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.  

Reanalyses contain estimates of atmospheric parameters (e.g. air temperature, pressure and wind at different altitudes), and surface parameters (e.g. rainfall, soil moisture content, and sea-surface temperature).  The estimates are produced as grid-box averages for all locations on earth. The reanalyses can made from data several decades old even when data was less available than currently.  

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


ERA5 is an improved and more comprehensive ECMWF climate reanalysis that is the fundamental initialising analysis for re-forecasts.    It replaced ERA-Interim in 2019.

ERA products are normally updated once per month and within three months of real-time.  Quality assurance processing is applied to ensure consistency by removal of biases in models and observations.   Preliminary daily updates of the dataset can be available to users within seven days of real time.   ERA5 is available for dates from 1979 and is being extended forward in near real time.

The main differences of ERA5 from ERA-interim are

  • 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).

ERA5 also provides:

  • 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 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  ERA-Interim data are available for dates from 1979 until 31st August 2019 and are no longer updated.


The system uses historical re-forecast runs on dates in past years relating to the date (i.e. month and day) of the current ENS 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 similar as for the operational ENS, but do not involve any data assimilation.  These perturbations derive from singular vectors 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 ENS runs.  

The set of re-forecast ensembles are 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 M-climate, ER-M-climate, and S-M-climate.

The procedures adopted for using re-forecasts allow for seasonal variations and model changes to be taken into account.  But it should be noted that 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 EFI data.  For an example of the effect, see Fig5.3.1 and Fig5.3.2.  Such effects have also appeared in the Extended Range ENS and Seasonal forecasts.


Fig5.3.1(left): Extreme Forecast Index (EFI) for 2m temperature for Days10-15, ENS forecast run data time 00UTC 20 June 2017. 

Fig5.3.2(right): Cumulative Distribution Function (CDF) for 2 m 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 corrwspomdimg 2 m CDF temperature curve (black).   This affects subsequent EFI and SOT fields.   Here the realistic real-time forecast of 2 m temperature CDF (red) over Lake Superior is thus incorrectly flagged as having a strongly negative EFI value in Fig5.3.1.  

More information is given in Documentation and a full description of ERA5.

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)

Updated 29/8/19 - ERA5


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