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• For medium range forecasts the anomaly correlation coefficient is evaluated between:
  
  • the mean of the anomaly of the forecast product relative to the medium range model climate (M-climate) and
  • the mean of the anomaly of the verifying HRES or CTRL analysis relative to the medium range model climate (M-climate).
• For extended range products the correlation is evaluated between:
  • the mean of the anomaly of the forecast product measured relative to the extended range model climate (ER-M-climate) and
  • the mean of the anomaly of the verifying the CTRL analysis relative to the extended range model climate (ER-M-climate).
• For seasonal products the correlation is evaluated between:
  • the mean of the anomaly of the forecast product relative to the a model climatology based on the ERA-interim re-analysis (based on the period 1993-2016) and
  • the mean of the anomaly of the verifying observations or reanalysis relative to the seasonal model climate (S-M-Climate). 

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 In the analysis below (Fig6.4.1 and Fig6.4.2), forecasts by all IFS models progressively correlate less well with observed values as forecast lead-time increases.  The unperturbed HRES and the ensemble unperturbed ensemble control (CTRL) perform similarly, and performs better than any individual perturbed member of ENS ensemble (PF in the diagrams).  The ENS ensemble mean smooths out many smaller scale features and performs best against the verifying analysis.  This supports preferential use of the ENS ensemble mean in practical forecasting.  Typically ACC falls to 0.6 at around day8 or day9 for HRES and ensemble control (CTRL), and at around day10 for the ensemble mean (EM).

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 Fig6.4.1: Anomaly Correlation Coefficients for 850hPa Temperatures. HRES in Red, Ensemble control (CTRL) in Purple, Ensemble mean (EM) in Green, An individual ensemble member (PF) in Cyan.  Note the ACC for CTRL and HRES are very similar, but the EM clearly out-performs them and is almost 2½ days better than any individual ENS member (e.g. ACC of PF at Day7 is still being attained by EM at Day9½) .

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 Fig6.4.2: Anomaly Correlation Coefficients for 500hPa Geopotential.  HRES in Red, Ensemble control (CTRL) in Purple, Ensemble mean (EM) in Green, An individual ensemble member (PF) in Cyan.   Note the ACC for CTRL and HRES are very similar, but the EM clearly out-performs them and is almost 2 days better than any individual ENS member (e,g, ACC of PF at Day7 is still being attained by EM at Day9) .

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 Fig6.4.3:  Time series of the annual running mean of anomaly correlations of HRES 500 hPa height forecasts evaluated against the operational analyses for the period 2000 until 2017.  Values are running 12month average scores.  Forecast lead-times in days ahead - 3(blue), 5(red), 7(green) and 10(yellow) - are shown for scores averaged over the northern extra-tropics (bold lines) and southern extra-tropics (thin lines).  The shading shows differences in scores between the two hemispheres at the forecast ranges indicated.   Currently 3-day, 5-day, 7-day, 10-day forecasts have attained approximately 98.5%, 92%, 80%, 50% anomaly correlation (ACC) respectively.

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