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The ECMWF Ensemble Prediction System (EPS)
is based upon the notion that erroneous forecasts result from a
combination of initial analysis errors and model deficiencies, the
former dominating during the first five days or so. Analysis errors
amplify most easily in the sensitive parts of the atmosphere, in
particular where strong baroclinic systems develop. These
errors then move downstream and amplify and thereby affect
the large-scale flow. To estimate the effect of possible initial
analysis errors and the consequent uncertainty of the forecasts,
small changes to the analysis (the Control analysis) are made,
creating an ensemble of 50 different, “perturbed”,
initial states. Model deficiencies are represented by a stochastic
process. In order to save computational time, the EPS members are
run on a lower resolution version of the deterministic model, the
EPS Control.
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