Constraining the Twomey effect from satellite observations: issues and perspectives
Atmospheric chemistry and physics, vol. 20, pp. 15079–15099
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 (Δ N d, 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, but rapid adjustments also contribute. These adjustments are essentially the responses of cloud fraction and liquid water path to Δ N d, ant and thus scale approximately with it. While the fundamental physics of the influence of added aerosol particles on the droplet concentration ( N d ) is well described by established theory at the particle scale (micrometres), how this relationship is expressed at the large-scale (hundreds of kilometres) perturbation, Δ N d, 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 updraught velocity and by droplet sink processes. These operate at scales on the order of tens of metres at which only localised observations are available and at which no approach yet exists 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 four key uncertainties in determining Δ N d, ant , namely the quantification of (i) the cloud-active aerosol – the cloud condensation nuclei (CCN) concentrations at or above cloud base, (ii) N d , (iii) the statistical approach for inferring the sensitivity of N d to aerosol particles from the satellite data and (iv) uncertainty in the anthropogenic perturbation to CCN concentrations, which is 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, non-direct relation to the concentration of aerosols, difficulty 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 utilising co-located polarimeter and lidar instruments, ideally including high-spectral-resolution lidar capability at two wavelengths to maximise vertically resolved size distribution information content. In terms of N d , 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 N d -to-CCN sensitivity, key issues are the updraught distributions and the role of N d 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.
Authors 24
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Johannes Quaas corresponding
Affiliation as printed
Leipzig Institute for Meteorology, Universität Leipzig, Leipzig, Germany
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Antti Arola corresponding
Finnish Meteorological Institute
Affiliation as printed
Finnish Meteorological Institute, Kuopio,Finland
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Goddard Institute for Space Studies
Affiliation as printed
NASA Goddard Institute for Space Studies, New York, USA
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Matthew W. Christensen corresponding
Affiliation as printed
Department of Physics, University of Oxford, Oxford, UK
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Hartwig M. Deneke corresponding
Leibniz Institute for Tropospheric Research
Affiliation as printed
Leibniz Institute for Tropospheric Research, Leipzig, Germany
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Annica M. L. Ekman corresponding
Stockholm University · Bolin Centre for Climate Research
Affiliation as printed
Department of Meteorology and Bolin Centre for Climate Research, Stockholm University, Stockholm, Sweden
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Graham Feingold corresponding
National Oceanic and Atmospheric Administration · NOAA Chemical Sciences Laboratory
Affiliation as printed
NOAA Earth System Laboratories, Chemical Science Laboratory, Boulder, USA
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Ann M. Fridlind corresponding
Goddard Institute for Space Studies
Affiliation as printed
NASA Goddard Institute for Space Studies, New York, USA
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Edward Gryspeerdt corresponding
Affiliation as printed
Space and Atmospheric Physics Group, Imperial College London, UK
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Space Research Organisation Netherlands
Affiliation as printed
SRON Netherlands Institute for Space Research, Utrecht, the Netherlands
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Zhanqing Li corresponding
Earth System Science Interdisciplinary Center · University of Maryland, College Park
Affiliation as printed
Earth System Science Interdisciplinary Center and Department of Atmospheric and Oceanic Science, University of Maryland, College Park, USA
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Antti Lipponen corresponding
Finnish Meteorological Institute
Affiliation as printed
Finnish Meteorological Institute, Kuopio,Finland
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Po‐Lun Ma corresponding
Pacific Northwest National Laboratory
Affiliation as printed
Pacific Northwest National Laboratory, Richland, USA
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Pacific Northwest National Laboratory
Affiliation as printed
Pacific Northwest National Laboratory , Richland , USA
Pacific Northwest National Laboratory, Richland, USA
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Athanasios Nenes corresponding
École Polytechnique Fédérale de Lausanne · Foundation for Research and Technology Hellas
Affiliation as printed
Institute of Chemical Engineering Sciences, Foundation for Research and Technology Hellas, Patras, Greece
School of Architecture, Civil & Environmental Engineering, École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland
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Joyce E. Penner corresponding
Affiliation as printed
Department of Climate and Space Sciences and Engineering, University of Michigan, Ann Arbor, USA
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Hebrew University of Jerusalem
Affiliation as printed
Institute of Earth Sciences, Hebrew University of Jerusalem, Jerusalem, Israel
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Leibniz Institute for Tropospheric Research
Affiliation as printed
Leibniz Institute for Tropospheric Research , Leipzig , Germany
Leibniz Institute for Tropospheric Research, Leipzig, Germany
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Kenneth Sinclair corresponding
Goddard Institute for Space Studies · Universities Space Research Association
Affiliation as printed
Department of Earth and Environmental Engineering, Universities Space Research Association (USRA), Columbia, MD 21046, USA
NASA Goddard Institute for Space Studies, New York, USA
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Odran Sourdeval corresponding
Centre National de la Recherche Scientifique · Université de Lille · Laboratoire d'Optique Atmosphérique
Affiliation as printed
Université de Lille, CNRS, UMR 8518 – LOA – Laboratoire d'Optique Atmosphérique, Lille, France
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Philip Stier corresponding
Affiliation as printed
Department of Physics, University of Oxford, Oxford, UK
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Matthias Tesche corresponding
Affiliation as printed
Leipzig Institute for Meteorology, Universität Leipzig, Leipzig, Germany
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Bastiaan van Diedenhoven corresponding
Goddard Institute for Space Studies
Affiliation as printed
NASA Goddard Institute for Space Studies, New York, USA
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Manfred Wendisch corresponding
Affiliation as printed
Leipzig Institute for Meteorology, Universität Leipzig, Leipzig, Germany
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