The Mistral at the exit of the Rhône valley

This paper examines the three-dimensional structure and dynamics of the Mistral at the Rhône valley exit on 28 June 2001. This summer Mistral event was documented in the framework of the ESCOMPTE field experiment. This study investigates the dynamical processes driving the circulation of the Mistral in the Rhône valley and particularly wake formation, vorticity generation and planetary boundary layer inhomogeneity at the scale of Rhône valley delta. Several important data sources are used (airborne Doppler lidar, wind profilers, radiosoundings and surface stations) as well as non-hydrostatic mesoscale simulations.

This paper investigates experimentally, numerically and theoretically the mechanism of wake formation. It shows that the flow impinging on the Alpine range and the Massif Central transition to supercritical all along the ridge line, including the Rhône valley and continue to accelerate in the lee regions until a hydraulic jump occurs. This leads to the formation of strong wakes behing and close to the peaks.

Wake formation in the lee of the Alps and the Massif Central causes large inhomogeneity of the planetary boundary layer. In the region of the wakes, enhanced convection associated to larger surface sensible heat fluxes is observed and simulated leading to deeper planetary boundary layer. In the Mistral jet, cold air advection inhibits the planetary boundary layer development.