Use Matplotlib to draw a contour plot

draw_contour.py

# coding: utf-8

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

import nuwe_pyeccodes

file_path = "some/file/path/to/gmf.gra.2018081800003.grb2"
message_number = 85


grib_file = nuwe_pyeccodes.GribFileHandler()
grib_file.openFile(file_path)
grib_message = None
for i in range(0, message_number):
    grib_message = grib_file.next()


left_lon = grib_message.getDouble('longitudeOfFirstGridPointInDegrees')
right_lon = grib_message.getDouble('longitudeOfLastGridPointInDegrees')
lon_step = grib_message.getDouble('iDirectionIncrementInDegrees')
nx = grib_message.getLong('Ni')
lon_array = np.arange(left_lon, right_lon + lon_step / 2, lon_step)

top_lat = grib_message.getDouble('latitudeOfFirstGridPointInDegrees')
bottom_lat = grib_message.getDouble('latitudeOfLastGridPointInDegrees')
lat_step = grib_message.getDouble('jDirectionIncrementInDegrees')
ny = grib_message.getLong('Nj')
lat_array = np.arange(top_lat, bottom_lat - lat_step / 2, -lat_step)

lons, lats = np.meshgrid(lon_array, lat_array)

values = grib_message.getDoubleArray('values')
grid_values = values.reshape(ny, nx)

ax = plt.subplot(111, projection=ccrs.PlateCarree(central_longitude=180))
ax.contourf(
    lons, lats, grid_values,
    transform=ccrs.PlateCarree(),
    cmap='rainbow'
)

ax.coastlines()
ax.gridlines()

plt.show()