Introduction

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.

How to download ERA-Interim

The data are archived in the ECMWF data archive MARS and datasets are available through both the Web interface and the ECMWF WebAPI,  which is the programmatic way of retrieving data from the archive.

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 IFS and data assimilation

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.

Data organisation

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

Stream:

• oper: sub-daily products
• wave: sub-daily ocean-waves products
• mnth: synoptic monthly means
• moda: monthly means of daily means
• mdfa: monthly means of daily forecast accumulations

Type:

• an: analyses
• fc: forecasts

Levtype:

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

In MARS: the date and time of the data is specified with three MARS keywords:

• date
• time
• step

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.

Spatial grid

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:

• GRIB1 format  (native format): native grid, different Gaussian grids, and regular lat/lon grids. Data will be interpolated to your chosen grid if different to the native one.
• NetCDF format: NetCDF only supports regular lat/lon grids. Data is transformed to a regular lat/lon grid, interpolated to your selected resolution, and converted from the native GRIB1 format to NetCDF.

Longitudes range from 0 to 360, which is equivalent to -180 to +180 in Geographic coordinate systems.

Spatial reference 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).

Earth model

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.

Temporal frequency

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.

Wave spectra

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.

#!/usr/bin/env python
from ecmwfapi import ECMWFDataServer
server = ECMWFDataServer()
server.retrieve({
    "class": "ei",
    "dataset": "interim",
    "expver": "1",
    "stream": "wave",
    "type": "an",
    "date": "2016-01-01/to/2016-01-31",
    "time": "00:00:00/06:00:00/12:00:00/18:00:00",
    "param": "251.140",
    "direction": "1/2/3/4/5/6/7/8/9/10/11/12/13/14/15/16/17/18/19/20/21/22/23/24",
    "frequency": "1/2/3/4/5/6/7/8/9/10/11/12/13/14/15/16/17/18/19/20/21/22/23/24/25/26/27/28/29/30",
    "target": "2d_spectra_201601",
})

#!/usr/bin/env python
from ecmwfapi import ECMWFDataServer
server = ECMWFDataServer()
server.retrieve({
    ......
    "grid": "0.75/0.75", # Spatial resolution in degrees latitude/longitude
    "format": "netcdf"
    "target": "2d_spectra_201601.nc"
})

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')

Instantaneous and Accumulated parameters

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.

Minimum/maximum since the previous post processing

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.

Monthly means

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:

• Synoptic Monthly Means (stream=mnth) are the monthly averages:
  
  • for each analysis time (at the four main synoptic hours - 00, 06, 12, and 18 UTC)
  • for each forecast start time (00 and 12 UTC) and step (3, 6, 9, 12 etc).
• Monthly Means of Daily Means (stream=moda) are only available for analysis and instantaneous forecast data.
• Monthly Means of Daily Forecast Accumulations (stream=mdfa) are similar to Monthly Means of Daily Means but are for accumulated fields (eg precipitation, radiation) and three step ranges are available.

See also Section 3 of the ERA-Interim archive documentation.

Data format

Model level fields are in GRIB2 format. All other fields are in GRIB1, unless otherwise indicated.?

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

Model levels: 1/to/60, which are described at https://www.ecmwf.int/en/forecasts/documentation-and-support/60-model-levels

Potential vorticity level:

PV=\pm 2PVU

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 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

Instantaneous, varying, surface and single level parameters table

Forecast accumulated surface and single level parameters

Forecast minimum/maximum surface and single level parameters

Vertical single level integrals for budgets

Wave and gridded ERS altimeter parameters

Monthly mean surface and single level parameters: Exceptions from Tables 1-4

Accumulated model full levels parameters to validate clear sky radiation

Accumulated model half or model full levels parameters to support chemical transport modelling

Accumulated model full levels net tendencies

Parameters on isentropic surfaces

Parameters on the PV = ± 2 PVU surface

Observations

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

Satellite Data

In-situ data, provided by WMO WIS

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 6.4 and 6.5 from Part IV, Physical processes section (Chapter 6, section 6.6.1b) in the documentation of the IFS for CY31R1. Use the 2m dew point temperature and surface pressure (which is approximately equal to the pressure at 2m) in these equations.
• Relative humidity should be calculated: RH = 100 * es(Td)/es(T)

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.

