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Following this link (https://confluence.ecmwf.int/display/CUSF/Atmospheric+variables+below+model+terrain), we can found atmospheric variables that are below the model terrain at lower levels (e.g. at 1000 hPa or 975 hPa). So, pressure level layers could be interpreted like the right-hand figure. To get all available layers from 0 to 500 m above ground level I have to consider the geopotential layer at the surface level (orography)., right? How is the correct way to do that?
However, model level definitions (https://confluence.ecmwf.int/display/UDOC/Model+level+definitions) are like the left-hand figure. In that case, I would only need two pressure level layers (1000 hPa and 975 hPa) independently of the terrain elevation.
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