Comparison of HF Radar and ADCP Surface Currents at the Delaware Bay Mouth

Abstract

Coastal surface currents measured remotely by high frequency (HF) radar are compared with those from in situ acoustic Doppler current profilers (ADCP) deployed in both Eulerian and Lagrangian configurations. Two 25 MHz CODAR SeaSondesreg, located at Cape May, NJ and Cape Henlopen, DE, continuously measure surface currents at the Delaware Bay mouth. The radars measure surface current hourly with a spatial resolution of approximately 1.5 km using ideal antenna patterns. Surface currents in this area are dominated by strong semidiurnal tidal exchange between the Delaware Bay and Atlantic Ocean, which can reach velocities of 190 cm/s. An upward looking 600 KHz ADCP with a vertical resolution of 0.5 m was deployed on a seafloor frame 1.5 km east of Cape Henlopen within the radar footprint for 38 days. A downward looking 600 KHz ADCP with a vertical resolution of 1 m was mounted on a research vessel and operated continuously along a transect within the radar footprint for 41 hours. Velocities from the uppermost complete bin of the seafloor mounted ADCP are compared with the nearest radar measurement. Velocities from the shallowest bin of the ship mounted ADCP are compared with the nearest radar measurement in both time and space. No interpolation was done. Regression analysis indicates correlation coefficients (R) between the radar and ADCPs of 0.8 to 0.9 for current magnitude and 0.7 to 0.8 for current direction. Complex correlation analysis yields correlation coefficients greater than 0.9 with the ADCP observations veering clockwise from the radar observations by 0.3deg to 0.6deg. The root mean square (RMS) difference of magnitudes indicates that radar measurements of surface current magnitude are 19.4% greater than those made by the moored ADCP and 8.2% less than those made by the ship ADCP.