Validation of the CMEMS-IBI wave model with data assimilation in a high resolution regional configuration.

One of the challenges of the Iberia-Biscay-Ireland (IBI) Monitoring Forecasting Centre in CMEMS phase 2 is the implementation of the assimilation of altimeter wave data in the wave forecast system.  In this work we explored the impact of the assimilation of altimeter wave data in the IBI domain.  We ran the Météo France version of the WAM wave model (MFWAM) in the IBI domain for 2018 and 2019, with data assimilated from the Jason 2 and 3, Saral, Cryosat 2 and Sentinel 3 altimeters.  This high-resolution (0.05 degree) configuration was forced by 0.05 degree ECMWF winds, and boundary conditions were provided by a 0.1 degree global model run.  We also included refraction from currents generated with the NEMO-IBI ocean circulation model.  We present results with and without wave–current interactions.  Validation against both buoy data and the HaiYang 2 altimeter shows that the assimilation of data leads to a marked reduction in scatter index and model bias compared to the run without data assimilation; the gains from including currents meanwhile are modest.  

The data assimilation scheme presently implemented in MFWAM uses an optimal interpolation algorithm where constant model and observational errors are assumed.  To add some sophistication, we experimented with non-constant background errors derived from a model ensemble.  Though the effect was small, the method suggests a way to improve the data assimilation performance without substantially altering the algorithm.