^[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. 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:

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:

• https://www.the-cryosphere.net/11/923/2017/tc-11-923-2017.pdf
• https://www.ecmwf.int/sites/default/files/elibrary/2013/13946-using-reanalyses-studying-eurasian-snow-cover-and-its-relationship-circulation-variability.pdf
• https://journals.ametsoc.org/doi/pdf/10.1175/JHM-D-12-012.1

Surface elevation and orography

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):

• First, download the surface geopotential (i.e. the geopotential of the land and sea surface), http://apps.ecmwf.int/datasets/data/interim-full-invariant/?param=129.128
• Then divide the surface geopotential by g=9.80665 to obtain the surface geopotential height in metres.

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.

Notes

• The surface geopotential is also available on model levels (at level=1), where it is archived in spectral form.
• Over oceans the surface geopotential field shows 'spectral ripples' (Know issue KI6, see ERA-Interim known issues page for details), which are a reflection of the fact that the ERA-Interim model is a spectral model and the grid point surface geopotential has been spectrally fitted.
• The model levels are hybrid pressure/sigma, eg for ERA-Interim. See Chapter 2 Basic equations and discretization of Part III. Dynamics and numerical procedures, of the ERA-Interim model documentation at   https://www.ecmwf.int/search/elibrary/part?solrsort=sort_label%20asc&title=part&secondary_title=31r1.
• The definition of the 60 model levels, for  ERA-Interim, and the corresponding half-level, ph, and full-level, pf, values of pressure (for a standard atmosphere with a surface pressure of 1013.250 hPa), geopotential and geopotential heights can be found at http://www.ecmwf.int/en/forecasts/documentation-and-support/60-model-levels

Known issues

Please see the ERA-Interim known issues page for guidance and workarounds.

How to cite ERA-Interim

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

To cite the source of the data, you may use the following data citation:

If no specific advice is given by the journals, it is usually recommended that the above data citation is put in the acknowledgements section.

Reports

2004-2007

• IFS Documentation CY31R1 (model used to produced ERA-Interim)

2008

• Technical Memo: 575: Variational bias correction in ERA-Interim

2009

• ERA Report series: 1: The ERA-Interim archive Version 1.0

2010

• ERA Report series: 2: On the quality of the ERA-Interim ozone reanalyses; Part 1 Comparisons with in situ measurements
• ERA Report series: 3: On the quality of the ERA-Interim ozone reanalyses; Part 2 Comparisons with satellites data
• ERA Report series: 4: List of observations assimilated in ERA-40 and ERA-Interim (v1.0)
• ERA Report series: 5: Evaluation of ERA-Interim and ERA-Interim-GPCP-rescaled precipitation over the U.S.A.
• ERA Report series: 6: Evaluation of ERA-Interim forcing fluxes from an ocean perspective

2011

• ERA Report series: 1: The ERA-Interim archive Version 2.0
• ERA Report series: 7: Data usage and quality control for ERA-40, ERA-Interim and the operational ECMWF data assimilation system
• ERA Report series: 8: Radiosonde temperature bias correction in ERA-Interim
• ERA Report series: 10: Forecast drift in ERA-Interim
• ERA Report series: 12: Atmospheric conservation properties in ERA-Interim

• ERA-­Interim Daily Climatology. Product description and basic validation. Compiled by Martin Janoušek, ECMWF,  January 2011

2014

• ERA Report series: 15: Estimating low-frequency variability and trends in atmospheric temperature using ERA-Interim (Revised 2014)

References

• Berrisford, P., P. Kållberg,  S. Kobayashi, D. Dee, S. Uppala, A. J. Simmons, P. Poli,  and H. Sato, 2011: Atmospheric conservation properties in ERA-Interim. Q.J.R. Meteorol. Soc., 137: 1381–1399. doi: 10.1002/qj.864
• Dee, D. P., and Coauthos, 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
• Further references available from the ECMWF e-library

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