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Model structure

The IFS atmospheric model formulation is based on a set of basic equations.  These are either:

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• the land-surface model (HTESSEL).  
• the lakes and coastal waters model (FLake).  This also provides information on water temperature changes and the development and decay of areas of ice.  The forecasts of ice cover impact upon heat flux and albedo.
• the Wave Model (ECWAM).
• the Dynamic Ocean Model (NEMO), and subprogram (SI3) which provides information on the development, decay and movement of areas of sea-ice.  The forecasts of ice cover impact upon heat flux and albedo.
  
  

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Fig2A.1.1-1: Sub-grid scale parameterised processes in the ECMWF model – Surface to Stratosphere.

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Basic prognostic equations are efficiently processed in spectral space and need only a relatively small proportion of computer time required for a forecast.  But many processes are computed in grid point space (e.g. rainfall) and this requires a larger, but relatively modest, proportion of computer time.   Processes in grid-point and in spectral space, and in the associated spectral transforms, are broadly similar in computer time.  However, the necessary transpositions between spectral and grid point spaces are a significant computing overhead.  Semi-lagrangian computations also take up an important proportion of processing time.

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Fig2A.1.1-2: Schematic pie-charts showing approximate proportions of computer processing time during execution of an IFS medium range and seasonal forecasts.

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