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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 vertical sponge (Vspng) have a look at the following wiki: Sponge Diffusion & Sponges
Global grid → adjust global pressure
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