[ferret_users] potential vorticity from OGCM

[prescilla kurien <prescy@xxxxxxxxxxxxxxxxx>]

Dear ferret users:
I am in need of calculating Potential vorticity from OGCM
I have found the same topic by Mr. Paulo
http://www.pmel.noaa.gov/maillists/tmap/ferret_users/fu_2001/msg00437.html

But I am having OGCM results which is continuously stratified.

The code I prepared is as below

use 2014_hori_U_AM500_AH250.nc
use 2014_hori_V_AM500_AH250.nc
set region/X=78E:94E/Y=6N:23N/L=8/K=10
use density.nc
LET zeta=(VVEL[d=2,x=@ddc])-(UVEL[d=1,y=@ddc])  ! relative vorticity
let deg2rad=(2*3.14)/360
let omega = 7.292e-5
let f=2*omega*sin(deg2rad*Y[g=UVEL[d=1]]) !planetary V
let rho_zero=1026 ! referece density
let grad=DENSITY[z=@ddc] ! density gradient
fill (grad*(zeta+f))/rho_zero

But the plot looks strange for me. I think my code went wrong somewhere.
If any of you have experience with vorticity calculation from
continuously stratified ocean,kindly help me..Thanks in advance 
Prescilla