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Preprints
https://doi.org/10.5194/acp-2020-279
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-279
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: review article 15 May 2020

Submitted as: review article | 15 May 2020

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This preprint is currently under review for the journal ACP.

Constraining the Twomey effect from satellite observations: Issues and perspectives

Johannes Quaas1, Antti Arola2, Brian Cairns3, Matthew Christensen4, Hartwig Deneke5, Annica M. L. Ekman6, Graham Feingold7, Ann Fridlind3, Edward Gryspeerdt8, Otto Hasekamp9, Zhanqing Li10, Antti Lipponen2, Po-Lun Ma11, Johannes Mülmenstädt11, Athanasios Nenes12,13, Joyce Penner14, Daniel Rosenfeld15, Roland Schrödner5, Kenneth Sinclair3,16, Odran Sourdeval17, Philip Stier4, Matthias Tesche1, Bastiaan van Diedenhoven3, and Manfred Wendisch1 Johannes Quaas et al.
  • 1Universität Leipzig, Leipzig Institute for Meteorology
  • 2Finnish Meteorological Institute
  • 3NASA Goddard Institute for Space Studies, New York
  • 4University of Oxford
  • 5Leibniz Institute for Tropospheric Research, Leipzig
  • 6Stockholm University, Department of Meteorology and Bolin Centre for Climate Research
  • 7NOAA Boulder
  • 8Space and Atmospheric Physics Group, Imperial College London
  • 9SRON Netherlands Institute for Space Research, Utrecht
  • 10University of Maryland, College Park
  • 11Pacific Northwest National Laboratory, Richland
  • 12School of Architeture, Civil & Environmental Engineering, Ecole Polytechnique Fédérale de Lausanne, Switzerland
  • 13Institute of Chemical Engineering Sciences, Foundation for Research and Technology Hellas, Patras, Greece
  • 14University of Michigan, Ann Arbor
  • 15Hebrew University of Jerusalem
  • 16Universities Space Research Association (USRA), Columbia, MD 21046, USA
  • 17Université de Lille, CNRS, UMR 8518 – LOA – Laboratoire d'Optique Atmosphérique, Lille, France

Abstract. The Twomey effect describes the radiative forcing associated with a change in cloud albedo due to an increase in anthropogenic aerosol emissions. It is driven by the perturbation in cloud droplet number concentration (ΔNd,ant) in liquid-water clouds and is currently understood to exert a cooling effect on climate. The Twomey effect is the key driver in the effective radiative forcing due to aerosol–cloud interactions which also comprises rapid adjustments. These adjustments are essentially the responses of cloud fraction and liquid water path to ΔNd,ant and thus scale approximately with it. While the fundamental physics of the influence of added aerosol particles on the droplet concentration (Nd) is well described by established theory at the particle scale (micrometres), how this relationship is expressed at the large scale (hundreds of kilometres) ΔNd,ant remains uncertain. The discrepancy between process understanding at particle scale and insufficient quantification at the climate-relevant large scale is caused by co-variability of aerosol particles and vertical wind and by droplet sink processes. These operate at scales on the order of 10s of metres at which only localized observations are available and at which no approach exists yet to quantify the anthropogenic perturbation. Different atmospheric models suggest diverse magnitudes of the Twomey effect even when applying the same anthropogenic aerosol emission perturbation. Thus, observational data are needed to quantify and constrain the Twomey effect. At the global scale, this means satellite data. There are three key uncertainties in determining ΔNd,ant, namely the quantification (i) of the cloud-active aerosol – the cloud condensation nuclei concentrations (CCN) at or above cloud base –, (ii) of Nd, as well as (iii) the statistical approach for inferring the sensitivity of Nd to aerosol particles from the satellite data. A fourth uncertainty, the anthropogenic perturbation to CCN concentrations, is also not easily accessible from observational data. This review discusses deficiencies of current approaches for the different aspects of the problem and proposes several ways forward: In terms of CCN, retrievals of optical quantities such as aerosol optical depth suffer from a lack of vertical resolution, size and hygroscopicity information, the non-direct relation to the concentration of aerosols, the impossibility to quantify it within or below clouds, and the problem of insufficient sensitivity at low concentrations, in addition to retrieval errors. A future path forward can include utilizing colocated polarimeter and lidar instruments, ideally including high spectral resolution lidar capability at two wavelengths to maximize vertically resolved size distribution information content. In terms of Nd, a key problem is the lack of operational retrievals of this quantity, and the inaccuracy of the retrieval especially in broken-cloud regimes. As for the Nd – to – CCN sensitivity, key issues are the updraught distributions and the role of Nd sink processes, for which empirical assessments for specific cloud regimes are currently the best solutions. These considerations point to the conclusion that past studies using existing approaches have likely underestimated the true sensitivity and, thus, the radiative forcing due to the Twomey effect.

Johannes Quaas et al.

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Johannes Quaas et al.

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Short summary
Anthropogenic pollution particles – aerosols – serve as cloud condensation nuclei and thus increase cloud droplet concentration and the clouds reflection of sunlight (a cooling effect on climate). This Twomey effect is poorly constrained by models, and requires satellite data for better quantification. The review summarizes the challenges in properly doing so, and outlines avenues for progress towards a better use of aerosol retrievals, and better retrievals of droplet concentrations.
Anthropogenic pollution particles – aerosols – serve as cloud condensation nuclei and thus...
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