| 摘要: | 水氣為大氣中最重要的溫室氣體，它吸收地球及大氣輻射並減少紅外輻射向外太空的散逸，對地球系統造成了溫室效應。而雲亦對地球系統有重要的影響，因為各模式間的演算法差異或是雲特性掌握不足等因素，讓模擬雲輻射的效應不一。由於水氣和雲的輻射作用會左右輻射能量收支，影響了氣候的變化，為了瞭解水氣和雲對輻射的重要性並鑑別出模式模擬的差異原因，進行了本研究。
本研究選擇常用的輻射傳輸模式CLIRAD和RRTMG為研究的對象。並以中緯度夏季和中緯度冬季的大氣剖面來模擬水氣和雲的輻射效應。在水氣的模擬中，調整每層厚24hPa內的水氣混合比15%，分析水氣的增加對大氣層頂(TOA)外出長波輻射(OLR)的影響。而雲的模擬中，先將雲區分為高雲、中雲、低雲，分析其對於TOA、地表及大氣的輻射效應，最後對影響雲輻射模擬的因素(粒徑、太陽天頂角)進行敏感度測試。
在水氣的模擬結果中，發現中對流層水氣增加時OLR減少較多，即暖化作用最為顯著，而上層和下對流層的水氣增加時OLR減少較少。在兩模式的模擬差異中，發現在中、下對流層水氣的暖化作用，CLIRAD較RRTMG大。而在雲的模擬結果中，發現雲的短波冷卻效應相對長波的加熱作用明顯。兩模式的模擬差異中以RRTMG模擬的厚高雲(冰晶)的短波冷卻效應明顯較CLIRAD大(12%左右的相對差異)；長波方面，各種雲的輻射效應在模式間的差異不明顯，僅約5%的相對差異。
至於在粒徑和太陽天頂角敏感度的測試中，發現隨粒徑的增大，雲的長、短波暖化/冷卻輻射效應則會隨之減弱。分析模式間的差異，發現短波的模擬以高的雲冰量和大粒徑的高雲最為顯著 (RRTMG的冷卻效應大於CLIRAD)。除了低雲在較高的雲水量下模擬的差異較為顯著外，長波的結果則相對短波具較佳的一致性。太陽天頂角的結果則發現，高雲的輻射效應兩模式皆大約在60度下達到極值，但低雲卻隨著角度的增加而減弱。
分析本研究的模擬結果中，了解到兩模式對於中對流層水氣增加造成的暖化作用較上對流層及下對流層水氣增加為敏感。又發現RRTMG模擬雲的非對稱因子較CLIRAD小，因此造成短波冷卻效應較CLIRAD大。而RRTMG忽略長波的散射作用造成了RRTMG雲的長波暖化效應比CLIRAD小。
;Atmospheric water vapor is the most important greenhouse gas. It absorbs earth and atmosphere’s radiation ,reduces the outgoing long wave radiation (OLR) , and heats on the Earth system. Clouds also have an important impact on the Earth systems. Due to differences between radiation models or inadequate understanding of cloud microphysical and optical properties, calculations of water vapor and cloud radiative effects using different radiation models lead to inconsistent results. Therefore, it’s important to investigate radiative effects of water vapor and clouds and identify the reasons for causing the differences in model simulations.
Two widely used radiative transfer models (CLIRAD and RRTMG) were chosen in this study for investigating water vapor and cloud radiative effects in two atmospheres typical of mid-latitude summer and mid-latitude winter. In the simulation of water vapor radiative effect, the water vapor mixing ratio was adjusted by 15 % within each layer of 24 hPa thick and the impacts of water vapor on OLR were investigated. In the simulation of cloud radiative effect, clouds were divided into three types of high, middle, low cloud and radiative effects of these clouds on top of atmosphere (TOA), the surface, and the atmosphere were analyzed. Finally, studies were carried out to understand the sensitivity of radiation to cloud particle size and the solar zenith angles.
In calculations of the water vapor greenhouse warming, it is found when water vapor increases in middle troposphere (400 – 800hPa), the OLR decreases significantly (significant greenhouse effect). However, the OLR is less sensitive to changes of water vapor in the upper and lower troposphere. Results also show that compared to the RRTMG model calculations, the CLIRAD calculations of the greenhouse effect due to middle and lower tropospheric water vapor is greater. In calculations of cloud radiative effects, results show that generally clouds have a shortwave cooling effect and a long wave heating effect. The shortwave cooling effect of thick high cloud calculated by RRTMG than that calculated by CLIRAD (12% relative difference). The difference in cloud long-wave warming effect between these two models is small (only about 5%).
In cloud particle size and solar zenith angle sensitivity tests, it found when the particle size increases, cloud shortwave and long-wave radiative effects decrease. Analyzing models results show that high cloud with higher cloud ice amount and larger particle size has largest difference in shortwave radiation simulation (shortwave cooling effect calculated by RRTMG is greater than that calculated by CLIRAD.). Results of low cloud long-wave calculations by these two models are consistent except in condition of higher cloud amount having much significant difference. results of solar zenith angle (SZA) test show that high cloud radiation effect has critical value at 60 degree and low cloud radiation effect decrease with SZA increase.
It is found that the OLR is more sensitive to the water vapor increase in middle troposphere than in the upper and lower troposphere. It is also found that RRTMG has a smaller cloud particle asymmetry factor than CLIRAD, causing a larger cloud cooling effect. Finally, RRTMG ignores longwave scattering effect that leads to a smaller cloud longwave radiative effect than CLIRAD. |
