DKRZ NCL color map example - use gsn_define_colormap()

Example script:

;-----------------------------------------------------------------------------------
; DKRZ NCL Example:  color_wks_use_gsn_define_colormap.ncl
;
; Description:   Demonstrate the use gsn_define_colormap() to set a color map
;      for all plots of a frame (file/page).
;
;      After a frame is advanced when gsnFrame is set to True or
;      frame(wks) is used the color map is fixed for that frame
;      but for the next frame it can change.
;
;      Two PNG files will be created
;
;         plot_color_wks_use_gsn_define_colormap.000001.png
;         plot_color_wks_use_gsn_define_colormap.000002.png
;
; 12.03.18 kmf
;-----------------------------------------------------------------------------------
begin
 f = addfile("./rectilinear_grid_2D.nc","r")
 var = f->tsurf(0,:,:)

;-- open workstation (frame)
 wks = gsn_open_wks("png","plot_color_wks_use_gsn_define_colormap")
 gsn_define_colormap(wks,"BlWhRe")  ;-- doesn't contain grays

;-- set common resources
 res = True
 res@gsnMaximize = True

 res@cnFillOn = True         ;-- turn on contour fill
 res@cnLinesOn = False       ;-- turn off contour lines
 res@cnLineLabelsOn = False  ;-- turn off line labels
 res@cnLevelSelectionMode = "ManualLevels" ;-- set contour levels manually
 res@cnMinLevelValF = 250.  ;-- minimum contour level
 res@cnMaxLevelValF = 310.  ;-- maximum contour level
 res@cnLevelSpacingF = 1    ;-- contour level spacing

 res@lbLabelStride = 4
 res@lbBoxMinorExtentF = 0.15  ;-- decrease the height of the labelbar

 res@tiMainFontHeightF = 0.02
 res@tiMainString = "plot"  ;-- title string

;-- this plot will be drawn directly and the first frame is created (first plot file or first page)
 plot = gsn_csm_contour_map(wks, var, res)

;-------------------------------------------------------------------------------------------
;-- now, we want to see what happens when we make some changes to the color map and draw the
;-- plots in memory to create a panel
;-------------------------------------------------------------------------------------------
 res@gsnDraw = False
 res@gsnFrame = False
 res@tiMainString = "plot 1" ;-- title string

 plot1 = gsn_csm_contour_map(wks, var, res)

;-- let's change the last two color indices 100 and 102 to light grays
 colors = gsn_retrieve_colormap(wks) ;-- retrieve color map for editing. dimensioned (103,3)
 colors(100,:) = (/ .60, .60, .60 /) ;-- replace the last color with a medium gray
 colors(102,:) = (/ .80, .80, .80 /) ;-- replace the last color with a light gray

;-- set color map to new colors array
 gsn_define_colormap(wks,colors) ;-- redefine colormap to workstation, color map now includes a gray

;-- see what happens with the second plot
 res@tiMainString = "plot 2" ;-- title string

;-- second plot created in memory!
 plot2 = gsn_csm_contour_map(wks, var, res)

;-- generate the panel, draw all plots and advance the frame
 gsn_panel(wks,(/plot1,plot2/),(/2,1/),True)

;-- You will see two identical plots in the panel even if you have expected that the upper
;-- plot should look like the first frame (no grays). This is caused by the creation of the
;-- plots in memory, the colors can be changed but will be valid after the changes to both
;-- plots. Only the advance of a frame will stop this behavior and you can change the color
;-- for the next frame.

end

Result: