ASSIMILATION OF WAVE SPECTRA FROM PITCH-AND-ROLL BUOYS IN A NORTH-SEAWAVE MODEL

A technique for the assimilation of spectral wave observations in wave models is presented and tested. The method uses the concept of spectr al partitioning to project the entire wave spectrum onto a few essenti al mean parameters. Model and observed partition parameters are assimi lated using an optimal interpolation (OI) technique. After data reduct ion, obtained by the partitioning, the cost of the assimilation is neg ligible compared to the cost of the model un itself. Therefore the opt imal interpolation of partitions (OI-P) method is a very attractive as similation technique for operational wave forecasting. The paper focus es on the assimilation of pitch-and-roll buoy spectra in a North Sea v ersion of the WAM wave model. Treatment of the (non-fully two-dimensio nal) buoy spectra is discussed. Appropriate choices for the OI weight functions are made. The problem of correlating wave partitions in diff erent spectra is addressed, which is essential for obtaining a robust a;nd efficient system. In order to assess the influence of spectral wa ve observations on the analysis of the sea state, the method is compar ed to a second scheme, optimal interpolation of integral parameters (O I-I), which can only be used to assimilate observations of significant wave height and mean wave period. First, tests with synthetic data ar e described, which illustrate advantages of the partitioning method me thods. Experiments with buoy observations for actual North Sea conditi ons show the benefits of the system, especially when several wave syst ems are present at the same time.