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For forecast ENS 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), or for .  For a sea grid point well offshore (where NEMO will is be used ).

Stage 1: Selection of ENS grid point relevant for a chosen location:

For land locations:

  • The nearest ENS grid point is selected from among the four ENS points surrounding the selected location.  Within these four grid points:
    • if there is at least one land grid point then the nearest land grid point is chosen (even though a sea grid point may be nearer).  A "land grid point" is one where the "fraction of land cover"  is greater than 50%.   
    • if there is no land grid point then the nearest ENS grid point is chosen (which will be a sea point).

For sea locations:

Flux information is governed by the "fraction of land cover" assigned to the ENS grid point (see Fig8.1.5.6A) .   Thus ENS grid points in rectangles:

to determine the surface fluxes of heat, moisture and momentum.

Energy flux information at each grid point is governed by the "fraction of land cover" assigned to the area surrounding it (see Fig8.1.5.6A).   Thus grid points in rectangles that are coloured:

  • 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
  • coloured dark green are land points and HTESSEL will supply 90-100% of the flux information.
  • coloured light green are land points (although with some water surface) and HTESSEL will supply 70-90% and Flake 10-30% of the flux information.
  • coloured turquoise are land points (although with rather more water surface) and HTESSEL will supply 50-70% and Flake 30-50% of the flux information.
  • coloured blue are sea points and FLake will supply 100% of the flux information.

Stage 2: Selection of HRES grid point values relevant to a selected location:

For all land and sea locations:

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  • (i.e. >50% water surface) so FLake will supply 100% of the flux information.

Users should note, for flux information: 

  • 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.


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Fig8.1.5.6A: An example of "fraction of land cover" values showing the proportion of land and water within each 9km x 9km square centred on each grid point.  At grid point X the fluxes of heat, moisture and momentum  will be determined by 70%-80% by HTESSEL and 20%30% by FLake.  At grid point Y the fluxes of heat, moisture and momentum will be determined by 100% by FLake, even though the grid point lies over land. 


Selection of ENS grid point relevant for a chosen location:

For land locations:

  • The nearest ENS grid point is selected from among the four ENS grid points surrounding the selected location.  Within these four grid points:
    • if there is at least one land grid point then the nearest land grid point is chosen (even though a sea grid point may be nearer).  A land grid point is one where the "fraction of land cover"  is greater than 50%.   
    • if there is no land grid point then the nearest ENS grid point is chosen (which will be a sea grid point)

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Flux information governed by the "fraction of land cover" assigned to the ENS grid points (see Fig8.1.5.6B).   Thus grid points in rectangles that are coloured:

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

Users should note, for flux information: 

  • Coastal waters (less than 50% water cover in a grid box) are treated as lakes (using Flake) rather than as oceans (using NEMO).  
  • Some water surfaces (e.g. The Great Lakes) are classed as lakes rather than sea and FLake is used exclusively
    • .


The process of selecting which gridpoints ENS that are used on meteograms is illustrated below, using relatively complex but informative examples.

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The Isle of Wight in southern England.   The island is approximately 40km long by 25km wide.  Coastal areas are strongly influenced by the sea while central parts are not.

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Fig8.1.5.6A: ENS grid points over part of southern England.  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 lake/coastal seas (where FLake will be used for lakes or shallow coastal water), or NEMO alone for sea grid points. Towns mentioned below are Ventnor (V), Bembridge (B), Freshwater (F) and the city of Portsmouth (P) and locations are marked by a cross.

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  • A coastal location - Bembridge (location B).  The ENS grid is scanned for the grid points surrounding the location (ENS grid points EFGH).  None is a land point and a sea point is chosen (Point E).  At this point the "fraction of land cover" is less than 50% and the surface energy fluxes are determined by FLake.  There will be no influence of land energy fluxes.  In fact any land location within grid box EFGH will be treated similarly, no matter how far away from the coast. 
  • A coastal location - Freshwater (location F).  The ENS grid is scanned for the grid points surrounding the location (ENS grid points MNPRMNRS) and the nearest land point is chosen (Point PR).  This is a land point where the "fraction of land cover" is 60%-70%.  Surface energy fluxes are determined 60%-70% by HTESSEL and 30%-40% by FLake.
  • A coastal city location - Portsmouth (location P).   The ENS grid is scanned for the grid points surrounding the location (ENS grid points ABCD) and the nearest land point is chosen (Point A).  This is a land point where the "fraction of land cover" is 100% and the surface energy fluxes are determined by HTESSEL. There  There will be no influence of a water surface.  HTESSEL does not take into account the urban nature of the city. 
  • An inland location - Newport (location N).  The ENS grid is scanned for the grid points surrounding the location (ENS grid points JHLKNPJR) and the nearest land point is chosen (Point J).  This is a land point where the "fraction of land cover" is 100% and the surface energy fluxes are determined by HTESSEL.
  • A coastal location - Ventnor (location V).  The ENS grid is scanned for the grid points surrounding the location (ENS grid points JHLK) and the nearest land point is chosen (Point J).  This is a land point where the "fraction of land cover" is 100% and the surface energy fluxes are determined by HTESSEL.  No adjustment is made for the influence of the sea and the effect of the sea may not be evident on ENS meteograms.
  • A location near land - offshore of Ventnor (location S).  The ENS grid is scanned for the grid points surrounding the location (ENS grid points JHLK) and the nearest sea point is chosen (Point L).  The surface energy fluxes are determined by FLake.  The influence of the sea will be more evident on ENS meteograms for location S than at location V. 

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  • a local effect (e.g. onset of a sea breeze).
  • the local prevalence of persistent cloud (e.g.sea fog and low cloud drifting onshore).
  • the influence of turbulent mixing with stronger winds.

It is for the user to assess critically the representativeness of the Meteogram displayed and to make adjustments in the light of local knowledge and experience.  

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