zero vector field error

hi,
I want to get the mean vector between 2 layers , so i create a new u,v to see the mean direction and magnitude. while the script error is as follows:

warning:StreamlinePlotInitialize: zero vector field; StreamlinePlot not possible:[errno=1103]

Any suggestions will be appreciate. Thanks
JinQ

--
JinQ,                          M.S. 
Ocean University of China
--
scripts:
load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_code.ncl"
load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_csm.ncl"
load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/contributed.ncl"
load "$NCARG_ROOT/lib/ncarg/nclscripts/wrf/WRFUserARW.ncl"
begin
;
; The WRF ARW input file. 
; This needs to have a ".nc" appended, so just do it.
  a = addfile("wrfout_d03_2010-09-09_12:00:00o.nc","r")
  type = "pdf"
;  type = "eps"
  wks = gsn_open_wks(type,"mean")
; Set some basic resources
  res = True
  res@gsnMaximize=True
  res@gsnDraw              = False    ; let gsn_panel do plotting
  res@gsnFrame             = False
 ;res@pmTickMarkDisplayMode="Always"
  res@tmXBTickSpacingF= 2
  res@tmYLTickSpacingF= 2
  res@tmXBLabelFontHeightF=0.025
  res@tmYLLabelFontHeightF=0.025
  res@tmXBMinorOn        = False           ; no lon minior tickmarks
  res@tmYLMinorOn        = False
  res@mpDataSetName         = "Earth..4"
  res@mpDataBaseVersion     = "MediumRes"
  res@mpOutlineSpecifiers   = (/"China:states","India","Taiwan"/)
  res@mpLimitMode         = "LatLon"
  res@mpMinLatF           = 22
  res@mpMaxLatF           = 28.1
  res@mpMinLonF           = 115.7
  res@mpMaxLonF           = 120.7
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
  times  = wrf_user_list_times(a)  ; get times in the file
  ntimes = dimsizes(times)         ; number of times in the file
; The specific pressure levels that we want the data interpolated to.
  pressure_levels = (/ 850., 200./)   ; pressure levels to plot
  nlevels         = dimsizes(pressure_levels)           ; number of pressure levels
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
  do it = 12,ntimes-36,12              ; TIME LOOP
    print("Working on time: " + times(it) )
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;   
    u  = wrf_user_getvar(a,"ua",it)        ; u averaged to mass points
    v  = wrf_user_getvar(a,"va",it)        ; v averaged to mass points
    p  = wrf_user_getvar(a, "pressure",it) ; pressure is our vertical coordinate
    z  = wrf_user_getvar(a, "z",it)        ; grid point height
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
      pressure0 = pressure_levels(0)
      pressure1 = pressure_levels(1)
;      z_plane  = wrf_user_intrp3d( z,p,"h",pressure,0.,False)
      u_plane0  = wrf_user_intrp3d( u,p,"h",pressure0,0.,False)
      v_plane0  = wrf_user_intrp3d( v,p,"h",pressure0,0.,False)
      u_plane1  = wrf_user_intrp3d( u,p,"h",pressure1,0.,False)
      v_plane1  = wrf_user_intrp3d( v,p,"h",pressure1,0.,False)
u_planeb=wgt_areaave(u_plane1,1.,1.,0)-wgt_areaave(u_plane0,1.,1.,0)
v_planeb=wgt_areaave(v_plane1,1.,1.,0)-wgt_areaave(v_plane0,1.,1.,0)
u_plane=new(dimsizes(u_plane0),float)
v_plane=new(dimsizes(v_plane0),float)
copy_VarMeta(u_plane0,u_plane)
copy_VarMeta(v_plane0,v_plane)
u_plane=(/u_planeb/)
v_plane=(/v_planeb/)
      u_plane = u_plane*1.94386     ; kts
      v_plane = v_plane*1.94386     ; kts
      u_plane@units = "kts"
      v_plane@units = "kts"
lat2d=a->XLAT(0,:,:)
lon2d=a->XLONG(0,:,:)
u_plane@lat2d=lat2d
u_plane@lon2d=lon2d
v_plane@lat2d=lat2d
v_plane@lon2d=lon2d
      ; Plotting options for Wind Vectors                
        opts = res         
        opts@vcGlyphStyle = "CurlyVector"  
    opts@vcMinDistanceF           =0.02
    opts@vcRefLengthF             = 0.045
;    opts@vcRefMagnitudeF          = 20.0
    opts@vcRefAnnoOrthogonalPosF  = -1.
        vector = gsn_csm_vector_map_ce(wks,u_plane,v_plane,opts)
        draw(vector)
        delete(opts)
    frame(wks)
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
  end do        ; END OF TIME LOOP
end

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Received on Tue Jan 10 00:15:11 2012