Variational Wave Data Assimilation in a Third-Generation Wave Model

Abstract

The adjoint of the wave model WAM, which runs operationally performing global wave forecast at the European Centre for Medium-Range Weather Forecasts, has been constructed. In this model, the nonlinear interactions are described by the discrete interaction approximation of Hasselmann et al., and the wind input and the dissipation are consistent with the theory of Janssen. The drag coefficient depends not only on the wind speed, but also on the wave-induced stress, reflecting in this way the dependence of winds on waves. The adjoint scheme constitutes a new (the first variational) method to assimilate arbitrary wave data into the WAM. Up to now, this had been done using an optimal interpolation technique (sequential method), and only for wave heights. The new assimilation scheme has been tested with a one-gridpoint version of the WAM. Assimilating only wave data-significant wave height and mean wave direction?is sufficient to reconstruct all wind fields, significant wave height, and two-dimensional wave spectra fields, respecting the wave model dynamics. To investigate the relation merits of the two methods, a number of realistic assimilation experiments have been performed showing the potential of the adjoint technique for wave data assimilation.