Numerical simulation of meso-gamma scale features of fohn at ground level in the Rhine valley

International audienceThis paper examines the impact of a mesoscale analysis (2.5 km grid distance) on the simulation of the meso-gamma scale aspects of föhn in the Rhine Valley. The föhn event, documented during IOP15 (5 November 1999) of the Mesoscale Alpine Programme, was standard in terms of intensity and was characterized by an important temporal variability. Many instruments operating in the Rhine valley target area are used to validate the simulation, in particular the airborne nadir-pointing lidar LEANDRE 2 (flown over the lower Rhine valley) as well as a wind profiler and a radio accoustic sounding system collocated in Rankweil, Austria. The large observational dataset acquired during the IOP allowed documentation of the entire föhn life cycle. For most of the IOP, a cold pool remained near the ground in the lower northern part of the valley. The non-hydrostatic model Meso-NH, used in a grid-nesting configuration with two nested models and initialized with a mesoscale analysis, allowed us to simulate realistically the location and depth of the cold pool. The relationship between the föhn intensity and the large-scale environment is also examined. The flow regime is a 'flow around' the Alps. The variability of this flow at the western tip of the Alps could explain some of the temporal changes observed at low level in the Rhine valley