Near-Surface Current Mapping by Shipboard Marine X-Band Radar: A Validation

The Lagrangian Submesoscale Experiment (LASER) involved the deployment of similar to 1000 biodegradable GPS-tracked Consortium for Advanced Research on Transport of Hydrocarbon in the Environment (CARTHE) drifters to measure submesoscale upper-ocean currents and their potential impact on oil spills. The experiment was conducted from January to February 2016 in the Gulf of Mexico (GoM) near the mouth of the Mississippi River, an area characterized by strong submesoscale currents. A Helmholtz-Zentrum Geesthacht (HZG) marine X-band radar (MR) on board the R/V F.G. Walton Smith was used to locate fronts and eddies by their sea surface roughness signatures. The MR data were further processed to yield near-surface current maps at similar to 500-m resolution up to a maximum range of; similar to 3 km. This study employs the drifter measurements to perform the first comprehensive validation of MR near-surface current maps. For a total of 4130 MR-drifter pairs, the root-mean-square error for the current speed is 4 cm s(-1) and that for the current direction is 12 degrees. The MR samples currents at a greater effective depth than the CARTHE drifters (1-5 mvs similar to 0.4 m). The mean MR-drifter differences are consistent with a wave- and wind-driven vertical current profile that weakens with increasing depth and rotates clockwise from the wind direction (by 0.7% of the wind speed and 15 degrees). The technique presented here has great potential in observational oceanography, as it allows research vessels to map the horizontal flow structure, complementing the vertical profiles measured by ADCP.