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Taking

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account of previous ECMWF ENS solutions -

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Lagged Ensembles

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Occasions when there is a clear divergence between the developments in the EM and the latest HRES occur increasingly rarely -  but if it does the forecasters is in a difficult position.  The HRES may even lie outside the ENS plume although maybe some agreement between the spread and jumpiness.

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Fig6.3.4A:  Schematic illustration of the relation between the ENS (blue lines) and the three latest HRES (latest HRES shown by the thickest, longest red line, earliest HRES shown by the thinnest, shortest red line) for the case where the latest few HRES are not contained in the ENS.  The HRES results consistently lie outside the ENS plume.

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Fig6.3.4B:  Schematic illustration of the relation between the latest ENS (blue lines), a previous ENS (green lines), and the three latest HRES (latest HRES shown by the thickest, longest red line, earliest HRES shown by the thinnest, shortest red line).  Here the three latest HRES solutions are more similar to a previous ENS solution suggesting the latest ENS is deficient in some way.  

An ensemble of NWP model runs starting from different data times is known as a lagged ensemble.   Forecasters might consider combining the ENS and the HRES in an extended ensemble including previous ENS results in a lagged Combining the current and recent ensembles into a larger ensemble (as in Fig6.1.31.2-1) can show possible consistency in forecasts and identify any different evolution.

4B).  It is inappropriate to rely on any individual ENS or HRES result, but if 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.  Nevertheless, there can be some agreement between the spread and jumpiness. Note that the y-axis refers to 'flow regime'.  Scenarios such as this are more commonly seen on meteograms, for local weather parameters, when resolution differences between HRES and ENS (e.g. affecting the representation of islands) can be pivotal. The discussion here does not relate to that aspect.

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Fig6.1.1.2-1:  Schematic illustration of the relation between the latest ensemble (blue lines) and two previous ensembles (green  and red lines)