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),
• the fraction of land,
• the fractional cover of different vegetation types.

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 model orography used by medium range ENS (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.  

Fig2.1.10: 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

The spectral representation of orography in the IFS, can:

• smooth model orography and local effects can be under-identified.
• 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.