Wave data assimilation in the WAM wave model

Using the adjoint technique, wave height data have been assimilated into the third generation ocean wave model WAM, which runs operationally at ECMWF. WAM describes the evolution of a two-dimensional wave spectrum and it is driven by surface winds. WAM considers three source functions: wind input, dissipation by white capping and non-linear wave-wave interactions between the different components of the spectrum. The wind input term is taken from the theory of Janssen (1991). According to this theory, the drag coefficient depends not only on the wind speed but also on the wave spectrum through the wave stress.

Up to now, a one-gridpoint version of the adjoint of the WAM has been constructed and tested. The assimilation scheme optimizes the driving wind sequence. The potential benefit of the scheme is investigated by performing some idealized experiments.