An Atmospheric radiative transfer model, code, or simulator calculates radiative transfer of electromagnetic radiation through a planetary atmosphere, such as the Earth's.

## Methods

At the core of a radiative transfer model lies the radiative transfer equation that is numerically solved using a solver such as a discrete ordinate method or a Monte Carlo method. The radiative transfer equation is a monochromatic equation to calculate radiance in a single layer of the Earth's atmosphere. To calculate the radiance for a spectral region with a finite width (e.g., to estimate the Earth's energy budget or simulate an instrument response), one has to integrate this over a band of frequencies (or wavelengths). The most exact way to do this is to loop through the frequencies of interest, and for each frequency, calculate the radiance at this frequency. For this, one needs to calculate the contribution of each spectral line for all molecules in the atmospheric layer; this is called a line-by-line calculation. For an instrument response, this is then convolved with the spectral response of the instrument. A faster but more approximate method is a band transmission. Here, the transmission in a region in a band is characterised by a set of pre-calculated coefficients (depending on temperature and other parameters). In addition, models may consider scattering from molecules or particles, as well as polarisation; however, not all models do so.

## Applications

Radiative transfer codes are used in broad range of applications. They are commonly used as forward models for the retrieval of geophysical parameters (such as temperature or humidity). Radiative transfer models are also used to optimize solar photovoltaic systems for renewable energy generation.[1] Another common field of application is in a weather or climate model, where the radiative forcing is calculated for greenhouse gases, aerosols, or clouds. In such applications, radiative transfer codes are often called radiation parameterization. In these applications, the radiative transfer codes are used in forward sense, i.e. on the basis of known properties of the atmosphere, one calculates heating rates, radiative fluxes, and radiances.

There are efforts for intercomparison of radiation codes. One such project was ICRCCM (Intercomparison of Radiation Codes in Climate Models) effort that spanned the late 1980s - early 2000s. The more current (2011) project, Continual Intercomparison of Radiation Codes, emphasises also using observations to define intercomparison cases. [2]

## Table of models

Name
Website
References
UV
Visible
Near IR
Thermal IR
mm/sub-mm
Microwave
line-by-line/band
Scattering
Polarised
Geometry
Notes
4A/OP Scott and Chédin (1981)

[3]

No No Yes Yes No No band or line-by-line Yes Yes freeware
6S/6SV1 Kotchenova et al. (1997)

[4]

No Yes Yes No No No band ? Yes non-Lambertian surface
ARTS Eriksson et al. (2011)

[5]

No No No Yes Yes Yes line-by-line Yes Yes spherical 1D, 2D, 3D GPL
BTRAM Chapman et al. (2009)

[6]

No Yes Yes Yes Yes Yes line-by-line No No 1D,plane-parallel proprietary commercial
COART Jin et al. (2006)

[7]

Yes Yes Yes Yes No No Yes No plane-parallel free
CRM No Yes Yes Yes No No band Yes No freely available Part of NCAR Community Climate Model
CRTM No Yes Yes Yes No Yes band Yes ?
DART radiative transfer model Gastellu-Etchegorry et al. (1996)

[8]

No Yes Yes Yes No No band Yes ? spherical 1D, 2D, 3D free for research with license non-Lambertian surface, landscape creation and import
DISORT Stamnes et al. (1988)[9] Yes Yes Yes Yes Yes radar Yes No plane-parallel or pseudo-spherical (v4.0) free with restrictions discrete ordinate, used by others
FARMS Xie et al. (2016)

[11]

λ>0.2 µm Yes Yes No No No band Yes No plane-parallel free Rapidly simulating downwelling solar radiation at land surface for solar energy and climate research
Fu-Liou Fu and Liou (1993)

[12]

No Yes Yes ? No No Yes ? plane-parallel usage online, source code available web interface online at [13]
FUTBOLIN Martin-Torres (2005)

[14]

λ>0.3 µm Yes Yes Yes λ<1000 µm No line-by-line Yes ? spherical or plane-parallel handles line-mixing, continuum absorption and NLTE
GENLN2 Edwards (1992)

[15]

? ? ? Yes ? ? line-by-line ? ?
KARINE Eymet (2005)

[16]

No No Yes No No ? ? plane-parallel GPL
KCARTA ? ? Yes Yes ? ? line-by-line Yes ? plane-parallel freely available AIRS reference model
KOPRA No No No Yes No No ? ?
LBLRTM Clough et al. (2005)

[17]

Yes Yes Yes Yes Yes Yes line-by-line ? ?
LEEDR Fiorino et al. (2014)

[18]

λ>0.2 µm Yes Yes Yes Yes Yes band or line-by-line Yes ? spherical US government software extended solar & lunar sources;

single & multiple scattering

LinePak Gordley et al. (1994)

[19]

