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Energy Fluxes - Surface (Mar 2023)

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:

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