Atmos. Chem. Phys. Discuss., 9, 16607-16682, 2009
© Author(s) 2009. This work is distributed
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Cirrus clouds in a global climate model with a statistical cirrus cloud scheme
M. Wang1,* and J. E. Penner1
1Department of Atmospheric, Oceanic, and Space Sciences, University of Michigan, USA
*now at: Pacific Northwest National Laboratory, Richland, Washington, USA

Abstract. A statistical cirrus cloud scheme that accounts for mesoscale temperature perturbations is implemented into a coupled aerosol and atmospheric circulation model to better represent both cloud fraction and subgrid-scale supersaturation in global climate models. This new scheme is able to better simulate the observed probability distribution of relative humidity than the scheme that was implemented in an older version of the model. Heterogeneous ice nuclei (IN) are shown to affect not only high level cirrus clouds through their effect on ice crystal number concentration but also low level liquid clouds through the moistening effect of settling and evaporating ice crystals. As a result, the change in the net cloud forcing is not very sensitive to the change in ice crystal concentrations associated with heterogeneous IN because changes in high cirrus clouds and low level liquid clouds tend to cancel. Nevertheless, the change in the net radiative flux at the top of the atmosphere due to changes in IN is still large because of changes in the greenhouse effect of water vapor caused by the changes in ice crystal number concentrations. Changes in the magnitude of the assumed mesoscale temperature perturbations by 25% alter the ice crystal number concentrations and radiative fluxes by an amount that is similar to that from a factor of 10 change in the heterogeneous IN number concentrations.

Citation: Wang, M. and Penner, J. E.: Cirrus clouds in a global climate model with a statistical cirrus cloud scheme, Atmos. Chem. Phys. Discuss., 9, 16607-16682, doi:10.5194/acpd-9-16607-2009, 2009.
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