The assimilation of altimeter data into the Australian wave model

We tested two established strategies for assimilating sea-surface wave heights into a spectral wave model. The first strategy modifies the energy of the spectrum, but allows the wave steepness to change. The second strategy modifies the energy and the mean frequency so as to maintain the wave steepness. We then used the second strategy to assimilate satellite altimetry data from GEOSAT into a hindcast run of the Bureau of Meteorology's operational wave model. Even though we did not modify the winds simultaneously with the waves, the wave height bias at a buoy site just off the western coast of Tasmania was reduced from –1.41 to –0.34 m and the rms from 1.64 to 0.93 m. We believe that some of these dramatic improvements can probably be attributed to the higher order numerics now incorporated into the operational wave model; changes made solely for the purpose of reducing numerical diffusion. As a result the energy initially assimilated into the wave model may now be retained much longer than would have been the case with lower order propagation numerics.