Vertical coordinate and resolution dependence of the second moment turbulent closure models and their limitations

Abstract

Coordinate and resolution dependence of three second moment turbulent closure models are studied using one-dimensional Navy Coastal Ocean Model (NCOM) experiments and large eddy simulations at Ocean Station Papa. Our results suggest that finer resolution near the base of the mixed layer is critical for better model performance. A mixed layer enhanced vertical grid is proposed that outperforms both the uniform and the stretched grids with significantly fewer vertical layers used. For the new grid, the model accuracy is strongly dependent on the resolution near the base of the mixed layer, and not affected much by the total number of vertical layers used. However, given the success of the new grid, the lack of representation for the near inertial gravity waves below the mixed layer has hampered the ability of second moment turbulent closure models on accurate representation of turbulent mixing in the water column. While both the Langmuir circulation and the variation of surface heat fluxes are shown to be able to significantly change the strength of the near inertial waves, they have negligible effect on the eddy viscosity in the transition layer.