NCL_native_2.py

This script illustrates the following concepts:

• Drawing filled contours over a mercator map

• Overlaying contours on a map without having latitude and longitude coordinates

• Turning on map tickmark labels with degree symbols

• Selecting a different color map

• Zooming in on a particular area on a mercator map

• Using best practices when choosing plot color scheme to accomodate visual impairments

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

• Original NCL script: https://www.ncl.ucar.edu/Applications/Scripts/native_2.ncl

• Original NCL plot: https://www.ncl.ucar.edu/Applications/Images/native_2_lg.png

# Import packages:

import numpy as np
import xarray as xr
import cartopy.crs as ccrs
import cartopy.feature as cfeature
import matplotlib.pyplot as plt
import matplotlib.ticker as mticker

import geocat.datafiles as gdf

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/1994_256_FSD.nc"),
                     decode_times=False)
t = ds.FSD.isel(time=0)

Plot:

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

# Generate axes using Cartopy and draw coastlines
ax = plt.axes(projection=ccrs.Mercator())
ax.coastlines(linewidths=0.5)
ax.add_feature(cfeature.LAND, facecolor="lightgray")

# Set extent to include latitudes from 34 to 52 and longitudes from 128
# to 144
ax.set_extent([128, 144, 34, 52], ccrs.PlateCarree())

# Plot data and create colorbar
pt = t.plot.contourf(ax=ax,
                     transform=ccrs.PlateCarree(),
                     vmin=0,
                     vmax=70,
                     levels=15,
                     cmap="inferno",
                     add_colorbar=False)

cbar_ticks = np.arange(0, 71, 5)
cbar = plt.colorbar(pt,
                    orientation='vertical',
                    extendrect=True,
                    ticks=cbar_ticks)

# Draw gridlines
gl = ax.gridlines(crs=ccrs.PlateCarree(),
                  draw_labels=True,
                  dms=False,
                  x_inline=False,
                  y_inline=False,
                  linewidth=1,
                  color="black",
                  alpha=0.25)

# Manipulate latitude and longitude gridline numbers and spacing
gl.top_labels = False
gl.right_labels = False
gl.xlocator = mticker.FixedLocator([130, 134, 138, 142])
gl.ylocator = mticker.FixedLocator([36, 38, 40, 42, 44, 46, 48, 50])
gl.xlabel_style = {"rotation": 0, "size": 15}
gl.ylabel_style = {"rotation": 0, "size": 15}

plt.title("Native Mercator Projection",
          loc="center",
          y=1.05,
          size=15,
          fontweight="bold")
plt.title(t.units, loc="right", y=1.0, size=14)
plt.title("free surface deviation", loc="left", y=1.0, size=14)

# Show the plot
plt.show()