ENH: Add shear and curvature vorticity calculations

[Z-Richard]

Description Of Changes

This PR implements the shear and curvature vorticity calculations. A test is added to ensure the sum of two terms equals the total vorticity computed from metpy.calc.vortictiy.

Checklist

• Tests added
• Fully documented

[kgoebber]

@Z-Richard this is great stuff. I've checkout out the PR and used the functions on a couple of analytic datasets that I have generated and it works as intended. I was wondering if you had a reference we could use with the formations of the equations that you have used in the code? We do attempt to reference appropriate sources to help our users identify the underpinning of the choices made in the package.

[Z-Richard]

@kgoebber The implementation follows the derivation by Professor Sharan Majumdar in a private communication with a friend of mine. The derivation is basically a conversion from natural coordinates to cartesian coordinates. I can ask for permission to upload the file here, but fortunately the derivation does not seem publicly available. They did indicate in the notes that the derivation is inspired by Bleck (1991): https://journals.ametsoc.org/view/journals/atsc/48/8/1520-0469_1991_048_1124_tefsac_2_0_co_2.xml?tab_body=pdf.

[Z-Richard]

Curvature and Shear Vorticity in Cartesian Coordinates.pdf
@kgoebber Here is the reference document that the code is based on.

[DWesl]

Could you add a test that the curvature vorticity of a purely sheared velocity field (say, $u=y$) is zero and similarly for the shear vorticity of a purely rotational velocity field ($u = -y; v = x$, for example)?

Perhaps these checks might work better as an example than a test, but I will leave the final placement for the maintainers.

[dopplershift]

@Z-Richard apologies for this lingering so long. Any problem if we include that document in our docs so we can cite it for the derivation?

[Z-Richard]

@dopplershift I don't think there is an issue as long as proper authorship is acknowledged.