Aggregate different netcdf variables and write output

Question

I have a netcdf files with 3 variables, nammed v1, v2, v3. I would like to aggregate those variables and create a new variable v4, as follow:

v4 = (v1*0.1)+(v2*0.2)+(v3*0.2)

I know how to aggregate 2 variables using cdo :

cdo expr,’sum=var1+var2;’ ifile ofile

But how could I handle more specific netcdf calculations like the one required?

Answer 1

CDO will be able to handle the formula you require:

cdo expr,’v4=(v1*0.1)+(v2*0.2)+(v3*0.2)’ ifile ofile

If you want to add the variable to the files, just do this:

cdo aexpr,’sum=var1+var2;v4=(v1*0.1)+(v2*0.2)+(v3*0.2)’ ifile ofile

Answer 2

By default, NCO propagates the metadata from the first variable on the RHS to the LHS so v4 gets the same attributes as v1 in Adrian's example. You can change or augment those attributes in the same ncap2 command using the at-sign @ to name attributes, e.g.,

ncap2 -O -s 'v4=(v1*0.1)+(v2*0.2)+(v3*0.2);v4@long_name="Fourth variable";v4@units="meters"' in.nc out.nc

Answer 3

you can also use ncap2 as an alternative

ncap2 -O -s "v4=(v1*0.1)+(v2*0.2)+(v3*0.2)" ifile.nc ofile.nc

Although I would usually use the cdo solution as suggested by Robert Wilson, the nice thing with nco is the ability to define all the metadata too:

ncatted -O -a units,v4,c,c,"units goes here" ofile.nc
ncatted -O -a long_name,v4,c,c,"long_name goes here" ofile.nc

EDIT: As Charlie correctly points out in his answer to this question, I should have highlighted the fact that ncap2 ports metadata from the source variables, so if metadata such as units already exists you need to either specify the new values in the ncap command as per his answer, or use ncatted in "modify" mode for those attributes.