center_finite_diff_n for relative vorticity

Dear ncl users,
Could I have some help? I am still learning ncl.
I am getting confused with using the center_finite_diff_n command to
calculate relative vorticity (RV) with non-cyclic data.
I want to calculate (RV) using center_finite_diff_n, instead of just
resorting to using the uv2vr_cfd command that is available, because I
want to calculate RV and then shear vorticity separately.
I am following example 6 at
http://www.ncl.ucar.edu/Document/Functions/Built-in/center_finite_diff_n.shtml.

1) Do dv/dx and dvdy need to be in the same loop or can they be
separate as I have done?
2) Since we want to try and avoid do loops in ncl, is there a way not
to use loops for this calculation
Any amount of advice, would be greatly appreciated.
Sincerely,
Bold

[ 02:42:16 ] > ncl tester.ncl
 Copyright (C) 1995-2010 - All Rights Reserved
 University Corporation for Atmospheric Research
 NCAR Command Language Version 5.2.1
 The use of this software is governed by a License Agreement.
 See http://www.ncl.ucar.edu/ for more details.
fatal:Undefined identifier: (dudy) is undefined, can't continue
fatal:Execute: Error occurred at or near line 30 in file tester.ncl

fatal:Variable (dudy) is undefined
fatal:Execute: Error occurred at or near line 33 in file tester.ncl

fatal:Variable (vor) is undefined
fatal:Execute: Error occurred at or near line 35 in file tester.ncl

[ 02:48:25 ] >

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"
;;; some contants
pi = 4.0*atan(1.0)
radian = pi/180.0
radius = 6370949.0
FILEIN = "era_uv.nc"
;=========================variables
a = addfile(FILEIN,"r")
U=a->U
u=U(0,{700},:,:)
V=a->V
v=V(0,{700},:,:)

lat = a->lat
lon = a->lon
nlat = dimsizes(lat)
mlon = dimsizes(lon)
dlon = (max(lon)-min(lon))/(mlon-1)
dlat = (max(lat)-min(lat))/(nlat-1)
; Arrays
dvdx = new ( (/nlat,mlon/), typeof(v), v@_FillValue)
dudx = new ( (/nlat,mlon/), typeof(v), v@_FillValue)
; loops
do nl=0,nlat-1
  dX = radius*cos(radian*lat(nl))*dlon
  dvdx(nl,:) = center_finite_diff_n (v(nl,:), dX, False, 0, 0)
end do
 do ml=0,mlon-1
   dY = radius*radian*dlat
   dudy(:,ml) = center_finite_diff_n (v(:,ml), dY, False, 0, 1)
 end do
vor = dvdx - dudy

printMinMax(vor,True)
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Received on Sun Dec 19 12:56:03 2010