Page History

...

The standard deviation, slope, orientation and anisotropy of the surface in each grid box can be important for assessment of sub-grid scale local wind effects, solar heating etc.  These are not yet included directly in the atmospheric models.  However, in mountainous areas the upscaled orography is supplemented by fields describing some characteristics of the sub-grid orography (sub-grid scale slope, orientation and anisotropy of the orography - GRIB fields are available for use by forecasters locally).  This allows better parameterisation of the effects of gravity waves and better representation of flow-dependent blocking of the airflow (e.g. cold air drainage trapped in valleys can effectively raise the local orography).  

Compare model orography used by medium range ENSENS model orography (resolution 9km),  and extended-range ENS model orography (resolution 36km).  

 In the diagrams, note:  

• these plots do not give a definitive representation of exactly where land and sea grid points lie.  Colour filling is shown up to a coastline rather than by allocation of a grid point as land or sea. For more information see the Land-Sea Mask section.
• the relatively coarse representation of orography for extended range forecasts.
• sea depth can be useful for explaining sea-surface temperature changes.  Model (and indeed real) sea ice cover and sea-surface temperatures tend to change more rapidly where the ocean is shallow.  This is because less energy is generally required to achieve a change in temperature.

...