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 (1Nd; 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 1Nd; 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) perturbation, 1Nd; 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 1Nd; ant,
namely the quantification of (i) the cloud-active aerosol – the
cloud condensation nuclei (CCN) concentrations at or above
cloud base, (ii) Nd, (iii) the statistical approach for inferring
the sensitivity of Nd 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 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.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:74864 |
| Date | 11 May 2021 |
| Creators | Quaas, Johannes, Arola, Antti, Cairns, Brian, Christensen, Matthew, Deneke, Hartwig, Ekman, Annica M. L., Feingold, Graham, Fridlind, Ann, Gryspeerdt, Edward, Hasekamp, Otto, Li, Zhanqing, Lipponen, Antti, Ma, Po-Lun, Mülmenstädt, Johannes, Nenes, Athanasios, Penner, Joyce E., Rosenfeld, Daniel, Schrödner, Roland, Sinclair, Kenneth, Sourdeval, Odran, Stier, Philip, Tesche, Matthias, van Diedenhoven, Bastiaan, Wendisch, Manfred |
| Publisher | Copernicus Publications |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:article, info:eu-repo/semantics/article, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
| Relation | 1680-7316, https://doi.org/10.5194/acp-20-15079-2020 |
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