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| Titel |
Parameterizations for convective transport in various cloud-topped boundary layers |
| VerfasserIn |
M. Sikma, H. G. Ouwersloot |
| 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 ; 15, no. 18 ; Nr. 15, no. 18 (2015-09-23), S.10399-10410 |
| Datensatznummer |
250120043
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| Publikation (Nr.) |
 copernicus.org/acp-15-10399-2015.pdf |
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| Zusammenfassung |
We investigate the representation of convective transport of
atmospheric compounds by boundary layer clouds. We focus
on three key parameterizations that, when combined, express
this transport: the area fraction of transporting clouds, the upward
velocity in the cloud cores and the chemical concentrations at
cloud base. The first two parameterizations combined represent the
kinematic mass flux by clouds.
To investigate the key parameterizations under a wide range of
conditions, we use large-eddy simulation model data for 10 meteorological situations, characterized by either shallow cumulus
or stratocumulus clouds. The parameterizations have not been previously tested with such large data sets. In the analysis,
we show that the parameterization of the area fraction of clouds currently
used in mixed-layer models is affected by boundary layer dynamics. Therefore,
we (i) simplify the independent variable used for this parameterization, Q1, by
considering the variability in moisture rather than in the saturation deficit and
update the parameters in the parameterization to account for this simplification.
We (ii) next demonstrate that the independent variable has to be evaluated
locally to capture cloud presence. Furthermore, we (iii) show that the area
fraction of transporting clouds is not represented by the parameterization
for the total cloud area fraction, as is currently assumed in literature. To capture
cloud transport, a novel active cloud area fraction parameterization is proposed.
Subsequently, the scaling of the upward velocity in cloud cores
by the Deardorff convective velocity scale and the parameterization
for the concentration of atmospheric reactants at cloud base from
literature are verified and improved by analysing six shallow cumulus cases.
For the latter, we additionally discuss how the
parameterization is affected by wind conditions. This study contributes to a more
accurate estimation of convective transport, which occurs at
sub-grid scales. |
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