Wave breaking over local topography during the MAP IOP 15 Mistral event: Observations and high-resolution numerical simulations

This study investigates the fundamental processes involved in the severe Mistral's windstorm occurring during the MAP IOP 15 (from 06 to 09 November 1999). It is based on numerical high-resolution simulations performed with the RAMS non-hydrostatic model at 3 km resolution. The simulation is found able to capture the flow complexity both upstream of the Alps and in coastal regions affected by the Mistral. The simulations accurately reproduce dynamical and thermodynamical fields observed by the observational network consisting of two UHF wind profilers set up near Marseille and Toulon and two radiosoundings at Lyon and Nimes. The model indicates that the Mistral is mainly governed by flow splitting and downslope accelerations occurring at different scales. Wave breaking are triggered above local topography resulting in hydraulic jumps that in turn induce two mountain wakes trailing from the Alps and the Massif Central. The flow is accelerated by channelling effects between these two wakes. The simulations therefore evidence that channelling effects by the Rhône valley do not explain the Mistral's dynammics which are fundamentally three-dimensional, multi-scaled and formed by a multitude of orographic processes.