For many years now the European Centre for Medium Range Weather Forecasts at Reading in Berkshire has been the world leader in the preparation of weather predictions for two to 10 days ahead.
Anxious to expand its horizons, however, the centre is now dabbling in "seasonal forecasting", attempts at predicting the weather pattern for several months into the future. Its latest weapon is DEMETER.
DEMETER is the somewhat contrived acronym for a project called Development of a European Multi-model Ensemble system for seasonal to inTERannual prediction. The key words, however, are "European", "multi-model" and "ensemble". Let's take "ensemble" first.
In the technique called "ensemble forecasting", a numerical model for predicting the future weather pattern is first run on computer with the real weather observations, and a prediction of the future pressure and temperature pattern is obtained in the normal way. But then the process is repeated a number of times, and each time some of the weather observations on which it is based are very slightly altered.
If the end result - a forecast for perhaps a week ahead - is very similar for each "run" of the ensemble, it is seen as an indication that the atmosphere is in a "predictable" state, and inferred that the forecast is likely to be correct. If the results differ widely, the atmosphere is seen as unstable and very sensitive to the absolute accuracy of the initial information; in these circumstances, the output of the computer must be treated with some caution.
"European" and "multi-model" are easier to understand. There are currently in Europe, having been developed at various institutions, six major coupled weather prediction computer models - "coupled" because they predict the behaviour of both the atmosphere and the oceans, and the interaction between them, and are therefore suitable for very long-range weather predictions.
All six models will now be installed at ECMWF, and used to produce "forecasts" based on "old" weather observations stored over the years at the centre. Ensemble predictions for several months into the "future" will be carried out on each one simultaneously before all are combined into one great "super-ensemble".
By this means it is hoped to develop a technique for producing seasonal forecasts that will exploit the different strengths of all the various models in different climatic situations. In addition, the ensemble process will also, it is hoped, provide an indication of the reliability of the predictions thus produced.
But why did the European Weather Centre go to such linguistically tortuous lengths to call their project DEMETER? Stay tuned. All will be revealed tomorrow.