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 use ERA5 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.

However, you might prefer to think of the ERA5 data as point data with a regular spacing, as shown on the right in plot (b): 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.

(a) Visualisation of regular lat/lon data as a continuous tiled surface

(b) Visualisation of regular lat/lon data as point matrix

 

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:

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:

For a list of spectral, Gaussian and equivalent lat/lon grids see the Open IFS FAQ > OpenIFS questions: general and runtime >   "What does the 'T' mean in 'T511', 'T1279' etc?"  and "How do I know the grid from from the 'T' number?"

 

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