Atmos. Chem. Phys. Discuss., 13, 11789-11825, 2013
www.atmos-chem-phys-discuss.net/13/11789/2013/
doi:10.5194/acpd-13-11789-2013
© 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.
Microphysical process rates and global aerosol-cloud interactions
A. Gettelman1, H. Morrison1, C. R. Terai2, and R. Wood2
1National Center for Atmospheric Research, 1850 Table Mesa Dr., Boulder, CO, 80305, USA
2Department of Atmospheric Sciences, Box 351640, University of Washington, Seattle, WA, 98195, USA

Abstract. Cloud microphysical process rates control the amount of condensed water in clouds and impact the susceptibility of precipitation to drop number and aerosols. The relative importance of different microphysical processes in a climate model is analyzed, and the autoconversion and accretion processes are found to be critical to the condensate budget in most regions. A simple steady-state model of warm rain formation is used to illustrate that the diagnostic rain formulations typical of climate models may result in excessive contributions from autoconversion, compared to observations and large eddy simulation models with explicit bin-resolved microphysics and rain formation processes. The behavior does not appear to be caused by the bulk process rate formulations themselves, because the steady state model with bulk accretion and autoconversion has reduced contributions from autoconversion. Sensitivity tests are conducted to analyze how perturbations to the precipitation microphysics for stratiform clouds impact process rates, precipitation susceptibility and aerosol-cloud interactions (ACI). With similar liquid water path, corrections for the diagnostic rain assumptions in the GCM based on the steady state model to boost accretion over autoconversion indicate that the radiative effects of ACI may decrease by 20% in the GCM for the same mean liquid water path. Links between process rates, susceptibility and ACI are not always clear in the GCM. Better representation of the precipitation process, for example by prognosing precipitation mass and number, may help better constrain these effects in global models with bulk microphysics schemes.

Citation: Gettelman, A., Morrison, H., Terai, C. R., and Wood, R.: Microphysical process rates and global aerosol-cloud interactions, Atmos. Chem. Phys. Discuss., 13, 11789-11825, doi:10.5194/acpd-13-11789-2013, 2013.
 
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