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Grid geometry
ERA5 data is produced and archived as spectral coefficients or on a reduced Gaussian grid, which has quasi-uniform spacing over the globe. Reduced Gaussian grids have a series of evenly spaced data points along each parallel (latitude), and parallels spaced at quasi-regular intervals. Near the poles there are only a few points along a parallel, but close to the equator there are many more data points along a parallel.
A reduced Gaussian grid is quite different from a Cartesian grid (a two-dimensional rectangular grid).
The ERA5 HRES (High Resolution) and EDA (Ensemble) data are archived in these grid representations:
- Some (upper air) parameters are archived as spectral coefficients (indicated in the data as 'sh', Spherical Harmonics) with a triangular truncation of T639 (HRES) and T319 (EDA)
- Some (including all the surface) parameters are archived on a reduced Gaussian grid with a resolution of N320 (HRES) and N160 (EDA). These grids are so called "linear grids", sometimes referred to as TL639 (HRES) and TL319 (EDA). N320, for example, refers to the number of latitude circles from pole to equator.
For wave data, both HRES and EDA are archived on a reduced latitude/longitude grid.
For a list of spectral, Gaussian and equivalent lat/lon grids see the Open IFS FAQ, section "OpenIFS questions: general and runtime", and there "What does the 'T' mean in 'T511', 'T1279' etc?" and "How do I know the grid from the 'T' number?"
When you download ERA5 data, the grid geometry of your output data depends on the selected data format:
- If you request the data in the native GRIB format it is delivered with the above grid geometry.
- If you request the data in NetCDF format, it is automatically converted and interpolated from the above grids to a regular lat/lon grid.
Coordinate system
All gridded data is made available in Decimal Degrees, lat/lon, with latitude values in the range [-90;+90] referenced to the equator and longitude values in the range [0;360] referenced to the Greenwich Prime Meridian. Some software applications automatically display these longitudes in the range [-180;+180].
Interpolation
When you download ERA5 data you have the option to interpolate the data to a custom grid and horizontal resolution (eg. 'grid':'0.5/0.5'). The
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:
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default interpolation method is bilinear for continuous parameters (e.g. Temperature) and nearest neighbour for discrete parameters (eg. Vegetation).
The lat/long equivalent of
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T639 is 0.28125 deg (360/(2*(
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639+1))).
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However, the GRIB1 format only supports three decimals, so we recommend that you
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round the resolution
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to 0.
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25 deg (giving 360/0.25=1440 equally-spaced grid points). Specifying a higher resolution is technically possible, e.g. as 'grid':'0.1/0.1', but this only oversamples the data and does not improve the accuracy of the data. Specifying a coarser resolution is also possible, but in this case, care should be taken to avoid aliasing.
If you request data in NetCDF format ('format':'netcdf'), interpolation to a regular grid is mandatory, because ECMWF's NetCDF implementation only supports regular grids. Specify the desired lat/lon grid with the 'grid' keyword, for example 'grid':'0.25/0.25'.
Visualisation of regular lat/lon data
If you use ERA5 data in a regular lat/lon grid, think of the data as point values, with a regular spacing, as shown in (a): here global ERA5 data was interpolated to a regular lat/lon grid with resolution r = 0.25 deg. The 'top left' ERA5 data point is always |
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For ERA-Interim data in all representations the assumed underlying earth model (the geodetic datum) is a sphere with with radius 6367.47km. This applies to ERA-Interim data in all supplied file formats (GRIB1, GRIB2, and NetCDF) and in both grid types (Gaussian and lat/lon). Longitudes range from 0 to 360, which is equivalent to -180 to +180 in Geographic coordinate systems.
at Longitude=0 ; Latitude=90, with further grid points spaced |
by r, and the 'bottom right' grid point at Longitude=360-r ; Latitude=-90. |
Many software applications by default visualise regularly spaced data as a continuous tiled surface, as in (b). If you use this visualisation, think of coordinates as referencing the centroids of the tiles. However, the ECMWF interpolation software does not conserve area integrals, so this visualisation can be misleading. We recommend you conceptualise ERA5 data as in (a). | (a) Visualisation of regular lat/lon data as point matrix
(b) Visualisation of regular lat/lon data as a continuous tiled surface
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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 ERA-Interim data if possible.
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.
For users of GIS software, The This GRIB1 sphere does not have an EPSG code.
Background 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.
Comparison with other ERA products
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). See ERA-Interim: What is the spatial reference
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- 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)
- , N80.
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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 and Contribution Agreement signed on 22/07/2021). 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. |
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