[gradsusr] vorticity potential

[morteza Miri]

hi 
I am a 
student Climatology.
Wanted to 
grads script calculate potential vorticity. With an example for 20 jun 1962,level 
500hp. I need to have the 
potential vorticity data.Where can I get?
I need potential vorticity datafor pressure level . 

but Site (ncep/ncar)
data are sigma levels.
   How I can convert variables in sigma to 
pressure
for nc data?
6e +06 or-5e 
+06 What Analysis is the the potential vorticity maps?
Thanks in advanc

**Let me to consider an exact level for computing PV  
(e.g. 200 hPa)
**The script looks like This:
****** Open the zonal
 and meridional wind, temperature and it calculates Potential Vorticity 
(PV)
 'reinit'
 'sdfopen uwind1962jun10.nc'
 'set defile 1'
 'define
 u = uwnd'
 'sdfopen vwind1962jun10.nc'
 'set defile 2'
 'define
 v = vwnd'
 'sdfopen air1962jun10.nc'
 'set
 defile 3'
 'define temp = air'
  'set time 10jun1962'
  'set 
lat 0 88'
  'set lon -19 120'
  'set lev 500'
* ########## 
Coriolis calculation
  'define coriol=2*7.29e-5*sin(lat*3.1415/180)'
*
 ########## Relative Vorticity calculation
  'define vort = 
hcurl(u,v)'
* ########## Potential Temperature (theta)calculation
 
 'define 
dt=temp(z-1)*pow(1000/lev(z-1),0.286)-temp(z+1)*pow(1000/lev(z+1),0.286)'
 
 'define
 dp=100*(lev(z-1)-lev(z+1))'
* ########## differential of 
theta/pressure
  'define dtdp=dt/dp'
* ########### PV calculation 
on pressure surface
  'define pv = -9.8*(coriol+vort)*dtdp'
 'set 
gxout contour'
 'd pv'
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