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| Titel |
Modelling atmospheric structure, cloud and their response to CCN in the central Arctic: ASCOS case studies |
| VerfasserIn |
C. E. Birch, I. M. Brooks, M. Tjernström, M. D. Shupe, T. Mauritsen, J. Sedlar, A. P. Lock, P. Earnshaw, P. O. G. Persson, S. F. Milton, C. Leck |
| 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 ; 12, no. 7 ; Nr. 12, no. 7 (2012-04-11), S.3419-3435 |
| Datensatznummer |
250011020
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| Publikation (Nr.) |
 copernicus.org/acp-12-3419-2012.pdf |
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| Zusammenfassung |
| Observations made during late summer in the central Arctic Ocean, as part of
the Arctic Summer Cloud Ocean Study (ASCOS), are used to evaluate cloud and
vertical temperature structure in the Met Office Unified Model (MetUM). The
observation period can be split into 5 regimes; the first two regimes had a
large number of frontal systems, which were associated with deep cloud.
During the remainder of the campaign a layer of low-level cloud occurred,
typical of central Arctic summer conditions, along with two periods of
greatly reduced cloud cover. The short-range operational NWP forecasts could
not accurately reproduce the observed variations in near-surface
temperature. A major source of this error was found to be the
temperature-dependant surface albedo parameterisation scheme. The model
reproduced the low-level cloud layer, though it was too thin, too shallow,
and in a boundary-layer that was too frequently well-mixed. The model was
also unable to reproduce the observed periods of reduced cloud cover, which
were associated with very low cloud condensation nuclei (CCN) concentrations
(<1 cm−3). As with most global NWP models, the MetUM does not have a
prognostic aerosol/cloud scheme but uses a constant CCN concentration of
100 cm−3 over all marine environments. It is therefore unable to represent
the low CCN number concentrations and the rapid variations in concentration
frequently observed in the central Arctic during late summer. Experiments
with a single-column model configuration of the MetUM show that reducing
model CCN number concentrations to observed values reduces the amount of
cloud, increases the near-surface stability, and improves the representation
of both the surface radiation fluxes and the surface temperature. The model
is shown to be sensitive to CCN only when number concentrations are less
than 10–20 cm−3. |
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