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For a list of spectral, Gaussian and equivalent lat/lon grids see the Open IFS FAQ > OpenIFS questions: general and runtime > , and there "What does the 'T' mean in 'T511', 'T1279' etc?" and "How do I know the grid from from the 'T' number?"
When you download ERA5 data, the grid geometry of your output data depends on the selected data format:
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