Errors in dy calculation before using center_finite_diff_n

Dear NCL-users,

I am trying to calculate dq/dx and dq/dy using "center_finite_diff_n" function.
For the same, I considered

dlon = (lon(2)-lon(1))*0.0174533
dlat = (lat(2)-lat(1))*0.0174533

do nl=0,nlat-1
   dX = 6378388.*cos(0.0174533*lat(nl))*dlon
   dqdx(:,nl:nl,:) = center_finite_diff_n (q(:,nl:nl,:), dX , True,0,2)
end do

do ml=0,mlon-1
   dY = 6378388.*lon(ml)*dlat ; I have also tried dY = 6378388.*cos(0.0174533*lon(ml))*dlat
   dqdy(:,:,ml:ml) = center_finite_diff_n (q(:,:,ml:ml), dY , True,0,1)
end do

The above calculates the dq/dx term reasonably but gives unrealistic dq/dy values.
I think probably the dY calculation is not correct.
What is the error in the above few lines?
Kindly point out the mistake and help me to solve the problem.

Thanking you.

Regards,
Abhik.
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Abhik Santra
CSIR Research Fellow,
Indian Institute of Tropical Meteorology,
Pune - 411008.
India
___________________________________________
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Received on Tue May 28 13:50:35 2013