Because the model is formulated in spherical coordinates, convergence of meridians near the earth's poles reduces the effective grid size in longitude. At high latitude, the solution may start to become unstable because of too large a time step. Instead of decreasing the time step, an alternative is to filter the solution at high (polar) latitudes to remove unstable wavenumber components. There are two filtering methods described below for suppressing these components. Option fourfil uses Fourier filtering and option firfil uses a finite impulse response filter to accomplish the same thing. Either one of these is typically applied polward of a reference latitude determined by the researcher. In the southern hemisphere, the reference latitude is given by filter_reflat_s and in the northern hemisphere, the reference latitude is given by filter_reflat_n. To save computation over the land mass of Antarctica, filtering is turned off southward of latitude rjfrst. These variables can be set through namelist. Refer to Section 14.4.6.
An alternative to filtering is to rotate the grid so that the convergence of meridians takes place outside to the model domain. Refer to Section 25.2.1 for details.