Impact of assimilating altimeter data on wave predictions in the western Iberian coast

•The assimilation of the altimeter data in a wave forecast system is made based on the optimal interpolation method.•A wave modelling system based on WAM, for the wave generation, and on SWAN for the coastal transformation are used.•The methodology proposed is applied to hindcasts and for validation purpose and then to operational forecast.•The corrections are applied to the output of the SWAN model.•The results show that the assimilation scheme works correctly and all statistical parameters are better predicted.

The present work describes the implementation of a methodology based on the optimal interpolation method for assimilating the altimeter data in a regional wave forecasting system. The main objective is to improve the wave predictions in the western Iberian coastal environment. The wave modelling system considered is based on WAM, for the wave generation, and on SWAN for the coastal transformation and delivers daily forecast products. An analysis scheme was first applied to the hindcast runs, when the observations and the simulations were blended within a time window of one day. The objective is to validate the methodology proposed and to evaluate the impact produced by this scheme on the accuracy of the wave predictions. Corrections are applied to the output of the SWAN model, and consequently, the data assimilation scheme is executed independently of the model simulations. As a second step, the data assimilation scheme is applied to operational runs, when the analysis fields are used as a first guess for the next simulations. The procedure considered uses the observations provided by the multi-satellite altimeter data. Both satellite data and in-situ observations are used for the quality assessment. The results show that the assimilation scheme works correctly and all the statistical parameters evaluated have better values in the case of the assimilated significant wave height scalar field. As expected, by increasing the amount of the data assimilated, the accuracy of the wave predictions is enhanced.