Abstract
Abstract. 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 are used to identify the optimum value of Smagorinsky diffusivity coefficient to best represent the mesoscale activity.
Highlights
The early ocean circulation models (Sarkisyan, 1962; Bryan, 1963) all used z coordinate vertical grids and a very coarse representation of the bottom topography
Similar constraints on the use of z coordinate have been found in ocean modelling in particular where both the shelf seas and the deep ocean are included in the model domain
There is a minimal loss of numerical levels (3 out of 18) in the shallowest grid cell in the western part of the transect. It order to compare performance of various grids, we carried out a number of idealised simulations, for which the solution is known, some idealised cases where solution is known only approximately and real world simulations where the results are compared with satellite-derived sea surface temperature
Summary
The early ocean circulation models (Sarkisyan, 1962; Bryan, 1963) all used z coordinate vertical grids and a very coarse representation of the bottom topography. Meteorologists who started using numerical models a decade earlier (Charney et al, 1950) have found that using a z coordinate system has certain computational disadvantages in the areas of varying topography, in particular in the vicinity of mountains To resolve this problem a terrain-following sigmacoordinate system was introduced (Philips, 1957). Ezer and Mellor (2004) compared sigma and z level grid simulations in a highly idealised dense water flow down a sloping bottom. The z level grid performed relatively poorly due to the step-like representation of topography Increasing both the horizontal and vertical resolution in the z. We evaluate efficiency and identify optimised parameters for the Smagorinsky algorithm for horizontal diffusion, which has become the preferred method in modern grid-point models (Becker and Burkhard, 2007)
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