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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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A reanalysis is necessary for initiation of each re-forecast.  Spacial resolution are generally different between reanalyses and the model version used to create the re-forecasts.

ERA5

ERA5 is an improved and more comprehensive ECMWF climate reanalysis and is the fundamental initialising analysis for re-forecasts.

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• a much improved representation of the troposphere.
• an improved representation of tropical cyclones.
• better global balance of precipitation and evaporation.
• better precipitation over land in the deep tropics.
• better soil moisture.
• more consistent sea surface temperatures and sea ice.

ERA-Interim

ERA-Interim is ECMWF's previous atmospheric reanalysis, based on a 2006 version of the IFS.  ERA-Interim data are available for dates from 1979 until 31st August 2019 but are no longer updated.  ERA5 replaced ERA-Interim in 2019.

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• • higher spacial resolution (137 levels, 31km; 62km for EDA).
  • higher output frequency of analysis fields (hourly, 3-hourly for EDA).
  • introduction of uncertainty estimates.
  • additional input observation types.
  • many more output parameters.
  • use of a much longer period of historical data (back to 1950).

Re-forecasts

The system uses historical re-forecast runs on dates in past years relating to the date (i.e. month and day) of the current ensemble run.   Re-forecasts are based on an ensemble of forecast members ideally using the same model techniques and physics as the current model.   The re-forecast ensemble uses the appropriate reanalysis field for initialisation.   Perturbations are applied to all but the control.  This is similar to the operational ensemble, but does not involve any data assimilation.  The perturbations derive from singular vectors (SVs) plus geographical averages of ensemble of data assimilations (EDA).  The EDAs are perturbations that have been computed operationally over the most recent 12 months.  This approach means that the flow-dependence inherent in operational EDA perturbations is missing in the re-forecasts.  Stochastic physics are also used during the re-forecast runs, as in operational ensemble runs.  

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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

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More information regarding differences between ERA-Interim and ERA5.

Additional sources of information

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

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