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Model levels: 1/to/137, which are described at https://www.ecmwf.int/en/forecasts/documentation-and-support/137-model-levels and ERA5: compute pressure and geopotential on model levels, geopotential height and geometric height. The model levels are hybrid pressure/sigma. See For more information, see the documentation of the underlying model, ECMWF's IFS, cycle 41R2CY41R2, Part III. Dynamics and numerical procedures, Chapter 2 Basic equations and discretisation.
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