Turbulente flukser ved reversert skjær i den geostrofiske vinden

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.