Boundary Layer Effects on Fronts over Topography

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 frontal intensity is weaker than for the frictionless front because weaker temperature gradients are created through vertical mixing. The same type of mixing acts to strengthen a cold front on the upwind slope and weaken it on the downwind slope. The divergence forcing is also frontogenetic on the upwind slope and frontolyic on the lee slope within the BL. The vertical mixing forcing is strongest near the top of BL and weaker within the BL due to weak temperature gradient within the BL. The divergent forcing is strongest within the BL and weak at the top. When BL effects are included, the final intensity of a front passing over a mountain is weaker than the front over flat topography. The translation of the front is slightly slower with the BL because of the overall reduced cross-frontal speed by surface friction. When moving over a mountain, a front with the BL has a more uniform speed than the frictionless front due to a more uniform flow within the BL.