Collaborative Research: Characterization of Langmuir Supercells in the Coastal Ocean

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Langmuir circulation (LC) arises from the interplay of wind forcing and wave dynamics, and consists of pairs of parallel counter-rotating vortices (or cells) oriented approximately in the direction of the wind. Over the continental shelf , where the water is shallow and relatively well-mixed, the passage of strong storms can induce large Langmuir cells, the largest of which can span the entire water column and have a strong influence on sediment re-suspension. These so-called Langmuir supercells (LS) play an important role as vectors for the transport of sediment and bioactive material on shallow shelves: their quasi-organized nature makes them more effective than bottom boundary layer shear turbulence at moving material out of the low-speed layer near bottom and into the bulk flow for lateral transport. This study will analyze existing observations from three different sites at 15, 26 and 40m depths as well as high resolution numerical simulations to determine the influence on LS of a number of factors such as large scale flows, tidal currents, less dense surface layers and misalignment between wind and wave direction. Continental shelves are biologically highly productive and LS have direct impact on processes affecting this productivity or distributing its products. Thus, it is essential that shelf models incorporate skillful parameterization of such dominant processes. This study will improve parameterizations used to account for Langmuir circulation in many coastal and regional models where the size of the domain precludes running the model at a high enough resolution to capture Langmuir circulations directly. One post-doctoral researcher, two graduate students and one undergraduate student in each summer will be trained in physical oceanography, turbulence dynamics and closures and numerical techniques. One of the students is from an underrepresented minority group.In the coastal ocean, LS has been observed under tidal and geostrophic flow components, surface heat fluxes and misaligned winds and waves serving to influence LS strength and coherency, as measured through bottom-mounted vertical-beam acoustic Doppler current profilers (VADCPs). Observation sites consist of the Rutgers LEO observatory in 15m of water at the inner shelf off the coast of southern New Jersey, the Navy's R2 tower in 26m of water at mid-shelf off the coast of Georgia, and an additional site north of Cape Hatteras in 40m of water. Analyses of the measurements at LEO and R2 have demonstrated differences in LS characteristics, with more coherent and intense LS occurring at LEO. New analysis of the Cape Hatteras data as well as large eddy simulations (LES) of LS flows closely following the VADCP measurements at the three sites will elucidate the dynamics by which tidal and geostrophic flows, surface buoyancy and misaligned wind and waves combine to affect LS and associated vertical transport. The LES will help provide key information missing from the field data sets, such as detailed velocity structure in the surface and bottom boundarylayers, required to make definite conclusions regarding the roles of the various forcing mechanisms influencing the LS at the different sites. LES will be used to obtain a scaling of vertical velocity fluctuations representative of the strength of non-local vertical mixing induced by LS under the various processes affecting its strength and coherency. The scaling of vertical velocity fluctuations will be used to inform a KPP (K-profile parameterization) developed by the PI accounting for the local transport of the overall Langmuir turbulence and the non-local transport induced by the LS. The updated LS KPP will be implemented in the one-dimensional vertical water column General Ocean Turbulence Model (GOTM) for validation via comparisons with the LES and the field measurements.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.