When a perturbation is present over Western Europe, southwest to south wind blows over the Alps. Two phenomena sign the Foehn occuring on their north side: the Foehn clearance - an increase of the low level temperature over a large region- and the Foehn wind jets closer to the foothills. The Foehn jets are well organized near the exit of some major valleys, north south oriented, like the Rhine valley.

A lot of measurements were performed in the Rhine valley during a strong foehn event (IOP2) of the MAP experiment, including aircraft in-situ measurements and high resolution soundings.

We simulate this event with the non-hydrostatic meso-NH model, using three nested models (10, 2.5 and 0.625m mesh) forced by the French operational model analyses. The highest resolution model, with a 3D turbulence scheme, correctly reproduces the Foehn circulations in the Rhine valley, in accordance with the data.

The simulation shows a succession of small scale wave breakings above the slopes of the secondary massifs. Moreover, the downslope air reduces the stability of the low layers by turbulent mixing due to the friction at the surface and the strong shear. This region of reduced stability, large of 3km in height, is capted by an inversion. Hydraulic jumps or smoother returns to higher altitude are recovered in the simulation (see figure: Parallel wind in two parallel cross-sections, in the Foehn jet and 10 km to the West).

The foehn jet propagation along the mountain slopes or in the valleys is interpreted at the light of the hydraulic theory, and the role of the shape of the Alpine crest around the Rhine valley is underlined.