Page History

...

There are 50 other equally valid members of the ensemble.  All are equally valid.  There is no reason to select the results from the unperturbed control member rather than any of the others.   And any member viewed in isolation cannot provide any estimate of forecast uncertainty or confidence.   

The higher resolution (currently 9km) brings advantages and disadvantages - smaller scale atmospheric features are modelled and forecast, and look beguilingly realistic.  Development of these atmospheric systems often is in response to inherent numerical instability (which affects all numerical models) and reliance on detail is inappropriate.  

The main strategy to adopt is to avoid over-interpreting non-predictable features.  Therefore the detail of the most recent ensemble members should not be used in isolation.   Run-to-run jumpiness can on the one hand be tackled as something negative that has to be dampened, but on the other hand as something positive which can enrich the forecast information by giving alternative scenarios.  Ensemble members can give an indication of the probability and the consistency of features of the forecast.

Use of the ensemble mean (EM)

...

Fig6.1.1 (same as Fig4.1.5): 1000hPa forecast from 12UTC 13 August 2010 T+156hr to 00UTC 16 August 00 UTC T+96 h, all valid at 00UTC 20 August 2010.   Full lines are the 1000 hPa geopotential EM overlaid by the probabilities of wind speeds >10m/s.  Probabilities are coloured in 20% intervals starting from 20%.  Compare with Fig4.1.3 and Fig4.1.4.

Use of several ensembles

Assessment based on the two latest forecasts

Normally use of the scale-predictability relation will highlight similarities in the latest two forecasts, reduce the error, dampen any jumpiness and thereby make the final forecast more trustworthy.  The scale-predictability relation also applies when the latest two forecasts are highly consistent.  Paradoxically, it is in cases of high consistency that forecasters might be lured into unfounded over-interpretation of non-predictable smaller synoptic features.

Assessment based on the last three or more forecasts

One way to take advantage of the skill of previous forecasts is to combine them, together with the latest forecast, into a consensus forecast.  Together, they can be regarded as a "ensemble of ensembles" or "lagged ensemble" that has started from slightly different initial conditions.  A consensus forecast will preserve those synoptic features which the individual ensemble forecasts have in common and can, therefore, be considered more reliable and predictable.  The spread of the "lagged ensemble” will define the degree of uncertainty and indicate possible alternative developments.