2.4.3 Model physics and numerics

The main advances in MOM 3 relative to MOM 2 are in the model's physics, numerics, and parallelization. The following is a brief outline of the additions to physics and numerics.

1.

Implementation of KPP vertical mixing scheme of Large, McWilliams, and Doney (1994) (Section 32.2.3).

2.

Implementation of partial bottom cell topography of Pacanowski and Gnanadesikan (1998) (Chapter 26).

3.

Implementation of bottom boundary layer of Gnanadesikan, Winton, and Hallberg (1998) (Chapter 36. Work on this option is ongoing.).

4.

Implementation of the Gent-McWilliams skew-flux of Griff ies (1998) (Section 34.1.6).

5.

Generalization of the isoneutral diffusion scheme of Griff ies et al. (1998) to allow for partial bottom cells (derivation in Appendix C).

6.

Streamlining of the isoneutral mixing schemes which results in a reduction in model run time relative to the MOM 2 implementation.

7.

Implementation of the Held and Larichev (1996) and Visbeck, Marshall, Haine, and Spall (1997) closures for the Redi and GM tracer diffusivities (Section 34.2).

8.

Implementation of the Roberts and Marshall (1998) biharmonic mixing scheme (Section 34.1.8).

9.

Implementation of an explicit free surface (Chapter 7 and Section 29.5).

10.

Implementation of fresh water fluxes into the explicit free surface, rather than virtual salt fluxes. Formulation is given in Chapter 7.

11.

Implementation of a specified spatially variable horizontal viscosity which includes the proper kinematic terms proportional to the spatial derivatives of the viscosity (Chapter 9 and Section 33.6).

12.

The meridional streamfunction diagnostic has been expanded so that the streamfunction can be computed using potential density as a vertical coordinate (Section 39.9).

13.

A diagnostic has been implemented which will map all the terms affecting the evolution of locally referenced potential density (Section 39.7).

14.

The old time manager has been replaced by a Fortran 90 time manager which defines time structures and overloads the standard numerical operations of plus, minus, times, and divide to work with structures. All manipulations involving time are now much simpler than before.

15.

An exchange module will be added to conserve quantities being passed between different latitude-longitude grids. The intent is for coupled air-sea applications. (planned but currently not implemented)

16.

Common blocks are being replaced by Fortran 90 modules. (For the barotropic portion only. There is a 30% slow down in speed when common blocks are removed from the baroclinic and tracer portions of the model. As Fortran 90 matures, the remaining ones will be replaced.)

17.

In addition to the Euler Backward and the forward mixing time steps every nmix time steps (usually nmix = 17), an option has been added for a Robert filter applied every time step (Section 21.4.4).

18.

The model topography can now be changed by editing the file kmt.dtawith a text editor.

19.

There is an option for an isotropic grid (one where $\Delta_y$compensates for the convergence of meridians to keep the grid cells square).

20.

The test case resolution has been changed from a $4^\circ\; x\; 3^\circ$ grid to a $3^\circ\; x\; 2.765^\circ$ grid to facilitate parallel processing tests with up to 64 processors.

21.

The mean radius of the earth has been changed from 6370 km to 6371 km.

22.

A parameterization for mixing tracers between unconnected regions of ocean has been added as a way to handle the tracer exchange between the Mediterranean and Atlantic as well as other regions where resolution is insufficient to allow realistic exchanges (Section 35.2).

23.

The older relaxation methods for solving elliptic equations have been removed in favor of the method of conjugate gradients.

24.

As an ongoing research topic, ways to speed up communication between processors are being explored. When improvements are implemented, changes are confined to a small communication package.