Observations of tidal and wind-driven currents, and acoustic backscatter using an acoustic Doppler current profiler on a surface-following buoy

The near-surface oceanic boundary layer is a challenging environment for current measurements: there are the practical difficulties of making measurements, and often the interpretation of the results is not straightforward. Acoustic Doppler current profilers (ADCPs) can contribute to easing both of these difficulties, but are not without their own set of problems. The authors have developed a surface following buoy fitted with an electromagnetic current meter at a depth of 1 meter, and a 1 MHz ADCP covering the upper 25 meters. Data on the near-surface current structure gathered from an 80 day experiment in the fall of 1993 are described. The profile of tidal currents is compared with an analytical model, and the wind-driven currents are compared to the local wind. In addition to measurements of current profiles, ADCP acoustic backscatter intensity can be used to estimate the coefficient of vertical eddy diffusion, Kv, using a simple model of bubble density in clouds generated by breaking waves. However, the authors find the acoustic environment in the near surface zone during winds in excess of 9 m s^-1 to be so dominated by bubbles at the depth of the transducer (0.6 m), that the effective signal-to-noise ratio is too low for estimating Kv. The degradation of the signal-to-noise ratio also reduces the accuracy of current measurements above this 9 m s^-1 wind speed threshold