A comparison of near-surface current measurements by ADCP and HF-Radar on the west Florida shelf

Surface currents (0.5 m) were measured during an eleven-day deployment of a pair of CODAR Ocean Sensors 25 MHz SeaSonde™ HF-Radars. The radar footprint overlooked of an array of acoustic Doppler current profilers on the West Florida Shelf (WSF) located between the 10 m to 30 m isobaths. An earlier study compared the hourly-averaged HF-Radar current vectors at grid points near the ADCP moorings, and found correlation coefficients (R) of 0.8 to 0.9 for the alongshelf components but 0.6 or less for the cross-shelf components, which are small in magnitude. The study noted that the alongshelf surface currents measured by the radar are about 30% larger than those of the ADCPs measured 2 to 3 m below the surface according to standard deviations and linear regression slopes. In this study we focus on the mooring located near the center of the radar coverage and explore the near surface shear and wind forcing. A fit of the M₂ tide, which is barotropic on the WSF, to the ADCP and radar vector time series yielded current tidal ellipse parameters that are in close agreement, suggesting that the low-frequency, upper-layer shear may be real. Alongshelf wind and radar current speed (at 0.5 m) were more strongly correlated than the alongshelf wind and ADCP current at 2.5 m depth. Both the principal axis analyses and the complex vector correlations indicate directional differences of 6° to 12° counterclockwise rotation (looking down) for currents with respect to wind, i.e., to the left of the wind, which is attributed primarily to the location of the coastal wind station.