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Guidelines

The following advice is intended to help users understand particular features of the ERA5 data:

• Sea surface temperature and sea-ice cover (see Table 2 above) are available at the usual times, eg hourly for the HRES, but their content is only updated once daily.
• Mean rates and accumulations at step=0 have values of zero because the length of the processing period is zero.

Known issues

Currently, we are aware of these issues with ERA5:

• ERA5 suffers from an overly strong equatorial mesospheric jet, particularly in the transition seasons.

• From 2000 to 2005, ERA5 has a poor fit to radiosonde temperatures in the stratosphere, with a cold bias in the lower stratosphere and a warm bias higher up.

• ERA5 uncertainty: although small values of ensemble spread correctly mark more confident estimates than large values, numerical values are over confident. The spread does give an indication of the relative uncertainty in space and time.

• Discontinuities in ERA5: ERA5 is produced by several parallel experiments, each for a different period, which are then appended together to create the final product. This can create discontinuities at the transition points.
• The Potential Evaporation field (pev, parameter Id 228251) is largely underestimated over deserts and high-forested areas. This is due to a bug in the code that does not allow transpiration in case no low vegetation type is present.
• ERA5 diurnal cycle for winds: the hourly data reveals a mismatch in the analysed near surface wind speed between the end of one assimilation cycle and the beginning of the next (which occurs at 9:00 and 21:00 UTC). This problem mostly occurs in low latitude oceanic regions, though it can also be seen over Europe and the USA. The forecast near surface winds show much better agreement between the assimilation cycles, at least on average, so our advice would be to use the forecast winds. We cannot rectify the problem in the analyses.
  
• Wind values are far too low on pressure levels at the poles in the Climate Data Store (CDS)
• Prior to 2014, the SST was not used over the Great Lakes to nudge the lake model. Consequently, the 2 metre temperature  has an annual cycle that is too strong, with temperatures being too cold in winter and too warm in summer.
• ERA5 large 10m winds: up to a few times per year, the analysed low level winds, eg 10m winds, become very large in a particular location, which varies amongst a few apparently preferred locations. The largest values seen so far are about 300 ms^-1.
  
• ERA5 rain bombs: from time to time, the rainfall (precipitation) can become extremely large in small areas.
• Ship tracks in the SST: prior to September 2007, in the period when HadISST2 was used, ship tracks can be visible in the SST.

Resolved issues

• Wrong values of U/V on pressure levels in the Climate Data Store (CDS)
  

How to cite ERA5

Please acknowledge the use of ERA5 as stated in the Copernicus C3S/CAMS License agreement:

• "5.1.2 Where the Licensee communicates or distributes Copernicus Products to the public, the Licensee shall inform the recipients of the source by using the following or any similar notice:

  'Generated using Copernicus Climate Change Service Information [Year]'.

• 5.1.3 Where the Licensee makes or contributes to a publication or distribution containing adapted or modified Copernicus Products, the Licensee shall provide the following or any similar notice:

  'Contains modified Copernicus Climate Change Service Information [Year]';

5.1.3 Any such publication or distribution covered by clauses 5.1.1 and 5.1.2 shall state that neither the European Commission nor ECMWF is responsible for any use that may be made of the Copernicus Information or Data it contains."

You may also cite the ERA5 dataset as follows:

Dataset citable as: Copernicus Climate Change Service (C3S) (2017): ERA5: Fifth generation of ECMWF atmospheric reanalyses of the global climate . Copernicus Climate Change Service Climate Data Store (CDS), date of access. https://cds.climate.copernicus.eu/cdsapp#!/home

References

Global reanalysis: goodbye ERA-Interim, hello ERA5.

Further ERA5 references are available from the ECMWF e-Library.

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