A comparison of acoustic turbulence profiling techniques in the presence of waves

In order to measure turbulent quantities in coastal waters, one must either avoid or confront the confounding effect of waves. In previous work, we have developed a method to cancel waves when using the variance technique to compute Reynolds stress from acoustic Doppler current profiler (ADCP) data. In this paper, we extend this wave cancellation methodology to measurements of turbulent kinetic energy and dissipation using velocities measured along a single acoustic beam. Velocity profiles were collected using a Teledyne/RDI 1,200 kHz ADCP and a Nortek AWAC. The AWAC has a vertical beam that was programmed by Nortek to deliver profiles of vertical velocity. Vertical velocities are desirable both because they eliminate sources of phase error in the wave cancellation procedure and because they constrain measurement uncertainty with respect to turbulent anisotropy. Results indicate that acoustic profiles taken in standard Doppler mode, to which the vertical beam of the AWAC was limited, were too noisy to resolve turbulence under the deployment conditions herein. Pulse-to-pulse coherent modes such as those available on the ADCP were sufficiently low noise to resolve turbulent signals; however, vertical beam data are not available for this device. Nevertheless, our wave cancellation methodology was successful in removing the overwhelming variance associated with waves from both instruments, allowing realistic estimates of Reynolds stress, turbulent kinetic energy, and dissipation from the ADCP. This method holds even more promise as low-noise operating modes are developed for vertical beam acoustic profiling instruments such as the AWAC.