Numerical study of factors controlling intensity and spatial extent of Langmuir Circulations in coastal zones

Langmuir turbulence is a microscale oceanographic phenomenon of the upper mixed layer that is relevant to mixing and vertical transport capacity. Langmuir turbulence is a manifestation of imposed aerodynamic stresses and the aggregate velocity profile due to orbital wave motion (the so-called Stokes profile), and results in the existence of streamwise-elongated counter-rotating cells. The Stokes profile is functionally dependent on depth, surface velocity, and wavenumber (McWilliams et al., 1997: J. Fluid Mech. 334, 1-30). In the past, many efforts to understand the factors that control the size and intensity of Langmuir turbulence have been based on field observations. Resultant field measurement datasets are inherently affected by a range of factors, which inhibits the isolation of individual system parameters (for example, the unsteady nature of atmospheric forcing associated with variable atmospheric pressure gradients and diurnal variability of surface heating). Therefore, we have used a comprehensive numerical study to investigate parameters affecting Langmuir turbulence. This has been accomplished by solving the grid-filtered Craik-Leibovich equations, absent Coriolis accelerations or buoyancy forcing. This simplicity, though physically unrealistic, provides a means to discern the influence of Stokes drift profile parameters on spatial extent and intensity of Langmuir turbulence. Moreover, our numerical research effort includes cases with (coastal) and without (deep) a bottom boundary layer, which further affects vertical structure. We investigated the length scale of Langmuir cells by considering cell-height and cell-width and found that Stokes drift velocity is mainly responsible for cell-width and wavenumber for cell-height. Wavenumber also had minor influence on cell-width. The strength of Langmuir cells is quantified via modified circulation (area integral of streamwise vorticity magnitude).