`[Model]`¶

This header defines the type of forward model (FM) that will be computed by TauREx3. There are several built-in forward model_type values:

transmission

Transmission forward model

emission

Emission forward model

directimage

Direct-image forward model

multi_transit

Composite transmission model combining multiple 1D regions

multi_eclipse

Composite emission model combining multiple 1D regions

multi_directimage

Composite direct-imaging model combining multiple 1D regions

custom

User-type forward model, See Custom Types

Both emission and direct image also include an optional keyword ngauss which dictates the number of Gaussian quadrate points used in the integration. By default this is set to ngauss=4.

Composite Forward Models¶

TauREx also includes built-in composite forward models for stitching together multiple 1D atmospheric regions into a single weighted spectrum. These were previously distributed through the taurex-multimodel plugin and are now available directly in the main package.

The parameter-file-driven entry points are:

`model_type`	Description
`multi_transit`	Weighted combination of multiple transmission regions
`multi_eclipse`	Weighted combination of multiple emission regions
`multi_directimage`	Weighted combination of multiple direct-image regions

Each region is defined through a separate parameter file listed in parfiles. TauREx reads the temperature, chemistry, pressure, and contribution sections from each file and combines the resulting spectra with the optional fractions list. For retrievals, provide N-1 fractions rather than N if you want TauREx to adapt the last region automatically. In that case the final fraction is inferred from the remaining weight so that the total remains unity. For a complete worked setup, see the multimodel notebook example in Notebook Examples.

Example composite transmission setup:

[Model]
model_type = multi_transit
parfiles = day.par, night.par
fractions = 0.7

The same pattern applies to multi_eclipse and multi_directimage.

Two internal helper models are also exposed for advanced use:

emission_radscale or eclipse_radscale for radius-scaled emission regions.
direct_radscale or directimage_radscale for radius-scaled direct-image regions.

Contributions¶

Contributions define what processes in the atmosphere contribute to the optical depth. These contributions are defined as subheaders with the name of the header being the contribution to add into the forward model.Any forward model type can be augmented with these contributions.

Examples¶

Transmission spectrum with molecular absorption and CIA from H2-He and H2-H2:

[Model]
model_type = transmission
    [[Absorption]]

    [[CIA]]
    cia_pairs = H2-He,He-He

Emission spectrum with molecular absorption, CIA and Rayleigh scattering:

[Model]
model_type = emission
ngauss = 4
    [[Absorption]]

    [[CIA]]
    cia_pairs = H2-He,He-He

    [[Rayleigh]]

The following sections give a list of available contributions

Molecular Absorption¶

[[Absorption]]

Adds molecular absorption to the forward model. Here the active molecules contribute to absorption. No other keywords are needed. No fitting parameters.

Collisionally Induced Absorption¶

[[CIA]]

Adds collisionally induced absorption to the forward model. Requires cia_path to be set. Both active and inactive molecules can contribute. No fitting parameters

Keywords¶

Variable	Type	Description
`cia_pairs`	`list`	List of molecular pairs. e.g. `H2-He, H2-H2`

Rayleigh Scattering¶

[[Rayleigh]]

Adds Rayleigh scattering to the forward model. Both active and inactive molecules can contribute. No keywords or fitting parameters.

Optically thick clouds¶

[[SimpleClouds]] or [[ThickClouds]]

A simple cloud model that puts a infinitely absorping cloud deck in the atmosphere.

Keywords¶

Variable	Type	Description
`clouds_pressure`	`float`	Pressure of top of cloud-deck in Pa

Fitting Parameters¶

Variable	Type	Description
`clouds_pressure`	`float`	Pressure of top of cloud-deck in Pa

Mie scattering (Lee)¶

[[LeeMie]]

Computes Mie scattering contribution to optical depth Formalism taken from: Lee et al. 2013, ApJ, 778, 97

Keywords¶

Variable	Type	Description
`lee_mie_radius`	`float`	Particle radius in um
`lee_mie_q`	`float`	Extinction coefficient
`lee_mie_mix_ratio`	`float`	Mixing ratio in atmosphere
`lee_mie_bottomP`	`float`	Bottom of cloud deck in Pa
`lee_mie_topP`	`float`	Top of cloud deck in Pa

Fitting Parameters¶

Parameter	Type	Description
`lee_mie_radius`	`float`	Particle radius in um
`lee_mie_q`	`float`	Extinction coefficient
`lee_mie_mix_ratio`	`float`	Mixing ratio in atmosphere
`lee_mie_bottomP`	`float`	Bottom of cloud deck in Pa
`lee_mie_topP`	`float`	Top of cloud deck in Pa

Mie scattering (Precomputed grids)¶

[[PyMieScattGridExtinction]]

Computes cloud extinction from precomputed \(Q_\mathrm{ext}\) grids, promoting the former taurex-PCQ plugin into the main TauREx codebase. This is useful when you want PyMieScatt-style cloud retrievals without paying

the cost of recomputing Mie efficiencies for every model evaluation.

Each species points to an HDF5 grid file containing:

radius_grid in microns
wavenumber_grid in \(cm^{-1}\)
Qext or Qext_grid with shape (n_radius, n_wavenumber)

The same contribution works in transmission, emission, and direct-image forward models because it supplies a wavelength-dependent extinction profile to the standard TauREx contribution pipeline.

