Auxiliary material for paper 2008GL037030 "Observation of swell dissipation across oceans" by Fabrice Ardhuin, ardhuin@shom.fr Service Hydrographique et Oc'eanographique de la Marine, 29609 Brest, France Bertrand Chapron Laboratoire d'Oc'eanographie Spatiale, Ifremer and Fabrice Collard CLS, division radar, 29280 Plouzan'e, France Introduction This data set contains storm descriptions and associated swell dissipation rates inferred from ENVISAT level 2 products. Swell parameters from individual SAR images are available at http://tinyurl.com/7q7jps : these are gathered in "tracks". After the storm center has been detected, with a position updated every 6 hours, SAR data are selected along great circle trajectories that correspond to the group speed that associated with a given peak period T and outgoing direction theta from the storm. Observations are selected if they occur within 3~hours and 100~km of the trajectory, provided that the SAR data contains a swell partition with a peak wavelength and direction within 50m and 20° of the expected values, and that the SAR-derived wind speed is less than 9m/s in order to use only the most accurate swell height estimations. The altimeter-derived wave heights are also included in the dataset. After grouping the tracks in ensembles with theta_min <= theta <= theta_max, we selected such ensembles that have enough (N >= 8) data points for distances larger than 4000 km from the storm, provided that these are spread over at least 3000 km of distance. The remaining 22 swell track ensembles gave reliable estimates of the swell attenuation. Tables "2008GL037030-ts01.txt" contains basic storm information and table "2008GL037030-ts02.txt" contains the derived swell dissipation rates, according to 3 different model. For each track ensemble, we investigate the variation of the swell height Hss with the spherical distance alpha. Semi-analytic functions Hss(alpha) were fitted to the observed values, using three theoretical models: one with a constant linear decay mu, the others with constant dissipation factors f_{e,s}, which takes into account the swell surface signficant orbital velocity u_{orb,s}}, or f_e, in which a numerical model is used to estimate the orbital velocity u_{orb} of the entire sea state. In each case the parameter \alpha, f_e or f_{e,s} was fitted together with the height Hss(alpha_0) at a distance x_0=R \alpha_0=4000km from the storm source. In practice, we scanned the possible values of Hss(alpha_0), from 1 to 12 m, and mu or f_e or f_{e,s} (from -2.0E-7 to 1.E-6 m^{-1} or -0.1 to 0.4, respectively), and the pair (Hss(alpha_0),\mu), (Hss(alpha_0), f_{e,s}), or (Hss(\alpha_0), f_e ) that gave the minimum root mean square difference with observations Hss(alpha_i) was retained. This procedure was repeated 400 times using a random pertubation of the observed values of Hss based on the observation error model. Therefore, the values of mu that is given is the median of these 400 values, and the 16 and 83 percentiles are given as mu_1 and mu_2. For the estimation of f_e, the orbital velocity of the entire sea state was taken to be at least 1.5 times that of the swell, in order to correct for the systematic underestimation of u_orb by the numerical model for the large swells wave heights (Ardhuin et al., JPO 2009, in revision). For H_s> 10m this negative bias is of the order of 10 to 15% of the observed value. The values of f_e are thus indicative, and are not expected to have a relative accuracy better than 50%. 1. 2008GL037030-ts01.txt 1.1 Column "number" : identification number for the swell track ensemble 1.2 Column "Storm time" : date (UTC) from which the trajectories are computed 1.3 Column "Latitude" : latitude in degrees of storm center from which the trajectories are computed 1.4 Column "Longitude" : longitude in degrees ... 1.5 Column "T" : wave period in seconds, used to detemine the group speed Cg=g*T/(4*pi) at which the trajectories are followed 1.6 Column "theta_min" : minimum outgoing direction from the storm center (in degrees, nautical) 1.7 Column "theta_max" : maximum outgoing direction from the storm center (in degrees, nautical) 1.8 Column "N" : number of SAR observations that satisfy the selection criteria 2. 2008GL037030-ts01.txt 2.1 Column "number" : identification number for the swell track ensemble 2.2 Column "mu" : median value of the dissipation coefficient mu units are 1.E-8 per meter 2.3 Column "mu_1" : 16% percentile of the he dissipation coefficient mu units are 1.E-8 per meter 2.4 Column "mu_2" : 83% percentile of the he dissipation coefficient mu units are 1.E-8 per meter 2.5 Column "Hss(alpha_0): Swell wave height at 4000 km from the source, fitted together with mu. 2.6 Column "eps_1" : r.m.s. error of the fit (mu,Hss(alpha_0)) normalized by the r.m.s. observed valeu (in percent) 2.7 Column "Re_s(alpha_0)": Swell Reynolds number at 4000 km from the source center 2.8 Column "f_{e,s}" : median value of the dissipation factor with only the swell orbital velocity taken into account 2.9 Column "f_e" : median value of the dissipation factor with the orbital velocity of the entire sea state based on a wave model 2.10 Column "eps_2 : r.m.s. error of the fit (f_e,H'ss(alpha_0)) normalized by the r.m.s. observed valeu (in percent) 2.11 Column "U_10" : mean SAR-derived wind speed in m/s, at the selected SAR observations