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The standard deviation, slope, orientation and anisotropy of the surface in each grid box can be important for assessment of sub-grid scale local wind effects, solar heating etc.  These are not yet included directly in the atmospheric models.  However, in mountainous areas the upscaled orography is supplemented by fields describing some characteristics of the sub-grid orography (sub-grid scale slope, orientation and anisotropy of the orography - GRIB fields are available for use by forecasters locally).  This allows better parameterisation of the effects of gravity waves and better representation of flow-dependent blocking of the airflow (e.g. cold air drainage trapped in valleys can effectively raise the local orography).  

Compare Compare HRES or medium range ENS model orography (resolution 9km),  and extended-range ENS model orography (resolution 36km).  

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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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