Chapter IV: Jason-2 and waves
Table of Contents
Tracking the storm with Jason-2
SWH estimates derived from a Jason-2 (descending) overpass, acquired at 01:30 UTC on 17 April, are shown in Fig. 9. SWH values from a WAM analysis of the same locations are presented along with the satellite observations.
Fig.9. SWH values obtained from Jason-2 overpass at 01:30 UTC on 17 April and WAM analysis for the same period.
Fig. 9 shows that highest SWH values are obtained in a large region (approximately with 5º x 5º) centered on 53ºN, -18ºE. In this area, Jason-2 observations reveal SWH values between 7 and 8 m, and these are consistent with the WAM estimations. Lower SWH estimates (less than 3 m) were observed to the south of Ireland and over the Bay of Biscay.
Fig.10. SWH estimations obtained from Jason-2 overpass at 01:30 UTC on 18 April and WAM analysis for the same period.
Fig. 10 and Fig. 11 show SWH values obtained from Jason-2 overpasses at 01:30 UTC and 16:30 UTC on 18 April, respectively. The analysis of Fig. 10 reports a different overpass of Jason-2, with highest SWH values in a region with the same dimensions as for the previous day, but centered on 48ºN, -17ºE and with observations near 8 m.
Fig.11. SWH values acquired from Jason-2 overpass at 16:30 UTC on 18 April and WAM analysis for the same period.
Fig. 11 exhibits an ascending overpass of Jason-2 on 18 April. Satellite measurements reveal the highest wave values to be over the Bay of Biscay, mainly in the coastal zone of the north of Spain, near 45ºN, -5ºE, where SWH values reached 7 m. In contrast with the previous observations from late April 18 (see Fig. 9 and 10), it is remarkable that the highest waves had propagated to the Bay of Biscay so soon after their generation. This is evident, as Jason-2 data showed SWH around 2 m at early April 17 in the region of the Bay of Biscay.
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).
Impacts on land
This severe storm caused considerable damage, mainly in the coastal regions of the north of Spain, with wave heights of up to 8 m. This was particularly intense in the region of Galicia where there were reports [7] [8] of large amounts of precipitation (40 mm in 12 hours), and strong wind observations (up to 90 km/h). Despite the strong winds there was no evidence of a storm surge event.