Assimilation of ERS-1 altimeter wave heights in an operational numerical wave model

Observed significant wave heights (SWH) from ERS-1 have been received and evaluated at the Norwegian Meteorological Institute (DNMI) since August 1991. The observations were found to be of high quality. Since January 1992 the data were assimilated in an operational numerical ocean wave model in real time. The first step of the assimilation consisted of a data analysis part in which SWH observations from the ERS-1 altimeter were filtered and interpolated to the model grid using SWH from the model as a first guess. This information was then transferred to the model wave spectra with the assumption that the relation of wind sea and swell energy was correct in the wave model. The wave model, with assimilation of ERS-1 data, was run in parallel with a ``nonassimilating'' operational wave model, both driven by winds from DNMI's numerical weather prediction model. The results were compared and impact of ERS-1 data was measured relative to independent buoy measurements in the North Sea. The impact proved to be variable but generally positive. The assimilated corrections of the spectra were shown to have impact on 12-24-h prognosis of the wave field.