Atmos. Chem. Phys. Discuss., 13, 5477-5507, 2013
www.atmos-chem-phys-discuss.net/13/5477/2013/
doi:10.5194/acpd-13-5477-2013
© Author(s) 2013. This work is distributed
under the Creative Commons Attribution 3.0 License.
Review Status
This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Monte Carlo-based subgrid parameterization of vertical velocity and stratiform cloud microphysics in ECHAM5.5-HAM2
J. Tonttila1,2, P. Räisänen1, and H. Järvinen2
1Finnish Meteorological Institute, P.O. Box 503, 00101, Helsinki, Finland
2Division of Atmospheric Sciences, Department of Physics, University of Helsinki, 00014, Helsinki, Finland

Abstract. A new method for parameterizing the subgrid variations of vertical velocity and cloud droplet number concentration (CDNC) is presented for GCMs. These parameterizations build on top of existing parameterizations that create stochastic subgrid cloud columns inside the GCM grid-cells, which can be employed by the Monte Carlo independent column approximation approach for radiative transfer. The new model version adds a description for vertical velocity in individual subgrid columns, which can be used to compute cloud activation and the subgrid distribution of the number of cloud droplets explicitly. This provides a consistent way for simulating the cloud radiative effects with two-moment cloud microphysical properties defined in subgrid-scale. The primary impact of the new parameterizations is to decrease the CDNC over polluted continents, while over the oceans the impact is smaller. This promotes changes in the global distribution of the cloud radiative effects and might thus have implications on model estimation of the indirect radiative effect of aerosols.

Citation: Tonttila, J., Räisänen, P., and Järvinen, H.: Monte Carlo-based subgrid parameterization of vertical velocity and stratiform cloud microphysics in ECHAM5.5-HAM2, Atmos. Chem. Phys. Discuss., 13, 5477-5507, doi:10.5194/acpd-13-5477-2013, 2013.
 
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