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Fig8.1.1-1: A example of postage stamps showing PMSL forecasts from ensemble control, and all 50 ensemble members, DT 12UTC 15 October 2023, VT T+144hr at 12UTC 21 October 2023.   Some large differences in the pressure pattern can be seen on individual ensemble members.  Each member has been allocated to a cluster, shown in a different colour heading above each frame for lead-times T+120 and above only.  The representative member of each cluster is here shown by arrows. The clusters are shown in Fig8.1.1-2 and Fig8.1.1-3.

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Fig8.1.1-5: Spaghetti Plot 500hPa geopotential height plotted at 560dam DT 12Z 26 May 2017, VT T+144hr at 01 June 2017.  The ensemble members (grey) have become more spread out, but retain the general pattern of an upper ridge over the northwest Atlantic and another over the North Sea but there are differences in position (or speed of movement eastwards) and amplitude.  The control ensemble member is shown in red.

Ensemble mean

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The ensemble mean (EM) forecast is a simple but average of the ensemble results.  It is an effective product because averaging reduces or removes atmospheric features that differ amongst the members.

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The ensemble mean is most suited to parameters like temperature and pressure, which usually have a rather symmetric Gaussian distribution at each point.  In the short range the ensemble mean is very similar to the ensemble control due to the symmetry (equal positive and negative) of the initial perturbations.   

Ensemble median

The ensemble mean is less suitable for wind speeds and precipitation because these have skewed distributions.  For these the ensemble median might be more useful.  The ensemble median is defined as the value of the middle ensemble member where the members have been ranked by value.   Ensemble median might be more suitable than the ensemble mean for wind speeds and precipitation as these have skewed distributions.

Fig8.1.1-6: Ensemble mean PMSL (red) and Spaghetti Plot of 990hPa isobars (grey) from ensemble DT 00UTC 10 June 2017 T+120 hr VT 00UTC 15 June 2017.  There is a wide diversity amongst ensemble members in the location and shape of the depressions.  The ensemble mean depression is smooth by comparison and less deep (the inner isobar has a value of 995hPa).   Because of averaging of the ensemble members, the pressure gradients and associated winds will generally be less strong in the ensemble mean field than in the ensemble members themselves.  Chart taken from ecCharts.

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