NCL_proj_3.py

This script illustrates the following concepts:

• Drawing filled contours over an orthographic map

• Changing the center latitude and longitude for an orthographic projection

• Turning off map fill

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

• Original NCL script: https://www.ncl.ucar.edu/Applications/Scripts/proj_3.ncl

• Original NCL plot: https://www.ncl.ucar.edu/Applications/Images/proj_3_lg.png

Import packages:

import numpy as np
import xarray as xr
import cartopy.crs as ccrs
import matplotlib.pyplot as plt

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)
t = ds.TS.isel(time=0)

Fix the artifact of not-shown-data around 0 and 360-degree longitudes

wrap_t = gv.xr_add_cyclic_longitudes(t, "lon")

Plot:

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

# Generate axes using Cartopy and draw coastlines with
ax = plt.axes(projection=ccrs.Orthographic(central_longitude=-120, central_latitude=50))

# Set extent to include latitudes between 0 and 90, and longitude between
# 0 and -180 only
ax.set_extent([0, -180, 0, 90], ccrs.PlateCarree())
ax.set_global()
ax.coastlines(linewidths=0.5)

# Plot data and add a colorbar
temp = wrap_t.plot.contourf(
    ax=ax, transform=ccrs.PlateCarree(), levels=11, cmap='coolwarm', add_colorbar=False
)

cbar_ticks = np.arange(210, 311, 10)
cbar = plt.colorbar(
    temp,
    orientation='horizontal',
    shrink=0.75,
    pad=0.05,
    extendrect=True,
    ticks=cbar_ticks,
)

cbar.ax.tick_params(labelsize=10)

# Use geocat.viz.util convenience function to add titles to left and right
# of the plot axis.
gv.set_titles_and_labels(
    ax,
    maintitle="Example of Orthogonal Projection",
    lefttitle="Surface Temperature",
    righttitle="K",
)

# Show the plot
plt.show()