Set Up AR6 Model Parameterization

[FrankErrickson]

Set up the parameters in FAIRv1.6.2 so they match the settings used for the AR6 runs. Currently basing this off of the AR6 Large FAIR Ensemble runs (see Cell #12 in particular):

CURRENT SETTINGS THAT MATCH

• ghg_forcing option that selects primary greenhouse gas forcing equations set to Meinshausen
• Diagnostics option that determines list of warming agents set to AR6
• gir_carbon_cycle option to select carbon cycle functions from Generalised Impulse Response Model v1.0.0 set to True
• aerosol_forcing option to select set of forcing equations set to aerocom+ghan2
• temperature_function option that selects forcing -> warming equations set to Geoffroy
• tropO3_forcing option that selects ozone forcing equations set to thornhill-skeie
• aCO2land land-use forcing coefficient set to -0.2/190
• stwv_from_ch4 forcing coefficient for stratospheric water vapor from methane oxidation set to 0.079047
• F_ref_BC reference forcing for black carbon on snow set to 0.08
• E_ref_BC reference emissions for black carbon on snow set to 6.095
• C_pi pre-industrial concentration values set to match AR6 results (CO2 sampled as uncertain, see note on C2F6 below)
• fossilCH4_frac values for calculating oxidized methane set to 0.0 (also see #2)
• aviNOx_frac values for calculating forcing from contrails based on fraction of NOx emissions from aviation set to 0.0.
• Uses AR6 natural CH4 & N2O emission scenarios (see #3)
• Uses AR6 volcanic & solar forcing scenarios (see #3 and Cell #9)
• Runs with AR6 historic emissions derived from "Run Large FAIR Ensemble" notebook (see Cell #6)
• E_pi option to set pre-industrial emission levels consistent with historic AR6 emissions scenario.
• scale terms for radiative forcing uncertainty default to 1.0 for all gases, but sampled as uncertain in AR6 Monte Carlo.

CURRENT SETTINGS THAT NEED TO BE ADJUSTED

UNCLEAR SETTINGS & NOTES

• The fixPre1850RCP parameter is set to FALSE for AR6. However, it doesn't appear any forcing equations in FAIR that use fixPre1850RCP are actually called for the AR6 runs. Perhaps just an oversight?
• The AR6 pre-industrial concentration for Hexafluoroethane (C2F6) is set to 32.28077001 ppt (I'm assuming units). This seems quite high, and there is a comment in the AR6 code stating, # where did this come from?. I still used this value for MimiFAIRv1.6.2, but it should be double-checked (also note, running it with this value seems to result in negligible differences... causing total radiative forcing in 2500 under RCP8.5 to differ by ~0.005 Wm^2).
• Note, the model results are quite sensitive to the parameterization of the indirect aerosol radiative forcing component (which are sampled as uncertain parameters for AR6). I selected values from the constrained sample that gave reasonable results as a placeholder in the default model.

[FrankErrickson]

Mostly addressed by 76e6e3eeb32f09191351a514974de223be72114c. Keeping issue open to continue tracking all of the AR6 settings.