mgrib(grib_field_position = 1,
grib_input_file_name = "t850.grb")
mcont(contour_grid_value_plot = "on",
contour_level_selection_type = "level_list",
contour_grid_value_height = 0.30,
contour_level_list = [-20.00, -10.00,...,10.00, 20.00],
contour_grid_value_plot_type = "both",
contour_line_colour = "grey",
contour_grid_value_marker_height = 0.20,
contour_line_thickness = 2,
contour_highlight = "off",
contour_label = "off",
contour_grid_value_lon_frequency = 10,
contour_grid_value_lat_frequency = 10,
legend = "off")