APPLICATION OF A 3RD-ORDER UPWIND SCHEME IN THE NCAR-OCEAN-MODEL

The National Center for Atmospheric Research (NCAR) Ocean Model has be en developed for use in NCAR's Climate System Modeling project, a comp rehensive development of a coupled ocean-atmosphere-sea ice-land surfa ce model of the global climate system. As part of this development, ne w parameterizations of diffusive mixing by unresolved processes have b een implemented for the tracer equations in the model. Because the str ength of the mixing depends upon the density structure under these par ameterizations, it is possible that local explicit mixing may be quite small in selected locations, in contrast to the constant diffusivity model generally used. When a spatially centered advection scheme is us ed in the standard model configuration, local overshooting of tracer v alues occurs, leading to unphysical maxima and minima in the fields. W hile the immediate problem is a local Gibbs's phenomenon, there is the possibility that such local tracer anomalies might propagate by advec tion and diffusion far from the source, causing inaccuracies in the tr acer fields globally. Because of these issues, a third-order upwind sc heme was implemented for the advection of tracers. Numerical experimen ts show that this scheme is computationally efficient compared to alte rnatives (such as the flux-corrected transport scheme) and that it wor ks well with other aspects of the model, such as acceleration (importa nt for spin-up efficiency) and the new mixing parameterizations. The s cheme mimimizes overshooting effects while keeping the dissipative asp ect of the advective operator reasonably small. The net effect is to p roduce solutions in which the large-scale fields are affected very lit tle while local extrema are nearly (but not completely) removed, leadi ng to physically much more realistic tracer patterns.