Boundary Layer Effects on Fronts over Topography

Abstract

Abstract A hydrostatic, primitive equation model with frontogenetical deformation forcing is used to study the effects of surface friction on fronts passing over a two-dimensional ridge. Surface friction is parameterized using a K-theory planetary boundary layer (BL) parameterization with implicitly defined diffusion coefficients, following Keyser and Anthes. Previous studies without surface friction, such as Williams et al., show that a cold front weakens on the upwind slope and intensifies on the lee slope. This is in part due to a superposition effect of mountain flow where colder temperatures exist over the crest and in part due to the divergence pattern caused by the basic flow over the mountain (divergence on the upwind slope and convergence on the lee slope). In Williams et al., the final intensity of a front after passing a symmetric mountain is the same as a front moving over flat land. For no-mountain simulations, the inclusion of the BL results in a more realistic frontal structure and the fron...