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Table of Contents

Reanalysis

Reanalysis combines observations made in the past with the current IFS model to provide a complete, consistent and model-compatible numerical representation of past weather and climate.   

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There is a potential problem related to inconsistencies between the model climate (initialised with ERA-5 and the ) and current real time forecast (Cy48r1).

The current real-time forecast based on (Cy48r1 which ) uses new glacier fields and multi-layer snow scheme which are not included in ERA-5.  This inconsistency will be addressed in 49r1.

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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, ER-M-climate, or S-M-climate) can nevertheless be different from the observed climate.

Some limitations of re-forecasts

Impact of differences between reanalysis and re-forecast systems

The model climate is generally compatible with model forecast output but there are still some local inconsistencies.  In particular:

  • 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). 

Forecasters should consider possible deficiencies in model climates when considering extreme forecast index EFI data.  For an example of the effect, see Fig5.3.4-1 and Fig5.3.4-2.  Such effects have also appeared in the extended range ensemble and seasonal forecasts.

 

Fig5.3.4-1: Extreme Forecast Index (EFI) for 2m temperature for Days10-15, ensemble forecast run DT 00UTC 20 June 2017.

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