The atmosphere — surface and sub-surface processes

Each land point is assigned characteristics according to the soil type and the vegetation type. These are important in the calculation of the heat, moisture and momentum fluxes at each grid point. If land is covered by snow then the properties, such as albedo, will be drastically altered.

The soil temperature is calculated in four separate levels. The temperature of the soil will change according to the radiation balance at the soil surface. Snow cover will act as an insulator to the soil.

Each land point has a value of the soil moisture content in four layers of different thickness which is altered according to how much evaporation is occurring and the amount of precipitation at that point. The vegetation plays an active role in the hydrology at the surface. When precipitation falls some is intercepted by and held in the canopy of the vegetation. The remainder is known as 'canopy throughfall' and falls to the soil's surface. This water is absorbed by the soil unless the intensity is too great or the soil is already saturated in which case surface run-off, into rivers and lakes, occurs. Soil water is primarily lost though evaporation through plants, in which case the term transpiration should be used. The amount of transpiration that can occur is limited by the soil moisture, as the soil dries it becomes progressively more difficult for plants to extract water.

Over the sea the roughness length, a representation of surface drag, is increased with increasing wind speed to represent the interaction with waves.