Collaborative Research: Detached Eddy Simulation of Nearshore Langmuir Circulation

Wind and wave-driven Langmuir circulation (LC) or Langmuir turbulence in the upper ocean mixed layer and in unstratified shallow continental shelf regions has been extensively studied via field measurements and numerical simulations. However, studies of Langmuir turbulence in the inner shelf under the influence of the surf zone are lacking. The overarching goal of this project is to investigate numerically the structure and intensity of Langmuir turbulence and its impact on cross-shore scalar transport in idealized shallow coastal shelf environments extending from the surf zone to the inner shelf.

This will require turbulent eddy-resolving simulations that can capture interactions between the Langmuir turbulence regime and common nearshore processes such as (1) pressure gradient-driven alongshore currents (e.g., tidal, and geostrophic currents), (2) transient rip currents and associated longshore currents generated by breaking waves and (3) cross-shore currents induced by diurnal surface heat fluxes. Simulations will extend over a surf-shelf transition cross-shore region of ~2 km, spanning depths between 0.1 m and ~15 m.

The eddy-resolving simulations will be based on novel application of the detached eddy simulation (DES) methodology, well-established in the aerospace engineering community, to be extended to coastal ocean turbulence. The unsteady DES will be able to capture rip currents emanating from the shore while modeling the wave-breaking turbulence and other wave-related phenomena such as wave bottom and surface streaming via well-established techniques developed by the nearshore circulation modeling community.

Nearshore regions are biologically highly productive and the turbulence dynamics to be studied have direct impact on physical, chemical, and biological processes affecting this productivity. Ultimately understanding nearshore turbulence dynamics is important for managing natural and economic coastal resources. This project will promote the use of DES within the physical oceanography community for continued application to understand the turbulence driven by simultaneously occurring mechanisms in the surf-shelf transition zone of the coastal ocean.

Results of the proposed project will be communicated at conferences (Ocean Sciences Meeting and the Annual Meeting of the American Physical Society / Division of Fluid Dynamics) and through publication in peer-reviewed journals. The project will support a graduate student and a postdoctoral fellow, who will train in physical oceanography, turbulence dynamics and closures and numerical techniques.

Although episodes of Langmuir turbulence on the inner shelf have been reported in the literature, interactions between this turbulence regime and classical nearshore processes have not been identified. This project aims to address this knowledge gap by uncovering, for example, interactions between LC and transient rip currents and the influence of LC on cross-shore scalar transport.

Specific objectives of the research include: (1) To determine how far can Langmuir turbulence extend to the nearshore before it is arrested by the surf zone turbulence due to wave-breaking and whether LCs can contribute to onshore transport induced by wind and waves. (2) To elucidate the potential generation of Langmuir turbulence via tilting of vertical vorticity associated with lateral shear induced by rip currents into wind-aligned LC. (3) To determine the role of Langmuir turbulence and the hybrid regime consisting of Langmuir turbulence and bottom-generated turbulence on cross-shore scalar exchange induced by rip currents and baroclinic cross-shore currents (induced surface heat fluxes).

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.