Taking account of solutions from ECMWF and other models - Grand Ensembles
An ensemble of model runs using different models but starting from the same data times.
Occasions when there is a clear divergence between the latest and previous developments in the ensemble or ensemble mean occur increasingly rarely - but if it does the forecasters is in a difficult position. Nevertheless, there can be some agreement between the spread and jumpiness.
An ensemble of NWP model runs starting from the same data time is known as a grand ensemble. Forecasters might consider combining the current and recent ensembles into a large grand ensemble (as in Fig6.3.4B). It is inappropriate to rely on the results of any individual ensemble member or ensemble run itself. But if there is some indication of extreme or hazardous weather the threat should be passed on to users, but with a very low probability.
Fig6.3.5: Schematic illustration of the relation between the latest ENS (green lines), the three latest CTRL (or HRES) (red lines), and results from other forecast models (blue lines). Here the three latest CTRL (or HRES) solutions are in agreement with ENS solution but two other forecast models show some more extreme troughs or higher maxima.
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