[gradsusr] question about meridional gradient computation in GRADS

[Lyndon Mark Olaguera]

Dear All,

I am trying to compute the meridional gradient of of potential temperature
(925hPa).
Then create a latitude-time plot.
I am getting very small magnitudes of the gradient which is not consistent
with the literature that I am following (
http://wwwoa.ees.hokudai.ac.jp/people/yamazaki/D/Dthesis_yOkada.pdf).

Here's the script that I am using:
'sdfopen /mnt/dat/wind/vwind/10m/pentad_ensmean_vwnd_surf_1981-2010.nc'

'set display color white'
'c'
'set gxout shaded'
'set mpdset hires'
'set grid off'
'set grads off'
'set parea 0.5 8 2 10.5'
'set font 1'
'set lat -90 90'
'set lon 0 360'
'set dfile 1'
'set t 1 73'
'define t=pottmp'
'set dfile 2'
'set t 1 73'
'define u=uwnd'
'set dfile 3'
'set t 1 73'
'define v=vwnd'

*'define dty = cdiff(t,y)'*
*'define Re = 6.37e6'*
*'define dy = Re*cdiff(lat,y)*(3.1416/180)'*
*'define mergrad = -1*v*dty/dy'*

'set t 1 73'
'set lat 15 45'
'set x 1'
'set xyrev on'
'set xaxis 1 73'
'define a = ave(mergrad,lon=110,lon=120)'
'd a'
'cbarn'
'printim test.png'
'quit'

Attached is the output image. In the literature,the unit is between -3 and
3, while in my case its x10-5.


Any suggestions on how I can do this correctly?


Lyndz
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