Forecast

The use of computers has played a key role in improving the accuracy and detail of weather forecasts, and in lengthening the forecast period for which useful guidance can be given. The calculations involved are both numerous and complex and must be performed quickly so that forecasts are available in good time. Consequently these models often require huge amounts of computer processing power.

The Role of Computer Models

Weather forecasts are based on the solution of a set of mathematical equations describing certain physical processes in the atmosphere. To solve these complex equations it is first necessary to divide the atmosphere up into boxes, with a grid point in the centre of each box. The properties of the atmosphere are then represented by what is happening at each of the grid points.

The array of grid points, the system of equations and the method of solving the equations is referred to as the model. In the present global model used by the Met Office, there is a spacing of roughly 40 km between each grid point in the horizontal. The grid points are also arranged in 50 vertical levels through the atmosphere.

The observations taken at a particular time can be used to compute values for each grid point of pressure, temperature, humidity and wind. This set of values (the computer analysis) then represents the atmosphere at the start of the forecast. Using the mathematical equations, a 15-minute forecast can be made of how these basic elements change. Once all the new values have been calculated, the process starts again with another 15-minute forecast being made. By repeating this procedure many times over, a forecast out to six days can be built up. The supercomputer at the Met Office takes around one hour to produce a six-day global forecast.

The computer model produces a global forecast twice a day using the midnight and midday observations as starting conditions. In order to provide more-detailed forecast charts out to 48 hours for the UK and parts of the Atlantic and Europe, the model is run again at 0600 and 1800 daily.

For local forecasts, the Met Office has developed a model which has an 11 km horizontal grid and covers the British Isles and the near continent. This 'mesoscale model' is especially good at taking into account the local effect of ranges of hills and the contrast between land and sea in its forecasts.

The Role of the Weather Forecaster

Despite greater computer power, improvements to the computer models, and other technological advances, there is still an important role for the forecaster. For the general development of weather systems, the model provides insight into how the atmosphere is behaving and developing, but it is only a guide. Good as it is, forecasters have to make allowances for the model's known problem areas - the handling of small-scale features, for example. The chief forecaster on duty modifies the computer output to correct for likely errors in the model output, such as removing spurious areas of rainfall. Forecasters also have to take into account any late observations and consult the latest satellite and radar pictures.

In providing specific services to individual customers, the local forecaster based at an airfield or regional office will take the process even further. Experience and local knowledge add the fine detail to the computer forecast, so that the best advice for a specific location (e.g. an oil rig) can be given. There is no doubt that the combination of man and computer together produces the best forecasting results.