The native horizontal spatial reference for ERA5 HRES data is a T639, N320 reduced Gaussian grid, equivalent to a horizontal resolution of about 31km or 0.3deg. When you download ERA5 data this is the default horizontal resolution, but optionally you can have the data interpolated to a custom horizontal resolution.
The native file format for ERA5 is GRIB, but when you download ERA5 data you can choose between the native GRIB format or have the data converted to NetCDF format. This determines your options for the output horizontal resolution:
- If you specify GRIB as output format you will get a GRIB2 file (for model level parameters) or GRIB1 file (for other parameters, unless otherwise indicated). By default the output will be on the native T639 grid, but you can specify a different Gaussian grid or a lat/long grid:
- If you specify a Gaussian grid other than the default T639 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 T639 is 0.28125 deg (360/(2*(639+1))). You could use this resolution for parameters stored in GRIB2 format, but GRIB1 format only supports three decimals, so we recommend you round the resolution; we recommend to 0.3x0.3 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 a regular lat/long grid. 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 T639 is 0.28125 deg (360/(2*(639+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 0.3x0.3 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 ERA5 data as point data with a regular spacing, as shown on the right: here global ERA5 data was downloaded with a regular lat/lon grid and a resolution r of 0.3 deg, and plotted on top of a satellite image with 0.25 degree image resolution. The 'top left' ERA5 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. |
For ERA5 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 has been interpolated from a combination of SRTM30 and other elevation datasets (for details see Part IV. Physical processes, of the IFS model documentation, Chapter 11.2.2 Surface elevation data at 30 arc seconds) The SRTM30 orography data is referenced in the horizontal with respect to the WGS84 ellipse (which defines the data major/minor axis) ...
<adapted for ERA5 until here>
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 T639 or on the N320 reduced Gaussian grid (depending on the parameter). See ERA-Interim: What is the spatial reference
- ERA-40 has a resolution of T159 (triangular truncation of 159), N80 (80 latitude circles, pole to equator), L60 (model levels).
- ERA-15 has a resolution of T106 with 31 vertical hybrid levels.
For a list of spectral, Gaussian and equivalent lat/lon grids see the Open IFS FAQ > OpenIFS questions: general and runtime > " and "
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