The presentation is based on the observations of a strong foehn episode during the MAP IOP8 (20 October 1999). Although our study uses all observations collected in the Rhine valley, the radial velocities from the Transportable Wind Lidar (TWL) received a particular attention. Together with high resolution simulations of the meso-NH non-hydrostatic model, the TWL gives a detailed movie of foehn penetration in the high Rhine valley. For the simulations, three nested, 2-way-interacting domains were used, at resolutions of 10, 2.5 and 0.625 kilometers.

The penetration of the flow at low altitudes varies along the valley axis, depending on the local topography and the stability of the original air-mass. The model’s high resolution produces a good description of the local onset of the foehn. The foehn winds are channelled close to the surface in both the upper Rhine valley and the north part of the Seez valley. The warm and fast foehn air confronts a stable air-mass in the Rhine valley towards the foot hill. The figure below is a vertical cross-section through this part of the Rhine valley, starting at the intersection with the Seez valley and directed towards the north. The foehn flow, characterized by a well mixed area and a significant turbulent kinetic energy (the dotted area represents a TKE higher than 0.8m²/s²), bumps into a stable air-mass, as revealed by the tangential wind and the contour-lines of the potential temperature field.

In the presentation, we will compare observations and simulations with a particular emphasis on TWL data. We will describe the local features and evolution of the foehn. The evolution of the interface between the two air-masses, its propagation, and the physical processes leading to the removal of the stable air-mass will receive a particular attention.