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

摘要

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.