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The land-sea mask (LSM) is an unchanging field containing the fraction of land within every grid box.  The proportion of land and water is calculated by using a satellite derived 300m resolution dataset, so this should be quite precise when aggregated to the 9km resolution (HRES and ENS 9km resolutionmedium range ensemble).

The Land-Sea mask values lie between 0 (grid box is fully covered with water) and 1 (grid box is fully covered with land).   A grid box is considered to be land if more than 50% of it is land, otherwise it's considered to be water (ocean or inland water, e.g. rivers, lakes, etc.).   This binary choice of assignment of land/water points means that globally land is slightly under-represented in the model.    The  The value of the land/water proportion strongly depends on the quality of used global land cover map and its horizontal resolution (current nominal resolution is ~300m). Difficulties also  

A grid box is considered to be:

sea if more than 50% of it is water (ocean or inland water, e.g. rivers, lakes, etc.).  

land if more than 50% of it is land.  The proportion of land and sea has implications in the assessment of surface energy fluxes within the grid box by HTESSEL and FLake.

Difficulties arise where small islands, parts of islands, or coasts cover half or less of a grid box.    Smaller  Thus:

• smaller lakes may not be captured

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• some lakes or inland seas vary in size and meteorological impact due to human or seasonal effects (e.g. the Sea of Azov where the depth varies substantially with the seasons).

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• some islands may be missed altogether (e.g. El Hierro in the Canary Islands).

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See also the discussion on the impact of the land-sea mask on the derivation of Meteograms which also uses the examples shown below.  

Diagrams etc still to do down from here.......................

Examples of grid point distribution and the effect on energy flux computation

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Grid boxes are coloured by the fraction of land cover - scales are on the right and apply to all figures in this sub-section.

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• Any location on and around the island within the green box will be represented by the single grid point near the centre.  Within the grid box the land fraction is about 90% and the water fraction about 10%.  Therefore HTESSEL will supply ~90% and FLake (rather than NEMO, because the land fraction >50%) supply ~10%.
• Locations on the island in the blue boxes will be represented by the corresponding grid points to the south or west.  Within these grid boxes the land fraction is about 10%, and the water fraction about 90%.  Therefore HTESSEL supply ~10% of the flux information and NEMO (rather than FLake, because the land fraction is <50% and the location is not a lake) will supply ~90%.
• Locations to the north of the island in the turquoise area will be represented by the grid points to the north and northeast.  Within these grid boxes the land fraction is about 60% land and the water fraction about 40%.   Therefore HTESSEL will supply ~60% of the flux information and FLake (rather than NEMO, because the land fraction >50%) will supply about 40%.

Fig2.1.7B: HRES* grid points over part of southern England.  More detail is represented than by the ENS grid, but note two points to the northeast of the island are considered as sea points (one of which encompasses the city of Portsmouth). 

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