Model Orography

Modelling the surface orography at an appropriate resolution is crucial to an effective forecast.  However, at some level, there always will be smoothing that misses important detail.   Model orography is derived from a data set with a resolution of about 1km, which contains:

  • the average elevation above mean sea level (MSL). This is taken from the smoothed model orography.
  • the fraction of land. This is used to assess the proportions of each grid square used by HTESSEL and FLake to derive heat, moisture and momentum fluxes. 
  • the fractional cover of different vegetation types.   This is used by HTESSEL to derive the different proportions of heat, moisture and momentum fluxes over the land.

This detailed data is upscaled (aggregated) to the coarser model resolution.

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 HRES or medium range ENS 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.

Importance of Model Orography

Orography has a direct bearing upon the drag on the lower layers of the model atmosphere.  Also, in some atmospheric situations, it influences the development of standing waves through the atmosphere and possibly inducing upper air drag if they break.  Orographic enhancement of precipitation depends upon the detail of the model orography.  

Fig2.1.3.2-1: Schematic of the preparation of the data sets to characterize the sub-grid scale orography.

  1. Global 1km resolution surface elevation data
  2. Reduce to 5 km resolution by smoothing
  3. Compute mean orography at model resolution grid points
  4. Subtract model orography (3) from 5km orography (2) grid points
  5. Compute standard deviation, slope, orientation and anisotropy for every grid box

Fig2.1.3.2-2: Schematic of the spectral representation of orography.   Green= true orography, purple=model orography.

Model orography matches true orography over large parts of the earth but is less exact in rugged mountainous regions. The spectral representation of orography can give negative heights over the sea near mountainous coasts.

Generally model orography matches true orography over large parts of the earth.  However, the spectral representation of orography in the IFS can:

  • smooth true orography, particularly in rugged mountainous areas where there are large variations in altitude over short distances.  Mountain peaks may be under-represented and narrow valleys may not be represented at all.
  • local effects can be under-identified where there are small scale variations in true or model orography, even where relatively low in altitude. 
  • lead to "topographic ripples" over adjacent sea/large lakes, which decay with offshore distance, and which are most prominent where there are steep-sided high mountains nearby.

See also selection of grid points for meteograms and model representation of orography.