STATISTICAL-ANALYSIS AND INTERCOMPARISON OF WAM MODEL DATA WITH GLOBAL ERS-1 SAR WAVE MODE SPECTRAL RETRIEVALS OVER 3 YEARS

Ocean wave spectra were retrieved from a set of ERS-1 synthetic apertu re radar (SAR) wave mode (SWM) spectra between January 1993 and Decemb er 1995. An assessment is given of the SWM data quality and the retrie val performance as well as the operational feasibility of the retrieva l algorithm. Sensitivity studies are performed to demonstrate the weak residual dependence of the retrieval on the first-guess input spectru m. The mean spectral parameters of the SWM retrievals are compared wit h spectral parameters from collocated wave model (WAM) spectra. The ti me series of SWM-retrieved and WAM-derived monthly mean significant wa ve heights H-s in various ocean basins show good overall agreement but with a small systematic underestimation of H-s by the WAM. A decompos ition of the wave spectra into wind sea and swell reveals an average 1 0% overprediction of the wind sea by the WAM while swell is underpredi cted by 20-30%. The positive wind-sea bias exhibits no clear wave heig ht dependence, while the negative swell bias decreases with swell wave height. This could be due to a too strong damping in the WAM at low f requencies. Detailed regional investigations point to the existence of smaller-scale phenomena, which may not be adequately reproduced by th e WAM at the present resolution of the wind forcing. Finally, an inter comparison is made of the observed and modeled azimuthal cutoff length scales, and global distributions are investigated. Ratios of the obse rved azimuthal cutoff wavenumber to the mean azimuthal wavenumber comp onent indicate that about 75% of the swell can be directly resolved by the SAR, while about 70% of the wind sea lies at least partially beyo nd the cutoff.