The reference grid paper is Voyer & Changeat (2026). If you want ready-made cloud grids instead of generating your own, the published dataset is available from Zenodo at 10.5281/zenodo.17456673.

Keywords¶

Variable	Type	Description
`species`	`list`	Names used to label each cloud species and its fit params
`mie_species_path`	`list`	Paths to the precomputed aerosol grid files
`mie_particle_radius_distribution` \| `str`		`normal`, `budaj`, or `deirmendjian` particle sampling
`mie_particle_mean_radius`	`list`	Mean particle radius in um
`mie_particle_logstd_radius`	`list`	Log-normal width used for `normal` and sampling control
`mie_particle_paramA/B/C/D`	`list`	Shape parameters for the `deirmendjian` distribution
`mie_particle_radius_Nsampling`	`int`	Number of radius samples used to integrate the distribution
`mie_particle_radius_Dsampling`	`float`	Width of the sampled radius interval in log space
`mie_particle_mix_ratio`	`list`	Particle number density in \(m^{-3}\)
`mie_midP`	`list`	Cloud centre pressure in Pa, or `-1` for the full column
`mie_rangeP`	`list`	Cloud vertical extent in log-pressure space
`mie_particle_altitude_distrib`	`str`	`exp_decay` or `linear` vertical particle profile
`mie_particle_altitude_decay`	`list`	Decay exponent used by `exp_decay`

Fitting Parameters¶

The contribution registers both shared and per-species fitting parameters. The main ones are:

Parameter	Type	Description
`Rmean_share`	`float`	Shared particle radius applied to all species
`Rlogstd_share`	`float`	Shared log-width for supported distributions
`X_share`	`float`	Shared particle number density
`midP_share`	`float`	Shared cloud centre pressure
`rangeP_share`	`float`	Shared log-pressure extent
`decayP_share`	`float`	Shared exponential-decay index
`Rmean_<species>`	`float`	Per-species particle radius
`Rlogstd_<species>`	`float`	Per-species log-width
`X_<species>`	`float`	Per-species particle number density
`midP_<species>`	`float`	Per-species cloud centre pressure
`rangeP_<species>`	`float`	Per-species cloud extent
`decayP_<species>`	`float`	Per-species exponential-decay index

Example parameter-file usage:

[Model]
model_type = transmission

    [[Absorption]]

    [[PyMieScattGridExtinction]]
    species = Mg2SiO4_glass, SiO2
    mie_species_path = /path/Mg2SiO4_glass.h5, /path/SiO2.h5
    mie_particle_radius_distribution = budaj
    mie_particle_mean_radius = 0.1, 0.5
    mie_particle_mix_ratio = 1e8, 5e7
    mie_midP = 1e5, 1e3
    mie_rangeP = 2.0, 1.0
    mie_particle_altitude_distrib = exp_decay
    mie_particle_altitude_decay = -4.0, -5.0

Mie scattering (BH)¶

[[BHMie]]

Computes a Mie scattering contribution using method given by Bohren & Huffman 2007

Keywords¶

Variable	Type	Description
`bh_particle_radius`	`float`	Particle radius in um
`bh_cloud_mix`	`float`	Mixing ratio in atmosphere
`bh_clouds_bottomP`	`float`	Bottom of cloud deck in Pa
`bh_clouds_topP`	`float`	Top of cloud deck in Pa
`mie_path`	`str`	Path to molecule scattering parameters
`mie_type`	`cloud` or `haze`	Type of mie cloud

Fitting Parameters¶

Parameter	Type	Description
`bh_particle_radius`	`float`	Particle radius in um
`bh_cloud_mix`	`float`	Mixing ratio in atmosphere
`bh_clouds_bottomP`	`float`	Bottom of cloud deck in Pa
`bh_clouds_topP`	`float`	Top of cloud deck in Pa

Mie scattering (Flat)¶

[[FlatMie]]

Computes a flat absorbing region of the atmosphere across all wavelengths

Keywords¶

Variable	Type	Description
`flat_mix_ratio`	`float`	Opacity value
`flat_bottomP`	`float`	Bottom of absorbing region in Pa
`flat_topP`	`float`	Top of absorbing region in Pa

Fitting Parameters¶

Parameter	Type	Description
`flat_mix_ratio`	`float`	Opacity value
`flat_bottomP`	`float`	Bottom of absorbing region in Pa
`flat_topP`	`float`	Top of absorbing region in Pa

[Model]¶

Composite Forward Models¶

Contributions¶

Examples¶

Molecular Absorption¶

Collisionally Induced Absorption¶

Keywords¶

Rayleigh Scattering¶

Optically thick clouds¶

Keywords¶

Fitting Parameters¶

Mie scattering (Lee)¶

Keywords¶

Fitting Parameters¶

Mie scattering (Precomputed grids)¶

Keywords¶

Fitting Parameters¶

Mie scattering (BH)¶

Keywords¶

Fitting Parameters¶

Mie scattering (Flat)¶

Keywords¶

Fitting Parameters¶

`[Model]`¶