Separation of upslope flow over a uniform slope

Abstract

Motivated by the importance of understanding mountain weather during periods of thermal convection, a laboratory study was conducted to investigate the separation of an upslope (anabatic) flow on a two-dimensional heated mountainous slope in the absence of a background mean flow. Three flow regimes were identified. In the first, at slope angles β larger than a critical value β c ≈ 20 • , the separated flow generated a rising plume completely fed by the anterior upslope flow. For this case, a simple model based on a balance between the opposing vorticities of baroclinicity and shear was proposed to predict the location of the separation point relative to the mountain base. The model also predicts the velocity and length scales at separation as well as those of the rising plume after separation. In the second regime, 10 • < β β c , the volume flow of the separated plume was not fully supplied by the upslope flow, requiring entrainment of additional ambient fluid at the base of the plume source. The third regime occurred when β 10 • , wherein the plume almost completely engulfed the slope, similar to a buoyant plume emanating from a source of finite dimensions, thus overshadowing the upslope flow. Measurements of the separation point conducted during the MATERHORN field research program were consistent with the results of the laboratory experiments and modelling.

Exchange Processes in the Atmospheric Boundary Layer Over Mountainous Terrain

“…Little progress has been made ever since. Poorly investigated issues include (1) the conditions under which upslope flow is disrupted by convective instability, leading to the formation of coherent turbulent structures (rolls, plumes) and eventually to enhanced mixing between the slope flow layer and the rest of the valley atmosphere [103]; (2) the response of the flow to the spatial variability of the lower boundary condition and of the atmosphere above the slope wind layer; (3) the morning and evening transitions, in particular the small-scale coherent structures that develop in these periods and that determine the phase lag of the wind reversal at different heights; (4) the relative contribution of upslope flow and horizontal entrainment to the mass flux within thermal plumes at mountain tops [104].…”

Separation of Upslope Flow over a Plateau

“…The vorticity of the upslope flow is countered by the baroclinic generation of vorticity (baroclinic torque), which facilitates flow separation [6]. In the case of flow separation, a thermal plume is formed, and deep convective clouds can often be observed directly over the mountain peak [7,8].…”

“…Though laboratory studies are advantageous, as they allow repetitive and well controlled measurements of high accuracy in order to deduce the important relations, only a few studies have been performed in such a controlled environment [16,17,23]. Recently, Hocut et al [7] conducted laboratory experiments on flow separation over smooth, heated slopes. The study was performed without background stratification, which represents the case of the upslope flow after the morning transition.…”

“…Hocut et al [7] focused on the flow over a uniform smooth slope. Terrain in nature, however, is more complex, as the natural topography includes breaking slopes, convex/concave slopes, roughness gradients because of the varying vegetation density, slopes topped with a plateau, and more.…”

“…Terrain in nature, however, is more complex, as the natural topography includes breaking slopes, convex/concave slopes, roughness gradients because of the varying vegetation density, slopes topped with a plateau, and more. Hence, the next logical step would be to moderately increase the complexity of the terrain that was used by Hocut et al [7], toward a representation of a more realistic case. To this end, this paper considers the case of a uniformly smooth slope with a plateau at the top, as observed in the natural settings [10].…”