I am trying to compute accumulated annual precipitation for year 2000-2001 based on the ERA Interim, but I am stuck with some unresolved questions:
If I download the synoptic monthly means of "total precipitation" from ECMWF for year 2000-2001 with "Select time" set to "00:00:00" and "12:00:00" respectively, and chose "Select step" as "3", I get a *.nc -file with 48 time steps. The file also contains a "scale_factor" and "add_offset". I assume that I must use those values in (add_offset + (downloaded data * scale_factor)) to get actual precipitation in meters? And I assume that if I sum the 48 time steps of data (corrected with add_offset and scale_factor) and divide by 2 (2 years of data) then I get the average accumulated precipitation per year. Is this correct?
What then puzzles me is that the "scale_factor" is very low (3.9E-8), meaning that the values I get will be almost equal to the "add_offset" which is 0.0013. Are there any obvious mistakes I make? Or do any of you know of a guide that is more elaborate than the documentation found at ECMWF?
Thanks a lot in advance for any ideas.