SAR wind measurements over open ocean in the Svalbard area

The objective of this study has been to investigate the possibility to retrieve ocean surface wind from SAR images in the Svalbard area and the marginal ice zone. The CMOD4 wind algorithm has been implemented for use with ENVISAT ASAR Wideswath, Image Mode and Alternating Polarisation images. The algorithm runs automatically on ASAR images obtained from ESA rolling archive every day, using wind direction from NCEP fields. During the project a number of wind fields derived from SAR has been collected for different wind situations in the Svalbard area. The most important wind phenomena that have been observed in the SAR data are the gap winds (or jet streams) coming out of the valleys and straits. Also local wind structures outside the ice edge such as polar lows and wind fronts have also been observed. To validate the SAR- derived wind fields, we used data from an oceanographic buoy in the Barents Sea, localized southwest of Bjørnøya. This is one of two buoy that have been in operation since March 2007, providing data in real time available from a website. It was first planned to use meteorological data from several met-stations around Svalbard, but the wind measurements for these buoys are biased by local topography. The wind data from the open ocean buoy southwest of Bjørnøya, taken near simultaneously with the SAR data, were extracted and compared with the SAR wind retrievals. In the period March – September we collected about 70 co-located data points which we used to make a quantitative comparison between the two data sets.

The result of this comparison is that there is reasonable good agreement between the wind speed from SAR and from in situ measurements. The SAR wind speed is slightly higher than the in situ wind speed. Since the CMOD algorithm produces wind speed at 10 m height above the sea surface and the buoy provides wind data at 2 m height, some discrepancy between the two methods is expected. The SAR wind maps are disseminated via a webpage which is temporarily password-protected. The web site will be open after we have completed a validation period in 2008. The possibilities to provide operational wind fields from SAR has been investigated. The wind fields are produced automatically every time there is a SAR image available over open ocean provided from ESA rolling archive. At present there is limited SAR coverage over the study area, with typically 2 – 4 images per week. This is useful for research and demonstration work, but for fully operational monitoring it is necessary to obtain SAR wind fields twice per day in the Svalbard area and the marginal ice zone. The SAR wind data are presently tested by met.no in the ArcChange and OOMM projects. Further studies of SAR wind is conducted in the ongoing ESA project “SAR ocean wind, waves and currents”.