Comparison of overflow simulations on different vertical grids using the Finite Element Ocean circulation Model

Abstract

The Finite Element Ocean circulation Model (FEOM) is applied to study the sensitivity of density driven overflows to the vertical discretization and bottom topography representation using the dynamics of overflow mixing and entrainment (DOME) setup. FEOM allows for hybrid grids combining σ, z + σ, full cell, partly shaved cell and fully shaved cell grids within the same numerical kernel thus isolating as far as possible effects of mesh geometry from those of model numerics. The sensitivity of diapycnal mixing, entrainment, plume thickness and plume meridional distribution to vertical discretization and partly to the subgrid process parameterization is explored. It is shown that simulations on pure σ grids or the combination of z + σ resolve the overflow processes best in terms of downslope plume propagation, plume thickness and dilution, and also have the least resolution dependence. Grids using z-levels generate excessive spurious mixing when resolution is insufficient. Applying partial cells improves the plume representation, but still requires higher horizontal and vertical resolution to converge to the σ grid results. It is demonstrated that increasing lateral viscosity causes the plume thickness to reduce whereas increasing lateral diffusivity has opposite effect. When keeping the Prandtl number constant, the increase in diffusivity and viscosity leads to an increase in mixing and plume thickness on z-level grids and also on σ-grids when lateral dissipation is oriented along geopotential surfaces. Using the along σ- diffusion helped to obtain correct plume thickness and entrainment on σ grids. Increasing the vertical mixing coefficients leads to an increase in diapycnal mixing and in downslope penetration as well.