Constraining the Twomey effect from satellite observations: Issues and perspectives
Abstract
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.
Authors 24
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Johannes Quaas corresponding
Affiliation as printed
Universität Leipzig , Leipzig Institute for Meteorology
Universität Leipzig, Leipzig Institute for Meteorology
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Finnish Meteorological Institute
Affiliation as printed
Finnish Meteorological Institute
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Goddard Institute for Space Studies
Affiliation as printed
NASA Goddard Institute for Space Studies , New York
NASA Goddard Institute for Space Studies, New York
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Affiliation as printed
University of Oxford
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Leibniz Institute for Tropospheric Research
Affiliation as printed
Leibniz Institute for Tropospheric Research , Leipzig
Leibniz Institute for Tropospheric Research, Leipzig
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Stockholm University · Bolin Centre for Climate Research
Affiliation as printed
Stockholm University , Department of Meteorology and Bolin Centre for Climate Research
Stockholm University, Department of Meteorology and Bolin Centre for Climate Research
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National Oceanic and Atmospheric Administration
Affiliation as printed
NOAA Boulder
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Goddard Institute for Space Studies
Affiliation as printed
NASA Goddard Institute for Space Studies , New York
NASA Goddard Institute for Space Studies, New York
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Affiliation as printed
Space and Atmospheric Physics Group , Imperial College London
Space and Atmospheric Physics Group, Imperial College London
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Space Research Organisation Netherlands
Affiliation as printed
SRON Netherlands Institute for Space Research , Utrecht
SRON Netherlands Institute for Space Research, Utrecht
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University of Maryland, College Park
Affiliation as printed
University of Maryland , College Park
University of Maryland, College Park
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Finnish Meteorological Institute
Affiliation as printed
Finnish Meteorological Institute
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Pacific Northwest National Laboratory
Affiliation as printed
Pacific Northwest National Laboratory , Richland
Pacific Northwest National Laboratory, Richland
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Pacific Northwest National Laboratory
Affiliation as printed
Pacific Northwest National Laboratory , Richland
Pacific Northwest National Laboratory, Richland
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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 Architeture , Civil & Environmental Engineering , Ecole Polytechnique Fédérale de Lausanne , Switzerland
Institute of Chemical Engineering Sciences, Foundation for Research and Technology Hellas, Patras, Greece
School of Architeture, Civil & Environmental Engineering, Ecole Polytechnique Fédérale de Lausanne, Switzerland
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Affiliation as printed
University of Michigan , Ann Arbor
University of Michigan, Ann Arbor
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Hebrew University of Jerusalem
Affiliation as printed
Hebrew University of Jerusalem
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Leibniz Institute for Tropospheric Research
Affiliation as printed
Leibniz Institute for Tropospheric Research , Leipzig
Leibniz Institute for Tropospheric Research, Leipzig
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Goddard Institute for Space Studies · Universities Space Research Association
Affiliation as printed
NASA Goddard Institute for Space Studies , New York
Universities Space Research Association (USRA) , Columbia , MD 21046 , USA
NASA Goddard Institute for Space Studies, New York
Universities Space Research Association (USRA), Columbia, MD 21046, USA
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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
Université de Lille, CNRS, UMR 8518 -LOA -Laboratoire d'Optique Atmosphérique, Lille, France
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Affiliation as printed
University of Oxford
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Affiliation as printed
Universität Leipzig , Leipzig Institute for Meteorology
Universität Leipzig, Leipzig Institute for Meteorology
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Goddard Institute for Space Studies
Affiliation as printed
NASA Goddard Institute for Space Studies , New York
NASA Goddard Institute for Space Studies, New York
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Affiliation as printed
Universität Leipzig , Leipzig Institute for Meteorology
Universität Leipzig, Leipzig Institute for Meteorology
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