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| Titel |
Simulation of low clouds in the Southeast Pacific by the NCEP GFS: sensitivity to vertical mixing |
| VerfasserIn |
R. Sun, S. Moorthi, H. Xiao, C. R. Mechoso |
| 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 ; 10, no. 24 ; Nr. 10, no. 24 (2010-12-23), S.12261-12272 |
| Datensatznummer |
250008979
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| Publikation (Nr.) |
 copernicus.org/acp-10-12261-2010.pdf |
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| Zusammenfassung |
| The NCEP Global Forecast System (GFS) model has an important systematic
error shared by many other models: stratocumuli are missed over the
subtropical eastern oceans. It is shown that this error can be alleviated in
the GFS by introducing a consideration of the low-level inversion and making
two modifications in the model's representation of vertical mixing. The
modifications consist of (a) the elimination of background vertical
diffusion above the inversion and (b) the incorporation of a stability
parameter based on the cloud-top entrainment instability (CTEI) criterion,
which limits the strength of shallow convective mixing across the inversion.
A control simulation and three experiments are performed in order to examine
both the individual and combined effects of modifications on the generation
of the stratocumulus clouds. Individually, both modifications result in
enhanced cloudiness in the Southeast Pacific (SEP) region, although the
cloudiness is still low compared to the ISCCP climatology. If the
modifications are applied together, however, the total cloudiness produced
in the southeast Pacific has realistic values. This nonlinearity arises as
the effects of both modifications reinforce each other in reducing the
leakage of moisture across the inversion. Increased moisture trapped below
the inversion than in the control run without modifications leads to an
increase in cloud amount and cloud-top radiative cooling. Then a positive
feedback due to enhanced turbulent mixing in the planetary boundary layer by
cloud-top radiative cooling leads to and maintains the stratocumulus cover.
Although the amount of total cloudiness obtained with both modifications has
realistic values, the relative contributions of low, middle, and high layers
tend to differ from the observations. These results demonstrate that it is
possible to simulate realistic marine boundary clouds in large-scale models
by implementing direct and physically based improvements in the model
parameterizations. |
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