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Introduction

Here we document the ERA5 dataset, which, eventually, will cover the period from January 1950. ERA5 data released so far covers the period from 1979 and continues to be extended forward in near real time.

ERA5 was produced using 4D-Var data assimilation in CY41R2 of ECMWF’s Integrated Forecast System (IFS), with 137 hybrid sigma/pressure (model) levels in the vertical, with the top level at 0.01 hPa. Atmospheric data are available on these levels and they are also interpolated to 37 pressure, 16 potential temperature and 1 potential vorticity level(s). "Surface or single level" data are also available, containing 2D parameters such as precipitation, 2m temperature, top of atmosphere radiation and vertical integrals over the entire atmosphere. The IFS is coupled to a soil model, the parameters of which are also designated as surface parameters, and an ocean wave model.

The ERA5 dataset contains one (31 km) high resolution realisation (HRES) and a reduced resolution ten member ensemble (EDA). Generally, the data are available at a sub-daily and monthly frequency and consist of analyses and short (18 hour) forecasts, initialised twice daily from analyses at 06 and 18 UTC. Most analysed parameters are also available from the forecasts. There are a number of forecast parameters, e.g. mean rates and accumulations, that are not available from the analyses.

How to download ERA5

The data are archived in the ECMWF data archive (MARS) and a pertinent sub-set of the data, interpolated to a regular latitude/longitude grid, has been copied to the Climate Data Store (CDS) disks.

Documentation is available on how to download ERA5 data. (Member State users can access the data directly from MARS, in the usual manner.) Data can be downloaded either from the relevant CDS download page or using the CDS API. The latter is suitable for larger data volumes and for accessing the ERA5 data in MARS.

Data format

Model level fields (see below) are in GRIB2 format. All other fields are in GRIB1, unless otherwise indicated.

In the CDS, there is the option of retrieving the data in netCDF format.

Data update frequency

Initial release data, i.e. data no more than three months behind real time, is called ERA5T.  In the event that serious flaws are detected in ERA5T, this data could be different to the final ERA5 data. In practice, though, this will be very unlikely to occur. Based on experience with the production of ERA5 so far (and ERA-Interim in the past), our expectation is that such an event would not occur more than once every few years, if at all. In the unlikely event that such a correction is required, users will be notified as soon as possible.

For the CDS, daily updates are available 5 days behind real time and monthly mean updates are available 5 days after the end of the month.

For MARS ERA5 data, monthly updates are available about two months after the month in question. In the future, ERA5T data will also be available in MARS.

For GRIB data, ERA5T can be identified by the key expver=0005 in the GRIB header. ERA5 is identified by the key expver=0001.

For netCDF data requests which return just ERA5 or just ERA5T data, there is no means of differentiating between ERA5 and ERA5T data in the resulting netCDF files.

For netCDF data requests which return a mixture of ERA5 and ERA5T data, the origin of the variables (0001 1 or 00055) will be identifiable in the resulting netCDF files (the netCDF variable names will have the terms "_0001" and "_0005" appended). See the link for more details.

The IFS and data assimilation

The model documentation for CY41R2 is at https://www.ecmwf.int/en/publications/search/?solrsort=sort_label%20asc&secondary_title=%22IFS%20Documentation%20CY41R2%22

The 4D-Var data assimilation uses 12 hour windows from 09 UTC to 21 UTC and 21 UTC to 09 UTC (the following day).

The model time step is 12 minutes for the HRES and 20 minutes for the EDA, though occasionally these numbers are adjusted to cope with instabilities.

Data organisation

The data can be accessed from MARS using the keywords class=ea and expver=0001 (expver=0005 for ERA5T). Subdivisions of the data are labelled using stream, type and levtype.

