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 Fig8.1.3.1-2: The most representative 500hPa members selected to describe the clustering of the forecast DT 00UTC 12 March 2017, T+120 to T+168 hours.  Here there are 3 clusters (one per row).  The most representative member or cluster scenario is the member of the cluster which has the minimum difference from the RMS of the cluster members.  On 500hPa cluster scenario charts, shading denotes the 500mb height anomaly , that is difference between the instantaneous 500mb of the ensemble member height field for that member (i.e. as is contoured) and from the long term climatological average 500mb height field for that time of year.

In Fig8.1.3.1-2 there are three clusters:

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The web site includes cluster products equivalent to Fig8.1.3.1-2 for each of the four predefined lead-time windows.  For additional information, the 1000hPa geopotential fields are also provided for each ensemble scenario to show the corresponding near-surface evolution.  Users should note that for these the clustering has been made on the 500hPa fields, not the 1000hPa fields.  Whilst the user should not treat the most representative members as deterministic solutions, it can nonetheless be helpful to examine the details of the evolution in such members, to see how a particular scenario can plausibly arise and evolve.  One good way to do this is to use the cyclone database products presented by the extratropical cyclone diagrams, specifically the animations, at 12 hour intervals, of synoptic patterns for individual members.

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