Set up a model grid

GEM offers the possibilities of running global uniform (up to version 4), global YinYang (starting version 4) and limited area (LAM) grids. Have a look at the grid types.

The grid is specified in the file 'gem_settings.nml' in the namelist 'grid'. This GEM grid specification parameters link explains how to define the parameters. Alternatively, see below.

You have to set variables for the

Grid type

First thing to do is to set 'Grd_typ_S' to the grid type you want:

Grid rotation (optional)

Next specify whether your grid is to be rotated or not (if not just skip this paragraph).
Four variables are used to define the rotation of the grid. They define two points:

• The first point will be the new center of your grid (at the grid(!) equator and the grid(!) 180° longitude).
• The second point is any other point on the grid(!) equator east of the center and will therefore set the rotation of the grid equator wrt the real equator in the first point.
  

Both points are specified in latitudes and longitudes of the real globe. For example:

To create a grid that is shifted but not rotated in the center point Grd_xlat2 and Grd_xlon2 should get set the following way:

   Grd_xlat2 = 0.,  
   Grd_xlon2 = Grd_xlon1 + 90.       =>  You need to set it to the value 'Grd_xlon1 + 90.', do not actually put 'Grd_xlon1' here!!!

Number of grid points and grid size

Global uniform grid
It is enough to specify the total number of points. The model will then calculate size of the grid boxes automatically.

Global Yin-Yang grid

One only needs to specify the core/free number of points in y-direction (Grd_nj) as well as the "overlap" in degrees (Grd_overlap). The model will calculate the number of points in x-direction as well as the size of the grid boxes automatically from that.

Note that there is no blending/merging area, also called Davies sponge, in Yin-Yang grids, which means that the free area is the same as the core area!

The overlap is needed because the two LAM grids will give different answers in the overlap region and the larger the overlap region the easier it is for the solver to find the common solution. The minimum overlap (degrees) should be 3 to 8 times the grid resolution (delta Y degrees).

To specify a Yin-Yang grid one should only specify 'Grd_nj' and 'Grd_overlap'. 'Grd_ni' will get calculated automatically using the following equation:

    Grd_ni = ( (Grd_nj - 1) * 3 ) + 1

This means the overlap is 3 times larger on the left and right border of each LAM grid than at the top and bottom.

Example to setup a 0.25° Yin-Yang grid

1. First calculate the size of the overlap when covering 8 times the grid resolution:

    Grd_overlap = 8 * 0.25° = 2°

2. Then calculate the resulting total number (including 2 times the overlap) of core/free points in y-direction + 1 point:

    Grd_nj = (90° + 2*2°) / 0.25° + 1 = 377

That's all!

FYI, Grd_nj does not have to match the FFT criteria.

To learn more about Yin-Yang grids have a look at the following wiki page:  Yin-Yang grid

LAM grid

Specify the total number of points and the size of the grid boxes in degrees.
If possible, insure that the total number of grid points in x-direction matches the FFT criteria. This will speed up your simulation by up to 10 %.

Location of limited area (just for LAM grids)

The only thing left now when setting up a LAM grid is to specify where the limited area is located on the rotated grid. Once does that by adjusting the following 4 parameters:

LAM grid

In the image above, the top left square represents the LAM area. Let's call it "LAM grid". Its size is given by 'Grd_ni' and 'Grd_nj'.

Global grid

The larger square on the lower right represents the rotated grid on the whole globe. Let's call it "global grid". It is defined by 'Grd_xlat1' ,  'Grd_xlon1', 'Grd_xlat2',  and 'Grd_xlon2' (for the rotation) and 'Grd_dx' and 'Grd_dy' (for the resolution).

Set location of LAM grid on global grid

Now one needs to decide where the "LAM grid" should be located on the "global grid".

In general, it is best to center the "LAM grid" over the grid(!) equator of the "global grid" to keep the grid boxes as square as possible. However, if the LAM grid region is not too large and fairly close to or even covering the "real" equator, some people prefer not to rotate the grid and therefore shift the "LAM grid"  a little north- or southward.

Imagine the "LAM grid" is a piece of paper and you want to pin it in a certain location on a global map. First you would put a pin in the piece of paper in the "reference point", defined by 'Grd_iref' and 'Grd_jref'. Then you would pin it at a certain location on the "global grid", defined by 'Grd_lon1' and 'Grd_lat1' (in longitude and latitude of the rotated grid!).

First one needs to choose any one grid point on the "LAM grid" by setting 'Grd_iref' and 'Grd_jref'. This point is called the "reference point". I find it easiest to choose the middle point of the grid (something that is not done in the example image above). Meaning set 'Grd_iref'  to 'Grd_ni/2' and 'Grd_jref'  to 'Grd_nj/2'. But you have to put the values!

Then one needs to set where this "reference point" should be located on the "global grid". The location is given by setting 'Grd_lonr' and 'Grd_latr' to the longitude and latitude of the reference point, in longitude and latitude with respect to the rotated grid. If one chose the middle point of the LAM grid to be the reference point and chose the center point of the global grid (set by 'Grd_lon1' and 'Grd_lat1'), then one should set:

    Grd_lonr  =  180.0,   Grd_latr  =  0.0

Click here to find out how to set blending and pilot area (halo).

FFT criteria

To fulfill the FFT criteria the number of grid points in x-direction, excluding the pilot zone, must be a multiple of 2, 3, and 5. Use the findfft GEMCLIM script to find the possible numbers of grid points fulfilling the FFT criteria.
Again you need to insure that the model environment is set.

Then you can use findfft:

This command will then print all "good" values you can use for 'Grd_ni' and 'Grd_nj' (can be different) between the range you specified with 'min' and 'max'.

Verify your grid

After having specified a grid in the file 'gem_settings.nml' you can have a look at the grid you specified using the grille script - see description below.

In the directory in which you have your file 'gem_settings.nml' simply type :

    grille  -xrec
or, if you just want to see the free grid:
    grille  -xrec  -grid free

'grille'

The command 'grille' reads the namelist 'grid' from an input file and :

    1. creates 3 files with the corresponding tictacs of the:
         a) model grid (tape1_model) - this file is needed as input to GenPhysX!!!
         b) core grid (tape1_core)
         c) free_grid (tape1_free)

    2. opens 'xrec' with the domain specified in 'grid' - only if requested - see table below.

These are the keys that can get used with the command:

After having specified a grid in the file 'gem_settings.nml' you can have a look at the grid you specified using the grille script.

In the directory in which you have your file 'gem_settings.nml' simply type :

  grille -xrec

This command will open 'xrec' with three fields of your grid: