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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.
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- 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.
Grid resolution
The ERA5 HRES (High Resolution) data has a native resolution of 0.28125 degrees (31km), and the EDA (Ensemble) has a resolution of 0.5625 degrees (62km).
The wave HRES (High Resolution) data has a native resolution of 0.36 degrees, and the wave EDA (ensemble) has a resolution of 1.0 degrees (EDA).
Projection
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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].
Earth model
For data in GRIB1 format the earth model is a sphere with radius = 6367.47 km, as defined in the WMO GRIB Edition 1 specifications, Table 7, GDS Octet 17
For data in GRIB2 format the earth model is a sphere with radius = 6371.2290 km, as defined in the WMO GRIB2 specifications, section 2.2.1, Code Table 3.2, Code figure 6.
For data in NetCDF format (i.e. converted from the native GRIB format to NetCDF), the earth model is inherited from the GRIB data.
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 default interpolation method is bilinear for continuous parameters (e.g. Temperature) and nearest neighbour for discrete parameters (eg. Vegetation).
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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 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 data if possible. This GRIB1 sphere does not have an EPSG code.
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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
- ERA-40 has a resolution of T159, N80.
- ERA-15 has a resolution of T106, 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 agreementAgreement 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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