Large-eddy Simulation of a Coastal Ocean Under the Combined Effects of Surface Heat Fluxes and Langmuir Supercells

Results are presented from large-eddy simulations (LES) of a wind-driven flow with Craik-Leibovch vortex forcing representative of the shallow coastal ocean under the influences of Langmuir (wind-wave) forcing and surface heating and cooling fluxes. In shallow water the Craik-Leibovich vortex force leads to the generation of Langmuir turbulence whose largest scales are Langmuir supercells engulfing the water column. Both unstable and stable stratification are imposed by constant surface heat fluxes and an adiabatic bottom wall. Surface buoyancy forcing relative to wind shear will be quantified in terms of a Rayleigh number in the case of surface cooling and a Richardson number in the case of surface heating. The impact of these heat fluxes on the Langmuir supercells will be assessed by analysis of cell strength and structure in a case in which a surface heat flux is absent, then with a range of positive and negative surface heat fluxes at the surface. In the case with surface cooling, the transition from Langmuir-dominated turbulence to convective instability-dominated turbulence with increasing Rayleigh number will be addressed and analyzed in terms of turbulent kinetic energy production rates. Further analysis of this transition will be made in terms of the Hoenikker number, the ratio of the buoyant forcing that drives thermal convection to the applied Craik-Leibovich vortex force that drives the Langmuir turbulence, in order to facilitate comparison to field observations. In the case with surface heating, the form and extent of turbulence suppression by stable stratification will be considered. It is intended that results may assist in determining the relative contribution of each mechanism to turbulence production or inhibition in field observations when both surface temperature fluxes and Langmuir supercells are present.