ASSIMILATION OF WAVE DATA INTO THE WAVE MODEL WAM USING AN IMPULSE-RESPONSE FUNCTION-METHOD

A new method for the assimilation of wave data into a third-generation wave model is presented, Deviations between observed and modeled wave spectra are used to derive corrections of the wind field which drives the wave model, The wave field can then be subsequently corrected by a new integration of the wave model with the improved wind field, A ba sic difficulty of such dynamically consistent wave data assimilations schemes which correct both wind and wave data is the nonsynchronous an d nonlocal nature of the wind field corrections: errors observed in th e wave spectrum at a given measurement time and location can be produc ed by errors in the wind field at much earlier times and far distant l ocations, Formally, these problems can be rigorously resolved by the a djoint modeling method, However, in practice, the adjoint technique re quires an order of magnitude more computer time than the integration o f the wave model itself, Here an alternative method is developed, The linearized wave model equation which relates small wind to wave spectr um changes is inverted, The central assumption of the inversion is tha t the wind impact functions representing the impulse response (Green's ) function of the wave evolution can be approximated by a S-function, Physically, this implies that the wind field perturbations responsible for observed perturbations in the wave spectrum can be regarded as st rongly localized in space and time for any given component of the spec trum, To obtain stable estimates, the corrections for different wave c omponents are averaged over wavenumber clusters representing different wave systems, For cases in which the linear approximation is inadequa te, the method can be applied iteratively, Tests of the concept and ap plication of the method for a number of synthetic wind field cases are encouraging,