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| Titel |
The importance of vertical velocity variability for estimates of the indirect aerosol effects |
| VerfasserIn |
R. E. L. West, P. Stier, A. Jones, C. E. Johnson, G. W. Mann, N. Bellouin, D. G. Partridge, Z. Kipling |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 14, no. 12 ; Nr. 14, no. 12 (2014-06-26), S.6369-6393 |
| Datensatznummer |
250118839
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| Publikation (Nr.) |
 copernicus.org/acp-14-6369-2014.pdf |
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| Zusammenfassung |
The activation of aerosols to form cloud droplets is dependent upon
vertical velocities whose local variability is not typically resolved
at the GCM grid scale. Consequently, it is necessary to represent
the subgrid-scale variability of vertical velocity in the calculation
of cloud droplet number concentration.
This study uses the UK Chemistry and Aerosols community model (UKCA)
within the Hadley Centre Global Environmental Model (HadGEM3), coupled
for the first time to an explicit aerosol activation parameterisation,
and hence known as UKCA-Activate. We explore the range of uncertainty
in estimates of the indirect aerosol effects attributable to the choice
of parameterisation of the subgrid-scale variability of vertical
velocity in HadGEM-UKCA. Results of simulations demonstrate that the
use of a characteristic vertical velocity cannot replicate results
derived with a distribution of vertical velocities, and is to be discouraged
in GCMs.
This study focuses on the effect of the variance (σw2)
of a Gaussian pdf (probability density function) of vertical velocity. Fixed values of σw
(spanning the range measured in situ by nine flight campaigns
found in the literature) and a configuration in which σw
depends on turbulent kinetic energy are tested. Results from the mid-range
fixed σw and TKE-based configurations both compare
well with observed vertical velocity distributions and cloud droplet
number concentrations.
The radiative flux perturbation due to the total effects of anthropogenic
aerosol is estimated at −1.9 W m−2 with σw = 0.1 m s−1,
−2.1 W m−2 with σw derived from
TKE, −2.25 W m−2 with σw = 0.4 m
s−1,
and −2.3 W m−2 with σw = 0.7 m s−1.
The breadth of this range is 0.4 W m−2, which is comparable to a substantial fraction of the total diversity of current aerosol forcing
estimates. Reducing the uncertainty in the parameterisation of σw
would therefore be an important step towards reducing the uncertainty
in estimates of the indirect aerosol effects.
Detailed examination of regional radiative flux perturbations reveals
that aerosol microphysics can be responsible for some climate-relevant
radiative effects, highlighting the importance of including microphysical
aerosol processes in GCMs. |
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