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. Since September 1991, thanks to the availability of a more powerful computer, together with the introduction of a more efficient way of solving the equations which was developed during the previous three years, the Centre has been able to increase the resolution of the discretization to being equivalent to having grid-points separated by only about 60 km around the globe. The new representation uses a net of points more evenly distributed geographically than before.