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For data in GRIB1 format the earth model is a sphere with radius = 6367.47 km, as defined in the WMO GRIB Edition 1 specifications, GDS Octet 17
For data in GRIB2 format the earth model is ...GRIB2 allows additional spheroids, including custom ones. For more information see a sphere with radius = 6371.2290 km, as defined in the WMO GRIB2 specifications, section 2.2.1, Code Table 3.2, Code figure 6.
For data in NetCDF format (i.e. converted from the native GRIB format to NetCDF), the earth model is inherited from the GRIB data.
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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 with the NetCDF format, our NetCDF implementation only supports regular grids, so when if you request data in NetCDF format ('format':'netcdf'), the data is automatically interpolated from the native Gaussian grid to a regular lat/long grid, which yuo have to specify (eginterpolation 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.
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