Fronts and the boundary layer - Some numerical studies

In this paper, we consider boundary-layer effects on atmospheric fronts. Fo r this purpose we developed a dry,hydrostatic, two-dimensional numerical mo del with turbulence parameterized with the 1 1/2-order eddy viscosity closu re of Mellor and Yamada. In the surface layer we use Monin-Obukhov similari ty and there is an interactive soil model to obtain lower boundary conditio ns for the atmospheric model. First, effects of turbulence are studied with out thermal forcing at the surface and comparisons are made between invisci d and turbulent conditions. Sensitivity studies with respect to surface rou ghness are also performed. Subsequently we study effects of surface thermal forcing on fronts. The results show that, during summer, fronts display st rong diurnal variation. Vertical velocities increase significantly during t he day while gradients in temperature and vorticity across the front streng then in the evening and at night. In winter the diurnal variation is much l ess pronounced. We attempt to find physical mechanisms responsible for such frontal behaviour and we compare our results with available observations.