Katabatic flow: a closed-form solution with spatially-varying eddy diffusivities

The Nieuwstadt closed-form solution for the stationary Ekman layer is generalized for katabatic flows within the conceptual framework of the Prandtl model. The proposed solution is valid for spatially-varying eddy viscosity and diffusivity (O’Brien type) and constant Prandtl number (Pr). Variations in the velocity and buoyancy profiles are discussed as a function of the dimensionless model parameters z0≡z^0N^2Prsin(α)|b^s|-1 and λ≡u^refN^Pr|b^s|-1, where z^ ₀ is the hydrodynamic roughness length, N^ is the Brunt-Väisälä frequency, α is the surface sloping angle, b^ _s is the imposed surface buoyancy, and u^ _ref is a reference velocity scale used to define eddy diffusivities. Velocity and buoyancy profiles show significant variations in both phase and amplitude of extrema with respect to the classic constant K model and with respect to a recent approximate analytic solution based on the Wentzel-Kramers-Brillouin theory. Near-wall regions are characterized by relatively stronger surface momentum and buoyancy gradients, whose magnitude is proportional to z₀ and to λ. In addition, slope-parallel momentum and buoyancy fluxes are reduced, the low-level jet is further displaced toward the wall, and its peak velocity depends on both z₀ and λ.