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The system uses historical re-forecast runs on dates in past years relating to the date (i.e. month and day) of the current ENS run.   Re-forecasts are based on an ensemble of forecast members ideally using the same model techniques and physics as the current model.   The re-forecast ensemble uses the appropriate reanalysis field for initialisation.   Perturbations are applied to all but the control similar as for the operational ENS, but do not involve any data assimilation.  These perturbations derive from singular vectors plus geographical averages of Ensemble of Data Assimilations (EDA).  The EDAs are perturbations that have been computed operationally over the most recent 12 months.  This approach means that the flow-dependence inherent in operational EDA perturbations is missing in the re-forecasts.  Stochastic physics are also used during the re-forecast runs, as in operational ENS runs.  

The set of re-forecast ensembles are based on previous dates which can stretch back several decades.   They differ in number and detail according to the IFS model configuration and are the basis for deriving the corresponding model climates.  These are described in the relevant section for M-climate, ER-M-climate, and S-M-climate.

The procedures adopted for using re-forecasts allow for seasonal variations and model changes to be taken into account.  But it should be noted that the model climates (M-climate, ER-M-climate, or S-M-climate) can nevertheless be different from the observed climate.

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