Stream:

• oper: HRES sub-daily
• wave: HRES waves sub-daily
• mnth: HRES synoptic monthly means
• moda: HRES monthly means of daily means
• wamo: HRES waves synoptic monthly means
• wamd: HRES waves monthly means of daily means
• enda: EDA sub-daily
• ewda: EDA waves sub-daily
• edmm: EDA synoptic monthly means
• edmo: EDA monthly means of daily means
• ewmm: EDA waves synoptic monthly means
• ewmo: EDA waves monthly means of daily means

Type:

• an: analyses
• fc: forecasts
• em: ensemble mean
• es: ensemble standard deviation

Levtype:

• sfc: surface or single level
• pl: pressure levels
• pt: potential temperature levels
• pv: potential vorticity level
• ml: model levels

In MARS: the date and time of the data is specified with three MARS keywords, date, time and step. For analyses, step=0 hours so that date and time specify the analysis time. For forecasts, date and time specify the forecast start time and step specifies the number of hours since that start time. The combination of date, time and forecast step defines the validity time. For analyses, the validity time is equal to the analysis time.

In the CDS: analyses are provided rather than forecasts, unless the parameter is only available from the forecasts. The date and time of the data is specified using the validity date/time, so step does not need to be specified. For forecasts, steps between 1 and 12 hours have been used to provide data for all the validity times in 24 hours, see Table 0 below.

Table 0: the mapping, for forecasts, between MARS date, time and step and the CDS date and time

Spatial grid

The ERA5 HRES atmospheric data has a resolution of 31km, 0.28125 degrees, and the EDA has a resolution of 63km, 0.5625 degrees. (Depending on the parameter, the data are archived either as spectral coefficients with a triangular truncation of T639 (HRES) and T319 (EDA) or on a reduced Gaussian grid with a resolution of N320 (HRES) and N160 (EDA). These grids are so called "linear grids", sometimes referred to as TL639 (HRES) and TL319 (EDA).)

The wave data are produced and archived on a different grid to that of the atmospheric model, namely a reduced latitude/longitude grid with a resolution of 0.36 degrees (HRES) and 1.0 degrees (EDA).

ERA5 data available from the CDS disks has been pre-interpolated to a regular latitude/longitude grid appropriate for that data.

The article Model grid box and time step might be useful.

Temporal frequency

For sub-daily data for the HRES (stream=oper/wave) the analyses (type=an) are available hourly. The short forecasts, run from 06 and 18 UTC, have hourly steps from 0 to 18 hours. For the EDA, the sub-daily non-wave data (stream=enda) are available every 3 hours but the sub-daily wave data (stream=ewda) are available hourly.

Wave spectra

The ERA5 wave model uses wave spectra with 24 directions and 30 frequencies (see "2D wave spectra (single)", Table 7),

import xarray as xr
import numpy as np
da = xr.open_dataarray('2d_spectra_201601.nc')
da = da.assign_coords(direction=np.arange(7.5, 352.5 + 15, 15))
da = da.assign_coords(frequency=np.full(30, 0.03453) * (1.1 ** np.arange(0, 30)))
da = 10 ** da
da = da.fillna(0)
da.to_netcdf(path='2d_spectra_201601_recoded.nc')

Mean rates and accumulations

The accumulations in the short forecasts (from 06 and 18 UTC) of ERA5 are treated differently compared with those in ERA-Interim and operational data (where they are from the beginning of the forecast to the forecast step). In the short forecasts of ERA5 the accumulations are since the previous post processing (archiving), so for:

• HRES: accumulations are over the hour ending at the forecast step
• EDA: accumulations are over the 3 hours ending at the forecast step
• Monthly means of daily means (stream=moda/edmo): accumulations have been scaled to have units that include "per day", see section Monthly means

Mean rate parameters in ERA5 are similar to accumulations except that the accumulations have been divided by the length of the processing period in seconds to produce temporally averaged rates, so the units include "per second". For "surface or single level" parameters, the mean rates (Table 4) provide similar information to the accumulations (Table 3), but with the different units.

Note that:

• For the CDS time, or validity time, of 00 UTC, the mean rates and accumulations are over the hour (3 hours for the EDA) ending at 00 UTC i.e. the mean or accumulation is during the previous day.
• Mean rates and accumulations are not available from the analyses.
• Mean rates and accumulations at step=0 have values of zero because the length of the processing period is zero.

Minimum/maximum since the previous post processing

The short forecasts of ERA5 contain some surface and single level parameters that are the minimum or maximum value since the previous post processing (archiving), see Table 5 below. So, for:

• HRES: the minimum or maximum values are in the hour ending at the forecast step
• EDA: the minimum or maximum values are in the 3 hours ending at the forecast step

Monthly means

In addition to the sub-daily data, most parameters are also available as monthly means. For the surface and single level parameters, there are some exceptions which are listed in Table 8.

