The file * prep.F* must set up the horizontal grid. This requires that the
domain boundaries and the model resolution be determined. For a
realistic North Atlantic run, the model domain is determined by
parameters read in from * Prep_parm.tag_root*. ** X0DEG** and
** Y0DEG** give the
longitude and latitude at the center of the grid cell in the
lower left extremity of the model. ** XRDEG** and ** YRDEG** give the distance
in degrees from the center of the leftmost cell to the center of the
rightmost cell and from the center of the southernmost to the center
of the northernmost cell, respectively. Because there are ** I0** grid cells in the
east-west direction, the east-west distance ** DXDEG** across each cell is
** XRDEG/( I0**-1) degrees. The north-south distance ** DYDEG**
depends on ** DXDEG** and the parameter ** DYDX**, which is read from the
* Prep_parm.tag_root*.
** DYDX** should be set to either 999 or 1. If ** DYDX** is set to 999, then a
uniform grid is used with ** DYDEG** = ** YRDEG**/(** J0**-1). More commonly, when a
spherical grid is used, the north-south extent of each grid cell is
taken to be proportional to the cosine of latitude so that dy/dx (=
*cos(phi)*** DYDEG / DXDEG**) is uniform over the model grid. If ** DYDX** is
not equal to 999, then it is adjusted to the value required to make (** J0**-1)
cells extend precisely over the north-south distance ** YRDEG**.

The grid spacing arrays vary only in the Y-direction. The arrays
** DX**(** J0**) and ** DY**(** J0**) give the X and Y grid spaces
centered on an A-grid point, and the arrays
** DXV**(** JP**) and ** DYV**(** JP**) give the X and Y grid spaces
centered on a C-grid advective V velocity point. These are shown in Fig. 4.
A warning is warrented here. Because the accuracy of the elliptic
solver used in DieCAST is reduced if ** DY** is larger than ** DX**, ** J0** should be chosen
large enough that ** DY** is no greater than ** DX**. If ** J0** is chosen such that
** DY** is greater than ** DX**, the program will print a warning for the user,
but it is up to the user to notice this warning and make appropriate
adjustments. Either ** J0**/** I0** should be increased or the domain
boundaries should be modified such that ** YRDEG**/** XRDEG** is decreased.

After the horizontal grid is established, the value of the Coriolis
parameter is determined at the mid-latitude of each row of cells and
stored in ** F**(** J0**).