qsat needed for humidity assimilation

[delippi]

Qsat or saturation_water_vapor_mixing_ratio_wrt_moist_air (previously saturation_specific_humidity) is needed for humidity observation assimilation to scale the RH ob errors to specific humidity. Specifically, it is needed in the ObsErrorFactorPressureCheck which is a ob error inflation function in ufo.

This is the error message when trying to assimilate specificHumidity obs in MPAS-JEDI

vars_nlevels: saturation_water_vapor_mixing_ratio_wrt_moist_air not available in MPAS domain or self % templated_fields

Qsat can be calculated from temperature and pressure. It is calculated using equations 2.17-2.19 in Rogers & Yau (1989).

There are a few possible solutions

1. Add as a model output variable
2. Implement, in MPAS-JEDI, a non-vader variable transform following what has been done in FV3-JEDI.
3. Implement vader in MPAS-JEDI.