Abstract
In boundary layer modelling, the Monin-Obukhov theory is widely used
to parmeterize the wind profiles and the momentum fluxes. During the
IPY-THORPEX campaign at Andøya at 3. March 2008, a flight was made
across an arctic front, dropping several sondes to measure atmospheric
parameters from about 7500 meters down to the surface. Due to the
horizontal gradient in potential temperature, the vertical shear in the
geostrophic wind was strongly reversed. This results in a low level jet with
high wind velocities close to the surface. The task of this master thesis is
to investigate the influence of this reversed shear on the momentum fluxes
in the turbulent atmospheric boundary layer. The main parameters are the
wind stress τ , the drag coefficient CD and the stability function φm ( z/ L).
A one-dimensional numerical boundary layer model is developed and run
with different geostrophic wind profiles. Geostrophic wind profiles with
reversed shear from the dropsonde observations are compared to runs with
constant geostrophic wind profiles. Equivalent runs are done with PALM,
a Large Eddy Simulation model (LES).
The simulations with reversed shear in the geostrophic wind gives a wind
stress profile which is curved and is decreasing faster with heigth at the
lowest few hundred meters than the results from the runs with a constant
geostrophic wind. Model runs with reversed shear gives a drag coefficient
which is slightly higher, and a stability function which is reduced compared
to the results from the runs with a constant geostrophic wind profile.