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| Titel |
On the Influence of Surface Heterogeneities onto Roll Convection |
| VerfasserIn |
M. Gryschka, C. Drüe, S. Raasch, D. Etling |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250030735
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| Zusammenfassung |
| Roll convection is a common phenomenon in atmospheric convective boundary layers
(CBL) with background wind. Roll convection is observed both over land and over
sea for different synoptic situations. There is still some debate about the different
types of roll convection and their causes or rather the necessary conditions for their
appearance.
The stability parameter ζ = -zi-L (zi: boundary layer height, L: Monin-Obukhov
stability length) is widely used as a predictor for roll convection, since numerous studies
suggest that convective rolls only appear when 0 < ζ < 20. In other words, roll development
becomes unlikely for strong surface heating and weak vertical wind shear. In contrast to those
studies the presence of roll convection in almost any polar cold air outbreak (as can be
seen in numerous satellite images as cloud streets) reveals that even for large ζ roll
convection can develop. Some studies report roll convection in cold air outbreaks for
ζ = 250.
Our large eddy simulations (LES) on roll convection suggests that the contrasting
results concerning the dependency of roll convection on ζ are due to two different
types of roll convection: One type which develops purely by self organization if
ζ < 20 (“free rolls”) and another type which is triggered by heterogeneities in surface
temperature and develops also for large ζ (“forced rolls”). We think that most of the
cloud streets observed in polar cold air outbreaks over open water are due to rolls
of forced type which are tied to upstream located heterogeneities in the sea-ice
distribution.
The results of this study suggests that the omission of surface inhomogeneities in
previous LES is the reason for the absence of rolls in all LES with strong surface heating
and weak vertical wind shear so far. In this contribution we will present a large
eddy simulation which successfully represents forced rolls under such conditions. |
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