Direct measurements of 4-dimensional variability in oceanic flow structures with a new towed phased array Doppler sonar

A Towed, Phased-Array Doppler Sonar (TPADS) has been developed. Using a phased array approach we form a fan of beams over a ~110 degree 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 ~15m. An asymmetric frontal jet with a scale of ~500m 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.