The gap and katabatic contributions for the MAP IOP 15 Mistral windstorm.

The gap and downslope contributions for the MAP IOP 15 Mistral windstorm are assessed with the 2D hydraulic theory and the non hydrostatic RAMS simulations. The Mistral is a northwesterly / northerly flow that blows in southeastern France and over the northwestern Mediterranean. The surface winds have reached more than 20 m/s during the MAP IOP 15 (6-8 November 1999) near Marseille and Toulon. The two-dimensional Local Hydraulic Theory (LHT) is derived to retrieve its depth evolution above the French Alps and along the Rhone valley that separates the Alps from the Massif Central. Its simple formulation allows a separation between the katabatic force and the gap force. Non-hydrostatic simulations with a 9 Km sized mesh, performed by the RAMS model, are conducted to define the limit of validity of the LHT and to provide some additional information on the three-dimensional dynamics of the Mistral. Both theoretical and numerical models are validated through comparisons with two UHF wind profilers set up in coastal zones near Marseille and Toulon . It is shown that the LHT is efficient to predict the evolution of the Mistral depth from the given upstream conditions provided by the soundings launched each 6 hours at Lyon. The LHT and RAMS are agreed to characterize the flow as a mix of gap and downslope effects downstream the Rhone valley and a pure downslope flow downstream the Alps. They also predict that a hydraulic jump moving westward with time is involved in the cessation of the winds. Furthermore, numerical simulations indicate that lee wakes and wave breaking excited by the two ridges play a major in the Mistral wind structure.