Representation of Geographical Features
Describing the earth's surface in the model is important as it enables a good representation of the energy exchanges appropriate to the geographical location. The energy fluxes themselves are modelled by HTESSEL, by FLake, and by using the NEMO model. The extent of land or sea governs which energy exchange algorithms should be used at each location, or what proportion of each is appropriate in coastal areas. Orography is important as it is used to derive the drag on the lower atmosphere and upper air drag due to standing waves etc. Orographic enhancement of precipitation depends upon the detail of the model orography. Modelling the surface orography and coasts at an appropriate resolution is crucial. In practice the data is taken from internationally available datasets at 1km resolution and interpolated to the resolution of the model HRES, ENS etc., but always, at some level, there will be smoothing that misses important detail. This is why the datasets are also used to derive statistical parameters such as standard deviations of the mean height, slopes and direction of unresolved orography at a given resolution, and these feed into the model via the subgrid-scale parametrisations.