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First thing to do is to set 'Grd_typ_S' to the grid type you want:
| Grd_typ_S='GU' | : | Global Uniform grid - not used anymore | |
| Grd_typ_S='GY' | : | Global YinYang grid | |
| Grd_typ_S='LU' | : | Limited area, Uniform |
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:
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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 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.
| Grd_overlap | : | Overlap in degrees(!) The overlap size should be at least 3 times the resolution of the grid resolution. In general, for higher resolution grids (ie: 0.25° or finer), an overlap of 8 times would be better for the solver. | |
| Grd_nj | : | ||
Core/ | |||
Free number of grid points in y-direction | Grd_overlap | : | Overlap in degrees(!) |
+ 1 point |
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:
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This means the overlap is 3 times larger on the left and right border of each LAM grid than at the top and bottom! This is done because the solver needs the grids to be 3 times larger in x- than in y-direction (minus 1 point). .
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 criteriaAn 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 should be 3 grid points.
To learn more about Yin-Yang grids have a look at the following wiki page: Yin-Yang grid
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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 %.
| Grd_ni | : | core(!) number of grid points in x-direction (including blending but excluding pilot area) | |
| Grd_nj | : | core(!) 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. Once does that by adjusting the following 4 parameters:
| 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" |
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'.
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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'.
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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 |