Geoscience Reference
In-Depth Information
4.5. Description of surface parameters used in a forecast model
4.5.1.
Digital elevation models
The majority of forecast models use the global elevation model GTOPO30,
which was made available by the US Geological Survey Centre for Earth Resources
Observation and Science (USGS/EROS) (http://edc.usgs.gov/products/elevation/
gtopo30/gtopo30.html).
The data is divided into 27 geographical domains, which cover 50° of latitude
and 40° of longitude, with a resolution of 30 seconds, which is equivalent to
approximately 1 km (Figure 4.17).
Figure 4.17.
The GTOPO30 digital elevation model for Western Europe
Different methods are used to compute the altitude of a model's grid:
- a grid-averaged orography;
- an “envelope orography” (one standard deviation of the subgrid-scale
orography is added to the grid-averaged orography);
- a mixed approach, where averaged orography is kept at larger scales, and
envelope orography is used at smaller scales.
Estimates of aerodynamic roughness length related to orography generally rely
on a parameterization based on sub-grid variability of topography.
