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| Titel |
Limited-are a modelling of stratocumulus over South-Eastern Pacific |
| VerfasserIn |
M. Andrejczuk, W. W. Grabowski, A. Gadian, R. Burton |
| 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.3511-3526 |
| Datensatznummer |
250011025
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| Publikation (Nr.) |
 copernicus.org/acp-12-3511-2012.pdf |
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| Zusammenfassung |
| This paper presents application of the Weather Research and Forecasting (WRF)
model to limited-area modeling of atmospheric processes over the subtropical
south-eastern Pacific, with the emphasis on the stratocumulus-topped boundary
layer. The simulations cover a domain from the VAMOS (Variability of the
American Monsoon Systems) Ocean-Cloud-Atmosphere-Land Study Regional
Experiment (VOCALS-REx) field project conducted in the subtropical
south-eastern Pacific in October and November 2008. We focus on a day where
the UK's BAe-146 research aircraft encountered Pockets of Open Cells (POCs)
at the very western edge of its flight track, rather than on the entire
campaign as investigated in previous limited-area modeling studies. Model
results are compared to aircraft observations with the main conclusion that
the simulated stratocumulus-topped boundary layer is significantly too
shallow. This appears to be a combination of an already too shallow boundary
layer in the dataset used to provide initial and lateral boundary conditions,
and the inability of the WRF model to increase the boundary-layer height.
Several sensitivity simulations, applying different subgrid-scale
parameterizations available in the model, a larger computational domain and
longer simulations, as well as a different dataset providing initial and
lateral boundary conditions were all tried to improve the simulation. These
changes appeared to have a rather small effect on the results.
The model does simulate the formation of mesoscale cloud-free regions
that one might consider similar to Pockets of Open Cells observed
in nature. However, formation of these regions does not seem to be
related to drizzle-induced transition from open- to closed-cell
circulations as simulated by LES models. Instead, the cloud-free
regions appear to result from mesoscale variations of the
lower-tropspheric vertical velocity. Areas of negative vertical
velocity with minima (a few cm s−1) near the boundary layer top seem
to induce direct evaporation of the cloud layer. It remains to be
seen in LES studies whether the mechanism seen in the model is
realistic or if it is simply an artifact of interactions between
resolved and parameterized processes. |
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