Evaluation of Radial Ocean Surface Currents Derived From Sentinel-1 IW Doppler Shift Using Coastal Radar and Lagrangian Surface Drifter Observations

Ocean surface radial velocities (RVLs) derived from the Sentinel-1 A/B Interferomic Wide (IW) mode Doppler frequency shift observations are regularly acquired over the Norwegian coastal zone. These data can be used to complement existing ocean observation systems with high-resolution (up to 1.5 x 1.5 km) spatial ocean surface current (OSC) maps. In this study, Sentinel-1 IW Level 2 OSC retrievals were obtained from 2 months (October-November 2017) of raw Doppler shift observations acquired over the Norwegian Coastal Current (NCC). The results were evaluated using coastal high-frequency radar (HFR) and Lagrangian ocean surface drifter observations. The analysis shows that distinct patterns of the NCC, with range directed currents reaching up to 0.7 m/s, can be detected in the SAR data. The mean bias between Synthetic Aperture Radar (SAR) and HFR observations was -0.08 m/s and the root mean square deviation (RMSD) was 0.25 m/s. In comparison, the agreement between the SAR-derived OSC and the trajectories from Lagrangian surface drifters showed a mean bias of 0.02 m/s and an RMSD of 0.24 m/s. The accuracy of the SAR OSC retrievals rely on precise wind-wave bias correction. Hence, the accuracy of the model wind field (speed and direction) is crucial. The sea state contribution must also be taken into account during the bias correction. A typical required accuracy of the OSC velocity is on the order of 0.1 m/s. Therefore, the comparisons demonstrate that the use of SAR for OSC retrieval is promising. Plain Language Summary Knowledge of ocean surface currents is crucial for tracking oil spills and marine debris (e.g., plastic), search and rescue operations, and fisheries. Traditionally, surface currents are studied using shipboard measurements or trajectory of buoys drifting within the water flow. Despite their accuracy, these measurements are also costly to collect and therefore irregular in time and space. The Doppler shift recorded by a radar placed on board of a satellite can be used for providing systematic snapshots of surface currents over vast areas of the ocean. However, these observations must be evaluated before application. In this study, we evaluated observations of the Norwegian Coastal Current acquired by the Sentinel-1 satellite in October-November 2017. Satellite observations were compared with collocated coastal radar and ocean surface drifters data. The analysis shows that distinct patterns of the surface current can be systematically detected in the Sentinel-1 data. The accuracy of the observations is within the range of user requirements. We underlined the importance of accurate information about wind field and ocean waves for analysis of the radar observation. The performed study demonstrates that the use of Sentinel-1 observations for the ocean surface current retrieval is promising.