波輻射加熱(溫室效應)，可進一步維持暖化後的海溫和氣溫型態，形成正回饋作用。
;In previous studies, there are various theories about tropical precipitation changes and mechanisms under global warming. The most well-known hypotheses are the “wet-get-wetter” theory from Chou and Neelin (2004) and Held and Soden (2006), and the “warmer-get-wetter” theory from Xie et al. (2010). In this study, the High Resolution Atmospheric Model (HiRAM) is used to investigate the changes of precipitation in the tropical region under the RCP8.5 warming scenario, and to further understand the physical mechanism affecting the precipitation changes.
The results show that the equatorial eastern Pacific is the most sensitive region in precipitation increase under the influence of global warming, while the precipitation reduction exhibits a hook-like distribution surrounding the above region. For the tropical atmosphere above oceans, slightly over 1/2 of the area follows the “wet-get-wetter” theory. From the moisture budget, we find that in WW and DD regions where follows the “wet-get-wetter” or “dry-get-drier” hypothesis generally holds, the precipitation change is caused by the cooperation of thermodynamic effect, (-⟨〖ω ̅∂〗_p q^′ ⟩), and the dynamic effect, (-⟨ω^′ ∂_p q ̅ ⟩). In regions of DW and WD disobeying the “wet-get-wetter” hypothesis, the dynamic effect dominates the moisture budget and shows opposite signs to the thermodynamic effect.
Besides, the difference in precipitation change mechanism in the target domain is compared, and changes in the Normalized Gross Moist Stability (NGMS) index and convective vertical structure are analyzed. The results show that there is an enhanced top-heavy structure of convection with positive NGMS change in the mean ascending region and an enhanced bottom-heavy structure of convection with negative NGMS change in the mean descending region. The differences in NGMS over the equatorial eastern Pacific may explain why the more stable mean descending region can have a similar size of precipitation changes as the more unstable mean ascending region under global warming.
Besides, the distribution of tropical precipitation changes is similar to the change of sea surface temperature (SST) and the cloud radiation effect (CRE). Accordingly, we try to use a positive Temperature-Cloud-Radiation feedback mechanism to explain the large positive precipitation changes over the equatorial Pacific under global warming. The results show that as the SST warms, more water vapor may exist in the atmosphere; as more water vapor exists in the atmosphere, more clouds may generate; as more clouds generate, the CRE effect may increase. The most significant region of the CRE effect causes the downward longwave radiation heating (greenhouse effect), which can further maintain the sea surface temperature and temperature patterns under warming and form a positive feedback loop. |
