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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.

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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 fairly quickly. Others fail to move from the BLO+ type, though nevertheless do 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.

Transitions between circulation patterns

Sometimes the transition from one regime to another may be fairly clear.  

Fig8.1.17-2: NAO-BLO diagram for the medium range showing transition of circulation pattern with time.  Colours indicate the elapse of time.  The initial NAO+ circulation pattern becomes a BLO+ circulation pattern by T+72hr, and finally becomes a NAO– circulation pattern by about T+168hr.

Rough practical guide to transitions

The frequency of transitions from one circulation type to another has been derived from a large number of reanalyses (36 years of ERA-interim data).  This gives a “climatology” of transitions between types. 

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At all forecast ranges, the model statistics and relative frequencies compare well with those from the analysis.  This indicates the IFS is well able to simulate transitions and suggests model bias in this context is not a major problem.

Additional Sources of Information

(Note: In older material there may be references to issues that have subsequently been addressed)

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See also the section regarding circulation patterns and regimes which gives further information on regimes within the sub-seasonal range.

(FUG Associated with Cy49r1)