Rules of Thumb

Rules of Thumb to run GEMCLIM

Find a "fast" grid size -> match the FFT criteria

See that the number of points of the core area in x-direction matches the FFT criteria to speed up the solver.
When using spectral nudging the number of points in both, x- and y-direction need to match the FFT criteria!

To match the FFT criteria the total number of points in the core area must be a multiple of 2, 3, and/or 5.
The command 'findfft' will give you a list of possible values for Grd_ni (and Grd_nj) in a certain range. Execute 'findfft -h' to get information about how to use the command. You have to set the model environment first to get access to 'findfft'.

Relation between time step and horizontal resolution
In general, the relation is linear. Here are some examples:
    Global 800x600: Δt=900s
    LAM 0.5 deg: Δt=1200s
    LAM 0.22 deg: Δt=600s
    LAM 0.11 deg: Δt=300s
    LAM 0.1375 deg: Δt=450s
    LAM 0.0225 deg: Δt=60s

Radiation time step: KNTRAD
The radiation scheme is rather costly. To speed up the simulation do not calculate new radiation tendencies at every time step but at least per hour, depending on the time step. For example:
Time step longer than 300s:
    KNTRAD = 1h
Time step equal 300s:
    KNTRAD = 30m
Time step equal 60s:
    KNTRAD = 15m

LAM: size of blending area / Davies sponge
The blending/merging/sponge zone should be at least 10 points wide.
The parameter to set the size of the blending zone is called: Grd_maxcfl
The final size of the blending zone will be Grd_maxcfl + 7!!!
So if you want a 10 point blending zone set Grd_maxcfl = 3.

Tile size
The number of tiles or MPI processes is set in the file configexp.cfg with the parameter 'GEM_ptopo'.
LAM:     Keep the tiles as square as possible.
Global uniform: Have as few tiles as possible in x-direction.
There must be 3 points in the free area of each tile. => LAM: If you have 10 points halo and 10 points blending your minimum tile size is 23 points.

Author: Katja Winger
Last update: January 2021