Publication Abstracts

Quaas et al. 2020, accepted

Quaas, J., A. Arola, B. Cairns, M. Christensen, H. Deneke, A.M.L. Ekman, G. Feingold, A. Fridlind, E. Gryspeerdt, O. Hasekamp, Z. Li, A. Lipponen, P.-L. Ma, J. Mülmenstädt, J. Penner, D. Rosenfeld, R. Schrödner, K. Sinclair, O. Sourdeval, P. Stier, M. Tesche, B. van Diedenhoven, and M. Wendisch, 2020: Constraining the Twomey effect from satellite observations: Issues and perspectives. Atmos. Chem. Phys., accepted.

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), 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 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 path forward can include utilizing polarimetric observations to resolve size and shape information of CCN, and combining this with lidar measurements to add vertical information. 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.

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BibTeX Citation

@unpublished{qu09000j,
  author={Quaas, J. and Arola, A. and Cairns, B. and Christensen, M. and Deneke, H. and Ekman, A. M. L. and Feingold, G. and Fridlind, A. and Gryspeerdt, E. and Hasekamp, O. and Li, Z. and Lipponen, A. and Ma, P.-L. and Mülmenstädt, J. and Penner, J. and Rosenfeld, D. and Schrödner, R. and Sinclair, K. and Sourdeval, O. and Stier, P. and Tesche, M. and van Diedenhoven, B. and Wendisch, M.},
  title={Constraining the Twomey effect from satellite observations: Issues and perspectives},
  year={2020},
  journal={Atmos. Chem. Phys.},
}

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RIS Citation

TY  - INPR
ID  - qu09000j
AU  - Quaas, J.
AU  - Arola, A.
AU  - Cairns, B.
AU  - Christensen, M.
AU  - Deneke, H.
AU  - Ekman, A. M. L.
AU  - Feingold, G.
AU  - Fridlind, A.
AU  - Gryspeerdt, E.
AU  - Hasekamp, O.
AU  - Li, Z.
AU  - Lipponen, A.
AU  - Ma, P.-L.
AU  - Mülmenstädt, J.
AU  - Penner, J.
AU  - Rosenfeld, D.
AU  - Schrödner, R.
AU  - Sinclair, K.
AU  - Sourdeval, O.
AU  - Stier, P.
AU  - Tesche, M.
AU  - van Diedenhoven, B.
AU  - Wendisch, M.
PY  - 2020
TI  - Constraining the Twomey effect from satellite observations: Issues and perspectives
JA  - Atmos. Chem. Phys.
ER  -

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