Maths and grid manipulation

Q&A
1

Hi,
I have a netcdf file (time, lon, lat) on which I’d like to run these operations :

  1. For each latitude of a subdomain, run a FFT2 : f(time,longitude) => ˆf(frequency,wavenumber) with

    • the total spectrum
    • the symmetrical part of the spectrum
    • the anti-symmetrical part of the spectrum

  2. Filter the summed total spectrum from (2) a little

  3. Divide (1) by (2) and plot

For (1)

I think grdfft does the job without specific options, so I assume I just need to run a loop such as

for lat in {lat1..lat2}; do
    gmt grdcut file.nc -Rlon1/lon2/$lat/$lat -JM10c -Gsubset_$lat.nc
    gmt grdfft subset_$lat.nc -Goutput_$lat.nc
done

Then for

  • the total spectrum :
    => mean( Z{lat1:lat2} )
gmt grdmath output_*.nc -S MEAN = total.nc
  • the anti-symmetric part :
    => mean( [Z{lat1:0} - Z{0:lat2}]/2 )
for lat in {lat1...0}; do
    gmt grdmath output_$lat.nc ouput_$($lat+xx).nc SUB 2 DIV = anti_$lat.nc
done
gmt grdmath anti_*.nc -S MEAN = anti.nc
  • the symmetric part :
    => mean( [Z{lat1:0} + Z{0:lat2}]/2 )
for lat in {lat1...0}; do
    gmt grdmath output_$lat.nc ouput_$($lat+xx).nc ADD 2 DIV = symm_$lat.nc
done
gmt grdmath symm_*.nc -S MEAN = symm.nc

Is this even correctly translated to grdmath ?

For (2)

a simple use of grdfilter with -Fb I assume, but which option corresponds best to a 1-2-1 filter ?

For (3)

And finally

gmt grdmath anti.nc filtered.nc DIV = anti_norm.nc
gmt grdmath symm.nc filtered.nc DIV = symm_norm.nc

gmt begin diagram png
gmt subplot 1x3 -R-15/15/0.001/0.5 -JX10c/10c+l
    gmt subplot set 
        gmt grdimage filtered.nc
    gmt subplot set
        gmt grdimage anti_norm.nc
    gmt subplot set
        gmt grdimage symm_norm.nc
gmt subplot end 
gmt end show

Do you have any input on steps (1) and (2) ?

Thanks,

[EDIT] : I forgot to mention an important part … the process is supposed to use a moving window of XX days for the FFT…