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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:
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- 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 sub-seasonal 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, orography 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.
Fig2A.1.3.2-1: Schematic of the preparation of the data sets to characterize the sub-grid scale orography.
- Global 1km resolution surface elevation data
- Reduce to 5 km resolution by smoothing
- Compute mean orography at model resolution grid points
- Subtract model orography (3) from 5km orography (2) grid points
- Compute standard deviation, slope, orientation and anisotropy for every grid box
Fig2A.1.3.2-2: Schematic of the spectral representation of orography. Green= true orography, purple=model orography.
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