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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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The current real-time forecast uses new glacier fields and multi-layer snow scheme which are not included in ERA5. This inconsistency will be addressed in 49r1.
Differences between ERA-Interim and ERA5
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 ERA-Interim data are available for dates from 1979 until 31st August 2019 but are no longer updated.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.
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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, SUBS-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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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.
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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More information regarding differences between ERA-Interim and ERA-5.
Additional sources of information
(Note: In older material there may be references to issues that have subsequently been addressed)
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