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The impact of using the upgrade processing of ASAR level 2 wave products in the assimilation system

Lotfi Aouf(1) , Jean-Michel Lefevre(1) , Daniele Hauser(2) , Bertrand Chapron(3) , and Fabrice Collard(4)

(1) Meteo France, 42 avenue Gaspard Coriolis, 31057 Toulouse cedex 1, France
(2) CETP/IPSL/CNRS, 10-12 avenue de l'Europe, 78140 Velizy, France
(3) Ifremer, B.P. 70, 29280 Brest, France
(4) Boost Technologies, 135 rue Claude Chappe, Technopole Brest Iroise, 29280 Plouzane, France

Abstract

Directional wave spectra retrieved from the imaging radar ASAR (Envisat) provide useful information on sea state for the wave community. This has a considerable advantage for the correction of the sea state, in particular when swell is dominant. It is now well known that such improvement induces to a better description of related physical processes at the ocean surface (currents circulation and air-sea heat fluxes). The validation study of using the ASAR level 2 wave products (Aouf et. al 2004) has mainly indicated that the fidelity controls and the use of an accurate wavelength cut-off in the optimal interpolation (OI) procedure represent the key points for a positive impact on the wave prediction. It is necessary to shed light on these points before the step forward to the operational use of ASAR wave data. This work presents the impact of using the ASAR level 2 wave products retrieved by the upgrade algorithms. The assimilation of ASAR directional wave spectra in the wave model WAM has been performed continuously for a period of one month (May 2004). Quality control procedure for ASAR data depending on the normalized variance of ASAR image, the wind speed at the ocean surface and the ratio of signal to noise is applied in a prior procedure before the assimilation. The choice of an optimal wavelength cut-off for the ASAR wave spectra is well discussed by examining different test cases. Furthermore, a check control on peak period of ASAR wave spectra gives an indication on the accuracy of the wavelength cut-off. In this paper, the comparison between the old and new ASAR wave mode processing is also investigated. The results show that the assimilation scheme works correctly and induces a significant impact on mean wave parameters. The fidelity controls indicate that only an average of 13 % of ASAR data are considered spurious and they were rejected from the analysis process. The comparison of the assimilation results with independent wave data (Jason-1 altimeter) exhibits a positive impact on the significant wave height for the two cases of wavelength cut-off (200 and 240 meters). The new ASAR wave mode processing improves significantly the quality of directional wave spectra, in particular for swell wave height less than 2 meters, and induces an optimistic use of wavelength cut-off of 200 meters. Further discussions and conclusions will be commented in the final paper.

 

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