For ERA-Interim (1st January 1979 to 31st August 2019) access through the ECMWF Web API stopped on 01 June 2023 Its successor ERA5 is available from the Climate Data Store (CDS) (What are the changes from ERA-Interim to ERA5?) and users are strongly advised to migrate to ERA5 (How to download ERA5). For those users who still need access to ERA-Interim after 01 June 2023 (subject to further notice), they can do so via the Climate Data Store (CDS) API. |
Here we document the ERA-Interim dataset, which, covers the period from 1st January 1979 to 31st August 2019.
ERA-interim was produced using cycle 31r2 of the Integrated Forecast System (IFS - CY31R2) 2006, with 60 vertical levels, 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.
Generally, the data are available at a sub-daily and monthly frequency and consist of analyses and 10-day forecasts, initialised twice daily at 00 and 12 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.
The data are archived in the ECMWF data archive MARS and datasets are available until 31 May 2023 through both the ECMWF Web interface and the ECMWF WebAPI, which is the programmatic way of retrieving data from the archive.
Until further notice, ERA-Interim is also available via the API of the C3S Climate Data Store.
Documentation is available on How to download ERA-Interim data from the ECMWF data archive (Member State users can access the data directly from MARS, in the usual manner).
The model documentation for CY31r2 (choose Cy31r1) is at https://www.ecmwf.int/en/publications/ifs-documentation.
The 4D-Var data assimilation uses 12 hour windows from 15 UTC to 03 UTC and 03 UTC to 15 UTC (the following day).
The model time step is 30 minutes.
The data can be accessed from MARS using the keywords class=ei. Subdivisions of the data are labelled using stream, type and levtype.
Stream:
Type:
Levtype:
In MARS: the date and time of the data is specified with three MARS keywords:
For analyses date and time, specify the analysis time and step equal to 0 hours. For forecasts date and time, choose the forecast start time and then 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.
The horizontal grid spacing of ERA-Interim atmospheric model and reanalysis system is around 80 km (reduced Gaussian grid N128) which became around 83km (
0.75^{\circ} |
) when interpolated to a regular lat/lon grid.
Depending on the parameter, the data are archived either as the full T255 spectral resolution and on the corresponding N128 reduced Gaussian grid, depending on their basic representation in the model. The coupled ocean-wave model data are produced and archived on a reduced
1.0^{\circ}\times 1.0^{\circ} |
latitude/longitude grid. For more information see ERA-Interim archive report Version 2.0, Section 2.
It is possible to specify the grid when downloading data. Available options are:
Longitudes range from 0 to 360, which is equivalent to -180 to +180 in Geographic coordinate systems.
The ECMWF model assumes the Earth is a perfect sphere, but the geodetic latitude/longitude of the surface elevation datasets are used as if they were the spherical latitude/longitude of the ECMWF model.
ECMWF data is referenced in the horizontal with respect to the WGS84 ellipse (which defines the major/minor axes) but in the vertical it is referenced to the Geoid (EGM96).
For data in GRIB1 format (as is the case with ERA-Interim data) the earth model is a sphere with radius = 6367.47 km, as defined in the WMO GRIB Edition 1 specifications, Table 7, GDS Octet 17
For data in NetCDF format (i.e. converted from the native GRIB format to NetCDF), the earth model is inherited from the GRIB data.
Analyses of atmospheric fields on model levels, pressure levels, potential temperature and potential vorticity, are available every 6 hours at 00, 06, 12, and 18 UTC. Forecasts run twice at 00 and 12 UTC and provide 3 hours output for surface and pressure level parameters up to 24 hours, with decreasing frequency to 10 days.
The ERA-Interim atmospheric model is coupled ocean-wave model resolving 30 wave frequencies and 24 wave directions at the nodes of its reduced
1.0^{\circ}\times 1.0^{\circ} |
latitude/longitude grid.
