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Vertical sponge & top piloting (LAM only)
Like other models, GEM gets unstable when Probably because waves get artificially reflected back into the atmosphere from the model lid. Therefore, a spurius wind jet forms at the equatorial stratopause. To remove this spurius wind jet, a so called "vertical sponge" is used at the model top to absorb these waves so they do not get reflected. For more information 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 instead nesting the top model layers. Which is also called "top piloting".
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