[gradsusr] gr2stn doesn't work

[Djordje Romanic]

Hello,

I am calculating the pressure gradients (PGs) from the NCEP/NCAR reanalysis
I MSLP data. The resolution of the data is 2.5 x 2.5 degrees. I want to
interpolate PGs to a particular coordinate that is not on the reanalysis
grid. I was thinking to use gr2stn function for that but it does not work.
This is my code:

'sdfopen 1948_1950.nc'
'define dx = 6.371e3 * cos(lat * 3.14159/180) * cdiff(lon * 3.14159/180, x)'
'define dy = 6.371e3 * cdiff(lat * 3.14159/180, y)'
'define pg = mag(cdiff(slp,x)/dx, cdiff(slp, y)/dy)'

'd gr2stn(pg,20.4667,44.8167)'  *This coordinate is just an example


This is the error message:

Error from GR2STN: 2nd argument is not a station expression
Operation Error:  Error from gr2stn function
  Error ocurred at column 1
DISPLAY error:  Invalid expression
  Expression = gr2stn(pg,20.4667,44.8167)

Please could you help me with this?

Thanks,
DjR
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