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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 "vertical sponge" is used at the model top to absorb these waves so they do not get reflected. For mor more information as and how to set up the vertical sponge (Vspng) have a look at the following wiki: Diffusion & Sponges
In LAM model one also has the option of nesting the top model layers. Which is also called "top piloting".
When top piloting is used the vertical sponge should NOT get used!!!
Global grid → adjust global pressure
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