Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

Surface energy fluxes

A vital part of the IFS is assessment of the thermal, moisture and momentum fluxes between the model atmosphere and the underlying surface.  For this reason it is necessary to have:

  • a fairly detailed representation of the characteristics of the land or sea surface within each grid box which can be updated by climatological changes or model output throughout the forecast period.
  • the energy interactions represented and modelled as well as possible.  

Energy exchanges are sub-grid scale so statistical methods and simplified mathematical-physical models have to be used but with some consequent impact on forecast accuracy.  Future higher and more appropriate resolution will allow air-surface interaction to be described more explicitly.

The IFS atmospheric models need information about the underlying boundary conditions to model their effect upon fluxes of momentum, radiation and moisture.  In IFS some conditions:

  • are fixed (e.g. orography and sub-grid scale orography, lakes, urban areas).
  • remain fairly constant with time (e.g. the extent of forests or sizes of lakes (lake extent that varies seasonally and from year to year is not accounted for yet)).
  • vary with the seasons (e.g. solar energy reaching the earth or the extent of vegetation (e.g. leaf area index but more detailed inter-annual variations in vegetation are not accounted for yet)).
  • are initially analysed but then vary considerably through the forecast period due to feedback from the forecasts themselves:
    • snow cover and depth (e.g. may alter due to forecast snowfall or melting).
    • soil moisture and temperature (e.g. may alter due to forecast drying winds, rainfall or radiative warming or cooling).
    • lake and sea thermal conditions and ice cover (e.g. formation or melting of ice).

The non-forecast fields can and do change over time, particularly over longer (decades or more) periods of time.

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 surface energy exchanges over lakes.  It gives information on surface roughness, moisture availability and heat flux at or near the surface and assesses the changes and impacts that occur during the forecast period.

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".  Energy fluxes over lake and coastal waters are modelled by Flake as a separate additional "tile".   Ocean waters are modelled within NEMO and are dealt with separately from HTESSEL. 


Evaluation of grid point data

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.

Energy flux information at each grid point is governed by the "fraction of land cover" assigned to the area surrounding it (see Fig8.1.4.1-1).   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 40-50% 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.

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.

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.


Fig2.1.4-1: An example over southern England 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. 



(FUG Associated with Cy49r1)