A NUMERICAL-MODEL FOR THE ONSET OF STRATIFICATION IN SHEAR-FREE TURBULENCE

The formation of a thermocline in a water column, where shear-free tur bulence is generated from both the surface and the bottom, and a stabi lizing buoyancy flux is imposed on the surface, was studied using a nu merical model with the aim of understanding the formation of a tidal f ront in the coastal area. The time evolutions of the distributions of density and turbulent kinetic energy calculated from the model shows t hat the emergence of a thermocline depends on the conditions determine d by the buoyancy flux at the surface Q, the eddy diffusivities mainta ined at the bottom and at the surface K(b) and K(s) and the height of the water column H. The criterion for the formation of a thermocline w as predicted as Rdelta4 approximately constant for large delta (delta > 0.5), but the dependence on delta decreases as delta tends to 0, whe re R = H-4Q/K(b)3, delta = 1 - D0/H is the depth of a thermocline in t he absence of bottom mixing. The depth of a thermocline was found to d ecrease as the bottom mixing increases for a given value of D0. The re sults were interpreted in comparison with previous studies.