Surface dynamics in LES of full-depth Langmuir circulation in shallow water

Abstract

Large-eddy simulations (LES) of wind-driven shallow water flows with and without full-depth Langmuir circulation (LC) are described and near-surface dynamics analyzed. LC consists of parallel counter-rotating vortices or cells that are aligned roughly in the direction of the wind and are generated by the interaction of the wind-driven shear with the Stokes drift velocity induced by surface gravity waves. Simulations do not resolve surface waves; thus the top of the domain is taken as a non-deforming, free-slip, wind shear-driven surface. In the absence of resolved surface waves, the LC-generating mechanism is parameterized via the well-known Craik–Leibovich vortex force (Craik and Leibovich 1976 J Fluid Mech. 73 401–26) appearing in the momentum equation. LES guided by the full-depth LC field measurements of Gargett and Wells (2007 J Fluid Mech. 576 27–61) shows that this large-scale, downwind-elongated structure changes surface log-layer dynamics in terms of mean downwind velocity and budgets of turbulent kinetic energy (TKE). For example, in terms of mean velocity, the mixing due to LC leads to a deviation from the classical surface log-law profile that is exhibited by wind-driven flow without LC. Furthermore, LC leads to a deviation from the classical balance between production and dissipation rates of TKE in the log-layer. Two key parameters controlling the extent of surface log-layer disruption caused by LC are the dominant wavelength (λ) of the surface waves generating LC and the turbulent Langmuir number, Lat, which is inversely proportional to wave forcing relative to wind forcing.