Under the direction of
Dr.
Jerome A. Smith of the Marine Physical Laboratory, one
of the goals of the Ocean Physics Group is to understand these
wave-current interactions. This "understanding"
can be measured in terms of our ability to predict and parameterize
the results.
The Oceanic Mixed Layer
When the wind blows across water, waves are generated. As
the waves grow they also break, transferring the momentum
gained from the wind to the water below. The action of wave
breaking also injects air and turbulence into the water, strongly
influencing the exchanges of gases and the mixing of the surface
layer of water. Finally, an instability arising through a
wave-current interaction acts to enhance a particular form
of motion in the mixed layer known as "Langmuir circulation".
To investigate the details of this motion, and how the form
and strength varies as a function of the wind and waves, Dr.
Smith developed and deployed an instrument that measures the
horizontal surface velocity over an area of the ocean large
enough to span several "rolls"- the "phased-array
Doppler sonar", or PADS. Recently, we developed a long-range
version (LRPADS) that can sample one component of velocity
roughly every 10 meters over a pieslice-shaped area about
1.6 km in radius by 42¡- a total area of about a square
kilometer.
The area is sampled every 2.5 to 3 seconds, resolving the
dominant surface waves (above) and, in the time-averaged view,
the surface expression of the underlying currents (figure
3).
Near Shore
A pair of similar instruments were deployed on the Atlantic
side of the barrier islands, at Duck, NC (just north of Kitty
Hawk). With this dual-PADS configuration, both horizontal
components of velocity can be assessed (in the intersecting
region covered by both instruments.
This allows direct estimation of the "vorticity"
in the water (the fluid-dynamical analog of angular momentum),
providing an additional dynamic constraint (vorticity is conserved,
along with momentum and energy, in the frictionless limit),
and so helping to improve our estimates of the overall flow
and evolution of the system.
More information and references can be found at http://jerry.ucsd.edu.
