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| Titel |
An investigation into the performance of four cloud droplet activation parameterisations |
| VerfasserIn |
E. Simpson, P. Connolly, G. McFiggans |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1991-959X
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| Digitales Dokument |
URL |
| Erschienen |
In: Geoscientific Model Development ; 7, no. 4 ; Nr. 7, no. 4 (2014-07-24), S.1535-1542 |
| Datensatznummer |
250115669
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| Publikation (Nr.) |
 copernicus.org/gmd-7-1535-2014.pdf |
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| Zusammenfassung |
| Cloud droplet number concentration prediction is central to large-scale weather and climate modelling. The benchmark cloud parcel model
calculation of aerosol particle growth and activation, by diffusion of
vapour to aerosol particles in a rising parcel of air experiencing
adiabatic expansion, is too computationally expensive for use in large-scale global models. Therefore the process of activation of aerosol
particles into cloud droplets is parameterised with an aim to strike
the optimum balance between numerical expense and accuracy. We present
a detailed systematic evaluation of three cloud droplet activation
parameterisations that are widely used in large-scale models and one recent update. In all
cases, it is found that there is a tendency to overestimate the
fraction of activated aerosol particles when the aerosol particle
"median diameter" is large (between 250 and 2000 nm) in a single lognormal mode
simulation. This is due to an infinite "effective simulation time"
of the parameterisations compared to a prescribed simulation time in
the parcel model. This problem arises in the parameterisations because
it is assumed that a parcel of air rises to the altitude where maximum
supersaturation occurs, regardless of whether this altitude is above the
cloud top. Such behaviour is problematic because, in some cases, large
aerosol can completely suppress the activation of drops. In some cases when
the "median diameter" is small (between 5 and 250 nm)
in a single lognormal mode the fraction of activated drops is
underestimated by the parameterisations. Secondly, it is found that in
dual-mode cases there is a systematic tendency towards underestimation
of the fraction of activated drops, which is due to the methods used by
the parameterisations to approximate the sink of water vapour. |
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