Land-Sea mask

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 of the medium range forecasts.  The Land-Sea mask values lie between 0 (grid box is fully covered with water) and 1 (grid box is fully covered with land).   This binary choice of assignment of land/water points means that globally land is slightly under-represented in the model.

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.  Thus:

• smaller lakes may not be captured.
• 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. Malfa or Stromboli).

See also selection of grid points for meteograms and selection of grid points for vertical profiles.  See the selection methods directly compared.

(FUG associated with Cy50r1)