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 medium 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). 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).
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 the discussion on the impact of the land-sea mask on the derivation of Meteograms which also uses the examples shown below.
Examples of grid point distribution and the effect on energy flux computation
Grid boxes are coloured by the fraction of land cover - scales are on the right and apply to all figures in this sub-section.
Coastal Area - Isle of Wight, Southern England.
Fig126.96.36.199-1: ENS grid points over part of southern England. Within each box all locations are considered to have the same values as forecast at the central grid point. The fluxes of heat, moisture and momentum at the grid point are calculated using the proportion of land (where the HTESSEL is used) and sea (shallow coastal sub-grid scale waters where FLake is used). Where the land fraction is <50% NEMO is used to provide oceanic fluxes unless the lake dataset specifically highlights the location as a lake (e.g. the Great Lakes) when FLake is used.
Consider the areas around the island shown in Fig188.8.131.52-1 (the Isle of Wight).
- 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%.
Fig184.108.40.206-2: ENS grid points around Lake Geneva. Only one grid box has less than 50% land (blue) and any land locations within that box will be considered as if over water. FLake is used here because the lake dataset tells the IFS that this is a lake and not open ocean (i.e. different to the Southern England case shown above). The other turquoise shades show the proportion of land cover within the each box and define the proportional influence of the FLake and HTESSEL for any land point within the grid box. For example, a point on the northeast coast of the lake will use ~60% HTESSEL and ~40% FLake for evaluation of fluxes.
Fig220.127.116.11-3: ENS grid points around the Canary Islands. Note several island points have a fairly high proportion of sea in their grid box, and some points on islands have no land within their grid box and are considered as sea points because their grid box has <50% land cover. All blue boxes are open ocean areas and NEMO is used to provide oceanic fluxes; all turquoise and dark green boxes use FLake and HTESSEL.
Small Islands - Isole Eolie
Fig18.104.22.168-4: ENS grid points around southwest Italy. Rectangles surrounding each grid point are coloured according to the "fraction of land cover" assigned to each grid point and shown by the scale on the right. Within each rectangle all locations are considered to have the same values. The fluxes of heat, moisture and momentum which in turn determine the surface values of temperature, dewpoint and wind at the grid point are calculated using the proportion of land (where HTESSEL will be used) and coastal water (where FLake will be used), or NEMO alone for grid points over open sea. Locations mentioned below are marked on the diagram. Note all the islands have no land points and FLake will be used exclusively to assess fluxes of heat, moisture and momentum which may not describe conditions sufficiently on land.