How to calcualte potential vorticity

Dear NCL users,
I want to compute the Potential vorticity with the help of NCEP reanalysis data. I don't know how to calculate it in ncl? Please if you have any script/example file send me. My script is given below:
Best regards,
kalim

********************
load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_code.ncl"
load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_csm.ncl"
load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/contributed.ncl"
begin
;************************************************
; variable and file handling
;************************************************
in = addfile("uwnd.2005.nc","r") ; open netcdf file
f = addfile("vwnd.2005.nc","r")
u = short2flt(in->uwnd(152,{200},:,:)) ; pull u off file
v = short2flt(f->vwnd(152,{200},:,:)) ; pull v off file
;************************************************
; calculate vorticity on a Gaussian Grid
; scale for visual convenience
;************************************************
scale = 1.e05
vrt = u ; retain coordinates
vrt = uv2vrG_Wrap(u,v) * scale
vrt@long_name = "vorticity"
vrt@units = "scaled"
f=2*7.292*1.e-05*sin(phi)
theta= 300
pv=g(vrt+f)*d(theta)/dp
;************************************************
; create plot
;************************************************
wks = gsn_open_wks("x11","vort") ; specifies a ps plot
gsn_define_colormap(wks,"BlWhRe") ; choose colormap
res = True ; plot mods desired
res@cnFillOn = True ; turn on color
res@gsnSpreadColors = True ; use full range of color map
res@lbLabelAutoStride = True ; nice label bar labels
res@cnLinesOn = False ; no contour lines
res@cnLevelSelectionMode = "ManualLevels" ; set manual contour levels
res@cnMinLevelValF = -5 ; set min contour level
res@cnMaxLevelValF = 5 ; set max contour level
res@cnLevelSpacingF = 0.5 ; set contour spacing
res@lbOrientation = "vertical"
res@mpMinLatF = 0 ; range to zoom in on
res@mpMaxLatF = 40.
res@mpMinLonF = 30.
res@mpMaxLonF = 120.

plot = gsn_csm_contour_map_ce(wks,vrt(:,:),res) ; create plot
end

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Received on Thu Oct 15 08:38:51 2009