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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 (a) on the right. In this case think of the data values as referring to the centroids of the tiles.However, the ECMWF interpolation software does not conserve area integrals, so interpolated values are best regarded as point values, with a regular spacing, as shown in (ba): here global ERA5 data was downloaded with interpolated to a regular lat/lon grid and a with resolution r = 0.25 deg, and plotted on top of a satellite image with 0.20 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. 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 a continuous tiled surfacepoint matrix
(b) Visualisation of regular lat/lon data as point matrix
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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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