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Titel |
The influence of small scale heterogeneities on the near surface turbulence structure and the energy balance closure |
VerfasserIn |
Linda Voß, Siegfried Raasch |
Konferenz |
EGU General Assembly 2011
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Medientyp |
Artikel
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Sprache |
Englisch
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 13 (2011) |
Datensatznummer |
250052895
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Zusammenfassung |
Inhomogeneities due to differences in the surface structure may trigger secondary circulations
in the convective boundary layer. They reach the top of the boundary layer if the wavelength
of the heterogeneity is in the order or larger than the boundary layer height. Turbulence
measurements with the eddy-covariance method are not able to detect these fluxes because of
their stationarity. These circulations are thus suspected to be partly responsible for the
non-closure of the energy balance in a height of 2 m (height of energy balance
stations).
Previous studies have shown that the mentioned circulations can influence tower
measurements but that they have nearly no influence in a height of 2 m. Therefore
heterogeneities with diameters of a few hundred meters are suspected to be responsible for
the observed non-closure. As the fluxes cannot be measured in field experiments, sinusoidal
heat flux heterogeneities with a wavelength of up to 500 m are simulated with the parallelized
LES model PALM and the developing circulations are analyzed.
The wavelength of the heterogeneity has a big influence on the strength of the circulation.
With growing wavelength the circulation gets stronger and reaches larger heights. However,
the generated circulations are clearly weaker than circulations induced by heterogeneities
with wavelengths in the order of the boundary layer height. Increasing the background
wind from 1 m s-1 to 6 m s-1 decreases the strength of the circulation for each
wavelength.
The energy imbalance is determined and investigations are made concerning the
influence of the wavelength on the spatially averaged imbalance and of the position
within a heat flux wave on the local imbalance. Increasing the wavelength of the
heterogeneity leads to just a small rise in the imbalance. The position within the wave has a
greater influence on the imbalance as there is nearly no imbalance in the part of
the wave where the vertical velocity of the circulation is zero and the imbalance
increases when the vertical velocity increases. The results of this study suggest
that small scale heterogeneities cannot be responsible for the non-closure of the
energy balance because the circulations are only in small parts of the heat flux
wave producing an imbalance of over 10 %, which is observed in field experiments. |
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