Atmos. Chem. Phys. Discuss., 11, 26741-26789, 2011
© Author(s) 2011. This work is distributed
under the Creative Commons Attribution 3.0 License.
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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.
The scale problem in quantifying aerosol indirect effects
A. McComiskey1,2 and G. Feingold2
1Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, USA
2NOAA Earth System Research Laboratory Chemical Sciences Division, 325 Broadway Boulder, CO 80305, USA

Abstract. A wide range of estimates exists for the radiative forcing of the aerosol effect on cloud albedo. We argue that a component of this uncertainty derives from the use of a wide range of observational scales and platforms. Aerosol affects cloud properties at the microphysical scale, or the "process scale" but observations are most often made of bulk properties over a wide range of resolutions, or "analysis scales". We show that differences between process and analysis scales incur biases in quantification of the albedo effect through the impact that data aggregation has on statistical properties of the aerosol or cloud variable, and their covariance. Measures made within this range of scales are erroneously treated as equivalent, leading to a large uncertainty in associated radiative forcing estimates. Issues associated with the coarsening of observational resolution particular to quantifying the albedo effect are discussed. Specifically, the omission of the constraint on cloud liquid water path and the separation in space of cloud and aerosol properties from passive, space-based remote sensors dampen the measured strength of the albedo effect. Based on our understanding of these biases we propose a new approach for an observationally-based, robust method for estimating aerosol indirect effects that can be used for radiative forcing estimates as well as a better characterization of the uncertainties associated with those estimates.

Citation: McComiskey, A. and Feingold, G.: The scale problem in quantifying aerosol indirect effects, Atmos. Chem. Phys. Discuss., 11, 26741-26789, doi:10.5194/acpd-11-26741-2011, 2011.
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