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Manfred H. Bauer, Georg J. Mayr, Ignaz Vergeiner, and Helmut Pichler

Abstract

The influence of the obstacle shape, expressed through the ratio of spanwise to streamwise extension β, on flow over and around a mesoscale mountain is examined numerically. The initial wind U as well as the buoyancy frequency N are constant; the earth’s rotation and surface friction are neglected. In these conditions the flow response depends primarily on the nondimensional mountain height H m = h m N/U (where h m is the maximum mountain height) and the horizontal aspect ratio β. A regime diagram for the onset of wave breaking, lee vortex formation, and windward stagnation is compiled. When β is increased, smaller H m are required for the occurrence of all three features. It is demonstrated that lee vortices can form with neither wave breaking above the lee slope nor upstream stagnation. For β ⩽ 0.5 a vortex pair can appear although the isentropes above the lee slope do not overturn for any H m. For β > 1, on the other hand, lee vortex formation is triggered by wave breaking. On the windward side two distinct processes can lead to a complete blocking of the flow: the piling up of heavier air ahead of the barrier and the upstream propagation of columnar modes, which are generated by the wave breaking process for β > 1. “High-drag” states and “downslope windstorms” exist above the threshold of wave breaking as long as no lee vortices appear (or, at least, as long as they are very small). Hence, the interval of H m where a high-drag state occurs becomes progressively larger for larger β. With the growth of lee vortices the maximum wind speed along the leeward slope is dampened. The normalized drag drops rapidly below its linear counterpart and asymptotically approaches zero.

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Michael Hill, Ron Calhoun, H. J. S. Fernando, Andreas Wieser, Andreas Dörnbrack, Martin Weissmann, Georg Mayr, and Robert Newsom

Abstract

Dual-Doppler analysis of data from two coherent lidars during the Terrain-Induced Rotor Experiment (T-REX) allows the retrieval of flow structures, such as vortices, during mountain-wave events. The spatial and temporal resolution of this approach is sufficient to identify and track vortical motions on an elevated, cross-barrier plane in clear air. Assimilation routines or additional constraints such as two-dimensional continuity are not required. A relatively simple and quick least squares method forms the basis of the retrieval. Vortices are shown to evolve and advect in the flow field, allowing analysis of their behavior in the mountain–wave–boundary layer system. The locations, magnitudes, and evolution of the vortices can be studied through calculated fields of velocity, vorticity, streamlines, and swirl. Generally, observations suggest two classes of vortical motions: rotors and small-scale vortical structures. These two structures differ in scale and behavior. The level of coordination of the two lidars and the nature of the output (i.e., in range gates) creates inherent restrictions on the spatial and temporal resolution of retrieved fields.

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