88 FIG. III-1. THREETOX modelling system. The concept of eddy viscosity/di?usivity is used to determine the turbulent stresses. The vertical eddy viscosity vt ? vf and eddy di?usivity ??? ? ?? are modelled as a product of turbulent velocity scale and turbulent length scale l, whereas vf is a constant background viscosity/di?usivity value when turbulence is suppressed by buoyancy forces at stable stratification. The turbulent velocity scale is proportional to the square root of kinetic turbulent energy k, therefore ?? = ????? and ??? = ??????. Here cµ and c?µ are stability functions [III-5]. The United Nations Educational, Scientific, and Cultural Organization equation of state in the form proposed in Ref. [III-6] was used. The two-equation k ? ? turbulence model is used to calculate k and its dissipation rate ? = ????? ? ???? ? ?. Here ??? is a constant. The horizontal turbulent viscosity/di?usivities are parameterized by a formula from [III-7]. At the free surface z = ? the kinematic boundary condition is: ?? ?? + ? ?? ?? + ? ?? ?? = ? (III-8) The surface ?uxes required by the model are those of momentum, heat and salt. Turbulent ?uxes of momentum at the surface are: ?? ?? ?? = (1 ? ?) ??? (?) ?? + ? ??? (?) ?? (III-9)