HYBRIDIZATION BETWEEN SIGMA-CO-ORDINATE AND Z-CO-ORDINATE FOR IMPROVING THE INTERNAL-PRESSURE GRADIENT CALCULATION IN MARINE MODELS WITH STEEP BOTTOM SLOPES

It is discussed in this paper how the pressure gradient error in gener al vertical co-ordinate models (in which the sigma-transformation is a special case) can be reduced by means of hybrid models. For a better understanding, the derivation of such a general vertical co-ordinate m odel from the Cartesian co-ordinate model is given. Two types of hybri dization between sigma- and z-co-ordinate models, each using one param eter specifying the degree of hydridization, are presented: (i) the mi xed layer transformation with a constant number of layers which are re fined near the surface and (ii) the z/sigma-transformation which intro duces steps near the bottom. In order to achieve good results with the models using other than sigma-co-ordinates, a profile-conserving mome ntum advection discretization is developed. The different co-ordinate transformations are tested with 2D barotropic and baroclinic flows ove r a topographic bump. Those models with nearly horizontal co-ordinate surfaces in the stratified area give the best correspondence with an i sopycnal reference solution. (C) 1997 John Wiley & Sons, Ltd.