Monthly means are available in two forms:

• Synoptic monthly means, for each particular time and forecast step (stream=mnth/wamo/edmm/ewmm) - in the CDS, referred to as "monthly averaged by hour of day".
• Monthly means of daily means, for the month as a whole (stream=moda/wamd/edmo/ewmo) - in the CDS, referred to as "monthly averaged". These monthly means are created from the hourly data for the reanalysis (HRES) and 3 hourly data for the ensemble members (EDA).

Monthly means for:

• forecast parameters are created using the first 12 hours of the twice daily short forecasts (beginning at 06 and 18 UTC).
• analysis and instantaneous forecast parameters are created from data with a valid time in the month, between 00 and 23 UTC, which excludes the time 00 UTC on the first day of the following month.
• accumulation and mean rate forecast parameters are created from data with a forecast period falling within the month. Therefore, monthly means of daily means for accumulations and mean rates are created from contiguous data with forecast periods spanning from 00 UTC on the first day of the month to 00 UTC on the first day of the following month.

The accumulations in monthly means of daily means (stream=moda/edmo) have been scaled to have units that include "per day", so for accumulations in these streams:

• The hydrological parameters are in units of "m of water per day" and so they should be multiplied by 1000 to convert to kgm^-2day^-1 or mmday^-1.
• The energy (turbulent and radiative) and momentum fluxes should be divided by 86400 seconds (24 hours) to convert to the commonly used units of Wm^-2 and Nm^-2, respectively.

Ensemble means and standard deviations

For the EDA sub-daily data (stream=enda/ewda), compared with HRES sub-daily data (stream=oper/wave), there are also ensemble means and standard deviations (type=em/es).

Ensemble standard deviation is often referred to as ensemble spread and is calculated as the standard deviation of the 10-members in the ensemble (i.e., including the control). It is not the sample stdv, so we divide by 10 rather than 9 (N-1). 

Ensemble means and standard deviations contain analysed parameters when step=0, otherwise they contain forecast parameters. However, only surface and pressure level data (levtype=sfc/pl) contain forecast steps beyond 3 hours. There are no monthly means for ensemble means and standard deviations.

Level listings

Pressure levels: 1000/975/950/925/900/875/850/825/800/775/750/700/650/600/550/500/450/400/350/300/250/225/200/175/150/125/100/70/50/30/20/10/7/5/3/2/1

Potential temperature levels: 265/275/285/300/315/320/330/350/370/395/430/475/530/600/700/850

Potential vorticity level: 2000

Model levels: 1/to/137, which are described at https://www.ecmwf.int/en/forecasts/documentation-and-support/137-model-levels.

Parameter listings

Tables 1-6 below describe the surface and single level parameters (levtype=sfc), Table 7 describes wave parameters, Table 8 describes the monthly mean exceptions for surface and single level and wave parameters and Tables 9-13 describe upper air parameters on various levtypes.

Information on all ECMWF parameters (e.g. columns shortName and paramId) is available from the ECMWF parameter database.

Note: If a parameter name listed in the tables below does not have a Variable name in CDS, this means that this particular parameter is only available from the ECMWF MARS tape archive using CDS-API (see Data Organisation section above).

Parameters described as "instantaneous"  below, are valid at the specified time.

Table 2: stream=oper/enda/mnth/moda/edmm/edmo, levtype=sfc: surface and single level parameters: instantaneous

¹

*GRIB2 format

Table 3: stream=oper/enda/mnth/moda/edmm/edmo, levtype=sfc: surface and single level parameters: accumulations

Accumulations are described in section Mean rates and accumulations. The accumulations in monthly means of daily means (stream=moda/edmo) have been scaled to have units that include "per day", so for accumulations in these streams:

• The hydrological parameters are in units of "m of water per day" and so they should be multiplied by 1000 to convert to kg m^-2 day^-1 or mm day^-1.
• Energy (turbulent and radiative) and momentum fluxes should be divided by 86400 seconds (24 hours) to convert to the commonly used units of W m^-2 and N m^-2, respectively.

