NCL_mask_1.py

This script illustrates the following concepts:

• Using “mask” to set land or ocean values in your data to missing

• Masking the ocean in a map plot

• Masking land in a map plot

• Spanning part of a color map for contour fill

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

• Original NCL script: https://www.ncl.ucar.edu/Applications/Scripts/mask_1.ncl

• Original NCL plot: https://www.ncl.ucar.edu/Applications/Images/mask_1_1_lg.png and https://www.ncl.ucar.edu/Applications/Images/mask_1_2_lg.png

Import packages:

import cartopy.crs as ccrs
import matplotlib.pyplot as plt
import numpy as np
import xarray as xr
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
                    )  # Disable time decoding due to missing necessary metadata
# Extract a slice of the data
ds = ds.isel(time=0, drop=True)

Data Masking:

# Use xarray.DataArray.where() function to mask out land and then ocean data
land_only = ds.TS.where(ds.ORO == 1.0)
ocean_only = ds.TS.where(ds.ORO == 0.0)
land_only = gv.xr_add_cyclic_longitudes(land_only, "lon")
ocean_only = gv.xr_add_cyclic_longitudes(ocean_only, "lon")

Plot Ocean Only:

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

# Generate axes using Cartopy and draw coastlines
projection = ccrs.PlateCarree()
ax = plt.axes(projection=projection)
ax.coastlines(linewidth=0.5, resolution="110m")

# Import an NCL colormap, truncating it by using geocat.viz.util convenience function
newcmp = gv.truncate_colormap(cmaps.BlAqGrYeOrRe, minval=0.1, maxval=1.0, n=22)

# Contourf-plot ocean-only data (for filled contours)
filled = ocean_only.plot.contourf(ax=ax,
                                  cmap=newcmp,
                                  levels=np.arange(260, 305, 2),
                                  xticks=np.arange(-180, 181, 30),
                                  yticks=np.arange(-90, 91, 30),
                                  transform=ccrs.PlateCarree(),
                                  add_colorbar=False,
                                  add_labels=False,
                                  vmin=260,
                                  vmax=304)

# Add horizontal colorbar
cbar = plt.colorbar(filled,
                    ax=ax,
                    orientation="horizontal",
                    aspect=30,
                    drawedges=True)
cbar.set_ticks(np.arange(262, 304, 4))

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

# Use geocat.viz.util convenience function to make plots look like NCL plots by using latitude, longitude tick labels
gv.add_lat_lon_ticklabels(ax)

# Use geocat.viz.util convenience function to add main title as well as titles to left and right of the plot axes.
gv.set_titles_and_labels(ax,
                         maintitle="Ocean Only",
                         lefttitle=ocean_only.attrs['long_name'],
                         lefttitlefontsize=14,
                         righttitle=ocean_only.attrs['units'],
                         righttitlefontsize=14)

# Show the plot
plt.show()

Plot Land Only:

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

# Generate axes using Cartopy and draw coastlines
ax = plt.axes(projection=projection)
ax.coastlines(linewidth=0.5, resolution="110m")

# Import an NCL colormap
newcmp = gv.truncate_colormap(cmaps.BlAqGrYeOrRe, minval=0.1, maxval=1.0, n=32)

# Contourf-plot land-only data (for filled contours)
filled = land_only.plot.contourf(ax=ax,
                                 cmap=newcmp,
                                 levels=np.arange(215, 316, 4),
                                 xticks=np.arange(-180, 181, 30),
                                 yticks=np.arange(-90, 91, 30),
                                 transform=ccrs.PlateCarree(),
                                 add_colorbar=False,
                                 add_labels=False,
                                 vmin=215,
                                 vmax=315)

# Add horizontal colorbar
cbar = plt.colorbar(filled,
                    ax=ax,
                    orientation="horizontal",
                    aspect=30,
                    drawedges=True)
cbar.set_ticks(np.arange(219, 304, 12))

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

# Use geocat.viz.util convenience function to make plots look like NCL plots by using latitude, longitude tick labels
gv.add_lat_lon_ticklabels(ax)

# Use geocat.viz.util convenience function to add main title as well as titles to left and right of the plot axes.
gv.set_titles_and_labels(ax,
                         maintitle="Land Only",
                         lefttitle=land_only.attrs['long_name'],
                         lefttitlefontsize=14,
                         righttitle=land_only.attrs['units'],
                         righttitlefontsize=14)

# Show the plot
plt.show()

/home/docs/checkouts/readthedocs.org/user_builds/geocat-examples/conda/latest/lib/python3.11/site-packages/geocat/viz/util.py:730: UserWarning: Overwriting the cmap 'trunc(BlAqGrYeOrRe,0.10,1.00)' that was already in the registry.
  mpl.colormaps.register(new_cmap, force=force)