A Study of Boundary-Layer Pressure Adjustments

Abstract

An analytical two-layer approximation of atmospheric flow is developed to study boundary-layer production of vertical motion. The model consists of a boundary layer topped by a free-flow layer. Both layers am time-dependent and possess different values of stratification. The boundary-layer equations are layer-integrated over a fixed depth and surface stress is parameterized using a linearized surface drag law. The dynamics of this modeled flow are quite different from the case of constant eddy viscosity where the boundary layer depth is unrealistically sensitive to the dynamics. Production of vertical motion is found to be most efficient in the region of the critical latitude where the forcing frequency is comparable to the natural internal frequency of the flow. This internal frequency depends not only on the Coriolis parameter, but also on stratification and boundary-layer properties. For a fixed value of these parameters, the boundary-layer produces vertical motion most efficiently when forced at a preferred horizontal length. With sufficiently stratified mesoscale flows, significant boundary-layer pressure adjustments develop which decrease the circulation strength. Stratification is less effective in flows with large horizontal length scale. Results are interpreted for the case of flow forced by oscillating, differential boundary-layer heating.