Topographical and climatological fields

The model orography is derived from a data set with a resolution of about 1 km which contains values of the mean elevation above the mean sea level, the fraction of land and the fractional cover of different vegetation types. This detailed data is aggregated (“upscaled”) to the coarser model resolution.

The resulting mean orography contains the values of the mean elevation above the mean sea level. In mountainous areas it is supplemented by sub-grid orographic fields, to enable the parametrization of the effects of gravity waves and provide flow-dependent blocking of the air flow. For example, cold air drainage in valleys makes the cold air effectively “lift” the orography.

The land-sea mask is a geographical field that contains the percentage of land and water between 0 (100% sea) and 1 (100% land) for every grid point. A grid point is defined as a land point if its value indicates that more than 50% of the area within the grid-box is covered by land, see section 2.4.6.

The albedo is determined by a combination of background monthly climate fields and forecast surface fields (e.g. from snow depth). Continental, maritime, urban and desert aerosols are provided as monthly means from data bases derived from transport models covering both the tropo­sphere and the stratosphere.

Soil temperatures and moisture in the ground are prognostic variables. There is a lack of observational data, so observed 2m temperature and relative humidity act as very efficient proxy data for the analysis.

The snow coverage depth is analysed every six hours from snow-depth observations, satellite snow extent and a snow-depth background field. The snow temperature is also analysed from satellite observations. They are forecast variables.

Sea surface temperature (SST) and ice concentration are based on analyses received daily from the Met Office (OSTIA, 5 km). It is updated during the model integration, according to the tendency obtained from climatology.

The temperature at the ice surface is variable and calculated according to a simple energy balance/heat budget scheme, where the SST of the underlying ocean is assumed to be -1.7°C.

The sea-ice cover, which is kept constant in the 10-day forecast integration, is relaxed towards climatology between days 10 and 30, with a linear regression. Beyond day 30 the sea-ice concentration is based on climatological values only (from the ERA 1979-2001 data).