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Note: HRES and Ensemble Control Forecast are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast output is equivalent to HRES output where shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

Ensemble mean and ensemble spread

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Fig8.1.2-1: 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 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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The spread around the ensemble mean as a measure of theoretical accuracy applies only to the ensemble mean forecast error.  It does not apply to the median or the Ensemble Control Forecast , even if they happen to lie mid-range within the ensemble.

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 Fig8.1.2-4: An example of forecast mean sea level pressure (taken from part of an ECMWF mean and spread chart) highlighting the difference between the HRES (green) and the ensemble mean (black).  Absolute spread of ensemble members is shown by shading.  The ensemble mean is the average over all ensemble members.  It smooths the flow more in areas of large uncertainty (large spread).  If there is large spread, the ensemble mean can be a rather weak pattern and may not represent any of the possible states.  The ensemble mean should always be used together with the spread to capture this uncertainty.  Note in particular the small depressions forecast by the HRES near 35W (shown by arrows) and the additional uncertainty (darker purple) within the ensemble members nearby.  This suggests at least some of the ensemble members show something similar to the Ensemble Control Forecast  although with timing and/or location differences. 

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• the standard deviation of the surface pressure pattern among the ensemble members is moderate (4hPa - 7hPa).  This implies some variation (and hence uncertainty) among ensemble members regarding MSLP values in this area, or in the location of any low pressure centres.  Some ensemble members may have developed a deeper low pressure centre or sharp pressure trough in the area while others may not have; this can be resolved by inspection of the corresponding postage stamp charts.  The large standard deviation is unsurprising as one would expect variability at longer lead-times. 
• the normalised standard deviation is relatively high (1·2 - 1·8).  This gives an indication of the variability among ensemble members regarding MSLP in this area compared to the variability expected at this forecast lead-time in this area.  Here there is more variability (or uncertainty) than from recent ensemble forecasts, probably due to the uncertainty in the depth and movement (or even existence) of low pressure centres developed (or not) by ensemble members.
• the ensemble mean PMSL shows a broad pressure trough over northern Britain.  This probably relates to the large normalised spread; it is likely that some ensemble members also have this feature.  The Ensemble Control Forecast shows development of a fairly deep depression (~987hPa) but the Ensemble Control Forecast should only be considered as one member of the ensemble. 

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• Variability within the ensemble members (as measured by standard deviation) usually can be expected to increase as forecast lead-times increase.
• Large normalised standard deviation states only that the variability (or uncertainty) of the ensemble members is more than from recent ensemble forecasts at this forecast lead-time and location.  It does not necessarily imply greater uncertainty.  One would anyway expect greater variability in ensemble results in the vicinity of a forecasted deep depression.

(FUG Associated associated with Cy509r1)