`model` is less descriptive than it could be. How about `uv_coeffs` here, if the only difference between ccm89 and gcc09 is the UV coefficients?

Also, since there are only a small number of options, the documentation on this line can be `model: {'ccm89', 'gcc09'}, optional`. See [numpydoc](https://github.com/numpy/numpy/blob/master/doc/HOWTO_DOCUMENT.rst.txt).