Yes Yes Yes Yes Yes Yes line-by-line No No spherical (Earth and Mars), plane-parallel freely available with restrictions web interface, SpectralCalc

[20]

Yes Yes Yes Yes No No band or line-by-line Yes Yes plane-parallel or pseudo-spherical GPL
MATISSE Caillault et al. (2007)

[21]

No Yes Yes Yes No No band Yes ? proprietary freeware
MCARaTS [22] GPL 3-D Monte Carlo
MODTRAN Berk et al. (1998)

[23]

<50,000 cm−1 Yes Yes Yes Yes Yes band or line-by-line Yes ? proprietary commercial solar and lunar source, uses DISORT
MOSART Cornette (2006)

[24]

λ>0.2 µm Yes Yes Yes Yes Yes band Yes No freely available
PUMAS Yes Yes Yes Yes Yes Yes Line-by-line and correlated-k Yes Yes plane-parallel and pseudo-spherical Free/online tool

[25]

No No Yes No No No No plane-parallel GPL
RFM No No No Yes No No line-by-line No ? available on request MIPAS reference model based on GENLN2
RRTM/RRTMG Mlawer, et al. (1997)

[26]

<50,000 cm−1 Yes Yes Yes Yes >10 cm−1 ? ? free of charge uses DISORT
RTMOM λ>0.25 µm Yes Yes λ<15 µm No No line-by-line Yes ? plane-parallel freeware
RTTOV Saunders et al. (1999)

[27]

λ>0.4 µm Yes Yes Yes Yes Yes band Yes ? available on request

(2008)[29]

(2015)[30]

Yes Yes Yes No No No line-by-line Yes Yes spherical 1D, 2D, 3D, plane-parallel available on request discrete and Monte Carlo options
SBDART Ricchiazzi et al. (1998)

[31]

Yes Yes Yes ? No No Yes ? plane-parallel uses DISORT
SCIATRAN Rozanov et al. (2005)

,[32]

`Rozanov et al. (2014)`

[33]

Yes Yes Yes No No No band or line-by-line Yes Yes plane-parallel or pseudo-spherical or spherical
SHARM Lyapustin (2002)

[34]

No Yes Yes No No No Yes ?
SHDOM Evans (2006)

[35]

? ? Yes Yes ? ? Yes ?
σ-IASI Amato et al. (2002)[36] No No Yes Yes Yes No band Yes No plane-parallel Available on request Semi-analytical Jacobians.
SMART-G Ramon et al. (2019)

[38]

Yes Yes Yes No No No band or line-by-line Yes Yes plane-parallel or spherical free for non-commercial purposes Monte-Carlo code parallelized by GPU (CUDA). Atmosphere or/and ocean options
Streamer, Fluxnet [39] Key and Schweiger (1998)

[40]

No No λ>0.6 mm λ<15 mm No No band Yes ? plane-parallel Fluxnet is fast version of STREAMER using neural nets
XRTM Yes Yes Yes Yes Yes Yes Yes Yes plane-parallel and pseudo-spherical GPL
Name Website References UV VIS Near IR Thermal IR Microwave mm/sub-mm line-by-line/band Scattering Polarised Geometry License Notes

### Molecular absorption databases

For a line-by-line calculation, one needs characteristics of the spectral lines, such as the line centre, the intensity, the lower-state energy, the line width and the shape.

NameAuthorDescription
HITRAN[41] Rothman et al. (1987, 1992, 1998, 2003, 2005, 2009, 2013, 2017) HITRAN is a compilation of molecular spectroscopic parameters that a variety of computer codes use to predict and simulate the transmission and emission of light in the atmosphere. The original version was created at the Air Force Cambridge Research Laboratories (1960's). The database is maintained and developed at the Harvard-Smithsonian Center for Astrophysics in Cambridge MA, USA.
GEISA[42] Jacquinet-Husson et al. (1999, 2005, 2008) GEISA (Gestion et Etude des Informations Spectroscopiques Atmosphériques: Management and Study of Spectroscopic Information) is a computer-accessible spectroscopic database, designed to facilitate accurate forward radiative transfer calculations using a line-by-line and layer-by-layer approach. It was started in 1974 at Laboratoire de Météorologie Dynamique (LMD/IPSL) in France. GEISA is maintained by the ARA group at LMD (Ecole Polytechnique) for its scientific part and by the ETHER group (CNRS Centre National de la Recherche Scientifique-France) at IPSL (Institut Pierre Simon Laplace) for its technical part. Currently, GEISA is involved in activities related to the assessment of the capabilities of IASI (Infrared Atmospheric Sounding Interferometer on board of the METOP European satellite) through the GEISA/IASI database derived from GEISA.

## References

Footnotes
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39. FluxNet
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41. HITRAN Site
42. GEISA Site
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