Longitude and latitude (in degrees) specifying the slice. If
None the center of the grid is used.
vmin, vmax : float
Lower and upper range for the colormesh. If either parameter is
None, a value will be determined from the field attributes (if
available) or the default values of -8, 64 will be used.
ax : axes or None.
Where to create the plot, if None the current axis is used.