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The land-sea mask (LSM) is a static field with containing the fraction of land in every grid box. The values are 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 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 arise where small islands, parts of islands, or coasts cover half or less of a grid box.   Smaller lakes may not be captured by the land-sea mask, and 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).  Some islands may be missed altogether (e.g. El Hierro in the Canary Islands).  See also the discussion on the impact of the land-sea mask on the derivation of Meteograms which also uses the examples shown below.

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