Gap-flow phenomena measured by Doppler lidar in the Wipptal during MAP

Doppler lidar has been used to study a variety of flow phenomena over mountain barriers, including various types of mountain wave, hydraulic jumplike structures, breaking waves, fronts, and thermally forced flows. The structure and characteristics of these features have also been shown by other instrumentation systems and modelled physically (e.g., in wind tunnels and water tanks) and numerically; the lidar analyses serve as an integrator, a visualization, and/or a verification of the results based on these other methods. An important question addressed by the Mesoscale Alpine Programme (MAP) is, how are these flows modified near the surface as they are channeled through a gap in the mountain barrier, and what other flow phenomena can be induced by the gap. ETL's scanning Doppler lidar was deployed to the Wipp valley (Wipptal) about halfway between Innsbruck, Austria and the major gap in the Alpine chain (Brenner pass) during October-November 1999 to address these questions. The lidar performed vertical and horizontal (azimuthal) scans to probe the structure of the flow in both planes, and often performed repeated scans to show the time dependence of the flows, or 3-D volume scans to show the full 3-D structure of the phenomena encountered. Many of these features showed a strong resemblance to similar features observed over barriers, including jumplike structures, various wave configurations, and turbulent eddies and vortices. Examples of each type of phenomenon will be presented, along with interpretive data from other instrumentation within and upstream of the valley.