Atmos. Chem. Phys. Discuss., 13, 31479-31526, 2013www.atmos-chem-phys-discuss.net/13/31479/2013/doi:10.5194/acpd-13-31479-2013© Author(s) 2013. This work is distributedunder 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.
Understanding the contributions of aerosol properties and parameterization discrepancies to droplet number variability in a Global Climate Model
R. Morales Betancourt1 and A. Nenes1,2
1School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia, USA
2School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia, USA

Abstract. Aerosol indirect effects in climate models strongly depend on the representation of the aerosol activation process. In this study, we assess the process level differences across activation parameterizations that contribute to droplet number uncertainty by using the adjoints of the Abdul-Razzak and Ghan (2000) and Fountoukis and Nenes (2005) droplet activation parameterizations in the framework of the Community Atmospheric Model version 5.1 (CAM5.1). The adjoint sensitivities of Nd to relevant input parameters are used to: (i) unravel the spatially resolved contribution of aerosol number, mass, and chemical composition to changes in $N_\mathrm{d}$ between present day and pre-industrial simulations; (ii) identify the key variables responsible for the differences in Nd fields and aerosol indirect effect estimates when different activation schemes are used within the same modeling framework. The sensitivities are computed online at minimal computational cost. Changes in aerosol number and aerosol mass concentrations were found to contribute to Nd differences much more strongly than chemical composition effects. The main sources of discrepancy between the activation parameterization considered were the treatment of the water uptake by coarse mode particles, and the sensitivity of the parameterized Nd accumulation mode aerosol geometric mean diameter. These two factors explain the different predictions of Nd over land and over oceans when these parameterizations are employed. Discrepancies in the sensitivity to aerosol size are responsible for an exaggerated response to aerosol volume changes over heavily polluted regions. Because these regions are collocated with areas of deep clouds their impact on short wave cloud forcing is amplified through liquid water path changes. Application of the adjoint-sensitivities illustrated the importance of primary organic matter emissions in controlling the droplet number concentration changes in several areas. The same framework is also utilized to efficiently explore droplet number uncertainty attributable to hygroscopicity parameter of organic aerosol (primary and secondary). Comparisons between the parameterization-derived sensitivities of droplet number against predictions with detailed numerical simulations of the activation process were performed to validate the physical consistency of the adjoint sensitivities.

Citation: Morales Betancourt, R. and Nenes, A.: Understanding the contributions of aerosol properties and parameterization discrepancies to droplet number variability in a Global Climate Model, Atmos. Chem. Phys. Discuss., 13, 31479-31526, doi:10.5194/acpd-13-31479-2013, 2013.

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