The native horizontal spatial reference for ERA Interim ERA5 data is a T255 reduced Gaussian grid, equivalent to a horizontal resolution of about 80km 31km or 0.75deg3deg. The native file format for ERA-Interim is GRIB, with all some parameters stored in GRIB1 formatas GRIB1 and others as GRIB2.
When you download ERA-Interim ERA5 data you can specify the output data to be in GRIB format or in NetCDF format. This determines your options for the horizontal resolution:
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