The assimilation of ERS-2 significant wave height data in the Australian region

The results from an experiment to assimilate satellite altimeter significant wave height (SWH) data into an ocean wave model are presented. Data from the European Remote-sensing Satellite 2 (ERS-2) are used. Statistical interpolation is used to construct analysed SWH fields using the altimeter data. The ocean wave model is AUSWAM, a version of WAM, which runs operationally at the Australian Bureau of Meteorology and provides up to 96-h forecasts of sea-state. Forcing is obtained from surface wind fields calculated from the operational atmospheric systems. Model spectra are adjusted using various assimilation strategies. Results were validated against a number of waverider buoys situated around the Australian coast. Over two 35-day periods, it was found that assimilation of the ERS-2 SWH data reduced the systematic bias in analysed SWH by approximately 10% (with a change of sign). The Scatter Index was reduced by 6% and the rms error by approximately 8%. The improvement in model skill is also retained throughout the forecast period. The assimilation is found to have the greatest impact at locations on the west coast of Australia, where the sea-state is dominated by swell. It is suggested that errors in the modelled wave fields arise from a combination of overpredicted wind speeds and excessive dissipation at low wavenumbers.