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For ERA-Interim the point interval on the native Gaussian grid is about 0.75 degrees (with the exception of Ocean-Wave data which are natively stored on the wave model’s reduced 1.0x1.0 degrees latitude/longitude grid). You can specify a custom grid on the data server web interface, or using the ECMWF WebAPI or using the MARS client (if you have access to it). On the web interface the default grid for ERA-Interim is lat/long, with a default resolution of 0.75x0.75 degrees (about 80km), approximating the irregular grid spacing on the native Gaussian grid.
For ERA5 test data the point interval on the native Gaussian grid is about 0.3 degrees. You can download ERA5 test data using Python and specify a custom grid and resolution in your script. You should set the horizontal resolution to no higher than 0.3x0.3 degrees (about 30km), approximating the irregular grid spacing on the native Gaussian grid.
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