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As forecast lead-time increases so forecasts progressively correlate less well with observed values.  The ensemble mean smooths out many smaller scale features and performs best against the verifying analysis.  This supports preferential use of the ensemble mean in practical forecasting.   Typically ACC falls to 0.6 at around day 8 or day 9 for ensemble control (CTRL), and at around day 10 for the ensemble mean (EM). 

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 Fig6.2.2-1: Variation of Anomaly Correlation Coefficients for 850hPa temperatures with forecast lead time during the period 1 July 2023 to 30 June 2024.  Ensemble Control (red), ensemble mean (green), an individual ensemble member (blue). 

The ensemble mean out-performs the ensemble control and is almost 2½ days better than any individual ensemble member (e.g. The ACC of a perturbed ensemble member at Day7 is being attained by ensemble mean at Day9½).  These graphs show results from Cy48r1 when the medium range ensemble and ensemble control had a resolution of 18km.  

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 Fig6.2.2-2: Variation of Anomaly Correlation Coefficients for 500hPa geopotential with forecast lead time during the period 1 July 2023 to 30 June 2024.  Ensemble Control (red), ensemble mean (green), an individual ensemble member (blue). 

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The ACC scores have steadily improved over the years at all lead-times.  This reflects the improvement of input data, analysis techniques and IFS model formulation (See Fig6.2.2-3).

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 Fig6.2.2-3:  Time series of 12 month running mean average ACC scores for HRES 500hPa geoptential forecasts evaluated against operational analyses during the period 1980 to 2024.  Forecast lead-times are shown for: 3 days (blue), 5 days (red), 7 days (green) and 10 days (yellow).   Scores were averaged over the northern extra-tropics (bold lines) and southern extra-tropics (thin lines).  Shading highlights the differences in scores between the two hemispheres.   Note non-uniform scale for ACC values.

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Fig6.2.2-4: The plot shows the forecast lead-time (in days) at which Anomaly Correlation Coefficient (ACC) of the HRES forecast dropped to 80% for:

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