GRDMATH and RPN

Q&A
1

Hi,

I have two big netCDF files on which I’d like to perform a calculation to return a third one.

The operation is : 1.25 * 1e-3 * sqrt(u^2 + v^2) * u ;

I tried the following syntax and only computed the 1st day (or so it seems).
gmt grdmath 1.25 0.0013 MUL $u $u MUL $v $v MUL ADD SQRT MUL $u MUL = w.nc

(u and v are 3D data with longitude[1440], latitude[721] and time[14610]).

Thanks,
Guillaume

2

Well the operation would be

gmr grdmath $u $u MUL $v $v MUL ADD SQRT $u MUL 0.00125 MUL = w.nc

but GMT won’t be able to compute over 3D grids, only 2D layers and then it has no way to reconstruct back 3D stack.

3

Hi Guillaume,

If you’re using Linux or UNIX, one get of jail as painlessly as possible way would be to install the NetCDF Operators (https://github.com/nco/nco) and then use the ncap2 operator to do your calculation.

Cheers,

Tim

4

Hi,

Thanks for the both of you. I’ll give a shot to ncap2…

edit : I ended up using Matlab and saving A LOT of memory with this command :

    var_ID = netcdf.defVar(ncid,paramID,'NC_SHORT',[dimlon dimlat dimtime]);

Best,
Guillaume