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Medium Range Regime Charts

Daily evolution of regime types through the medium range period is shown using a regime projection diagram (Fig8.1.17-1) in phase space diagram format.   The forecast regime type is plotted at 24hr intervals for each individual ensemble member.  This shows the spread of the members (a measure of uncertainty) and changes of regime type (gradual or rapid) as the forecast proceeds.

The regime charts show the distribution of the forecast Euro-Atlantic flow type presented diagrammatically in NAO/BL phase space in two-dimensional probability density function format.  The x-axis is for NAO+/-, and the y-axis for BLO+/-.  An objective assessment of each ensemble member forecast, during each 24hr period, supplies a measure of both the NAO and BL components for each member.   These are plotted on the diagram, colour coded according to forecast lead time.  See Fig8.1.17-1.

The regime Projection Diagrams are derived using Mirror 2-regime scheme.

Real time example of Medium Range Two-dimensional Regime diagram.  


Fig8.1.17-1: NAO-BLO phase space diagrams (regime projection diagram) for the medium range period Day0 to Day8 from the medium range ensemble (DT 00UTC 23 April 2019).  Shown are the analysis for the previous 8 days (black spots and black line) and the ensemble mean trajectory (red line).  Coloured markers show the ensemble members at 24-hour intervals (colours and times shown above the chart).  No clear assessment of regime type should be made for those few members which lie within the central circle.  The spread of the cluster at a given time is a measure of uncertainty at that time.

In Fig8.1.17-1 the Medium Range forecasts show a weakening BLO+ type before a transition, mostly by around Day7, to a moderate NAO- regime type.  Some ensemble members make the transition more quickly; others fail to move from the BLO+ type though these do nevertheless show a trend towards NAO-.  Comparing the latest regime projection diagram with those from previous medium range forecasts allows an assessment of run-to-run consistency.  


See also the section regarding Extended Range Regime Charts which gives further information on regimes within the Extended Range.

Read more about the use of "Weather regimes" at ECMWF (2020 article)


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