RFM Application to Planetary Atmospheres

04FEB21

Introduction

Default (terrestrial) values of adjustable RFM parameters
Symbol Value Units Description Application
rE 6367.421 km Radius of curvature Circular geometry
g 9.80665 ms-2 Gravitational acceleration Plane-parallel geometry
Mair 28.964 kg kmol-1 Molar mass of air Plane-parallel geometry
Cp 29012.0 J K-1 kmol-1 Molar heat capacity of air Cooling rate
Tsp 2.7 K Cosmic background temperature Microwave calculations

The atmospheric profile within the RFM is entirely user-specified so the main adaptation for non-terrestrial atmospheres is straightforward.

However, depending on the type of calculation, there are other parameters for which the RFM assumes terrestrial values by default, summarised in the table on the right.

These can all be altered using the *PHY section of the RFM driver file.

Adjustable Parameters

The radius of curvature is perhaps the most obviously Earth-specific parameter. This is required for ray-tracing in the circular geometry e.g., for limb-viewing, but not required for plane-parallel atmosphere calculations (ie not required if using NAD, ZEN or FLX Flags).

For plane-parallel atmospheres, the default behaviour of the RFM is to assume hydrostatic equilibrium to derive absorber amounts in each layer from pressure and mixing ratio, independent of altitude coordinate (any user-supplied *HGT profile being ignored). The conversion requires both gravitational acceleration and the molar mass of air so, for other planets, these two values should be altered. However, using the HYD Flag, it is also possible bypass the hydrostatic assumption and use the user-supplied *HGT profile instead, in which case the absorber amount can be calculated independently of these parameters (presumably you will have used appropriate values of g and Mair to match your height and pressure profiles beforehand).

Cooling rate calculations (COO Flag) are, effectively, a conversion from radiation loss to temperature change for an atmospheric layer, and therefore require some value of the molar heat capacity of air.

The cosmic background temperature is used to calculate the Planck function representing cold space. By its very definition it is not specific to the Earth, but is included here since it can be adjusted in the same way as the other parameters.

Refractive Index

Strictly speaking, the RFM refraction and Ralyeigh extinction modules are also Earth-specific, but these are primarily dependent upon molecular number density rather than detailed composition, so should be adequate for other atmospheres.

The function refrac_fnc.f90 returns the refractivity (=refractive index - 1) using a modified form of Edlen's formula printed in Kaye & Laby, which is a function of (p/T) (i.e., density) and wavelength. In reality, at microwave frequencies, the refractivity also depends on water vapour, but this is not included in the refractivity model. The wavelength dependence is weak, refractivity increasing by only a few percent between long wavelengths and the visible region of the spectrum, most of the variation being at short wavelengths.

Rayleigh (i.e. molecular) scattering, which again only becomes significant at visible wavelengths (varies as 1/λ4), is handled in subroutine spcrex_sub.f90, and based on Bodhaine et al (1999). Apart from reproducing the Kaye & Laby refractivity formula, this also includes the King Factor (or depolarisation) calculated in subroutine rexpth_sub.f90, which is based on atmospheric composition. This subroutine includes default concentrations for four components (N2, O2, CO2 and Ar) which may be replaces by any user-supplied atmospheric profiles. However, the influence of the King Factor is generally close to 1.

Spectroscopic Data

The RFM has three basic spectral widths which are determined by the requirement to model Earth's atmosphere
Fine Grid 0.0005 cm-1
This is the default internal spectral sampling when spectrally convolved output is required. It is set by the requirement to capture Doppler broadened lines in the mid-infrared which are approximately 0.001 cm-1 half width. Doppler widths are typically of the order of line centre frequency x 10-6 so need to be reduced for longer wavelengths in any case. It can be easily altered (see *FIN section)
Widemesh 1.0 cm-1
This is the range (from the line centre) over which the lineshape varies rapidly, determined by the pressure half-width at maximum pressure, so should be increased for high-pressure atmospheres. It is set by the FEXC in rfmcon_dat.f90.
Inclusion window 25.0 cm-1
This is the distance from line centre for which any line wing contribution is included. This distance is set for compatibility with the MT-CKD H2O continuum model. This should be larger for high pressure atmospheres. It is set by FEXC in rfmcon_dat.f90.
The RFM aside, it should be remembered that most spectroscopic data are also intrinsically terrestrial, e.g., the standard HITRAN line list is largely determined by molecular transitions relevant to Earth's atmosphere and the standard line width assumes 'air' as the broadening gas.

RFM v5.10 allows line parameters to be supplied in an alternative format which allows 'self' as well as 'air' terms for additional line parameters. This may be useful for atmospheres where the main absorber is also the main constitutuent (eg CO2 on Mars).

There is a HITEMP version of HITRAN, for transitions which become significant at higher temperatures, and the GEISA database contains some extra molecules. See RFM Spectroscopic Data for use of these alternative datasets

The HITRAN Collision Induced Absorption data has recently been extended to allow for a number of different broadening gases. RFM v5.10 onwards allows for a completely general specification of CIA molecules.