The effect of various vertical discretization schemes and horizontal diffusion parameterisation on the performance of a 3D ocean model: the Black Sea case study

Results of a sensitivity study are presented from various configurations of the NEMO ocean model in the Black Sea. The standard choices of vertical discretization, viz. z-levels, s-coordinates and enveloped s-coordinates, all show their limitations in the areas of complex topography. Two new hybrid vertical coordinate schemes are presented: the ‘s-on-top-of-z’ and its enveloped version. The hybrid grids use s-coordinates or enveloped s-coordinates in the upper layer, from the sea surface to the depth of the shelf break, and z-coordinates are set below this level. The study is carried out for a number of idealised and real world settings. The hybrid schemes help reduce errors generated by the standard schemes in the areas of steep topography. Results of sensitivity tests with various horizontal diffusion formulations allow to identify the optimum value of Smagorinsky diffusivity coefficient to best represent the mesoscale activity.