# ERA Interim, how to handle total precipitation

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.

First of all, these netCDF files follow the CF Metadata Conventions, which describe the use of scale_factor and add_offset in section 8.1 Packed data of the conventions description. In short, you're applying them correctly:
However, I think that you've selected the wrong fields for your purpose. The total precipitation variable is an accumulation since the start of the re-forecast, and you've selected a monthly mean of accumulations over the first 3 hours of the 00Z re-forecasts and the same thing from the 12Z re-forecasts. This means that you're missing the accumulated precipitation from hours 3 to 12 of the re-forecasts, so your annual mean estimates might only be 1/4 of any ballpark values you're expecting. I suspect that you need to choose step=12 rather than step=3, as described in the ERA FAQ.