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The set of re-forecast ensembles is based on previous dates which can stretch back several decades.  They differ in number and detail according to the IFS model configuration and are the basis for deriving the corresponding model climates.  These are described in the relevant section for medium range M-climate, sub-seasonal range SUBS-M-climate, and seasonal S-M-climate.

The procedures adopted for using re-forecasts allow for seasonal variations and model changes to be taken into account.  But note the model climates (M-climate, SUBS-M-climate, or S-M-climate) can nevertheless be different from the observed climate.

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• the land surface scheme used by ERA-Interim differs from the current IFS model version.  An equivalent "offline land surface reanalysis" that is compatible with the current model version may be used instead. 
• reanalysis is not performed over open water surfaces and climatological reference data is used instead.       
• real-time IFS contains a lake model (FLake) while ERA-Interim did not.   Representation of temperature in the model climate may diverge in a systematic way from the actual changes within the forecast period.  These effects are particularly important over and around large lakes or inland waters (e.g. the Great Lakes - notably Lake Superior, Aral Sea, Caspian Sea and some others). 

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Fig5.3.4-2: Cumulative Distribution Function (CDF) for 2m temperature for Days10-15 in the middle of Lake Superior (red), with M-climate (black).  The initialisation techniques are different for real-time forecasts (using lake surface temperature observed by satellites), and for the re-forecasts (for which this information is not available).  This can lead to the model climate developing anomalously warm or cold lake surfaces and corresponding 2 m CDF temperature curve (black).   This affects subsequent extreme forecast index (EFI) and shift of tails (SOT) fields.   Here the realistic real-time forecast of 2m temperature CDF (red) over Lake Superior is thus incorrectly flagged as having a strongly negative EFI value in Fig5.3.4-1(left).  

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• Watch a comprehensive lecture on estimation of the model climate (re-forecasts).

(FUG Associated associated with Cy50r1)