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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
https://doi.org/10.5194/acp-2017-680
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
04 Aug 2017
Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).
On the representation of aerosol activation and its influence on model-derived estimates of the aerosol indirect effect
Daniel Rothenberg1, Alexander Avramov2, and Chien Wang1 1Center for Global Change Science, Massachusetts Institute of Technology, Cambridge, MA, USA
2Department of Environmental Sciences, Emory University, Atlanta, GA, USA
Abstract. Interactions between aerosol particles and clouds contribute a great deal of uncertainty to the scientific community's understanding of anthropogenic climate forcing. Aerosol particles serve as the nucleation sites for cloud droplets, establishing a direct linkage between anthropogenic particulate emissions and clouds in the climate system. To resolve this linkage, the community has developed parameterizations of aerosol activation which can be used in global climate models to interactively predict cloud droplet number concentrations (CDNC). However, different activation schemes can exhibit different sensitivities to aerosol perturbations in different meteorological or pollution regimes. To assess the impact these different sensitivities have on climate forcing, we have coupled three different core activation schemes and variants with the CESM-MARC. Although the model produces a reasonable present day CDNC climatology when compared with observations regardless of the scheme used, ΔCDNC between the present and pre-industrial era regionally increase by over 100 % in zonal mean when using the most sensitive parameterization. These differences in activation sensitivity lead to a spread of over 0.8 W m−2 in global average shortwave indirect effect (AIE) diagnosed from the model, a range which is as large as the inter-model spread from the AeroCom inter-comparison. Model-derived AIE strongly scales with the simulated pre-industrial CDNC burden, and those models with the greatest pre-industrial CDNC tend to have the smallest AIE, regardless of their ΔCDNC. This suggests that present day evaluations of aerosol-climate models may not provide useful constraints on the magnitude of AIE, which will arise from differences in model estimates of the pre-industrial aerosol and cloud climatology.

Citation: Rothenberg, D., Avramov, A., and Wang, C.: On the representation of aerosol activation and its influence on model-derived estimates of the aerosol indirect effect, Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2017-680, in review, 2017.
Daniel Rothenberg et al.
Daniel Rothenberg et al.
Daniel Rothenberg et al.

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Short summary
Estimates of how much the particles we emit into the atmosphere cool the climate depend on how those particles influence the number of cloud droplets in the atmosphere. Those estimates are strongly influenced by how many droplets a given climate model predicts under clean conditions, even more-so than how much human emissions increase droplet concentrations. Because of this, observations of particles influencing clouds in clean conditions could help constrain their climate-cooling potential.
Estimates of how much the particles we emit into the atmosphere cool the climate depend on how...
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