ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-10-8037-2010 Two-moment bulk stratiform cloud microphysics in the GFDL AM3 GCM: description, evaluation, and sensitivity tests Salzmann M. 1 5 Ming Y. 2 Golaz J.-C. 2 Ginoux P. A. 2 Morrison H. 3 Gettelman A. 3 Krämer M. 4 Donner L. J. 2 Atmospheric and Oceanic Sciences Program, Princeton University, 300 Forrestal Road, Princeton, NJ 08544, USA Geophysical Fluid Dynamics Laboratory, NOAA, P.O. Box 308, Princeton, NJ 08542, USA National Center for Atmospheric Research, Boulder, Colorado, USA Forschungszentrum Jülich GmbH, ICG-I, 52425 Jülich, Germany now at: Max Planck Institute for Meteorology, Bundesstraße 53, 20146 Hamburg, Germany 30 08 2010 10 16 8037 8064 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/10/8037/2010/acp-10-8037-2010.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/10/8037/2010/acp-10-8037-2010.pdf

A new stratiform cloud scheme including a two-moment bulk microphysics module, a cloud cover parameterization allowing ice supersaturation, and an ice nucleation parameterization has been implemented into the recently developed GFDL AM3 general circulation model (GCM) as part of an effort to treat aerosol-cloud-radiation interactions more realistically. Unlike the original scheme, the new scheme facilitates the study of cloud-ice-aerosol interactions via influences of dust and sulfate on ice nucleation. While liquid and cloud ice water path associated with stratiform clouds are similar for the new and the original scheme, column integrated droplet numbers and global frequency distributions (PDFs) of droplet effective radii differ significantly. This difference is in part due to a difference in the implementation of the Wegener-Bergeron-Findeisen (WBF) mechanism, which leads to a larger contribution from super-cooled droplets in the original scheme. Clouds are more likely to be either completely glaciated or liquid due to the WBF mechanism in the new scheme. Super-saturations over ice simulated with the new scheme are in qualitative agreement with observations, and PDFs of ice numbers and effective radii appear reasonable in the light of observations. Especially, the temperature dependence of ice numbers qualitatively agrees with in-situ observations. The global average long-wave cloud forcing decreases in comparison to the original scheme as expected when super-saturation over ice is allowed. Anthropogenic aerosols lead to a larger decrease in short-wave absorption (SWABS) in the new model setup, but outgoing long-wave radiation (OLR) decreases as well, so that the net effect of including anthropogenic aerosols on the net radiation at the top of the atmosphere (netradTOA = SWABS-OLR) is of similar magnitude for the new and the original scheme.

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