I want to calculate daily mean of hourly temperature data from ERA5 data for 1975-2005. I have merged the downloaded hourly NETCDF files to create a single NETCDF file containing temperature and total precipitation data for all years. I think "daymean" operator would be suitable for this task but then I see another operator which is "ydaymean" for multi-year data in cdo. How are they both different and do they give different results? In which case using one can be beneficial over the other?
cdo -daymean -selvar,2 metre temperature infile outfile
or
cdo -ydaymean -selvar,2 metre temperature infile outfile
daymean is the simply the daily mean, whereas ydaymean gives you the daily mean over all years (the average of all Jan1, the average of all Jan2 etc) - just look at the dates in the output file and don't forget to check the online documentation, it is very well written.