A Description of the NCOM Model


Paul Martin and Alan Wallcraft

Naval Research Laboratory

Stennis Space Center, MS



NCOM is a three-dimensional, free surface, baroclinic ocean model that was developed for coupling with the Coupled Ocean/Atmospheric Mesoscale Prediction System (COAMPS). NCOM is based on the primitive equations and the hydrostatic, Boussinesq, and incompressible approximations. The model uses an Arakawa C grid and second-order, centered, spacial finite differences, but has options to use some higher-order differences including 3rd-order upwind advection. The temporal differencing is leapfrog with an Asselin filter to supress timesplitting. The propagation of surface waves and vertical mixing are treated implicitly. A choice of the Mellor-Yamada Level 2 or Level 2.5 turbulence models is provided for the parameterization of vertical mixing.

The horizontal grid is orthogonal curvilinear. The vertical grid uses sigma coordinates for the upper layers and z-level (constant depth) coordinates for the lower layers and the depth at which the model changes from sigma to z-level coordinates can be specified by the user. Hence, sigma coordinates can be used for just the surface layer, for several layers between the surface and a specified depth, or for all the layers.

An arbitrary number of levels of nesting is implemented by using dynamic memory allocation and by passing model variables through subroutine argument lists so that the same subroutines can calculate the different nested grids. Domain decomposition with MPI or SHMEM is used for running on distributed memory, multi-processor computers.