The native horizontal spatial reference for ERA Interim data is a T255 reduced Gaussian grid, equivalent to a horizontal resolution of about 80km or 0.75deg. The native file format for ERA-Interim is GRIB, with all parameters stored in GRIB1 format.
When you download ERA-Interim data you can specify the output data to be in GRIB format or in NetCDF format. This determines your options for the horizontal resolution:
- If you specify GRIB as output format you will get a GRIB1 or GRIB2 file (depending on the parameter). By default the output will be on the native T255 grid, but you can specify a different Gaussian grid or a lat/long grid:
- If you specify a Gaussian grid other than the default T255 the data will be interpolated to your chosen resolution. The default interpolation method is bilinear for continuous parameters (e.g. Temperature) and nearest neighbour for discrete parameters (eg. Vegetation).
- If you specify a lat/lon grid the data will be interpolated to your chosen resolution.. 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))). You could use this resolution for parameters stored in GRIB2 format, but GRIB1 format only supports three decimals, so we recommend you in any case round the resolution; we recommend to 0.75x0.75 deg.
- If you specify NetCDF as output format: Our NetCDF implementation only supports regular grids, so when you extract data it is automatically interpolated from the native Gaussian grid to lat/long. The default interpolation method is bilinear for continuous parameters (e.g. Temperature) and nearest neighbour for discrete parameters (eg. Vegetation). Regarding resolution, the lat/long equivalent of T255 is 0.703125 deg (360/(2*(255+1))). You could use this resolution for parameters stored in GRIB2 format, but GRIB1 format only supports three decimals, so we recommend you in any case round the resolution; we recommend to 0.75x0.75 deg.
| To visualise the data many software applications by default plot regular lat/lon data as a continuous surface. However, you might prefer to think of the data as point data with a regular spacing. In global lat/lon data the 'top left' point will be at Longitude=0 ; Latitude=90, with further grid points spaced according to your selected resolution r, and the 'bottom right' grid point at Longitude=360-r ; Latitude=-90. For example, if you download ERA-Interim data with a 0.75 degree Lat/Lon grid and plot it onto a satellite image with 0.25 degree resolution: |
For ERA-Interim data in all representations the assumed underlying earth model (the geodetic datum) is a sphere with with radius 6367.47km. The surface of the ECMWF model is defined by the orography which indicates the geopotential (from which you can calculate the geopotential height by dividing by 9.80665) of that surface relative to mean sea level. The orography has been interpolated from a higher resolution (DTM/DEM) dataset, GTOPO30, plus some fixes for Antarctica and Greenland, see Chapter 10 Climatological data, of Part IV. Physical processes, of the ERA-Interim model documentation. The original (~1km) orography data is referenced in the horizontal with respect to the WGS84 ellipse (which defines the data major/minor axis) but the geodetic lat/lon of the orography dataset are used as if they were the spherical lat/lon of the ECMWF model. In the vertical the data is referenced to the Geoid (EGM96).
For GIS users:
Some software applications do not recognise the spatial reference information embedded in the data file and may require you to manually assign a spatial reference. In this case use a 6367.47km sphere for all data if possible. This GRIB1 sphere does not have an EPSG code.
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.
Further information:
The different file formats handle spatial reference information differently:
- GRIB1 only allows two possibilities - a spherical earth with radius 6367.47 km or an oblate spheroid earth with the major axis 6378.160 km, minor axis 6356.775 km and f = 1/297.0, as specified in the WMO GRIB Edition 1 specifications.
- GRIB2 allows additional spheroids, including custom ones. For more information see the WMO GRIB2 specifications, section 2.2.1 .
- ERA-Interim data provided in NetCDF format is converted on demand from the native GRIB format to NetCDF, so the spatial reference is inherited from the GRIB file.
At ECMWF the ERA-Interim data is produced and stored as a set of grid points on a reduced Gaussian grid on a sphere with radius 6367.47 km, as specified in the WMO GRIB Edition 1 specifications.
Note that other ERA products have different resolution:
ERA-Interim is produced and stored as spectral coefficients with a truncation of T255 or on the N128 reduced Gaussian grid (depending on the parameter), both on a sphere with radius 6367.47 km, as specified in the WMO GRIB Edition 1 specifications.
- 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.
For a list spectral, gaussian and eqivalent lat/lon grids see the Open IFS FAQ:
Further reading:
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)
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