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| Titel |
Impact of Forced Roll Convection on Turbulent Fluxes in Cold Air Outbreak Situations |
| VerfasserIn |
Jens Fricke, Micha Gryschka, Siegfried Raasch |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250082694
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| Zusammenfassung |
| Roll convection is a common phenomenon in atmospheric convective boundary layers (CBL)
with background wind. It is often argued that the presence of roll vortices may increase
vertical transports which should be considered in parameterizations of weather and climate
models. At least, in case of roll convection within cold air outbreaks (CAOs), our study does
not support this idea.
Gryschka et al. [2008, Geophys. Res. Lett., 35] showed with large eddy simulations
(LES) that in strong CAOs, roll vortices develop only under the presence of heterogeneities in
the sea ice distribution. These rolls extended up to several hundred kilometers downstream
over open water. Since they are generated at the heterogeneities, they called them forced rolls.
Roll vortices due to pure self organization of the flow occurred only in weak and
moderate CAOs and were called free rolls in their study (not triggered by the sea ice
distribution).
Due to these findings, we were able to simulate CAOs with and without rolls under the
same meteorological conditions. We performed a parameter study of strong CAOs with LES
using non-cyclic lateral boundary conditions in flow direction. For each case, we carried out
two simulations, one with a sharp ice edge so that no rolls are generated and one with an
idealized heterogeneous ice edge in order that forced rolls arise. We chose twelve cases with
varying geostrophic wind speeds and temperature differences between ice and water.
Furthermore, we varied the wavelength of the rolls which can be controlled by the
heterogeneity.
Our results show no significant differences in the development of the CBL and total
vertical fluxes between the roll and non-roll cases. However, the rolls take over a part of the
turbulent transport. In some cases, the vertical transport of momentum by rolls is larger than
the transport by small scale turbulence.
These findings suggest that there is no need to consider roll convection in parameterizations
of weather and climate models in case of CAOs. |
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