Penetration and Interruption of Alpine Foehn (PIANO): Evaluation of processes with multiple Doppler wind lidars

A prominent example of a downslope windstorm is foehn in the Alps. Its fully developed stage has been extensively investigated in the framework of international field campaigns and mesoscale model simulations. However, less scientific attention was paid to the penetration and interruption of this severe wind in valleys. Hence, there is lack in understanding the underlying processes of these complex, transient phases. These periods are accompanied by small-scale processes such as turbulent interaction between foehn and relatively colder air in valleys. Therefore, turbulence-resolving numerical simulations or observations are needed to capture them explicitly. The research project “Penetration and Interruption of Alpine Foehn (PIANO)” uses a combination of both to deepen the knowledge of foehn breakthrough and breakdown. The region around the city of Innsbruck, Austria, located in the Inn Valley was chosen as the target area for the PIANO field campaign which took place in fall 2017. This conference contribution will focus on selected observations, while numerical simulations and an overview of the PIANO project are presented in separate contributions.

To quantify three-dimensional turbulent processes, measurements with high resolution in space and time are mandatory. This requirement can be satisfied by a combination of several scanning Doppler wind lidars. For this reason four StreamLine Doppler lidars manufactured by Halo Photonics were deployed in the PIANO field campaign. Three of them were installed in the city of Innsbruck on tall buildings forming a triangle with a horizontal separation of about 1300 m. A fourth instrument was located at a lower level along one side of the triangle. Coordinated dual Doppler coplanar scans were conducted in both horizontal and vertical planes to deduce the three-dimensional kinematic structure. Simultaneously, conical scans and vertical stares were performed to derive vertical profiles of mean and turbulent wind quantities.

In this contribution we will present results from a case study of the second Intensive Observation Period (IOP2) from 4 to 5 November 2017. During IOP2 a slow but gradual lowering of the transition zone between foehn and cold-pool occurred during the nighttime and was terminated by the foehn breakthrough in the morning hours. The focus will be on the transition zone between the foehn flow and the colder valley air. In this layer shear-flow instabilities form that contribute to the erosion of the cold pool. This study evaluates the relative role of shear-induced mixing and cold-pool displacement. Furthermore, their temporal and spatial inhomogeneity is assessed.