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Sea, lakes and soil each have their own characteristics regarding energy exchange with the atmosphere.  These are modelled by HTESSEL (Tiled ECMWF Scheme for Surface Exchanges over Land incorporating land surface Hydrology) and Flake (A sub-tile of HTESSEL dealing with surface energy exchanges over lakes).  HTESSEL uses a system of "tiles" to describe the characteristics of each of the various and time-varying surface conditions within each grid box.  Energy fluxes over land are modelled using several differing land "tiles" each representing a type of surface.  Energy fluxes over lake and coastal waters are modelled by Flake by Flake as a separate additional "tile" within HTESSEL.   Ocean waters are modelled within NEMO and are dealt with separately from HTESSEL.  

HTESSEL, Flake and NEMO give information on heat flux, moisture availability and surface roughness, at or near the surface.    But changes to the surface, and the associated impacts, can and do occur during the forecast period.

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For forecast ensemble temperature data, all locations within each grid box surrounding a grid point are considered to have the same values as that forecast at the central grid point.  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 within the surrounding area (where HTESSEL will be used) and lake/coastal seas (where FLake will be used).  For a sea grid point well offshore NEMO is be used to determine the surface fluxes of heat, moisture and momentum.

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• dark green are land points and HTESSEL will supply 90-100% of the flux information.
• mid-green are land points (but with 10-20% water surface) so HTESSEL will supply 80-90% and FLake 10-20% of the flux information.
• light green are land points (but with 20-30% water surface) so HTESSEL will supply 70-80% and FLake 20-30% of the flux information.
• turquoise are land points (but with 30-40% water surface) so HTESSEL will supply 50-60% and FLake 30-40% of the flux information.
• cyan are land points (but with 40-50% water surface) so HTESSEL will supply 50-60% and FLake 40-50% of the flux information.
• blue are sea points (i.e. >50% water surface) so FLake will supply 100% of the flux information in coastal waters.  NEMO will supply 100% of the flux information in oceanic waters.
  

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• At coastal locations where there is less than 50% land cover in a grid box the water proportion is treated as a lake (using FLake) rather than as an ocean (which would use NEMO).  

• Some water surfaces (e.g. The Great Lakes) are classed as lakes rather than sea and FLake is used exclusively.

Some influences of the adjacent sea areas or mountains may be over- or under-represented.  This can significantly affect the forecast parameter (temperature, wind, etc).    Users should assess differences in meteograms and vertical profiles for coastal, island or mountainous regions.   In particular consider:

• • the impact of the grid point(s) relative to the land-sea mask, especially where surface winds might blow onshore.
  • the variation of the altitude of the land, especially when compared with the model representation of orography.   Forecast values at the grid point nearest to the location are adjusted for altitude using a standard lapse rate assumption.  The difference in temperature can be considerable.

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

Examples of grid point location and energy flux computation

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• If land fraction is <50% then NEMO provides oceanic fluxes unless the lake dataset specifically highlights the location as a lake (e.g. the Great Lakes) when FLake is used.
•  If land fraction is >50% then HTESSEL is used for the proportion of land and FLake is used for the proportion of lake or coastal water.  The proportions are given by the colouring.

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• Locations on the green boxes the land fraction is ~95% and the water fraction ~5%.  Therefore HTESSEL will supply ~95% and FLake (rather than NEMO, because the land fraction >50%) will supply ~5%.
• Locations on the island in the blue 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 on the island in the turquoise boxes where the land fraction is about 60% 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%.

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• If land fraction is <50% then FLake provides lake fluxes.
•  If land fraction is >50% then HTESSEL is used for the proportion of land and FLake is used for the proportion of lake water. The proportions are given by the colouring.

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• If land fraction is <50% then NEMO provides oceanic fluxes.
•  If land fraction is >50% then HTESSEL is used for the proportion of land and FLake is used for the proportion of coastal water.  The proportions are given by the colouring.

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 HTESSEL and FLake according to the proportion of land and sea. 

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• If land fraction is <50% then NEMO provides oceanic fluxes.
•  If land fraction is >50% then HTESSEL is used for the proportion of land and FLake is used for the proportion of coastal water.  The proportions are given by the colouring.

 All the minor islands have no land points and NEMO will be used exclusively to assess fluxes of heat, moisture and momentum which may not describe conditions sufficiently on land. Many of these islands are mountainous and variations between temperature or winds forecast for the grid point and occurring at inland loc

(FUG Associated associated with Cy50r1).