Fwd: Wind barb spacing

Hi NCL users,

I am using "vcMinDistanceF" to control wind barb spacing from GFS data.
according to my understanding when i want plot the whole world map

-180 to 180 longitude is mapped to 0-1 in NDC space
and -90 to 90 latitude is mapped to 0-1 in NDC space

if so and if i want to draw a wind barb each 45°, vcMinDistanceF shoul=
d be
0.125
and when testing this i get a kind of random spacing (please see attached
image)

So is there any solution to control barb spacing according geographical
distance (in degrees).

Thank you for help.
below is you find my ncl script used :

;************************************************
load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_code.ncl"
load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_csm.ncl"
;************************************************
begin

;************************************************
; read in GRIB2 file
;************************************************
  a = addfile("gfs.t00z.pgrb2f00.grib2","r")
;************************************************
;;; get variables: temperature, longitude, latitude

    t2m = a->TMP_P0_2L108_GLL0(:,:)
    lon = a->lon_0
    lat = a->lat_0
    uu= a->UGRD_P0_L6_GLL0
    vv= a->VGRD_P0_L6_GLL0


if (any(uu.lt.1000.)) then
       x1D = ndtooned (uu) ; convert to 1D array
       i50 = ind(x1D.lt.1000.) ; all indices x1D > 50
       x1D(i50) = -30 ; set all array syntax
       uu = onedtond(x1D, dimsizes(uu)); return to the original ar=
ray
       delete (x1D) ; x1D no longer needed
       delete (i50) ; i50 no longer needed
   end if

if (any(vv.lt.1000.)) then
       x1D = ndtooned (vv) ; convert to 1D array
       i50 = ind(x1D.lt.1000.) ; all indices x1D > 50
       x1D(i50) = -30 ; set all array syntax
       vv = onedtond(x1D, dimsizes(vv)); return to the original ar=
ray
       delete (x1D) ; x1D no longer needed
       delete (i50) ; i50 no longer needed
   end if

;

; create plot
;************************************************
wks_type="png"

  wks_type@wkWidth = 1384
   wks_type@wkHeight = 1384
  wks_type@wkBackgroundOpacityF = 1.0


  wks = gsn_open_wks(wks_type,"wind_barb") ; open a ncgm file
  gsn_define_colormap(wks,"gui_default")

setvalues wks
 "wkBackgroundColor" : (/1.,1.,1./)
end setvalues


  res = True ; plot mods desired
res@pmTickMarkDisplayMode = "Always"
  res@mpProjection = "Mercator" ; choose projection
  res@mpGridAndLimbOn = True ; turn on lat/lon lines
  res@mpPerimOn = True ; turn off box around plot
  res@mpGridLatSpacingF = 45. ; spacing for lat lines
  res@mpGridLonSpacingF = 45. ; spacing for lon lines
  res@mpFillOn = False
   res@mpLimitMode = "LatLon"
    res@mpMinLatF = 85.0
    res@mpMaxLatF = -85.0
    res@mpMinLonF = -180
    res@mpMaxLonF = 180

  res@cnFillOn = False ; color plot desired
  res@cnLineLabelsOn = False ; turn off contour lines

  res@vpXF = 0 ; make plot bigger
  res@vpYF = 1
  res@vpWidthF = 1
  res@vpHeightF = 1


  res@vcGlyphStyle = "WindBarb" ; select wind barbs
  res@vcRefMagnitudeF = 1 ; make vectors larger
  res@vcRefLengthF = 0.03 ; ref vec length
  res@vcWindBarbLineThicknessF = 8.0 ;Thickness


  res@vcWindBarbTickSpacingF = 0.125 ;

  res@vcMinDistanceF = 0.125 ; thin out windbarbs
  res@vcMonoWindBarbColor = True
  res@vcWindBarbColor = 1

  res@mpOutlineOn = True ; turn on map outline
   plot1=gsn_csm_vector_map(wks,uu,vv,res)



------------------------------
Cordialement,
Best regards,
Rabah Hachelaf
     ____
    ( )
   ( )
  (___ __)
   /////////

Received on Tue Jul 22 11:39:06 2014