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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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Orography has a direct bearing upon the drag on the lower layers of the model atmosphere.  Also, in some atmospheric situations, it influences the development of standing waves through the atmosphere and possibly inducing upper air drag if they break.  Orographic enhancement of precipitation depends upon the detail of the model orography.  

Fig2.1.103.2-1: Schematic of the preparation of the data sets to characterize the sub-grid scale orography.

1. Global 1km resolution surface elevation data
2. Reduce to 5 km resolution by smoothing
3. Compute mean orography at model resolution grid points
4. Subtract model orography (3) from 5km orography (2) grid points
5. Compute standard deviation, slope, orientation and anisotropy for every grid box

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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:

• smooth

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• true orography, particularly in rugged mountainous areas where there are large variations in altitude over short distances.  Mountain peaks may be under-represented and narrow valleys may not be represented at all.
• local effects can be under-identified where there are small scale variations in true or model orography, even where relatively low in altitude. 
• lead to "topographic ripples" over adjacent sea/large lakes, which decay with offshore distance, and which are most prominent where there are steep-sided high mountains nearby.

See also selection of grid points for meteograms and model representation of orography.