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When you download ERA5 data you optionally can have the data interpolated 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).
Note that if you request data in NetCDF format ('format':'netcdf'), interpolation to a regular grid is mandatory. This is because ECMWF's NetCDF implementation only supports regular grids. Specify the grid as e.g. 'grid':'0.5/0.5'
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 you in any case round the resolution; we recommend to at least 0.3x0.3 deg. You can in principle specify 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.
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':'0.5/0.5'.
Visualisation of regular lat/lon data
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