Modeling turbulent fluxes in heterogeneous thermally stratified boundary layers

A common challenge in numerical models of atmospheric boundary layer flows (e.g., large-eddy simulations and weather prediction models) is the parameterization of grid-averaged surface fluxes. These fluxes are often calculated from the simulated velocity and scalar fields using Monin-Obukhov similarity theory. Since this approach is strictly only valid over homogeneous surfaces, its application to heterogeneous boundary layers can result in significant errors. Particularly challenging is the estimation of surface heat fluxes in heterogeneous stably stratified flows, where the assumption of constant surface-layer flux breaks down. Wind-tunnel experiments and large-eddy simulations have been carried out to test surface flux parameterizations in stably stratified boundary layers over heterogeneous surface temperature fields. Models based on Monin-Obukhov similarity are found to grossly underestimate the heat flux magnitude over relatively cold patches. This is due to the inability of these models to capture flux enhancement associated with local advection. Motivated by these results, a new flux parameterization is introduced based on local similarity theory and the assumption of linear (instead of constant) flux distribution. The new model yields improved predictions of local as well as areally-averaged surface fluxes.