Validation of High Spatial Resolution Wave Data From Envisat RA-2 Altimeter in the Gulf of Cádiz

Extending the applications of satellite altimetry to the coastal zone requires validated, quality controlled data. We present a validation study in the Gulf of Cádiz (SW Iberian Peninsula), an area of relevant social, economic, and strategic importance. We compare against in-situ data seven years (Dec. 2002-Jan. 2010) of significant wave height (SWH) measurements by the Envisat RA-2 altimeter from two sources: 1) standard geophysical data records (GDR, 1 Hz) and 2) the processor developed in the COASTALT (development of radar altimetry data processing in the coastal zone) project (18 Hz). The comparison is made along two Envisat passes (one descending, i.e., north to south, one ascending). For the descending pass (land to ocean transition) the COASTALT processor improves SWH retrieval in the coastal fringe. In particular, the COASTALT SWH product displays accuracies in the sub-coastal strip (12-20 km from the coastline) of a very similar magnitude to those further offshore, representing a clear improvement over GDR. The bias and standard deviation of the difference with regard to the in-situ measurements in the coastal fringe is about 60% and 40% lower, respectively, when COASTALT data are used instead of the standard GDR product. For the ascending pass, the differences in the ocean to land transition are less marked, probably due to the altimeter keeping a good lock on the sea surface until relatively close proximity to the coastline. This validation case shows that this new coastal-oriented product gets closer to the shoreline than before, while also making available higher-resolution along-track estimates.