On the definition of appropriate eddy viscosity and eddy diffusivities for the surface layer of thermally driven slope winds

Up-slope and down-slope winds are basic components of a variety of flows composing the system of thermally driven circulations over complex terrain. They commonly develop over inclines under daytime heating and nighttime cooling, respectively. The basic mechanisms of these flows are nowadays quite understood, after many research efforts performed in the last decades, including both intensive field measurements and high-resolution numerical model simulations. However, differently from the case of flat horizontal terrain, where Monin-Obukhov similarity theory provides a well-established conceptual framework, for the above winds the scientific community has not yet reached a consensus on unifying theories explaining the properties and structure of turbulence associated with them, e.g. in terms of generally applicable scaling laws. The latter would be beneficial for providing appropriate parameterizations in numerical models for weather and climate prediction, as well as for simulations of other atmospheric processes, such as pollutant transport, or for the evaluation of surface energy and water budgets for hydrological, glaciological, and ecological purposes.

Here we concentrate on the evaluation of the mixing properties of the surface layer associated with slope winds. In this layer strong slope-normal gradients are known to occur, and K-closures are expected to be reasonable closures for turbulent fluxes of momentum, heat and mass, provided appropriate expressions are found for eddy viscosity and eddy diffusivities. In the present contribution some solutions for K coefficients are proposed, based on the analysis of data from various field campaigns and similarity arguments. The resulting formulations are tested on the basis of their success in reproducing observed structures of first and second order moments.