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:
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:
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].
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).
The lat/long equivalent of T639 is 0.28125 deg (360/(2*(639+1))). However, the GRIB1 format only supports three decimals, so we recommend that you round the resolution to 0.25 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'.
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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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.
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
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. |
