The Performance of High-Frequency Doppler Sonars in Actively Breaking Wave Crests

Breaking ocean waves influence wave dynamics, momentum transfer, air-sea exchange, ocean albedo, and ambient noise generation, all of which are impacted by the transient, two-phase flow in a whitecap. Lasting O(1s) or so, actively breaking whitecaps contain air fractions up to 0.6, bubbles ranging in size O(10-1000) μm and turbulent dissipation rates O(1) W·kg^-1. Strong fluid turbulence, high air fractions, large bubbles, and short duration make active whitecaps a challenging process to study. This paper presents a model for the performance of high-frequency Doppler sonar (0.5-2 MHz) when used to probe the interior of actively breaking whitecaps. The results suggest that the ability of high-frequency sonars to penetrate the interior of bubble plumes in whitecaps becomes limited for air fractions greater than 0.03-0.06 and plumes become completely impenetrable for air fractions greater than 0.08-0.17. This severely limits their usefulness as a tool to probe the interior of breaking waves. Moreover, the bias introduced by the terminal rise velocity of large bubbles interacting with fluid turbulence within the wave crest will need to be accounted for when interpreting any backscatter signals that are returned from the plume interior. At this time, in situ methods such as optical fiber probes, conductivity cells, and cameras remain the best option for field studies of the interior of breaking oceanic waves.