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The analysis essentially uses data within a time window, currently 09–21UTC for the 12UTC model run; 21–09UTC for the 00UTC model run.  In practice the main forecasts of the ensemble use the data cut-off (i.e. the last time) brought back to 15UTC (for the 12UTC run) and 03UTC (for the 00UTC run).  This is in order to be able to deliver forecasts to customers in a timely manner (see the continuing sequence of analyses).  All observational data are processed similarly, including radiances from satellites.  A weakly coupled sea-ice atmosphere assimilation is used in the surface analyses of the 4D-Var, and in the ensemble of data assimilations (EDA).  Before the introduction of cycle 45r1 in June 2018  a remotely-generated sea ice cover analysis (OSTIA) had been used directly.

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Meanwhile the Ensemble of Data Assimilation (EDA) could be thought of as being equivalent to the above process, but activated several times producing slightly different smooth and continuous but equally probable sequences.  Where there are lots of observations the several sequences might end up almost the same, but where data is sparse one could end up with much more variability between sequences.  And where there is large dynamic instability (e.g. a developing frontal wave rather than the centre of an anticyclone) this 'spread' might be even greater.  In the real EDA this is essentially also what happens; more spread is found amongst members in data sparse areas and when there is larger innate uncertainty in actual atmospheric developments.

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Fig2A.6-5:  Scale-selective re-centring has been introduced in the EDA in Cy50r1.  This is to improve the realism of initial conditions in the ENS, particularly for tropical cyclones.  Re-centring is now only applied to large-scale upper-air fields, centring them on the control forecast, while small-scale structures come directly from the EDA.  This helps avoid unrealistic 'double-centred' tropical cyclones from appearing at the start of the ENS forecasts (see Fig2A.6-5).

Additional Sources of Information

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• Read more on 4D-Var.
• Read more on the use of EDA in 4D-Var.
• Read more on data assimilation and interpolation of data.
• Watch a webinar on Data assimilation and 4D-Var (30sec delay).

(FUG Associated associated with Cy50r1)

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