CHAOS THEORY is one of
the major scientific developments of the twentieth century. Chaotic systems
are governed by precise deterministic evolution equations, but have unpredictable
and seemingly random behaviour. Chaos can occur when these equations are
both non-linear and unstable to small perturbations.
Chaos in the atmosphere
The atmosphere is a chaotic system, and as a result, small
errors in our estimate of the current state can grow to have a major impact
on the subsequent forecast. Because of the limited number of observations
available and the uneven spread of these around the globe, there is always
some uncertainty in our estimate of the current state of the atmosphere.
In practice this limits detailed weather prediction to about a week or
so ahead.
Ensemble prediction
Accepting the findings from chaos theory about the sensitivity
of the prediction to uncertainties in the initial conditions, it is becoming
common now to run in parallel a set, or ensemble, of predictions from
different but similar initial conditions. The ECMWF Ensemble Prediction
System (EPS) provides a practical tool for estimating how these small
differences could affect the forecast. The ECMWF weather prediction model
is run 51 times from slightly different initial conditions. To take into
account the effect of uncertainties in the model formulation, each forecast
is made using slightly different model equations.
The 51 scenarios can be combined into an average forecast
(the ensemble-mean) or into a small number of alternative forecasts (the
clusters), or they can be used to compute probabilities of possible future
weather events.
The EPS can be used as a quantitative tool for risk assessment
in a range of weather-sensitive commercial and humanitarian activities.
The potential economic value of the EPS can be much higher than that of
a forecasting system based on only a single deterministic forecast.
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