Uncertainty in aerosol-cloud radiative forcing is driven by clean conditions

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https://acp.copernicus.org/articles/23/4115/2023/

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https://doi.org/10.5194/acp-23-4115-2023
 http://hdl.handle.net/11603/26196

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https://orcid.org/0000-0001-9149-1789

Date

2023-04-05

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journal articles
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Citation of Original Publication

Gryspeerdt, Edward, Adam C. Povey, Roy G. Grainger, Otto Hasekamp, N. Christina Hsu, Jane P. Mulcahy, Andrew M. Sayer, and Armin Sorooshian. “Uncertainty in Aerosol–Cloud Radiative Forcing Is Driven by Clean Conditions.” Atmospheric Chemistry and Physics 23, no. 7 (April 5, 2023): 4115–22. https://doi.org/10.5194/acp-23-4115-2023.

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This work was written as part of one of the author's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. Law.
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Abstract

Atmospheric aerosols and their impact on cloud properties remain the largest uncertainty in the human forcing of the climate system. By increasing the concentration of cloud droplets (Nd), aerosols reduce droplet size and increase the reflectivity of clouds (a negative radiative forcing). Central to this climate impact is the susceptibility of cloud droplet number to aerosol (β), the diversity of which explains much of the variation in radiative forcing in global climate models. This has 5 made measuring β a key target for developing observational constraints of the aerosol forcing. While the aerosol burden of the clean, pre-industrial atmosphere has been demonstrated as a key uncertainty for the aerosol forcing, here we show that the behaviour of clouds under these clean conditions is of equal importance for understanding the spread in radiative forcing estimates between models and observations. This means that the uncertainty in the aerosol impact on clouds is, counterintuitively, driven by situations with little aerosol. Discarding clean conditions produces a close agreement 10 between different model and observational estimates of the cloud response to aerosol, but does not provide a strong constraint on the radiative forcing from aerosol-cloud interactions. This makes constraining aerosol behaviour in clean conditions an important goal for future observational studies.