Large-Eddy Simulation of the Diurnal Cycle of Deep Equatorial Turbulence

TLDR: In this article, the authors studied the deep diurnal cycle of turbulence at the equator using the technique of large-eddy simulation (LES) based on a scale-separation hypothesis, and found that boundary-layer turbulence penetrates as deep as 50 m below the mixed layer during nighttime cooling.

Abstract: The deep diurnal cycle of turbulence at the equator is studied using the technique of large-eddy simulation (LES). Based on a scale-separation hypothesis, the LES model includes the following large-scale flow terms: the equatorial undercurrent (EUC), zonal pressure gradient, upwelling, horizontal divergence, zonal temperature gradient, and mesoscale eddy forcing terms for the zonal momentum and the heat equations. The importance of these terms in obtaining a quasi-equilibrium boundary layer solution is discussed. The model is forced with a constant easterly wind stress and diurnal cooling and heating. It is found that boundary-layer turbulence penetrates as deep as 50 m below the mixed layer during nighttime cooling. The diurnal variation of turbulence dissipation and mixed layer depth are within the range of observations. The gradient Richardson number (Ri) of the mean flow shows a diurnal cycle but the amplitudes decrease with depth. Within the mixed layer and just below the layer, Ri can be lo...