relhum

[cyschneck]

PR Summary

Covers relhum to calculate relative humidity given temperature, mixing ratio, and pressure as part of Meteorology functionality

Related Tickets & Documents

Closes #165

PR Checklist

General

• PR includes a summary of changes
• Link relevant issues, make one if none exist
• If adding a new page, select which type:
  • NCL Entry
  • Receipt
• Clear Outputs

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[jukent]

Looks great

[kafitzgerald]

I assume this came from the GeoCAT-comp docstring and/or the here in the NCL code for relhum_ice, but I don't know that it's a good reference for relhum. As far as I can tell the connection is to the form of equation used to approximate saturation vapor pressure (maybe) and not either the overall RH equation or a specific instance of the equation used to calculate saturation vapor pressure.

In the NCL code the comments for the relhum function reference CCM Processor routines: https://github.com/NCAR/ncl/blob/c4de4acd24af095abcd5e705d174fd7eb553f186/ni/src/lib/nfpfort/relhum_dp.f#L16

There are some old CCM Processore User's Guides that reference similar relhum equations:
https://opensky.ucar.edu/islandora/object/technotes%3A394?search_api_fulltext=processor
https://opensky.ucar.edu/islandora/object/technotes%3A151?search_api_fulltext=processor

These don't have have the (1-eps) or 0.378 term though that's in the NCL and GeoCAT code and interestingly something folks were recently confused about over on WRF-Python..

The User's Guides also don't have a reference for the table values used or how they were derived. Mike shared some old code with me, but I wasn't finding much there either for references.

It seems like a long story and perhaps related to the many different definitions / approximations.

I think the short answer here is probably to cite the NCL code and maybe leave an open issue somewhere.

[cyschneck]

I updated the description, and made this issue in the meantime (#190)

[kafitzgerald]

I think we should remove the references the 1996 journal article (I'm fairly sure this isn't even the right reference for the saturation vapor pressure definition much less RH), but otherwise this looks great to me.