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Sensitivity of a GCM Climate to Enhanced Shortwave Cloud Absorption

J. T. KiehlNational Center for Atmospheric Research, Boulder. Colorado

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J. J. HackNational Center for Atmospheric Research, Boulder. Colorado

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M. H. ZhangInstitute for Terrestrial and Planetary Atmospheres, Marine Science Research Center, State University of New York, Stony Brook, New York

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R. D. CessInstitute for Terrestrial and Planetary Atmospheres, Marine Science Research Center, State University of New York, Stony Brook, New York

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Abstract

Recent studies by Cess et al. and Ramanathan et al. find that clouds absorb significantly more shortwave radiation than currently modeled by general circulation models. Initial calculations for the global annual shortwave energy budget imply that including the additional shortwave cloud absorption leads to an additional 22 W m−2 absorption in the atmosphere, with an equivalent reduction of shortwave flux at the surface. The present study investigates the climate implications of enhanced cloud absorption with the use of the National Center for Atmospheric Research Community Climate Model. The GCM response to this forcing is to warm the upper troposphere by as much as 4 K. The additional shortwave heating in the upper troposphere reduces the strength of the Hadley circulation by 12% and leads to lower surface wind speeds in the Tropics. In turn, these lower wind speeds lead to a 25 W m−2 reduction in surface latent heat flux.

Abstract

Recent studies by Cess et al. and Ramanathan et al. find that clouds absorb significantly more shortwave radiation than currently modeled by general circulation models. Initial calculations for the global annual shortwave energy budget imply that including the additional shortwave cloud absorption leads to an additional 22 W m−2 absorption in the atmosphere, with an equivalent reduction of shortwave flux at the surface. The present study investigates the climate implications of enhanced cloud absorption with the use of the National Center for Atmospheric Research Community Climate Model. The GCM response to this forcing is to warm the upper troposphere by as much as 4 K. The additional shortwave heating in the upper troposphere reduces the strength of the Hadley circulation by 12% and leads to lower surface wind speeds in the Tropics. In turn, these lower wind speeds lead to a 25 W m−2 reduction in surface latent heat flux.

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