October 31, 2012
Navy Oceanographers Improving Wave Forecasts
Lee Rannals for redOrbit.com — Your Universe Online
Oceanographers from the U.S. Navy are using ocean wave research to help improve forecasts.
Naval oceanography can provide information critical for combat disciplines, and today U.S. Naval Research Laboratory (NRL) physicists at the Remote Sensing Division will continue to improve the integrity of these forecasts.
"Current state-of-the-art ocean wave forecasting numerical models do not take this mechanism into account," Dr. Ivan Savelyev said in a statement. "This can potentially account for some forecasting errors and directly impact the Navy's ability to predict wave heights in oceans anywhere in the world."
The parameterization of turbulence production in the upper ocean has relied on the assumptions of wall turbulence, where the wind-generated surface friction velocity acts as a moving boundary.
In the top few feet of the ocean another resource term is added due to turbulence injection by breaking waves.
There are currently a number of competing theories and hypotheses attempting to describe a third mechanism, which would account for energy flux from nonbreaking waves to the upper ocean turbulence. However, scarce results struggle to establish the existence of the energy transfer.
NRL researchers are investigating both experimental and numerical approaches to the third mechanism. Turbulent velocities at the water surface were measured in a laboratory wave tank with high precision using the thermal marking velocimetry technique.
Numerically, a fully nonlinear model for the wave motion was coupled with Large Eddy Simulation for the turbulent motion.
Both numerical and laboratory results confirm the turbulence production due to wave motion and the turbulent kinetic energy was found to be a function of time, wave steepness, wave phase, and initial turbulent conditions.
Also, they found turbulent motion near the surface was horizontal due to the formation of near-surface eddies, elongated in the direction of wave propagation.
"The mechanism responsible for the observed production of turbulence appears to be the result of interaction between pre-existing turbulence and wave motion," Savelyev said in the statement. "More specifically, the horizontal shear, associated with wave-induced Stokes drift, stretches and intensifies existing vortices, giving rise to 'streaky' turbulent patterns we observed at the water's surface."
The new mechanism reveals a function of existing turbulence in the upper ocean. This helps to reveal the added need to couple atmosphere-wave-ocean dynamics within a joint forecasting system.