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## Grid Boundaries

The east/west boundaries are periodic, and the north/south boundaries are solid walls, with a sponge layer from S to S, and N to N. At latitudes higher than (this cutoff value is set by the local variable `y0` in subroutine `parinit`), this sponge layer reduces heat and moisture flux, and increases momentum damping, with a linear drop-off. This layer is used to reduced excessive boundary precipitation. The code contains options for damping out other dynamical and physical components in this sponge layer. However, in the standard release, only `spngh1` and `spngh3` are active; sponge layer terms `spngh2` and `spngh4` are commented out. Respectively, these latter terms increase diffusion and reduce precipitation ( ) at latitudes higher than . The functional descriptions of the sponge layer are set in subroutine `parinit`.

Sponge layer functions `spngh1` and `spngh3` are defined as:

where is the latitude (in degrees) at the jth grid point. In Figure 2.1, this is the location where the `u` (and `T`) variables are located.

At the north-south boundaries, meridional velocity v is set to zero. Thus, at the northern boundary, both `v0(i,NY)` and `v1(i,NY)` are set to zero. To describe the southern boundary, an extra point has been added at the southern-most extent of arrays `v0` and `v1`. Thus, the y-direction of these two arrays (and arrays which depend on them, such as `psi0`) are actually dimensioned 0:NY instead of 1:NY, as the other arrays (such as `u0` and `u1`) are. An exception is `vort0`, which one might think is dimensioned 0:NY, but actually is dimensioned 1:NY. Then, this southern-most point of both `v0` and `v1` (i.e. `v0(i,0)` and `v1(i,0)`) are set to zero. As a reminder, `v0(i,0)` and `v1(i,0)` describe and , respectively.

Note that though the east/west boundaries are periodic, numerically this is implemented without the use of ghostpoints.

Next: Spherical Geometry Up: Model Code and Numerics Previous: Model Code and Numerics

Johnny Wei-Bing Lin
Wed Aug 26 16:58:31 PDT 1998