Acoustic backscattering at a tidal intrusion front

Strong spatial gradients and rapidly evolving, three-dimensional structure make estuarine fronts difficult to sample. Echosounders can be used near fronts to provide nearly synoptic images of water column processes and, with sufficient bandwidth, can provide quantitative information about dynamical variables derived from forward and inverse methods using acoustic backscattering measurements. This manuscript discusses measurements using broadband (50-420 kHz) echosounders from the James River (Virginia, USA) tidal intrusion front. The dominant backscattering mechanisms observed at the site include bubbles, turbulent microstructure, interfaces associated with stratification, suspended sediment, and biota. Existing analytical models are used to interpret contributions from these sources with acoustic inversions providing quantitative information about the physical structure and processes that compare favorably with conventional, in situ measurements. Supporting data sets for this analysis include measurements of temperature, salinity, velocity, and turbidity; X-band radar images of sea surface roughness; aerial optical imagery; Lagrangian measurements of waves, turbulence, and velocity structure; and Regional Ocean Modeling System circulation model simulations. A notable advantage of acoustic remote sensing is the ability to resolve processes at considerably higher spatial resolution (< 1 m horizontal; < 5 cm vertical) than other in situ sampling approaches.