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|Keywords: ||輻射傳輸;水氣與輻射;雲輻射效應;輻射傳輸模式;radiative transfer;water vapor and radiation;cloud radiative effect;radiative transfer model;CLIRAD;RRTMG|
|Issue Date: ||2014-08-11 18:12:12 (UTC+8)|
;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.
|Appears in Collections:||[大氣物理研究所 ] 博碩士論文|
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