ERA-Interim production stopped on 31st August 2019
For the time being and until further notice, ERA-Interim (1st January 1979 to 31st August 2019) shall continue to be accessible through the ECMWF Web API. Keeping in mind that ERA-Interim is published with an offset of about three months from the dataset's reference date, ERA-Interim August 2019 data will be made available towards the end of October 2019.
ERA5 is now 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 migrate from ECMWF Web API to CDS API).
Grid geometry and horizontal resolution
At ECMWF the ERA-Interim data is archived in two representations, depending on the parameter:
- Some parameters are archived as spectral coefficients with a truncation of T255 (see Wikipedia: Spherical harmonics)
- Some parameters are archived on the N128 reduced Gaussian grid (see Wikipedia: Gaussian grid)
The equivalent horizontal resolution for both representations is about 80km.
For a list of spectral, gaussian and eqivalent lat/lon grids see the Open IFS FAQ, specifically 'What does the 'T' mean in 'T511', 'T1279' etc?' and 'How do I know the grid from the 'T' number?'
When you download ERA-Interim data you can specify the output data to be delivered in a grid other than the native grid. Available options depend on the file format chosen:
- If you download in GRIB1 format (the native format): You have a choice of grids, including the native form as described above, different Gaussian grids, and regular lat/long grids. Native spectral data will be transformed to the chosen grid. For data natively on the N128 grid, if you specify a Gaussian grid other than the native N128, the data will be interpolated to your chosen grid.
- If you download in NetCDF format: The ECMWF NetCDF implementation only supports regular lat/lon grids, so data is on the fly 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.
All gridded data is made available in spherical coordinates, i.e. unprojected. Some GIS software applications call this 'Geographic Coordinate Systems'.
As earth model a sphere with radius 6367.47 km is used, as defined in the WMO GRIB Edition 1 specifications, Table 7, GDS Octet 17
ERA-Interim data can be downloaded from ECMWF in NetCDF format, in this case the data is converted on the fly from the native GRIB1 format to NetCDF, and the earth model is inherited from the GRIB1 data.
When you download ERA-Interim data you can optionally have the data interpolated to a custom grid and horizontal resolution, by specifying for example 'grid':'1.5/1.5'. The default interpolation method is bilinear for continuous parameters (e.g. Temperature) and nearest neighbour for discrete parameters (eg. Vegetation).
The lat/long equivalent of T255 is 0.703125 deg (360/(2*(255+1))), but GRIB1 format only supports three decimals, so we recommend using a resolution of 0.75x0.75 deg or lower. A higher resolution is technically possible, e.g. as 'grid':'0.25/0.25', but this only oversamples the data and does not improve the accuracy of the data.
Visualisation of regular lat/lon data
If you use ERA-Interim data in a regular lat/lon grid, many software applications by default visualise the data as a continuous tiled surface, as in plot (a) on the right. In this case think of the data values as referring to the centroids of the tiles.
However, the ECMWF interpolation software does not conserve area integrals, so interpolated values are best regarded as point values, with a regular spacing, as shown on the right in plot (b): here global ERA-Interim data is used with a regular lat/lon grid and a resolution r = 0.75 deg. The 'top left' ERA-interim data point is always at Longitude=0 ; Latitude=90, with further grid points spaced by r, and the 'bottom right' grid point at Longitude=360-r ; Latitude=-90.
(a) Visualisation of regular lat/lon data as a continuous tiled surface
(b) Visualisation of regular lat/lon data as point matrix
For GIS users
Some GIS software applications do not recognise the spatial reference information embedded in the data file and may require you to manually assign spatial reference metadata. In this case use a 6367.47km sphere for all ERA-Interim data if possible. The spheres used as earth models do not have EPSG codes.
In practice, considering that global meteorological models operate in spatial resolutions of at least multiple kilometres and with significant spatial uncertainty, for most users assigning one of the commonly used geodetic datums (WGS1984, ETRS1989, etc.) to the downloaded data is a 'good enough' solution.
Note that other ERA products have different resolution:
- ERA-40 has a resolution of T159 (triangular truncation of 159), N80 (80 latitude circles, pole to equator), L60 (model levels), 23 pressure levels and 15 isentropic levels.
- ERA-15 has a resolution of T106 with 31 vertical hybrid levels.
The technical documentation for ERA-Interim: 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. ECMWF, ERA Report Series (specifically section 2)