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Model Structure

The IFS atmospheric model formulation (HRES, ENS, etc) is based on a set of basic equations.  These are either:

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These processes are mostly unresolved due to not well resolved because of their small scales compared to model resolution.  Because of To deal with this, they are handled by physical parameterisation in a statistical way that describes the mean effect of sub-grid processes.

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Other models running at the same time describe energy, moisture and momentum fluxes from the underlying land or sea water surfaces.  These are:

• a Land-Surface Model the land-surface model (HTESSEL).  
• the lakes , including a program for lakes and coastal waters model (FLake) that itself .  This also provides information on water temperature changes and the development , and decay and movement of areas of ice.  The forecasts of ice which cover impact upon heat flux and albedo.
• the Wave Model (ECWAM).
• the Dynamic Ocean Model (NEMO), including .  This includes a program (LIM2) which provides information on the development, decay and movement of areas of sea ice.  The forecasts of ice which cover impact upon heat flux and albedo.
  
  

Fig2.1.1-1: 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 this is reflected in a need only a relatively small proportion of computer time required .  However, for a forecast.  But many processes are computed in grid point space (e.g. rainfall) which and this requires a larger, but still relatively smallmodest, proportion of computer time.   This Processes in grid-point and processes in spectral space, together with and in the associated spectral transforms, are approximately broadly similar .  The in computer time.  However, the necessary transpositions between spectral and grid point spaces is are a significant computing overhead in computer time.  Semi-lagrangian computations also take up an important proportion of processing time.

Fig2.1.1-2: A schematic pie-chart showing approximate proportions of computer processing time during execution of an atmospheric model forecast based on T799 (regular 25km resolution on regular grid) and 91 levels (.  The current resolution for ENS is T_co640 1279 (18km 9km resolution) and 137 levels).   Parameterised physical processes consume about 30% of computer processing time.  Computations in grid point space and spectral space together take about 20% of computer processing time while rather more time (~27%) is taken in transposing data from one space to the other.

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