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Strange behaviour masking (cdo mul)

Added by Matthias Demuzere over 7 years ago

Hi all,

I just bumped into a strange masking behaviour.
Basically I have two files:
- an input file in.nc
- a mask file 1_GHCN_mask_noAnt.nc (1 for land, missing data for ocean)

Both are on the same grid.

The idea is to mask in.nc with the 1_GHCN_mask_noAnt.nc:
cdo mul /media/Matthias_2/data/cmip5/griddata/1_GHCN_mask_noAnt.nc in.nc out.nc

Although Antartica is fully masked out in the mask, I still get zeros in the out.nc?
Though I would like to have all nan-areas in mask being nans in out.nc

I have used this before for other variables, and I never had this issue?
Seems to be so that 0 * NaN = 0? Is this right?
And if so, how can I deal with this?

Please note that zero's may occur over the land pixels, so these should be kept in the out.nc file.

Thanks!
M.


Replies (4)

RE: Strange behaviour masking (cdo mul) - Added by Ralf Mueller over 7 years ago

Matthias Demuzere wrote:

Hi all,
[...]

I have used this before for other variables, and I never had this issue?
Seems to be so that 0 * NaN = 0? Is this right?

yes, it is, documented on page 16 of the pdf-docu or here (first 2 paragraphs)

And if so, how can I deal with this?

you could

  1. use the ifthen operator or
  2. use zeros in your mask and do div instead of mul. this created missing values in the results field

hth
ralf

RE: Strange behaviour masking (cdo mul) - Added by Matthias Demuzere over 7 years ago

Hi Ralph,

Thanks! The ifthen option is indeed a nice one ...
I'll go with that!

Cheers,
M.

RE: Strange behaviour masking (cdo mul) - Added by Ralf Mueller over 7 years ago

you could also use the recent features of the expr operator

    (1-4/4)