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Fig2.6-2: The recurring cycle of the LWDA 4D-Var analysis. Consider the 12UTC LDWA:  The 00UTC LWDA generates an 00UTC analysis but also provides an 06UTC analysis because it has access to data times between 03UTC and 09UTC.  This 06UTC analysis becomes the basis of an 06UTC short forecast.  This short forecast provides the first guess (at T+3, 09UTC) and then background fields for use by the SCDA 4D-Var process.  The SCDA 12UTC analysis is used as the first guess for use by the LWDA 4D-Var process.  LWDA then uses T+12hr and T+15hr as background fields to the end of the assimilation period (at T+15, 21UTC).  The data arrival cut-off time is 02UTC (14hr after the nominal time (12UTC) of the analysis) in order to ensure all observations with data times between 09UTC and 21UTC are captured.   The cycle is repeated for subsequent analysis cycles 12hr later.

The early delivery

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assimilation (SCDA)

The early delivery system is also known as the Short Cutoff Daily Analysis Assimilation (SCDA).  The system extracts information from the products of the stand-alone LWDA system without waiting for the full LWDA analysis, yet providing sufficient information to enable ECMWF forecasts to be delivered without significant loss of effectiveness.  The early-delivery analyses assimilations do not propagate information from cycle to cycle.  Each analysis is reinitialized with the best available model fields from the LWDA assimilation.

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