CDO

With CDO release 1.7.1 the expr (and the related exprf, aexpr, aexprf) recieve on important update: the ability to include meta data into user defined computations

• Calculating with horizontal coordinates: With the two new functions clon(V) and clat(V) the user has access to the longitudes and latitudes of a given variable V. Suppose the both coordinate fields are given in degrees, this can be used to calculate a global wind field based on a global sin pattern

  cdo shaded,device=png -expr,'wind_u=sin(2*rad(clat(topo)))' -topo analytic_forcing 

  which leads to westward winds on northern and eastward winds on southern hemisphere
  
  If this is not enough, lets combine it with a cosin longitude pattern:

  cdo shaded,device=png -setunit,m/s -expr,'wind_u=sin(2*rad(clat(topo)))*cos(2*rad(clon(topo)))' -topo analytic_forcing

  to get circular regions with different wind directions
  
• Calculating with vertical coordinates: With the new function clev(V), the vertical coordinates of a variable V can be read. Lets compute the potential temperator! Instead of looping over the list vertical levels

  for level in $(cdo showlevel -select,name=temp ifile); do
    cdo -f nc -expr,"pot_temp=temp*((100000/${level})^0.287)" -sellevel,${level} ifile potTemp_${level}.nc
  done
  cdo merge pottemp_?????.nc pottemp_??????.nc pottemp.nc
  rm pottemp_??????.nc pottemp_?????.nc 
  

  the whole procedure can be written in a single line without temporal files

  cdo expr,"pottemp=temp*((100000/clev(temp))^0.287);"  ifile pottemp.nc

• Calculating with temporary data: Variables starting with an underscore, will not be written into the result file. This can increase the readabiliy of the formulars, while keeping the output file clean

  cdo-dev -f nc -expr,'_xSquare = sin(rad(clat(topo)))^2; _ySquare = cos(rad(clat(topo)))^2; constant1=sqrt(_xSquare + _ySquare)' -topo constant_1.nc
  
  cdo infov constant_1.nc
      -1 :       Date     Time   Level Gridsize    Miss :     Minimum        Mean     Maximum : Parameter name
       1 : 0000-00-00 00:00:00       0   259200       0 :      1.0000      1.0000      1.0000 : constant1     
  

• Calculate with cdo-internal functions: Vertical and horizontal operations like fldmean, vertmean, fldstd or vertmax can now be used in formulars. Lets scale a temperature (K) with its maximum value

  cdo -f nc -infov -aexpr,'h=temp/fldmax(temp)'  -temp
      -1 :       Date     Time   Level Gridsize    Miss :     Minimum        Mean     Maximum : Parameter name
       1 : 0001-01-01 00:00:00       0   259200       0 :      220.85      276.72      317.30 : temp          
       2 : 0001-01-01 00:00:00       0   259200       0 :     0.69603     0.87210      1.0000 : h
  

For further options, please have a look here