Surface energy fluxes

Assessment of the thermal, moisture and momentum fluxes between the model atmosphere and the underlying surface is vital.  It is necessary to have:

• a fairly detailed representation of the characteristics of the land or sea surface within each grid box.  
• energy interactions both represented and modelled as well as possible.  

These can be updated by climatological changes or model output as the forecast progresses throughout the forecast period.

Some thermal, moisture and momentum fluxes:

• are fixed (e.g. orography and sub-grid scale orography, lakes, urban areas).  However, these can and do change over time, more particularly over longer (decades or more) periods.
• remain fairly constant with time (e.g. the extent of forests or sizes of lakes (seasonal or annual variation of lake extent 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.  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).

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 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.

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

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 Fig8.1.4.1-2).   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.

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

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.

Fig2A.1.4-1: 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.  Sea grid points open circles, land grid points (>50% land in the surrounding box) solid circles.  Within each rectangle all locations are considered to have the same values as 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 or sea in the surrounding box.

• 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.

Consider the areas around the island shown in Fig2A.1.4-1 (the Isle of Wight). 

• 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%.

Lake area - Lake Geneva

Fig2A.1.4-2: ENS grid points around Lake Geneva.  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 as 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 or sea in the surrounding box.

• 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.

Island Areas - Canary Islands.

Fig2A.1.4-3: ENS grid points around the Canary Islands.  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 as 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 or sea in the surrounding box.

• 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. 

Small Islands - Isole Eolie

Fig2A.1.4-4:  Isole Eolie are the group of islands to the north of Sicily.  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 as 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 or sea in the surrounding box.  

• 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 with Cy50r1).