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• over different geographical areas
• for different parameters
• for each forecast time
• over different forecast intervals.
• or a combination of the above.

Currently clustering summarises the range of synoptic flow patterns from the ensemble over a restricted area in two forms: 

• Weather scenario clustering is performed on medium range ensemble output over a restricted area covering Europe and the northwest Atlantic.  It highlights the differences between medium range scenarios in terms of the large-scale flow.  
• Weather regime clustering summaries are based on one of four climatological regimes which have been derived from reanalysed data over a rather larger area covering Europe and the north Atlantic. It also provides information about the possible transitions between regimes during the forecast

Clustering is a compromise between the advantage of condensing a large amount of ensemble output against 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 which typifies the members within that cluster.

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• (MRM or cluster scenario) which is the ensemble member closest to its centre

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Weather regime clusters are one of four climatological regimes which have been derived from reanalysed data.

Current use of clustering

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• 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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• .