Towards Retrieving Reliable Ocean Surface Currents in the Coastal Zone From the Sentinel‐1 Doppler Shift Observations

Abstract

AbstractRecent developments on calibration and partitioning of the signal between the wave and current contributions significantly improve the accuracy of geophysical retrievals from Sentinel‐1 Synthetic Aperture Radar‐based Doppler shift measurements in the open ocean. In this study, we revise the Sentinel‐1B Interferometric Wide products acquired from December 2017 to January 2018 along the coastal zone of northern Norway. We find that the satellite attitude is responsible for 30% of the variation in the Doppler shift observations, while the antenna pattern can describe an additional 15%. The residual variation after recalibration is about 3.8 Hz, corresponding to 0.21–0.15 m/s radial velocity (RVL) depending on the incidence angle. Using recalibrated Sentinel‐1 observations, collocated with near‐surface wind from MetCoOp‐Ensemble Prediction System and sea state from MyWaveWAM, we develop an empirical function (CDOP3SiX) for estimating the sea‐state‐induced Doppler shift. CDOP3SiX improves the accuracy of sea state contribution estimates under mixed wind fetch conditions and demonstrates that the Norwegian Coastal Current can be detected in the Sentinel‐1 derived ocean surface current RVL maps. Moreover, two anticyclonic mesoscale eddies with radial velocities of about 0.5 m/s are detected. The surface current patterns are consistent with the collocated sea surface temperature observations. The Doppler shift observations from Sentinel‐1 can therefore be used to study ocean surface currents in the coastal zone with a 1.5 km spatial resolution.