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

Abstract from the manuscript associated with this submission (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.