Diurnal Currents in the Bohai Sea Derived From the Korean Geostationary Ocean Color Imager

Hourly measurements during 9:00 to 16:00 local time are a unique capability of the Korean Geostationary Ocean Color Imager (GOCI) in monitoring fast-evolving ocean features. In this paper, we derive ocean surface currents in the Bohai Sea from GOCI data using the maximum cross correlation (MCC) feature tracking method and compare the results with altimetry-inversed tidal currents produced from the Oregon State University Tidal Inversion Software (OTIS) after the latter is validated using in situ measurements. The performance of the GOCI-based MCC method is assessed and the discrepancies between the GOCI and OTIS currents are evaluated through a series of sensitivity studies, including studies of different ocean color products, MCC and filter parameters, and time interval between image pairs, in order to find the best setups for optimal MCC performance. Our results show that a box size of 10-15 km and a search range of 4-8 km for every 1 h of time difference, combined with subsequent filters that use the cutoff correlation coefficient of 0.5 and 3 or 4 neighbors within 15 cm/s difference, gives optimal results. Under these setups, the bias errors of GOCI-derived current maps are about 8 cm/s for current vectors and 3° for current directions, respectively. The noise errors are about 20 cm/s in current velocity and 20° in current direction with correlation coefficients of ~0.75 and ~0.9 for the current velocity and current direction, respectively. These findings are useful in developing an hourly surface current product from GOCI measurements for the Bohai Sea.