Boundary layer effects on fronts over topography

A hydrostatic, primitive equation model with frontogenetical deformation fo rcing is used to study the effects of surface friction on fronts passing ov er a two-dimensional ridge. Surface friction is parameterized using a K-the ory planetary boundary layer (BL) parameterization with implicitly defined diffusion coefficients, following Keyser and Anthes. Previous studies witho ut surface friction, such as Williams et al., show that a cold front weaken s 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 bas ic flow over the mountain (divergence on the upwind slope and convergence o n 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 real istic frontal structure and the frontal intensity is weaker than for the fr ictionless front because weaker temperature gradients are created through v ertical 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 forcin g 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. T he divergent forcing is strongest within the BL and weak at the top. When B L effects are included, the final intensity of a front passing over a mount ain 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 frictionle ss front due to a more uniform flow within the BL.