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Evaluation of grid point data

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Example of grid point data on meteograms

Fig814Fig8.1.4-2: 10-day medium-range meteogram for Oslo from HRES/Ensemble Control (blue line) and ENS members (box and whiskers) data time 00UTC 26 June 2023.   The map shows a close up of Oslo city.  The nearest land grid point to central Oslo is at 59.93N 10.83E which lies some 5km away from and some 141m higher than Oslo city centre.  This grid point may well be representative of Haugerud on the fringes of Oslo, but temperatures are reduced to near sea level using 6.5K/km lapse rate.

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There are wide variations in orography within the islands (the islands are quite mountainous) and the representativeness of a grid point can be uncertain.  Local uncertainty in forecast temperatures at altitude can have a large impact of model precipitation especially over mountainous islands and coasts.  

Example4: Isolated small islands.

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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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