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

by C. Cagnazzo, E. Manzini, M. a. Giorgetta, P. M. De F. Forster, J. J. Morcrette
Atmospheric Chemistry and Physics ()
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In order to improve the representation of ozone absorption in the\nstratosphere of the MAECHAM5 general circulation model, the spectral\nresolution of the shortwave radiation parameterization used in the\nmodel has been increased from 4 to 6 bands. Two 20-years simulations\nwith the general circulation model have been performed, one with\nthe standard and the other with the newly introduced parameterization\nrespectively, to evaluate the temperature and dynamical changes arising\nfrom the two different representations of the shortwave radiative\ntransfer. In the simulation with the increased spectral resolution\nin the radiation parameterization, a significant warming of almost\nthe entire model domain is reported. At the summer stratopause the\ntemperature increase is about 6 K and alleviates the cold bias present\nin the model when the standard radiation scheme is used. These general\ncirculation model results are consistent both with previous validation\nof the radiation scheme and with the offline clear-sky comparison\nperformed in the current work with a discrete ordinate 4 stream scattering\nline by line radiative transfer model. The offline validation shows\na substantial reduction of the daily averaged shortwave heating rate\nbias (1?2 K/day cooling) that occurs for the standard radiation parameterization\nin the upper stratosphere, present under a range of atmospheric conditions.\nTherefore, the 6 band shortwave radiation parameterization is considered\nto be better suited for the representation of the ozone absorption\nin the stratosphere than the 4 band parameterization. Concerning\nthe dynamical response in the general circulation model, it is found\nthat the reported warming at the summer stratopause induces stronger\nzonal mean zonal winds in the middle atmosphere. These stronger zonal\nmean zonal winds thereafter appear to produce a dynamical feedback\nthat results in a dynamical warming (cooling) of the polar winter\n(summer) mesosphere, caused by an increased downward (upward) circulation\nin the winter (summer) hemisphere. In addition, the comparison of\nthe two simulations performed with the general circulation model\nshows that the increase in the spectral resolution of the shortwave\nradiation and the associated changes in the cloud optical properties\nresult in a warming (0.5?1 K) and moistening (3%?12%) of the upper\ntropical troposphere. By comparing these modeled differences with\nprevious works, it appears that the reported changes in the solar\nradiation scheme contribute to improve the model mean temperature\nalso in the troposphere.

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