Two-moment bulk stratiform cloud microphysics in the GFDL AM3 GCM: description, evaluation, and sensitivity tests 1Atmospheric and Oceanic Sciences Program, Princeton University, 300 Forrestal Road, Princeton, NJ 08544, USA
2Geophysical Fluid Dynamics Laboratory, NOAA, P.O. Box 308, Princeton, NJ 08542, USA
3National Center for Atmospheric Research, Boulder, Colorado, USA
4Forschungszentrum Jülich GmbH, ICG-I, 52425 Jülich, Germany
*now at: Max Planck Institute for Meteorology, Bundesstraße 53, 20146 Hamburg, Germany
Received: 16 Feb 2010 – Published in Atmos. Chem. Phys. Discuss.: 08 Mar 2010 Abstract. 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.
Revised: 29 Jul 2010 – Accepted: 09 Aug 2010 – Published: 30 Aug 2010
Citation: Salzmann, M., Ming, Y., Golaz, J.-C., Ginoux, P. A., Morrison, H., Gettelman, A., Krämer, M., and Donner, L. J.: Two-moment bulk stratiform cloud microphysics in the GFDL AM3 GCM: description, evaluation, and sensitivity tests, Atmos. Chem. Phys., 10, 8037-8064, doi:10.5194/acp-10-8037-2010, 2010.