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ECMWF analyses the data available within two periods and for different reasons.  The sequence of model data assimilation using 4D-Var that is followed by IFS forecast system is explained in more detail below.    

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• The 12UTC 4D-Var LWDA uses observations with data times between 09UTC and 21UTC to produce the best available 12UTC analysis.
• Also, the LWDA uses the observations with data times between 15UTC and 21UTC to produce an 18UTC analysis.  This is the basis for a short 18UTC forecast to T+15hr (to 09UTC).
• Forecast fields T+03hr to T+09hr from the 18UTC forecast (21UTC to 03UTC) are used as first guess and background fields for the 00UTC SCDA.
• The first guess for the 00UTC 4D-Var LWDA analyses is provided by the analysis from the 00UTC SCDA which has already used all available information with data times between 21UTC and 03UTC (T+03hr and T+09hr from the 18UTC forecast), including incorporation of late arrival data by Continuous Data Assimilation (CoDA).
• Background fields during the rest of the 4D-Var analysis is provided by T+12hr and T+15hr output from the 18UTC forecast (06UTC and 09UTC).

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• The 00UTC 4D-Var LWDA uses observations with data times between 21UTC and 09UTC to produce the best available 00UTC analysis.
• Also, the LWDA uses the observations with data times between 03UTC and 09UTC to produce an 06UTC analysis. This is the basis for a short 06UTC forecast to T+15hr (to 21UTC).
• Forecast fields T+03hr to T+09hr from the 06UTC forecast (09UTC to 15UTC) are used as first guess and background fields for 12UTC SCDA.
• The first guess for the 12UTC 4D-Var LWDA analyses is provided by the analysis from the 12UTC SCDA which has already used all available information with data times between 09UTC and 15UTC (T+03hr and T+09hr from the 06UTC forecast), including incorporation of late arrival data by Continuous Data Assimilation (CoDA).
• Background fields during the rest of the 4D-Var analysis is provided by T+12hr and T+15hr output from the 06UTC forecast (18UTC and 21UTC).

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The analysis starts 4 hours after the nominal model data time.  However, Continuous Data Assimilation (CoDA) allows any further observations which arrive up until 4hr25min to be included while the analysis process is running (the data is incorporated within the nested analysis loops).  This early cut-off is nevertheless sufficiently late to allow inclusion of more than about 85% of all global observations available within the 6 hour data window centred on the nominal data time of the forecast runs.  Thus:

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• Read more about continuous data assimilation (CoDA).
• Read more about continuous long window data assimilation (Co-LWDA). 
• Read more on EDA members.
• ECMWF documentation on data assimilation cycles (Section 11.2).

(FUG Associated associated with Cy50r1)