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. 

(FUG Associated with Cy49r1)