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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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Volume 18, issue 11 | Copyright
Atmos. Chem. Phys., 18, 7961-7983, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 06 Jun 2018

Research article | 06 Jun 2018

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 Daniel Rothenberg et al.
  • 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 (CDNCs). 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 (two-Moment, Multi-Modal, Mixing-state-resolving Aerosol model for Research of Climate (MARC) coupled with the National Center for Atmospheric Research's (NCAR) Community Earth System Model (CESM; version 1.2)). Although the model produces a reasonable present-day CDNC climatology when compared with observations regardless of the scheme used, ΔCDNCs between the present and preindustrial era regionally increase by over 100% in zonal mean when using the most sensitive parameterization. These differences in activation sensitivity may lead to a different evolution of the model meteorology, and ultimately to a spread of over 0.8W 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 intercomparison. Model-derived AIE strongly scales with the simulated preindustrial CDNC burden, and those models with the greatest preindustrial 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 the AIE, which will arise from differences in model estimates of the preindustrial aerosol and cloud climatology.

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Estimates of how much the particles we emit into the atmosphere cool the climate depend on how those particles influence the relative number of cloud droplets. Those estimates are strongly influenced by how many droplets a given climate model predicts under clean conditions, even more so than how much that 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...