Comment on acp-2022-642

Abstract

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.

Authors 8

1. Edward Gryspeerdt corresponding

   Imperial College London

   Affiliation as printed

   Grantham Institute - Climate Change and the Environment, Imperial College London, London SW7 2AJ, United Kingdom

2. Adam C. Povey

   University of Oxford · National Centre for Earth Observation

   Affiliation as printed

   National Centre for Earth Observation, Department of Physics, University of Oxford, Oxford, OX1 3PU, United Kingdom

3. R. G. Grainger

   University of Oxford · National Centre for Earth Observation

   Affiliation as printed

   National Centre for Earth Observation, Department of Physics, University of Oxford, Oxford, OX1 3PU, United Kingdom

4. Otto Hasekamp Aachen Netherlands Institute for Space Research (SRON

   Space Research Organisation Netherlands

   Affiliation as printed

   Netherlands Institute for Space Research (SRON, NWO-I), Utrecht, The Netherlands

5. Nai-Yung Christina Hsu

   Goddard Space Flight Center

   Affiliation as printed

   Climate and Radiation Laboratory, NASA Goddard Space Flight Center, Greenbelt, USA

6. Jane Patricia Mulcahy

   Met Office

   Affiliation as printed

   Met Office, Exeter, EX1 3PB, UK

7. A. M. Sayer

   Goddard Space Flight Center · National Aeronautics and Space Administration · University of Maryland, Baltimore County

   Affiliation as printed

   GESTAR II, University of Maryland Baltimore County, Baltimore, MD, USA

   Ocean Ecology Laboratory, Goddard Space Flight Center, National Aeronautics and Space Administration, Greenbelt, MD, USA

8. Armin Sorooshian

   University of Arizona

   Affiliation as printed

   Department of Chemical and Environmental Engineering, University of Arizona, Tucson, Arizona, USA