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The model levels on GEM are terrain following at the surface and pressure levels at the model top. The model levels are generally closer to each other at the surface and further apart higher up. The model is very sensitive to the location of these model levels. They must neither be too far apart nor too close and the transition from terrain following to pressure levels must be very smooth. There are ways to set up these model levels. Several sets of model levels have already been created and can get selected by setting 'CLIMAT_etaname' in the file configexp.cfg. The possible settings for this parameter can get found in the file:
~/modeles/GEMDM/*/bin/Climat_eta2preshyb
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Vertical sponge & top piloting (LAM only)
Like other models, GEM gets unstable when waves get artificially reflected back into the atmosphere from the model lid. Therefore, a so called "sponge" is used at the model top to absorb these waves so they do not get reflected. For mor information as to set up the sponge have a look at the following wiki: Sponge
Global grid → adjust global pressure
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