Inferring the vertical turbulent diffusion coefficient from backscatter measurements with a multibeam sonar

A 240 kHz multibeam sonar was deployed on a sub‐surface mooring in order to collect acoustic backscatter measurements from the bubbles in a ships wake in August, 2002. The sonar was located at a depth of 30 m and oriented so that its swath of beams was orthogonal to the ships track. Measurements proportional to the acoustic scattering cross‐section per unit volume were made and used to estimate the vertical turbulent diffusion coefficient in the ships wake using the same methodology that Thorpe used with a single‐beam echosounder underneath breaking waves [J. Phys. Ocean. 14, 855–863 (1984)]. The wake was viewed as a distributed source of bubbles that are transported downward from the surface by turbulence. A steady state representation of the bubble field was used, where it was assumed that the bubble population exponentially decayed with depth, and that a bubble conservation equation could be written that only accounted for turbulent diffusion, buoyancy, and gas dissolution. Estimates of the vertical turbulent diffusion coefficient in the wake of a research vessel will be presented. [Work supported by ONR Code 321US. First author supported by a National Defense Science and Engineering Graduate Fellowhip.]