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To estimate the effect of possible initial analysis errors and the consequent uncertainty of the forecasts, an ensemble is formed of many different “perturbed” initial states and one unperturbed analysis (the control member, CTRL).

Currently :

• Ensemble Control Forecast (ex-HRES), or equally a forecast from any individual ensemble member, may be considered as a possible detailed solution during the first 15 days.

• Medium range ensemble consists of 50 members to investigate uncertainty and variability of detail in the forecast during the first 15 days.
• Sub-seasonal range 46 day ensemble consists of 100 members and the unperturbed control member to give probability of general conditions at longer ranges.
• Seasonal range 7 month or 13 month ensemble of 50 members and the unperturbed control member to give probability of general conditions at longer ranges.

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Processing the ensemble of forecasts is computationally expensive. The medium range ensemble  ensemble currently has 50 members and 9km resolution.  The sub-seasonal range ensemble has 100 members so in order to save computation time the ensemble members are run with a lower resolution, currently 36km.

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Fig5-2: The plume diagram shows schematically the the spread of the ensemble for the whole forecast range (orange shaded area). The ensemble mean (red line) lies more or less in the middle of the ensemble spread.  Any individual ensemble member (blue line) can lie anywhere within the spread.  The Ensemble Control Forecast (ex-HRES) solution (green line) does not constitute a part of the plume and can even on rare occasions be outside the plume (theoretically on average 4% of the time).

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