DKRZ PyNGL masking example

Example script:

#
#  File:
#    TRANS_masking.py
#
#  Synopsis:
#    Illustrates how to mask data
#
#  Categories:
#    map plot
#    contour plot
#
#  Author:
#    Karin Meier-Fleischer, based on NCL example
#
#  Date of initial publication:
#    September 2018
#
#  Description:
#    This example shows how to mask data.
#
#  Effects illustrated:
#    o  Drawing contours on a map
#    o  Using manual levels
#    o  Change map resolution
#    o  Using area specifiers
#
#  Output:
#    A single visualization is produced.
#
#  Notes: The data for this example can be downloaded from
#    http://www.ncl.ucar.edu/Document/Manuals/NCL_User_Guide/Data/
#
'''
  Transition Guide Python Example: TRANS_masking.py

  -  Drawing contours on a map
  -  Using manual levels
  -  Change map resolution
  -  Using area specifiers

  18-09-11  kmf
'''
from __future__ import print_function
import Ngl, Nio

#-----------------------------------------------------------------------
#-- Function: main
#-----------------------------------------------------------------------
def main():

    #-- countries where contours should appear
    #-- too small countries: "Monaco","San Marino","Vatican City"
    mask_specs = ["Albania","Andorra","Armenia", \
                  "Austria","Azerbaijan","Belarus","Belgium", \
                  "Bosnia and Herzegovina","Bulgaria","Croatia", \
                  "Cyprus","Czech Republic","Denmark","Estonia", \
                  "Finland","France","Georgia","Germany","Greece", \
                  "Hungary","Iceland","Ireland","Italy","Kazakhstan", \
                  "Latvia","Liechtenstein","Lithuania","Luxembourg", \
                  "Macedonia","Malta","Moldova","Montenegro", \
                  "Netherlands","Norway","Poland","Portugal","Romania", \
                  "Russia","Serbia","Slovakia","Slovenia", \
                  "Spain","Sweden","Switzerland","Turkey","Ukraine", \
                  "United Kingdom"]

    #--  open file and read variables
    f    = Nio.open_file("rectilinear_grid_3D.nc", "r")
    var  = f.variables["t"][0,0,:,:]
    lat  = f.variables["lat"][:]
    lon  = f.variables["lon"][:]

    #-- start the graphics
    wks = Ngl.open_wks("png","plot_TRANS_masking_py")

    minlat =  30.0
    maxlat =  80.0
    minlon = -12.0
    maxlon =  35.0

    #-- resource settings
    res                    =  Ngl.Resources()
    res.nglFrame           =  False

    res.cnFillOn           =  True              #-- turn on contour level fill
    res.cnFillPalette      = "cmp_b2r"          #-- choose color map
    res.cnLinesOn          =  False             #-- don't draw contour lines
    res.cnLineLabelsOn     =  False             #-- turn off contour line labels
    res.cnLevelSelectionMode = "ManualLevels"   #-- use manual contour line levels
    res.cnMinLevelValF     =  250.0             #-- contour min. value
    res.cnMaxLevelValF     =  300.0             #-- contour max. value
    res.cnLevelSpacingF    =    2.5             #-- contour interval
    res.cnFillDrawOrder    = "Predraw"          #-- contours first; then fills

    res.lbBoxMinorExtentF  =  0.2               #-- height of labelbar boxes
    res.lbOrientation      = "horizontal"       #-- horizontal labelbar
    res.lbLabelFontHeightF = 0.014

    res.mpDataBaseVersion  = "MediumRes"        #-- alias to Ncarg4_1
    res.mpDataSetName      = "Earth..4"
    res.mpLimitMode        = "LatLon"
    res.mpMinLatF          =  minlat
    res.mpMaxLatF          =  maxlat
    res.mpMinLonF          =  minlon
    res.mpMaxLonF          =  maxlon
    res.mpGridAndLimbOn      =  False

    res.mpFillOn              = True            #-- turn on map fill
    res.mpOutlineBoundarySets = "National"
    res.mpFillBoundarySets    = "NoBoundaries"
    res.mpAreaMaskingOn       =  True
    res.mpMaskAreaSpecifiers  =  mask_specs
    res.mpFillAreaSpecifiers  = ["land","water"]
    res.mpSpecifiedFillColors = ["gray65","gray85","gray65"]  #-- Land,Ocean,InlandWater

    res.sfXArray           =  lon
    res.sfYArray           =  lat

    #-- viewport resources
    res.nglMaximize        =  False             #-- don't maximize plot
    res.vpYF               =  0.9               #-- start y-position of viewport
    res.vpWidthF           =  0.65              #-- width of viewport
    res.vpHeightF          =  0.65              #-- height of viewport

    #-- create the contour plot
    plot = Ngl.contour_map(wks,var,res)

    #-- write variable long_name and units to the plot
    txres               = Ngl.Resources()
    txres.txFontHeightF = 0.018

    Ngl.text_ndc(wks,f.variables["t"].attributes['long_name'],0.29,0.92,txres)
    Ngl.text_ndc(wks,f.variables["t"].attributes['units'],    0.82,0.92,txres)

    Ngl.frame(wks)

#-- call  main function
if __name__ == '__main__':
    main()