23 Sep 2022
23 Sep 2022
Status: this preprint is currently under review for the journal ACP.

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

Edward Gryspeerdt1, Adam C. Povey2, Roy G. Grainger2, Otto Hasekamp3, N. Christina Hsu4, Jane P. Mulcahy5, Andrew M. Sayer6,7, and Armin Sorooshian8 Edward Gryspeerdt et al.
  • 1Grantham Institute - Climate Change and the Environment, Imperial College London, London SW7 2AJ, United Kingdom
  • 2National Centre for Earth Observation, Department of Physics, University of Oxford, Oxford, OX1 3PU, United Kingdom
  • 3Netherlands Institute for Space Research (SRON, NWO-I), Utrecht, The Netherlands
  • 4Climate and Radiation Laboratory, NASA Goddard Space Flight Center, Greenbelt, USA
  • 5Met Office, Exeter, EX1 3PB, UK
  • 6Ocean Ecology Laboratory, Goddard Space Flight Center, National Aeronautics and Space Administration, Greenbelt, MD, USA
  • 7GESTAR II, University of Maryland Baltimore County, Baltimore, MD, USA
  • 8Department of Chemical and Environmental Engineering, University of Arizona, Tucson, Arizona, USA

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 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 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 a key goal for future observational studies.

Edward Gryspeerdt et al.

Status: open (until 23 Oct 2022)

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Edward Gryspeerdt et al.

Edward Gryspeerdt et al.

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Short summary
The impact of aerosols on clouds is one of the largest uncertainties in the human forcing of the climate. Aerosol can increase the concentrations of droplets in clouds, but observational and model studies produce widely varying estimates of this effect. We show that these estimates can be reconciled if only polluted clouds are studied, but that this is insufficient to constrain their climate impact. The uncertainty in aerosol impact on cloud is currently driven by cases with not much aerosol.