In boundary layer modelling, the Monin-Obukhov theory is widely usedto parmeterize the wind proﬁles and the momentum ﬂuxes. During theIPY-THORPEX campaign at Andøya at 3. March 2008, a ﬂight was madeacross an arctic front, dropping several sondes to measure atmosphericparameters from about 7500 meters down to the surface. Due to thehorizontal gradient in potential temperature, the vertical shear in thegeostrophic wind was strongly reversed. This results in a low level jet withhigh wind velocities close to the surface. The task of this master thesis isto investigate the inﬂuence of this reversed shear on the momentum ﬂuxesin the turbulent atmospheric boundary layer. The main parameters are thewind stress τ , the drag coefﬁcient CD and the stability function φm ( z/ L).A one-dimensional numerical boundary layer model is developed and runwith different geostrophic wind proﬁles. Geostrophic wind proﬁles withreversed shear from the dropsonde observations are compared to runs withconstant 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 windstress proﬁle which is curved and is decreasing faster with heigth at thelowest few hundred meters than the results from the runs with a constantgeostrophic wind. Model runs with reversed shear gives a drag coefﬁcientwhich is slightly higher, and a stability function which is reduced comparedto the results from the runs with a constant geostrophic wind proﬁle.