NCL_lb_4.py

This script illustrates the following concepts:

• Drawing a horizontal colorbar

• Changing the colorbar labels

• Adding a title to a colorbar

• Changing the font of the colorbar’s labels

• Making the colorbar label fonts smaller

• Centering the labels inside each box in a colorbar

• Adding a vertical title to a colorbar

See following URLs to see the reproduced NCL plot & script:

• Original NCL script: https://www.ncl.ucar.edu/Applications/Scripts/lb_4.ncl

• Original NCL plot: https://www.ncl.ucar.edu/Applications/Images/lb_4_lg.png

Import packages:

import numpy as np
import xarray as xr
import cartopy.crs as ccrs
from cartopy.mpl.gridliner import LongitudeFormatter, LatitudeFormatter
import matplotlib.pyplot as plt
import cmaps

import geocat.datafiles as gdf
import geocat.viz as gv

Read in data:

# Open a netCDF data file using xarray default engine and load the data into xarrays
ds = xr.open_dataset(gdf.get("netcdf_files/atmos.nc"), decode_times=False)

# Extract slice of data
V = ds.V.isel(time=0, lev=3)

# Fix the artifact of not-shown-data around 0 and 360-degree longitudes
V = gv.xr_add_cyclic_longitudes(V, "lon")

Plot:

# Generate figure (set its size (width, height) in inches)
fig = plt.figure(figsize=(10, 6))

# Generate axes using Cartopy and draw coastlines
ax = plt.axes(projection=ccrs.PlateCarree())
ax.coastlines(linewidths=0.5, alpha=0.6)

# Use geocat.viz.util convenience function to set axes limits & tick values
gv.set_axes_limits_and_ticks(ax,
                             xlim=(-180, 180),
                             ylim=(-90, 90),
                             xticks=np.linspace(-180, 180, 13),
                             yticks=np.linspace(-90, 90, 7))

# Use geocat.viz.util convenience function to add minor and major tick lines
gv.add_major_minor_ticks(ax, labelsize=10)

# Use geocat.viz.util convenience function to make latitude, longitude tick labels
gv.add_lat_lon_ticklabels(ax)
# Remove degree symbol from tick labels
ax.yaxis.set_major_formatter(LatitudeFormatter(degree_symbol=''))
ax.xaxis.set_major_formatter(LongitudeFormatter(degree_symbol=''))

# Use geocat.viz.util convenience function to add titles
gv.set_titles_and_labels(ax,
                         lefttitle=V.long_name,
                         righttitle=V.units,
                         lefttitlefontsize=12,
                         righttitlefontsize=12)

# Import an NCL colormap
cmap = cmaps.wgne15

# Specify which contour levels to draw
contour_lev = np.arange(-20, 28, 4)
# Plot filled contour
contour = V.plot.contourf(ax=ax,
                          transform=ccrs.PlateCarree(),
                          cmap=cmap,
                          levels=contour_lev,
                          add_colorbar=False,
                          add_labels=False)
# Plot line contour
V.plot.contour(ax=ax,
               transform=ccrs.PlateCarree(),
               colors='black',
               linewidths=0.5,
               linestyles='solid',
               levels=contour_lev,
               add_colorbar=False,
               add_labels=False)

# Create horizontal colorbar
# By changing the kwarg `pad`, the colorbar can be moved closer to or farther
# away from the axis parallel to it. `pad` defaults to 0.15 for horizontal
# colorbars. `extendrect` and `extendfrac` format the ends of the colorbar,
# default is pointed ends to show there are values beyond the given contour
# levels
cbar = plt.colorbar(contour,
                    ax=ax,
                    orientation='horizontal',
                    shrink=0.5,
                    pad=0.11,
                    extendrect=True,
                    extendfrac='auto',
                    aspect=11,
                    drawedges=True)

# Turn off automatically created ticks and tick labels
cbar.ax.set_xticklabels([])
cbar.ax.get_xaxis().set_ticks([])

# Draw text in the center of each box
# The origin of the colorbar xaxis starts at right edge of the leftmost box
# The offset moves the test left to the center of each box
# The i/11 comes from the 11 main boxes on the colorbar since the boxes on each
# end are the extensions for values that fall outside the colorbar rand
offset = 3 / 22
for i in range(1, 14):
    cbar.ax.text(i / 11 - offset,
                 0.45,
                 i,
                 horizontalalignment='center',
                 verticalalignment='center',
                 fontweight='bold',
                 transform=cbar.ax.transAxes)

# Draw the colorbar units
cbar.ax.text(1.15,
             0.5,
             V.units,
             horizontalalignment='center',
             verticalalignment='center',
             transform=cbar.ax.transAxes)

plt.tight_layout()
plt.show()