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WEATHER FORECASTS lose skill because of the growth of inevitable uncertainties
in the initial conditions, and because numerical models describe in only
an approximate way the exact laws of physics. The ECMWF Ensemble Prediction
System (EPS) provides a practical tool for estimating how these errors
could affect the forecast. The first version of the EPS was implemented
operationally in December 1992.
Initial condition perturbations
The ECMWF weather prediction model is run 51 times from
slightly different initial conditions. One forecast, called the EPS control
forecast, is run from the operational ECMWF analysis. 50 additional integrations,
the perturbed members, are made from slightly different initial conditions
which are designed to represent the uncertainties inherent in the operational
analysis. The initial perturbations are generated using the singular vector
technique.
Model perturbations
To take into account the effect of uncertainties in the
model formulation, each forecast is made using slightly different model
equations.
Operational EPS forecasts
Multi-analysis forecasts
To investigate the effect of analysis differences on medium-range forecasts,
the ECMWF model is run routinely from initial conditions of other operational
forecast centres.
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