We have then a Limited Area Model. These models can produce a very detailed forecast, but they are useful only in the range several hours to about two days into the future - what is happening outside the treated area influences the weather inside it, the more so the longer the forecast interval in which we are interested. In this time the future state of the atmosphere at any point can be influenced by phenomena at very distant geographical locations. Many applications of medium-range forecasting, for example ship routeing, or pollution dispersion, are not confined to limited areas of the globe. The discretization we can afford depends on the power of the computer we have available and how efficiently we use this power.
Some important factors influencing the evolution of the atmosphere occur on a very small scale.
These include the heating of the soil by the sun, the turbulence of the air near the ground and at high levels in the atmosphere, for example when air flows over mountains, and cumulus cloud systems. These cannot be represented properly by the discretization we can afford in even the most powerful computers available. We must represent their effects by taking into account their influence on the behaviour of the parameters of the large scales. The the resolution of the discretization of the Centre's current model is equivalent to having gridpoints separated by about 60 km around the globe. The points are evenly distributed geographically.