2.1 Model Background

The DieCAST suite of models is characterized by a Z-level stretched vertical coordinate, and by a horizontal grid based on spherical coordinates with non-uniform latitudinal increments. A useful added feature of CANDIE is partial bottom cells for more accurate representation of bathymetry. The geophysical fluid dynamics computations are computed in a Free Stream model with low dissipation numerics. The models have a rigid lid and an elliptic pressure solver is used to calculate the surface pressure. The model uses a ``swamp layer" approximation over land. This is a single layer a few centimeters thick with a large bottom drag coefficient, which allows creeping flow over land (Zuur and Dietrich, 1990). This is not viscosity and applies only over the land points and not over the ocean points. It gives very slow creeping flow over land, after a brief transient which sets up a pressure field that effectively blocks flow over land as if were a real barrier. This is a good approximation because the large drag is not applied over the ocean points. The latest DieCAST approach iterates the rigid-lid pressure solver to rapidly converge the flow over land to exactly zero, which even in the rather severe test problem of Von Karman vortex shedding in the wake of an island, gives almost identical results to the original ``swamp" layer approach (Dietrich and Ko, 1994).

The ``swamp layer" approach enables the elliptic solver to use the entire grid domain, and so irregular coastlines and islands can be accomodated by the model. The first version of the model was the Sandia Ocean Modeling System (SOMS) and is described by Dietrich et al. (1990). Next the DieCAST (Dietrich - Center for Air Sea Technology) model was developed from SOMS and is described by Dietrich and Ko (1994).

There are two web sites that describe the DieCAST model


These sites have a DieCAST model users list, references describing the numerics of the SOMS and DieCAST models, bibliographies of the scientific research done with them, and information on recent model oceanographic applications. A copy of the DieCAST user's manual by Dietrich and Brian Sanderson can be obtained from these sites. While the DieCAST and CANDIE models are different, their DieCAST user's manual is of use for background information.

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