Laboratory simulation of rotating atmospheric boundary layer flows over obstacles

Abstract

The present study fits in the frame of a research program concerning in general the dynamics of airflow in the atmospheric boundary layer and in particular the influence of terrestrial rotation on the movements of air masses interacting with natural extended obstacles (mountains). The experiment has been performed by the method of hydraulic simulation, using schematic models at reduced scale in a channel placed on a rotating platform. We only considered the case of a neutral atmosphere and studied the wake of an obstacle with semi-circular section and the reciprocal interaction of two obstacles of this kind placed perpendicularly to the flow. In this last case we investigated the influence of the distance between the obstacles on their wake length and on the vorticity conditions inside the wakes. Among the various results we obtained, the modifications of the velocity profiles over the reliefs and their dependence on the rotation velocity are particularly interesting. We discuss here our results from two different points of view, namely the purely hydraulic one (which includes the effects of different rotation velocities) and the atmospheric one, according to which the model simulates—with given reduction scales—an actual situation characterized by a fixed value of the Coriolis parameter. As to the first approach to the problem, we found that: 1) The roll with horizontal axis, which is observed behind an obstacle, becomes narrower and narrower as the rotation velocity of the platform increases, while its stability in time and its definition in space increase. In general, it may be said that rotation plays a stabilizing role for vortex dimensions and velocity profiles. 2) The transversal velocity behind the obstacles may attain values about twice the mean longitudinal velocity of the flow. 3) When rotating, the roll is thicker at its left edge than on the right one, due to the transversal flux which provides fluid supply. 4) When the thickness of the boundary layer is increased by making the channel bottom rough, the above-mentioned phenomena are emphasized; moreover, with a second obstacle placed in the flow (and not too far from the first) the transversal velocity components increase, due to a canalization effect. 5) The accelerations of the low layers over the obstacles are strongly amplified by rotation, due to the action of Coriolis' force. As for the second approach, we checked the extent to which the simulation procedures adopted for our laboratory flows, and for their boundary conditions, can be representative of the features of atmospheric phenomena. In order to do that, we compared the dynamical structure of the low flow layers over the obstacle with the analogous structure observed in the field and in wind tunnel by other authors, as well as with the predictions of a few theoretical models. Inside the lowest part of the planetary boundary layer, where the overshooting due to the relief is confined, a good consistency was found among all these results, in particular for what concerns the maximum of overflow velocity.