Extension of the Mixed-Layer Concept to Steep Topography

The conceptual model of a well-mixed, dry-adiabatic boundary layer driven by surface heating and separated from the free atmosphere by a capping inversion has been successfully applied in flat terrain. It can be extended by linearization to gently sloping terrain provided thermally induced flows are taken into account. Little is known however about the characteristics of convective boundary layer growth in steep terrain. Which of the assumptions and simplifications implicit in the mixed-layer concept are still valid, and which have to be dropped? It is shown both theoretically and by numerical simulation for an area in the Colorado Rockies that the parameterization of entrainment at the top of the mixed-layer must be extended. Specifically, the assumption of a one-way turbulent transport which considers the entrainment of outside air into the mixed-layer but does not allow for detrainment, is no longer valid. In steep terrain, convergence of upslope flows over ridges and mountain tops leads to a substantial flux of mixed-layer air into the free atmosphere (venting). This happens primarily in areas where the mixed-layer top rises steeply. The fact that it may lead to a discontinuity in the mixed-layer top sheds some light on the mechanism of elevated mixed-layer formation.