Download from ERA-Interim Wave data can be downloaded using the same mechanisms as atmospheric data. Please see How to download ERA-Interim For wave spectra you need to specify the additional parameters 'direction' and 'frequency'. If you want to download the data in NetCDF format, please add the 'format' and 'grid' parameters. Decoding 2D wave spectra in GRIB To decode wave spectra in GRIB format we recommend ecCodes. Wave spectra are encoded in a specific way that other tools might not decode correctly. In GRIB, the parameter is called 2d wave spectra (single) because in GRIB, the data are stored as a single global field per each spectral bin (a given frequency and direction), but in NetCDF, the fields are nicely recombined to produce a 2d matrix representing the discretized spectra at each grid point. The wave spectra are encoded in GRIB using a local table specific to ECMWF. Because of this, the conversion of the meta data containing the information about the frequencies and the directions are not properly converted from GRIB to NetCDF format. So rather than having the actual values of the frequencies and directions, values show index numbers (1,1) : first frequency, first direction, (1,2) first frequency, second direction, etc .... Also note that it is NOT the spectral density that is encoded but rather log10 of it, so to recover the spectral density, expressed in m^2 /(radian Hz), one has to take the power 10 (10^) of the NON missing decoded values. Missing data are for all land points, but also, as part of the GRIB compression, all small values below a certain threshold have been discarded and so those missing spectral values are essentially 0. m^2 /(gradient Hz). Decoding 2D wave spectra in NetCDF The NetCDF wave spectra file will have the dimensions longitude, latitude, direction, frequency and time. However, the direction and frequency bins are simply given as 1 to 24 and 1 to 30, respectively. The direction bins start at 7.5 degree and increase by 15 degrees until 352.5, with 90 degree being towards the east (Oceanographic convention). The frequency bins are non-linearly spaced. The first bin is 0.03453 Hz and the following bins are: f(n) = f(n-1)*1.1; n=2,30. The data provided is the log10 of spectra density. To obtain the spectral density one has to take to the power 10 (10 ** data). This will give the units 2D wave spectra as m**2 s radian**-1 . Very small values are discarded and set as missing values. These are essentially 0 m**2 s radian**-1. This recoding can be done with the Python xarray package, for example:
Units of 2D wave spectra Once decoded, the units of 2D wave spectra are m2 s radian-1 |
Instantaneous parameters represent an average over the model time step (30min). Accumulated parameters are accumulated from the start of the forecast, ie. from 00 UTC or 12 UTC to the time step selected. All the analysed fields are instantaneous instead forecast data could be either instantaneous or accumulated, depending on the parameter. More detailed information on parameters are shown in Parameter listing.
In ERA-Interim there are some parameters named '...since previous post-processing', for example 'Maximum temperature at 2 metres since previous post-processing'. This represents the maximum temperature between the previous archived forecast 'Step' and the forecast 'Step'. For example, 'Maximum temperature at 2 metres since previous post-processing' with start time 00 UTC and Step=9, is the maximum 2m temperature in the 3-hour period between 06 UTC and 09 UTC.
ERA-interim sub-daily data are monthly averaged on data with valid times or accumulation periods that fall within the calendar month in question. The different monthly means are:
See also Section 3 of the ERA-Interim archive documentation.
Model level fields are in GRIB2 format. All other fields are in GRIB1, unless otherwise indicated.
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
Model levels: 1/to/60, which are described at L60 model level definitions
Potential vorticity level:
PV=\pm 2PVU |
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 is available from the ECMWF parameter database.
Parameters described as "instantaneous" below, are valid at the specified time.
Instantaneous, invariant, surface and single level parameters table
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Instantaneous, varying, surface and single level parameters table
1
2
3Forecasts are only available up to a range of 12-hours |
Forecast accumulated surface and single level parameters
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Forecast minimum/maximum surface and single level parameters
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Vertical single level integrals for budgets
|
Wave and gridded ERS altimeter parameters
1Forecasts are also available at a range of 3-hours 2Available from late 1991 3Available for 30 frequencies and 24 directions |
Monthly mean surface and single level parameters: Exceptions from Tables 1-4
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Accumulated model full levels parameters to validate clear sky radiation
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Accumulated model half or model full levels parameters to support chemical transport modelling
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Accumulated model full levels net tendencies
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Parameters on isentropic surfaces
1Gaussian grid 2Spherical harmonics 3Only PV is archived at 320 K |
Parameters on the PV = ± 2 PVU surface
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The observations (satellite and in-situ) used as input into ERA-Interim are listed in tables below.
Satellite Data
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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 (sp) from which you can calculate specific and relative humidity at 2m:
The relative humidity can be calculated 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.
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. The method for calculating snow cover depends on the particular version of the IFS and for ERA-Interim is computed directly using snow water equivalent (ie parameter SD (141.128)) as:
snow_cover (SC) = min(1, RW*SD/15 ) where RW is density of water equal to 1000 and RSN is density of snow (parameter 33.128). |
The Physical depth of snow where there is snow cover is equal to RW*SD/(RSN*SC) where RSN = density of snow (parameter 33.128). For more in depth information see:
In ERA-Interim, and often in meteorology, altitudes (the altitude of the land and sea surface, or specific altitudes in the atmosphere) are not represented as geometric altitude (in metres above the spheroid), but as geopotential height (in metres above the geoid). However, ECMWF archive the geopotential (in m2/s2), not the geopotential height.
