Constraining the Twomey effect from satellite observations : issues and perspectives

Medientyp: E-Artikel

Titel: Constraining the Twomey effect from satellite observations : issues and perspectives

Beteiligte: Quaas, Johannes [Verfasser:in]; Arola, Antti [Verfasser:in]; Cairns, Brian [Verfasser:in]; Christensen, Matthew [Verfasser:in]; Deneke, Hartwig [Verfasser:in]; Ekman, Annica M. L. [Verfasser:in]; Feingold, Graham [Verfasser:in]; Fridlind, Ann [Verfasser:in]; Gryspeerdt, Edward [Verfasser:in]; Hasekamp, Otto [Verfasser:in]; Li, Zhanqing [Verfasser:in]; Lipponen, Antti [Verfasser:in]; Ma, Po-Lun [Verfasser:in]; Mülmenstädt, Johannes [Verfasser:in]; Nenes, Athanasios [Verfasser:in]; Penner, Joyce E. [Verfasser:in]; Rosenfeld, Daniel [Verfasser:in]; Schrödner, Roland [Verfasser:in]; Sinclair, Kenneth [Verfasser:in]; Sourdeval, Odran [Verfasser:in]; Stier, Philip [Verfasser:in]; Tesche, Matthias [Verfasser:in]; van Diedenhoven, Bastiaan [Verfasser:in]; Wendisch, Manfred [Verfasser:in]

Erschienen: Göttingen: Copernicus Publications, [2021]

Sprache: Englisch

DOI: https://doi.org/10.5194/acp-20-15079-2020

Schlagwörter: Twomey effect ; satellite observations

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Beschreibung: The Twomey effect describes the radiative forcingassociated with a change in cloud albedo due to an increasein anthropogenic aerosol emissions. It is driven by the perturbationin cloud droplet number concentration (1Nd; ant)in liquid-water clouds and is currently understood to exerta cooling effect on climate. The Twomey effect is the keydriver in the effective radiative forcing due to aerosol–cloudinteractions, but rapid adjustments also contribute. Theseadjustments are essentially the responses of cloud fractionand liquid water path to 1Nd; ant and thus scale approximatelywith it. While the fundamental physics of the influenceof added aerosol particles on the droplet concentration(Nd) is well described by established theory at the particlescale (micrometres), how this relationship is expressed at thelarge-scale (hundreds of kilometres) perturbation, 1Nd; ant,remains uncertain. The discrepancy between process understandingat particle scale and insufficient quantification atthe climate-relevant large scale is caused by co-variability ofaerosol particles and updraught velocity and by droplet sinkprocesses. These operate at scales on the order of tens of metres at which only localised observations are available and atwhich no approach yet exists to quantify the anthropogenicperturbation. Different atmospheric models suggest diversemagnitudes of the Twomey effect even when applying thesame anthropogenic aerosol emission perturbation. Thus, observationaldata are needed to quantify and constrain theTwomey effect. At the global scale, this means satellite data.There are four key uncertainties in determining 1Nd; ant,namely the quantification of (i) the cloud-active aerosol – thecloud condensation nuclei (CCN) concentrations at or abovecloud base, (ii) Nd, (iii) the statistical approach for inferringthe sensitivity of Nd to aerosol particles from the satellitedata and (iv) uncertainty in the anthropogenic perturbationto CCN concentrations, which is not easily accessible fromobservational data. This review discusses deficiencies of currentapproaches for the different aspects of the problem andproposes several ways forward: in terms of CCN, retrievalsof optical quantities such as aerosol optical depth suffer froma 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 problemof insufficient sensitivity at low concentrations, in additionto retrieval errors. A future path forward can includeutilising co-located polarimeter and lidar instruments, ideallyincluding high-spectral-resolution lidar capability at twowavelengths to maximise vertically resolved size distributioninformation content. In terms of Nd, a key problem is the lackof operational retrievals of this quantity and the inaccuracy ofthe retrieval especially in broken-cloud regimes. As for theNd-to-CCN sensitivity, key issues are the updraught distributionsand the role of Nd sink processes, for which empiricalassessments for specific cloud regimes are currently the bestsolutions. These considerations point to the conclusion that past studies using existing approaches have likely underestimatedthe true sensitivity and, thus, the radiative forcing dueto the Twomey effect.

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