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Fig2.1.3.2-2: Schematic of the spectral representation of orography.   Green= true orography, purple=model orography.

Model orography matches true orography over large parts of the earth but is less exact in rugged mountainous regions. The spectral representation of orography can give negative heights over the sea near mountainous coasts.

Generally model orography matches true orography over large parts of the earth.  However, the spectral representation of orography in the IFS can:

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