In order to calculate the geopotential height of the land and sea surface (the so called surface geopotential height, or orography):
In order to define the surface geopotential in ERA-Interim, the ECMWF model uses surface elevation data interpolated from GTOPO30, with some fixes for Antarctica and Greenland. See Chapter 10 Climatological data, of Part IV. Physical processes, of the ERA-Interim model documentation at https://www.ecmwf.int/search/elibrary/part?solrsort=sort_label%20asc&title=part&secondary_title=31r1.
Please see the ERA-Interim known issues page for guidance and workarounds.
Guidelines
In addition to the terms and conditions of the license(s), users must:
Copernicus Climate Change Service (2023): ERA-Interim atmospheric reanalysis. Copernicus Climate Change Service (C3S) Climate Data Store (CDS), DOI: 10.24381/cds.f2f5241d (Accessed on DD-MMM-YYYY)
Copernicus programme:
[Generated using/Contains modified] Copernicus Climate Change Service information [year]. Neither the European Commission nor ECMWF is responsible for any use that may be made of the Copernicus information or data it contains.
Products:
Dee, D.P., Uppala, S.M., Simmons, A.J., Berrisford, P., Poli, P., Kobayashi, S., Andrae, U., Balmaseda, M.A., Balsamo, G., Bauer, P., Bechtold, P., Beljaars, A.C.M., van de Berg, L., Bidlot, J., Bormann, N., Delsol, C., Dragani, R., Fuentes, M., Geer, A.J., Haimberger, L., Healy, S.B., Hersbach, H., Hólm, E.V., Isaksen, L., Kållberg, P., Köhler, M., Matricardi, M., McNally, A.P., Monge-Sanz, B.M., Morcrette, J.J., Park, B.K., Peubey, C., de Rosnay, P., Tavolato, C., Thépaut, J.N. and Vitart, F. (2011): ERA-Interim global atmospheric reanalysis. Copernicus Climate Change Service (C3S) Climate Data Store (CDS), DOI: 10.24381/cds.f2f5241d (Accessed on DD-MMM-YYYY)
2004-2007
2008
2009
2010
2011
ERA-Interim Daily Climatology. Product description and basic validation. Compiled by Martin Janoušek, ECMWF, January 2011
2014
Please use this as the main scientific reference to ERA-Interim:
Dee, D. P., Uppala, S. M., Simmons, A. J., Berrisford, P., Poli, P., Kobayashi, S., Andrae, U., Balmaseda, M. A., Balsamo, G., Bauer, P., Bechtold, P., Beljaars, A. C. M., van de Berg, L., Bidlot, J., Bormann, N., Delsol, C., Dragani, R., Fuentes, M., Geer, A. J., Haimberger, L., Healy, S. B., Hersbach, H., Hólm, E. V., Isaksen, L., Kållberg, P., Köhler, M., Matricardi, M., McNally, A. P., Monge-Sanz, B. M., Morcrette, J.-J., Park, B.-K., Peubey, C., de Rosnay, P., Tavolato, C., Thépaut, J.-N. and Vitart, F. (2011), The ERA-Interim reanalysis: configuration and performance of the data assimilation system. Q.J.R. Meteorol. Soc., 137: 553–597. doi: 10.1002/qj.828
For a more technical documentation of the contents of the ERA-Interim dataset please use:
Berrisford, P, Dee, DP, Poli, P, Brugge, R, Fielding, K, Fuentes, M, Kållberg, PW, Kobayashi, S, Uppala, S, Simmons, A (2011): The ERA-Interim archive Version 2.0. ERA Report Series 1, http://www.ecmwf.int/en/elibrary/8174-era-interim-archive-version-20
This document has been produced in the context of the Copernicus Climate Change Service (C3S). The activities leading to these results have been contracted by the European Centre for Medium-Range Weather Forecasts, operator of C3S on behalf of the European Union (Delegation agreement signed on 11/11/2014). All information in this document is provided "as is" and no guarantee or warranty is given that the information is fit for any particular purpose. The users thereof use the information at their sole risk and liability. For the avoidance of all doubt, the European Commission and the European Centre for Medium-Range Weather Forecasts have no liability in respect of this document, which is merely representing the author's view. |