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Abstract
In the atmospheric boundary layer, turbulent structures are responsible for the transport
of heat and momentum. In numerical weather forecast models a horizontal grid
resolution of several kilometers is used, so that small-scale turbulent elements of some
hundred meters (microturbulence) cannot be resolved explicitly. Therefore, a parameterization
is used in those models in order to calculate the turbulent transport from the
explicitly known mean variables. Due to this interrelation the quality of the turbulence
parameterization plays a crucial role for the quality of the weather forecast.
In the last years it turned out that thermal inhomogeneities of the land surface can
lead to persisting circulations being much larger than the small-scale turbulence. It was
discovered that these circulations have influence on the characteristica of the turbulence
in the bounday layer and therefore also on the transport of heat and momentum. Usual
parameterizations do not take into account these effects and are based on a homogeneous
spectrum of turbulence.
Numerical investigations of the influence of inhomogeneities on the boundary layer
lead to first estimates how to adapt current turbulence parameterizations. But in these
studies only idealistic surface conditions (stripe- and checkerboard-like heterogeneities),
idealistic model forcings and idealistic boundary layer conditions were applied. Moreover,
these studies used a limited parameter space. Therefore, the first part of this thesis
deals with a significant extension of these former idealistic studies. With the help of the
generated results it is possible to unite current contrary interpretations of the influence
of the inhomogeneity on the boundary layer and to create a general theory.
In the main part of this thesis a totally new approach towards the investigation of
realistic inhomogeneities is chosen. In the large-eddy simulation model PALM different
data sets coming from the LITFASS-2003 experiment are used for a realistic model
forcing, but also for a comparison to the results of PALM. With the use of ensembleaverages
of time-averaged, threedimensional model data it is possible to extract the
structure of thermally induced mesoscale circulations over a realistic heterogeneous land
surface and to extract the heat fluxes resulting from these circulations.
In contrast to results from existing publications the horizontally averaged heat fluxes
induced by the realistic heterogeneity in LITFASS-2003 depend on the daytime and
are relatively small compared to the heat fluxes aroused by the small-scale turbulence.
Nevertheless, a comparison with local in-situ measurements in the lower and mid-part
of the boundary layer shows that the heterogeneity-driven heat fluxes might lead to an
error in these measurements.
In contrast to most of the idealistic studies a change in the mean profiles of heat
fluxes is only observed in some extraordinary situations. Therefore, no significant global
effect of the thermally induced mesoscale circulations on the turbulent heat fluxes exists,
so that an adaption of current parameterization of turbulent fluxes in numerical weather
forecast models seems not to be necessary.
Keywords: thermal heterogeneity, mesoscale circulation, large-eddy simulation
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