Assimilation de données par méthode variationnelle utilisant l’adjoint d’un code de circulation côtière tridimensionnel

The optimal control technique using an adjoint method is applied to fit the parameters of a 3-D primitive equation coastal model due to a data assimilation procedure. Two of the main unknowns of coastal modelling operations - open boundary conditions and turbulent mixing processes in stratified conditions - are successively considered. The aim of the boundary control procedure is to identify boundary forcing to ensure the best fit between data and model results by minimising a function that measures model and data discrepancies. A methodology for proceeding from the continuous to the discrete formulation of a control problem is given. The robustness of the method is tested for the typical barotropic case of a semi-enclosed basin Kelvin wave. The data assimilation method of optimal control is then applied to the turbulence closure problem in the framing of coastal oceanic circulation. Numerical simulation of a schematic destruction of an halocline and an upwelling event resulting from a wind stress imposed at the sea surface in a strongly stratified two-layer case are realized. The adjoint method is used to find out an optimal set of viscosity and diffusivity coefficients which permits the retrieval of the closest computed simulation to hydrodynamic synthetic observations.