DKRZ NCL Example masked data by shapefile

Example script:

;--------------------------------------------------------------------
; DKRZ NCL Example:  mask_data_by_shapefile.ncl
;
; Description:    use a shapefile to mask your data
;
; Shapefile:      DEU_adm1.shp downloaded from
;
;                 http://www.gadm.oarg/country/Germany
;
; You have to download the shapefile_utils.ncl file from:
;
; https://www.ncl.ucar.edu/Applications/Scripts/shapefile_utils.ncl
;
; 13.07.18 kmf
;--------------------------------------------------------------------
load "./shapefile_utils.ncl" ;-- down load the file and save it in current dir

begin
;-- shapefile name
 shp_filename = "$HOME/data/Shapefiles/DEU_adm/DEU_adm1.shp"

;-- open file and read variable
 f = addfile("$HOME/data/CMIP5/atmos/tas_day_MPI-ESM-LR_rcp45_r1i1p1_20060101-20091231_0.1deg.nc","r")
 var = f->tas(0,:,:)

;-- define sub-region, here for Germany
 minlat = 46.8
 maxlat = 55.3
 minlon = 6.0
 maxlon = 15.5

;-- open workstation
 wks_type = "png"
 wks_type@wkWidth = 1200
 wks_type@wkHeight = 1200
 wks = gsn_open_wks(wks_type,"plot_shapefiles_masking")

;-- resource settings
 res = True
 res@gsnDraw = False    ;-- don't draw plot yet
 res@gsnFrame = False   ;-- don't advance frame yet
 res@gsnMaximize = True ;-- maximize plot in frame

 res@cnFillOn = True
 res@cnLinesOn = False
 res@cnLineLabelsOn = False
 res@cnFillPalette = "BlueYellowRed"
 res@cnLevelSelectionMode = "ManualLevels" ;-- manually set contour levels
 res@cnMinLevelValF = 260.0   ;-- contour min lev
 res@cnMaxLevelValF = 280.0   ;-- contour max lev
 res@cnLevelSpacingF = 0.5    ;-- contour spacing

 res@mpDataBaseVersion = "HighRes"  ;-- use HighRes map
 res@mpDataResolution = "Fine"      ;-- we need a finer resolution
 res@mpProjection = "Mercator"      ;-- use Mercator projection
 res@mpLimitMode = "Corners"        ;-- map limit mode
 res@mpLeftCornerLatF = minlat+0.5  ;-- min lat
 res@mpRightCornerLatF = maxlat-0.5 ;-- max lat
 res@mpLeftCornerLonF = minlon-0.5  ;-- min lon
 res@mpRightCornerLonF = maxlon+0.5 ;-- max lon
 res@mpCenterLatF = ((minlat+maxlat)*0.5)-5.0
 res@mpCenterLonF = ((minlon+maxlon)*0.5)-5.0
 res@mpGridLineColor = "grey40"
 res@mpFillOn = True
 res@mpOutlineOn = True
 res@mpGeophysicalLineColor = "black"
 res@mpOceanFillColor = (/ 0.824, 0.961, 1.0 /)
 res@mpInlandWaterFillColor = (/ 0.824, 0.961, 1.0 /)
 res@mpLandFillColor = (/ 0.7, 0.7, 0.7 /)

 res@lbBoxMinorExtentF = 0.16   ;-- decrease height of labelbar boxes
 res@pmLabelBarOrthogonalPosF = -0.08  ;-- move labelbar to the left side of plot res@lbBoxMinorExtentF = 0.2 ;-- decrease height of labelbar boxes

 res@pmTickMarkDisplayMode = "Always"
 res@tmXTOn = False

 res@tiMainString = "Masked by shapefile"

;-- create plot
 plot = gsn_csm_contour_map(wks,var,res)

;-- use shapefile to mask these dummy arrays based on shapefile outline
 var_mask = shapefile_mask_data(var,shp_filename,True)

;-- create contours of masked data
 plot_mask = gsn_csm_contour_map(wks,var_mask,res)

;-- add plot_mask to plot
 lnres = True
 lnres@gsLineColor = "black"
 lnres@gsLineThicknessF = 1.0

 plot@lines = gsn_add_shapefile_polylines(wks, plot_mask, shp_filename, lnres)

;-- draw the plot and advance the frame
 draw(plot_mask)
 frame(wks)

end

Result: