Scalar transport in large-eddy simulation of Langmuir turbulence in shallowwater

Large-eddy simulations (LES) of wind-driven shallow water flows with Langmuir turbulence have been conducted and scalar transport and surface scalar transfer dynamics analyzed. In these flows, the largest scales of the Langmuir turbulence consist of full-depth Langmuir circulation (LC), parallel downwind-elongated, counter-rotating vortices acting as a secondary structure to the mean flow. Langmuir turbulence is generated by the interaction of the wind-driven shear current with the Stokes drift velocity induced by surface gravity waves. In the absence of resolved surface waves, the Langmuir turbulence-generating mechanism is parameterized via the well-known Craik–Leibovich vortex force (Craik and Leibovich, 1976) appearing in the momentum equation. Simulations do not resolve surface waves, thus the top of the domain is taken as a non-deforming, free-slip, wind shear-driven surface. LES guided by the full-depth LC field measurements of Gargett and Wells (2007) shows that Langmuir turbulence plays a major role in determining scalar transport throughout the entire water column and scalar transfer at the surface. Langmuir turbulence affects scalar transport and its surface transfer through (1) full-depth, large-scale LC and (2) near-surface, small-scale eddies. Two key parameters controlling the extent of these two mechanisms are (1)the ratio λ˜=λ/H, where λλ is the dominant wavelength of the surface waves generating the turbulence and H is the mean water column depth and (2)the turbulent Langmuir number, Lat, which is inversely proportional to wave forcing relative to wind forcing. Results from simulations with varying combinations of λ˜ and Lat are analyzed in order to understand the effect of these two parameters on scalar dynamics.

► Impact of shallow water Langmuir turbulence on scalar transport is studied.
► Impact of Langmuir turbulence on surface mass transfer is also considered.
► Langmuir turbulence is characterized by full-depth Langmuir circulation.
► Focus is placed on the impact of mixing caused by full-depth Langmuir cells.
► Focus is also on near-surface small eddies enhancing vertical turbulence.