Turbulence generation by unresolved orography

Slope flows, resulting from the interplay between buoyancy and gravitational forces, are well-known to govern a plethora of local weather phenomena. In particular, orographic features and the associated surface roughness can induce turbulent mixing in the planetary boundary layer. While orographic drag models have been proposed to understand the effects of turbulence and waves due to orography, numerical simulations locally rely on closures based on the Monin-Obukhov Similarity Theory. The validity of these models and their interaction regarding turbulence production due to orography at unresolved scales is questionable. We study the turbulence generation by small-scale orography under the influence of stable stratification and weak mixing. To bypass the common complications with surface modeling, we use direct numerical simulation featuring a shallow valley to study the problem at a reduced scale. To imitate the intricate boundary conditions, an Immersed Boundary Method is used that features fully resolved three-dimensional roughness elements in the form of a local valley. However, modeling such flows also poses challenges due to the numerous parameters governing the triggering of turbulence. In this presentation, we introduce a scaling framework orographic for the problem and a viable numerical set-up along with the first results from preliminary studies at intermediate scale separation.

* This work is funded by the ERC Starting Grant ”Turbulence-Resolving Approaches of the Intermittently Turbulent Atmospheric Boundary Layer [trainABL]” of the European Research Council (funding ID 851347).