 Table 4: stream=oper/enda/mnth/moda/edmm/edmo, levtype=sfc: surface and single level parameters: mean rates

The mean rates in Table 4 provide similar information to the accumulations in Table 3, except that they are expressed as temporal averages instead of accumulations, and so have units of "per second". The hydrological parameters are in units of "kg m^-2 s^-1" and so they can be multiplied by 86400 seconds (24 hours) to convert to kg m^-2 day^-1 or mm day^-1.

Table 5: stream=oper/enda, levtype=sfc: surface and single level parameters: minimum/maximum

Table 6: stream=oper/enda/mnth/moda/edmm/edmo, levtype=sfc: surface and single level parameters: vertical integrals (not available for type=em/es) and total column: instantaneous

Table 7: stream=wave/ewda/wamo/wamd/ewmm/ewmo: wave parameters: instantaneous

*for 30 frequencies and 24 directions

Table 8: stream=mnth/moda/edmm/edmo, levtype=sfc or wamo/wamd/ewmm/ewmo: monthly mean surface and single level and wave parameters: exceptions from Tables 1-7

Table 9: stream=oper/enda/mnth/moda/edmm/edmo, levtype=pl: pressure level parameters: instantaneous

Table 10: stream=oper/enda/mnth/moda/edmm/edmo, levtype=pt: potential temperature level parameters: instantaneous

Table 11: stream=oper/enda/mnth/moda/edmm/edmo, levtype=pv: potential vorticity level parameters: instantaneous

Table 12: stream=oper/enda/mnth/moda/edmm/edmo, levtype=ml: model level parameters: instantaneous

*Only archived on level=1.

Table 13: stream=oper/enda/mnth/moda/edmm/edmo, levtype=ml: model level parameters: mean rates

*These parameters provide data for the model half levels, at the interfaces of the model layers.

Observations

The observations (satellite and in-situ) used as input into ERA5 are listed below.

Satellite Data

* reprocessed dataset
+ when different than the satellite agency

In-situ data, provided by WMO WIS

Snow data

Computation of near-surface humidity and snow cover

Near-surface humidity

Near-surface humidity is not archived directly in ERA datasets, but the archive contains near-surface (2m from the surface) temperature (T), dew point temperature (Td), and surface pressure^[1] (sp) from which you can calculate specific and relative humidity at 2m.

• Specific humidity can be calculated over water and ice using equations 7.4 and 7.5 from Part IV, Physical processes section (Chapter 7, section 7.2.1b) in the documentation of the IFS for CY41R2. Use the 2m dew point temperature and surface pressure (which is approximately equal to the pressure at 2m) in these equations. The constants in 7.4 are to be found in Chapter 12 (of Part IV: Physical processes) and the parameters in 7.5 should be set for saturation over water because the dew point temperature is being used.
• Relative humidity should be calculated: RH = 100 * es(Td)/es(T)

 Relative humidity can be calculate with respect to saturation over water, ice or mixed phase by defining es(T) with respect to saturation over water, ice or mixed phase (water and ice). The usual practice is to define near-surface relative humidity with respect to saturation over water.

^[1] Access to surface pressure varies by dataset. For example, for ERA-Interim surface pressure is available from the Web Interface and from the WebAPI, while for ERA-40 surface pressure is not available from the Web Interface, but only via the WebAPI.

Snow Cover

In the ECMWF model (IFS), snow is represented by an additional layer on top of the uppermost soil level. The whole grid box may not be covered in snow. The snow cover gives the fraction of the grid box that is covered in snow.

For ERA5, the snow cover (SC) is computed as follows:

ERA5 physical depth of snow where there is snow cover is equal to RW*SD/(RSN*SC).

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 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.

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

• From 2000 to 2007, 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.

• 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.
  
• 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.
  
• 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.
• 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.
• Wind values are far too low on pressure levels at the poles in the Climate Data Store (CDS)
  
• 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.
• Wave parameters (Table 7 above) for the three swell partitions: these parameters have been calculated incorrectly. The problem is most evident in the swell partition parameters involving the mean wave period: Mean wave period of first swell partition, Mean wave period of second swell partition and Mean wave period of third swell partition, where the periods are far too long.
• The instantaneous turbulent surface stress components (eastward and northward) and friction velocity tend to be too small.

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.

Global stratospheric temperature bias and other stratospheric aspects of ERA5 and ERA5.1

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

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