Modified downwelling sfc flux calculation

[helgehr]

Thanks for this amazing resource!
I modified the flux computation (only cells 33/34/35 are changed) in this notebook because I noticed that it was not consistent with the definition of the signs of up/downwelling fluxes. Because the signs where flipped for both sides of the equation in cell [34], the result at the end was correct before.
I don't know why so many output cells are marked as changed, if you give me a hint how I can avoid that, I could open a more "clean" pull request.

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[brian-rose]

Thanks!

I think the only way to get a more "clean" diff would be to strip all the output of the notebook before committing to version control. That would work well if all the notebooks were lightweight and the output could easily be regenerated, but that's not the case for all the notebooks in the repository.

So don't worry about it.

I will take a look at the sign issue when I have a bit of time. I appreciate the contribution!