expr - the swiss army knife for computations with variables of a single file
expr is an operator for arithmetic expressions on multiple variables. It supports many function from the C math library as show in the documentation. A classic example is the computation of absolute value of velocity from its components u and v:
cdo expr,'abs_velocity=sqrt(u*u+v*v)' <ifile> <ofile>
Units are not handled by expr, but this can be done manually
cdo setunit,'m/s' -expr,'abs_velocity=sqrt(u*u+v*v)' <ifile> <ofile>
A more complex expression is shown in the tutorial:
cdo -expr,'ps=1013.25*exp((-1)*(1.602769777072154)*log((exp(z/10000.0)*213.15+75.0)/288.15))' 3dheights.nc out.nc
This is the essential part or the pressure computation of the stdatm operator. It possible to write multiple variables at once by separating expression with a semicolon. The temperature computation can be added like this:
cdo -expr,'P=1013.25*exp((-1)*(1.602769777072154)*log((exp(topo/10000.0)*213.15+75.0)/288.15));T=213.0+75.0*exp((-1)*topo/10000.0)' 3dheights.nc out.nc
This can be combined with the topo operator to compute surface pressure and temperature for a realistic height (over land):
cdo -f nc -expr,'P=1013.25*exp((-1)*(1.602769777072154)*log((exp(topo/10000.0)*213.15+75.0)/288.15));T=213.0+75.0*exp((-1)*topo/10000.0)-273.15' -setrtomiss,-100000,-0.0001 -topo out.nc
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If you already know expr, please reply your favorite example!