Surface Currents and Tidal Ellipses from Hourly Variations of Suspended Sediment from Geostationary Ocean Color Imager Data

The significant capability of Geostationary Ocean Color Imager (GOCI) observations in space and time has enabled us to retrieve hourly variations of surface currents under clear sky conditions, which had long been impossible from numerous other satellite images. One of the most commonly used methods to retrieve these current vectors was the normalized maximum cross-correlation (NMCC) method, in which a location was estimated with a NMCC coefficient from sequential images. NMCC currents typically show good agreement with in-situ measurements in some cases; however, in many other cases, the level of agreement has been very poor, specifically with highly random current vectors. To resolve such potential problems and for a better definition of the limitations of NMCC currents, we applied this method to time-varying suspended sediment measurements to estimate surface flows in the coastal regions of Korea with strong tidal currents. Preliminary current vectors were filtered out by applying a series of quality-control procedures. The current vectors resulting from the tests were compared with the currents from a tide model and from the Ekman drift. It was found that the NMCC currents were more similar to the currents caused by both tide and wind rather than by the tide only. A high degree of discrepancy was detected in the tidal frontal zone, where the fundamental assumption of horizontal movement was violated due to the dominant vertical tidal mixing in the shallow region. The rotations of the current vectors with time within a day were clarified by a comparison of the time-varying tilting angles of tidal ellipses. The emphasis on the limitations of the NMCC method in this study provide information about how to use this method for a better understanding of surface flows using hourly satellite data.