Direct Measurements of Four-Dimensional Variability in Oceanic Flow Structures with a New Towed Phased Array Doppler Sonar

Abstract A towed phased array Doppler sonar (TPADS) has been developed. Using a phased array approach, we form a fan of beams over a ∼110° aperture that allows us to measure radial ocean velocities in three dimensions. The use of modern analog-to-digital converters developed for medical ultrasound enables a compact instrument that can be mounted on a small towed body, which allows us to isolate the instrument from the motion of the ship. The compact nature of the instrument also enables significant flexibility in deployment parameters. Here, we describe this instrument and show two case studies from early deployments. The first shows the three-dimensional shear field near the bottom in a region of complex bathymetry with slopes near critical to the internal tide. We capture the three-dimensional structure of shear layers that slope along internal tide characteristics. In the second case study, we deploy TPADS in a surface scan mode that enables high-precision measurement of ocean surface velocities. We show measurements of surface velocity across a sharp density front with horizontal resolutions of ∼15 m. An asymmetric frontal jet with a scale of ∼500 m is measured. TPADS promises to be an important tool in the study of small-scale and three-dimensional instabilities on the ocean surface and in the bottom boundary layer, which are hard to observe with conventional acoustic Doppler techniques. Significance Statement Ocean processes with scales from tens to hundreds of meters play a major role in vertical transport of heat, momentum, and nutrients. These processes have a complex three-dimensional structure and are hard to measure with traditional techniques. The resulting observational gap makes it difficult to validate ocean models at these scales. We have developed a tool to measure three-dimensional ocean velocity structure. Our instrument uses measurements of transmitted, reflected, and Doppler-shifted sound to estimate ocean velocity across 31 beams that extend outward 200 m in a fan shape. Advanced electronics allow us to make this instrument relatively compact. This makes it flexible, allowing us to deploy it in unique ways to target processes of interest.