On the combined assimilation of RA-2 and ASAR wave data for the improvement of wave forecasting
Lotfi Aouf(1) , Jean-Michel Lefevre(1) , Daniele Hauser(2) , and Bertrand Chapron(3)
42 avenue Gaspard Coriolis,
31057 Toulouse cedex 1,
(2) CETP/IPSL/CNRS, 10-12 avenue de l'Europe, 78140 Velizy, France
(3) Ifremer, B.P. 70, 29280 Brest, France
The increasing number of wave observations from satellites related to sea state represents a stimulating challenge for wave modelers in order to improve operational wave forecasting. It is now well known that a better estimate of the sea state has an important contribution to a better description of the physical processes at the ocean surface, such as currents circulation, air-sea heat fluxes and surface stress. Improving the wave forecast is also an essential need for several activities at sea (off-shore activity, ship navigation and coastal survey). The RA2 altimeter provides the total wave height at the free surface, while the imaging radar ASAR gives the directional wave spectra limited to azimuthal wavelength cut-off. The use of these two wave observations conjointly is well appropriate to the correction of both wind sea and swell parts of the sea state. As their orbit tracks are separated with an average distance of 200 km, the coverage of observations over sea points is also improved. In our previous studies, it was found that the use of spectral information improved the estimate of wave parameters, in particular for swell (Aouf et al. 2004).
This work presents the impact of using continuously the combined assimilation of RA2 and ASAR data in the wave model WAM for long period of two months, in February and March 2004. To remove corrupted ASAR data, quality controls depending on the retrieved parameters from ASAR (wind speed, the ratio of signal to noise and the normalized variance of images), are performed in a prior procedure before assimilation. The validation of the assimilation results with independent wave observations is performed for the periods of analysis and forecast. An important point related to the choice of optimal wavelength cut-off for the ASAR wave spectra is well discussed by investigating different test cases.
The results showed that the assimilation system works correctly and the estimate of mean wave parameters is significantly improved. The impact of the assimilation stays effective for more than three days after the end of assimilation. The filtering procedure seems to work efficiently for the rejection of spurious ASAR data. In other respects, statistical analysis in comparison with Jason-1 altimeter and buoys data has clearly indicated a significant positive impact for the significant wave height and mean period of waves. Further, the analysis for specific ocean areas (intertropical and high latitudes areas) exhibited the benefit of using both RA2 and ASAR wave data. Longer period of assimilation and validation is still needed to prepare the assimilation system for operational use.