I couldn't overlay a rectangle on a lambert projection map

Dear all,


I tried to overlay a box on the terrain from WRF output in Lambert projection.


The terrain height displayed quite well, but the box didn't show up at all (as the attached figure showing).


What should be mentioned is that when I use "box = gsn_add_polygon(wks,plot,lon,lat,lnres)‍" to displace the original code :
  "do n = 0 , 3
    box(n)=gsn_add_polyline(wks,plot,lat(n:n+1),lon(n:n+1),lnres)
  end do",


a filled box showed up successfully. I didn't figure it out.‍



Here is my complete code:




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/wrf/WRFUserARW.ncl"


;************************************************
begin
;************************************************
; read in netCDF file
;************************************************
  a = addfile("wrfout_d03_2011-06-13_18_00_00.nc","r")
  ter = a->HGT(0,:,:) ; Read the variable to memory, HGT: terrain height, units = m
  mlat = wrf_user_getvar(a,"XLAT",0)
  mlon = wrf_user_getvar(a,"XLONG",0)
  
  ter@lat2d = mlat ; Special method for setting 2D lat/lon coords
  ter@lon2d = mlon




;************************************************
; create plot
;************************************************
  wks = gsn_open_wks("eps" ,"terd03_lambert") ; open a ps file
  gsn_define_colormap(wks,"topo_15lev") ; choose colormap


  res = True ; plot mods desired
  res@gsnDraw = False
  res@gsnFrame = False


  ;res@mpProjection = "LambertConformal"; choose projection
  res@mpProjection = "LambertEqualArea"


  res@mpFillOn = False ; turn off map fill




  res@cnFillOn = True ; turn on color




  res@cnLinesOn = False ; turn off contour lines
  res@gsnSpreadColors = True ; use full colormap
  res@gsnSpreadColorStart = 3 ; start with the second color
  ;res@lbLabelAutoStride = True ; nice lb labels


  ; Set contour levels.
  res@cnLevelSelectionMode = "ExplicitLevels"
  res@cnLevels = (/ 50, 100, 150, 300, 450, 600, 750, \
                                900, 1050, 1200, 1350/)
                                                                




  res@gsnAddCyclic = False ; regional plot
  res@gsnMaximize = True ; enlarge plot
  
  res@mpDataBaseVersion = "HighRes"
  res@mpDataSetName = "Earth..4"
  res@mpGridAndLimbOn = False ; Turn off lat/lon lines
  res@mpOutlineOn = True ; Turn on map outlines
  res@mpGeophysicalLineColor = "Black" ; Change the outline line color
  res@mpGeophysicalLineThicknessF= 2. ; double the thickness of geophysical boundaries
  res@mpOutlineSpecifiers = (/"China","Taiwan","Disputed area between India and China","India:Arunachal Pradesh"/)
  res@pmTickMarkDisplayMode = "Always" ; Turn on map tickmarks
  res@tmXTLabelsOn = False ; do not draw bottom labels
  res@tmXTOn = False ; no bottom tickmarks
  res@tmYRLabelsOn = False ; do not draw bottom labels
  res@tmYROn = False ; no bottom tickmarks
  
  ;---Zoom in on
  res@mpLimitMode = "LatLon"
  res@mpMinLatF = min(mlat)-1
  res@mpMaxLatF = max(mlat)+1
  res@mpMinLonF = min(mlon)-1
  res@mpMaxLonF = max(mlon)+1
  res@mpCenterLonF = 117.547
  res@mpCenterLatF = 29.591




  ;res@gsnMaskLambertConformal = True ; turn on lc masking
  ;res@gsnMaskLambertConformalOutlineOn = False ; turns off outline


 
; subset data going into the plot template so that the colorbar reflects only
; the data viewed vice the entire data set
  plot = gsn_csm_contour_map(wks,ter,res) ;create plot
  
;************************************************
; create points for box
;************************************************
                lnres = True
                lnres@gsLineColor = "Black"
                lnres@gsLineThicknessF = 2.5
lat = (/28.3, 29.5, 29.5, 28.3, 28.3/)
lon = (/116.6, 116.6, 119.2, 119.2, 116.6/)
;box = gsn_add_polygon(wks,plot,lon,lat,lnres)


box = new(4,graphic)
                
; draw each line separately. Each line must contain two points.
  do n = 0 , 3
    box(n)=gsn_add_polyline(wks,plot,lat(n:n+1),lon(n:n+1),lnres)
  end do
        
  draw(plot)
  frame(wks)


end

‍

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