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Evaluation of grid point data

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Modelling the surface orography at an appropriate resolution is crucial to an effective forecast.  However, at some level, there always will be smoothing that misses important detail.   

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Fig8.1.4.1-14: Schematic of the spectral representation of orography.   Model orography matches true orography over large parts of the earth but is less exact in rugged mountainous regions.  See also Section on Model Orography for further points regarding orography.

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

• smooth 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.  It is quite possible that the model sea surface level is negative!

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