Asynchronous-coupled large-eddy simulation of Langmuir turbulence and the atmospheric surface layer

Coupled flows of air and water near the ocean surface ocean affect ocean sequestration of anthropogenic quantities (e.g., CO2), dispersion of biomass, and a host of other issues of scientific importance. When the direction of ocean waves and the prevailing atmospheric winds are coaligned, counter-rotating roll cells emerge within the ocean layer. These cells – known as Langmuir turbulence – manifest on the ocean surface as elongated streaks.

It is common practice to assume uniform atmospheric forcing on the ocean surface, when in fact the atmosphere itself is composed of eddying gusts that induce spatial variability in wind stresses. High-fidelity computer simulations will be used to characterize how realistic spatial heterogeneity in atmospheric stress affects evolution of Langmuir turbulence.

This project offers opportunities for fundamental research in multiphase, multiscale, large-scale turbulence. The results of this project will be used to promote outreach activities directed to improve diversity and inclusion in graduate-level STEM programs. The investigator will continue to organize an annual one-day fluid dynamics symposium for graduate students throughout Texas and Oklahoma.

Langmuir turbulence in the ocean mixed layer is the product of imposed atmospheric surface drag and the aggregate drift due to wave orbital motion. Langmuir turbulence is a microscale phenomenon that regulates vertical (air-sea) exchanges of momentum, heat, and other quantities. Langmuir cells – counter-rotating vortices, meandering spatially about a predominant transport direction aligned with the prevailing wind direction – exhibit lengths of tens to thousands of meters.

Langmuir turbulence is generally defined with the turbulent Langmuir number, which is the square root of atmospheric shear velocity divided by Stokes drift magnitude. The imposed atmospheric stress exhibits dramatic spatial and temporal variation about the aggregate value used to compute the turbulent Langmuir number. Such variability is driven by the passage of coherent parcels of momentum in the aloft atmospheric surface layer – so called large-scale motions, which themselves meander spatially and exhibit lengths up to thousands of meters.

High fidelity asynchronously-coupled large-eddy simulations of the atmospheric surface layer-ocean mixed layer will be used to quantify how such large-scale atmosphere motions modulate the spatial and dynamical characteristics of deep ocean (unaffected by bottom-boundary layer) Langmuir turbulence.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.