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Clustering

Clustering is the process where The clustering process groups individual ensemble members that are "similar" according to some measure (or norm) are be grouped together.  This compresses the large amount of information produced by ensemble members and highlights the most predictable parts.  Clustering can be performed:

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• Weather scenario clustering (up to day10 of medium range forecast output). It is performed on medium range ensemble forecast output over a restricted area covering .  The area covers Europe and the northwest Atlantic.  It highlights the differences between medium range scenarios in terms of the large-scale flow.  
• Weather regime clustering (beyond day10 of medium range forecast output).  These summaries are based on one of four climatological regimes which have been derived from reanalysed data over an area covering .  The area covers Europe, eastern Canada, and the north Atlantic.  This is used beyond day10 of the medium range ensemble forecast output to provides It provides information about the possible transitions between regimes.
• Weather regime clustering (seasonal forecast output).  These summaries are based on one of six climatological regimes which have been derived from reanalysed data over a slightly larger area.  The area covering covers Europe, eastern Canada, the North Pole region and the north Atlantic.  This is used with sun-seasonal range ensemble forecast output and It provides information about the possible transitions between regimes.

Clustering is a compromise between the :

•  the advantage of condensing a large amount of ensemble output

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• .
• the disadvantage of losing possibly important information.

   A convenient overview of ensemble members forecast information can be gained from using:

• a cluster mean (the average over all members in a cluster).
• a most representative member (MRM or cluster scenario) which .  This is the ensemble member closest to its the centre of the cluster centre and typifies the members within that cluster.  Each most representative member is then attributed to one of the weather regime clusters.

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