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Discussion papers
https://doi.org/10.5194/acp-2019-533
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-533
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 02 Jul 2019

Research article | 02 Jul 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Surprising similarities in model and observational aerosol radiative forcing estimates

Edward Gryspeerdt1, Johannes Mülmenstädt2, Andrew Gettelman3, Florent F. Malavelle4, Hugh Morrison3, David Neubauer5, Daniel G. Partridge4, Philip Stier6, Toshihiko Takemura7, Hailong Wang8, Minghuai Wang9,10,11, and Kai Zhang8 Edward Gryspeerdt et al.
  • 1Space and Atmospheric Physics Group, Imperial College London, UK
  • 2Institute for Meteorology, Universität Leipzig, Germany
  • 3National Center for Atmospheric Research, Boulder, USA
  • 4Department of Mathematics, University of Exeter, UK
  • 5Institute for Atmospheric and Climate Science, ETH Zurich, Switzerland
  • 6Atmospheric, Oceanic and Planetary Physics, Department of Physics, University of Oxford, UK
  • 7Research Institute for Applied Mathematics, Kyushu University, Japan
  • 8Atmospheric Sciences and Global Change Division, Pacific Northwest National Laboratory, USA
  • 9Institute for Climate and Global Change Research, Nanjing University, China
  • 10School of Atmospheric Sciences, Nanjing University, China
  • 11Collaborative Innovation Center of Climate Change, China

Abstract. The radiative forcing from aerosols (particularly through their interaction with clouds) remains one of the most uncertain components of the human forcing of the climate. Observation-based studies have typically found a smaller aerosol effective radiative forcing than in model simulations and were given preferential weighting in the IPCC AR5 report. With their own sources of uncertainty, it is not clear that observation-based estimates are more reliable. Understanding the source of the model-observational difference is thus vital to reduce uncertainty in the impact of aerosols on the climate.

These reported discrepancies arise from the different decompositions of the aerosol forcing used in model and observational studies. Applying the observational decomposition to global climate model output, the two different lines of evidence are surprisingly similar, with a much better agreement on the magnitude of aerosol impacts on cloud properties. Cloud adjustments remain a significant source of uncertainty, particularly for ice clouds. However, they are consistent with the uncertainty from observation-based methods, with the liquid water path adjustment usually enhancing the Twomey effect by less than 50 %. Depending on different sets of assumptions, this work suggests that model and observation-based estimates could be more equally weighted in future synthesis studies.

Edward Gryspeerdt et al.
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Short summary
The aerosol radiative forcing is a key uncertainty in our understanding of the human forcing of the climate, with much of this uncertainty coming from aerosol impacts on clouds. Observations-based estimates of the radiative forcing are typically smaller than those from global models, but it is not clear they are more reliable. This work shows how the forcing components in global climate models can be identified, highlighting similarities between the two methods and areas for future investigation
The aerosol radiative forcing is a key uncertainty in our understanding of the human forcing of...
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