Broadband acoustic quantification of stratified turbulence

Abstract

High-frequency broadband acoustic scattering techniques have enabled the remote, high-resolution imaging and quantification of highly salt-stratified turbulence in an estuary. Turbulent salinity spectra in the stratified shear layer have been measured acoustically and by in situ turbulence sensors. The acoustic frequencies used span 120–600 kHz, which, for the highly stratified and dynamic estuarine environment, correspond to wavenumbers in the viscous-convective subrange (500–2500 m−1). The acoustically measured spectral levels are in close agreement with spectral levels measured with closely co-located micro-conductivity probes. The acoustically measured spectral shapes allow discrimination between scattering dominated by turbulent salinity microstructure and suspended sediments or swim-bladdered fish, the two primary sources of scattering observed in the estuary in addition to turbulent salinity microstructure. The direct comparison of salinity spectra inferred acoustically and by the in situ turbulence sensors provides a test of both the acoustic scattering model and the quantitative skill of acoustical remote sensing of turbulence dissipation in a strongly sheared and salt-stratified estuary.