Set up a 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

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. Note that you have to set the model environment first.

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:

   
 LA
 :
 Latitudes

LO
 :
 Longitudes

ME
 :
 Mountain heights


Grid type

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

   
 Grd_typ_S='GU'    :
 Global Uniform grid

Grd_typ_S='GY' :
 Global YinYang grid

Grd_typ_S='LU' :
 Limited area, Uniform


Grid rotation

Next specify wether 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:
Both points are specified in latitudes and longitudes of the real globe.
I.e.:

  Grd_xlon1 =  260.00 , Grd_xlat1 =   50.00 ,
  Grd_xlon2 =   -10.00 , Grd_xlat2 =    0.00 ,

For LAM grids it is recommended to set Grd_xlat1 and Grd_xlon1 to the center of your domain
and set Grd_xlat2 = 0. and Grd_xlon2 = Grd_xlon1 + 90.
 









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.

   
 Grd_ni
:
 total number of grid points in x-direction

Grd_nj :
 total number of grid points in y-direction


Global YinYang grid

It is enough to specify the core number of points in y-direction as well as the "overlap" in degrees.


    Grd_nj : core number of grid points in y-direction

Grd_overlap : overlap



LAM grid
Specify the total number of points and the size of the grid boxes in degrees.
If at all possible, insure that the total number of grid points in x-direction matches the FFT criteria.

   
 Grd_ni
:
 core number of grid points in x-direction
(including blending but excluding pilot area)

Grd_nj :
 total number of grid points in y-direction
(including blending but excluding pilot area)

Grd_dx :
 grid size in x-direction in degrees

Grd_dy :
 grid size in y-direction in degrees


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.

   
 Grd_iref :
 reference point in limited area region, x-grid coordinate

Grd_jref :
 reference point in limited area region, y-grid coordinate

Grd_latr :
 location of reference point in "grid latitude"

Grd_lonr :
 location of reference point in "grid longitude"




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:

  
findfft   -min   :
minimum number of grid points, default 150


-max
 : 
 maximum number of grid points, default 250

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'.



Author: Katja Winger
Last update: January 2021