Impact of an improved shortwave radiation scheme in the MAECHAM5 General Circulation Model

[摘要] In order to improve the representation of ozone absorption in thestratosphere of the MAECHAM5 general circulation model, the spectralresolution of the shortwave radiation parameterization used in the model hasbeen increased from 4 to 6 bands. Two 20-years simulations with the generalcirculation model have been performed, one with the standard and the otherwith the newly introduced parameterization respectively, to evaluate thetemperature and dynamical changes arising from the two differentrepresentations of the shortwave radiative transfer. In the simulation withthe increased spectral resolution in the radiation parameterization, asignificant warming of almost the entire model domain is reported. At thesummer stratopause the temperature increase is about 6 K and alleviates thecold bias present in the model when the standard radiation scheme is used.These general circulation model results are consistent both with previousvalidation of the radiation scheme and with the offline clear-sky comparisonperformed in the current work with a discrete ordinate 4 stream scatteringline by line radiative transfer model. The offline validation shows asubstantial reduction of the daily averaged shortwave heating rate bias(1–2 K/day cooling) that occurs for the standard radiation parameterizationin the upper stratosphere, present under a range of atmospheric conditions.Therefore, the 6 band shortwave radiation parameterization is considered tobe better suited for the representation of the ozone absorption in thestratosphere than the 4 band parameterization. Concerning the dynamicalresponse in the general circulation model, it is found that the reportedwarming at the summer stratopause induces stronger zonal mean zonal winds inthe middle atmosphere. These stronger zonal mean zonal winds thereafterappear to produce a dynamical feedback that results in a dynamical warming(cooling) of the polar winter (summer) mesosphere, caused by an increaseddownward (upward) circulation in the winter (summer) hemisphere. In addition,the comparison of the two simulations performed with the general circulationmodel shows that the increase in the spectral resolution of the shortwaveradiation and the associated changes in the cloud optical properties resultin a warming (0.5–1 K) and moistening (3%–12%) of the upper tropicaltroposphere. By comparing these modeled differences with previous works, itappears that the reported changes in the solar radiation scheme contribute toimprove the model mean temperature also in the troposphere.

[学科分类] 大气科学