Formulation, implementation and examination of vertical coordinate choices in the Global Navy Coastal Ocean Model (NCOM)

A 1/8 degrees global version of the Navy Coastal Ocean Model (NCOM) is described with details of its formulation, implementation, and configuration of the vertical coordinate. NCOM is a baroclinic, hydrostatic, Boussinesq, free-surface ocean model that allows its vertical coordinate to consist of a coordinates for the upper layers and z-levels below a user-specified depth. This flexibility allows implementation of a hybrid sigma-z coordinate system that is expected to mitigate some of the weaknesses that can be associated with either pure coordinate option. For the global NCOM application, the sigma-z coordinate is used to allow terrain-following a coordinates in the upper ocean, providing better resolution and topographic fidelity in shelf regions where flow is most sensitive to its representation. Including z coordinates for deeper regions efficiently maintains high near-surface vertical resolution in the open ocean. Investigation into the impact of the selected coordinate system focuses on differences between atmospherically-forced free-running (no assimilation) global solutions using sigma-z and pure z coordinates. Comparisons with independent temperature observations indicate that global NCOM using the sigma-z coordinate has improved skill relative to its z coordinate implementation. Among other metrics, we show that in comparison with time series of surface temperature from National Oceanic Data Center (NODC) buoys, mostly located in coastal regions, root mean squared differences (RMSD) improved for 63% and correlation improved for 71% of the stations when sigma-z coordinates were used instead of pure z. For the exclusively open-ocean Tropical Atmosphere-Ocean (TAO) buoys, differences between the simulations were small, with the sigma-z showing smaller RMSD for 45% of the stations and higher correlation for 65% of the stations. Additional comparisons using temperature profile observations further confirm a tendency for improved performance using the hybrid sigma-z coordinates. Published by Elsevier Ltd.