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| Titel |
How Turbulence should be represented in Atmospheric Models at the kilometric Scale. |
| VerfasserIn |
Rachel Honnert, Valéry Masson, Fleur Couvreux |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250031733
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| Zusammenfassung |
| Turbulence is well-represented by atmospheric models for very fine meshes, from
10 to 100 m, for which turbulent movements are mainly resolved, and for very
large meshes, greater than 2 km, for which they are entirely parametrized. But what
happens at intermediate scales, the so-called "Terra Incognita" from Wyngaard (2004)
?
Here an original method is presented which makes it possible to calculate the subgrid and
resolved parts of five parameters at different scales : Turbulent Kinetic Energy (TKE), heat
and moisture fluxes, as well as potential temperature and mixing ratio variances. They are
established at intermediate scales in the case of shear-free convective boundary layers. The
Theorem of Similarity allows the determination of the dimensionless variables of the
problem. It is determined that a new dimensionless variable needs to be added to the
Deardorff free convective scaling variables : the scaled mesh size, Îx-
h. Similarity functions
for the subgrid and resolved parts are assumed to be the product of the similarity function of
the total (subgrid plus resolved) parameters and a "partial" similarity function that
depends only on Îx-
h. In order to determine this "partial" similarity function form,
Large Eddies Simulations (LES) of boundary layers have been performed. They are
based on experimental data from field-experiments. The resolved and subgrid parts
of the parameters at coarser resolutions are then derived from means of the LES
fields.
The evolution of the subgrid and resolved parts with Îx-
h is as follows : at high resolutions,
parameters are mainly resolved. Then the more coarse the mesh, more eddies are subgrid.
Finally, for very large meshes, turbulence is entirely subgrid. A scale therefore exists for
which the subgrid and resolved parts are equal. This is obtained for Îx-
h = 0.2 in the case of
TKE, 0.4 for the potential temperature variance and 0.8 for the mixing ratio variance.
This means that the vertical wind structures are smaller than those for the potential
temperature which are smaller than those for the mixing ratio (De Roode et al.
(2004)).
The defaults of a mesoscale model at intermediate scales are succinctly described. In the
case of free convective boundary layer, the model produces too many resolved movements, as
the turbulence scheme does not sufficiently represent the impact of the subgrid thermal.
This is not true when a mass-flux scheme is introduced. However in this case, a
completely subgrid thermal is modeled leading to an overestimation of the subgrid part. |
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