Jason-2 against model and buoy data
The altimeter observations and WAM data presented here (Fig. 9, 10 and 11) have shown that wave model results and satellite measurements are, in general, in good agreement. The three overpasses provided more than 1400 individual observations, and reveal a mean difference of 0.035 m between the wave model and the Jason-2 estimations.
Nevertheless, it is possible to identify two major exceptions. The first, at 01:30 UTC on 18 April (Fig. 10), in an area of roughly 5º x 5º centered on 48ºN, -17ºE, in which the remote sensing values were lower (by about 1.5 to 2.0 m) than the WAM results. The second, which occurred on the following satellite overpass over our region of interest, was where a zoom at southern Bay of Biscay revealed a deviation between model and remote sensing data (Fig. 12). The differences in SWH values in this region of the northern Spanish coast reach up to 2 m, the model results having forecasted lower values than the altimeter radar observations.
Fig.12. SWH values from Jason-2, WAM and buoy data for 16:30 UTC on 18 April in (a) the Bay of Biscay and (b) near Bilbao (north of Spain).
Fig. 12 also shows the SWH value recorded at the Bilbao buoy at the time when the Jason-2 data was acquired (16:30 UTC on 18 April). The in-situ and near coastal measurements reveal SWH of 6.7 m, thus revealing the underestimation of wave height from WAM (5.9 m) and the overestimation from the radar altimeter (7.1 m). Altimetry and WAM data represent the near coastal average (between coastline and 30 km – about 0.3º offshore) and the buoy data is an hourly average between 16:oo and 17:00 UTC. These results illustrate the overestimations from the radar (in 0.4 m), and the underestimation of the wave model (0.8 m)" in the last sentence of chapter 4.b).