# Atmospheric radiative transfer codes

An atmospheric radiative transfer model, code, or simulator calculates radiative transfer of electromagnetic radiation through a planetary atmosphere.

## 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. 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.

## Table of models

| Name | Website | References | UV | Visible | Near IR | Thermal IR | mm/sub-mm | Microwave | line-by-line/band | Scattering | Polarised | Geometry | License | Notes |
| 4A/OP | Archived 2011-07-21 at the Wayback Machine | Scott and Chédin (1981) | No | No | Yes | Yes | No | No | band or line-by-line | Yes | Yes | | freeware | |
| 6S/6SV1 | | Kotchenova et al. (1997) | No | Yes | Yes | No | No | No | band | ? | Yes | | | non-Lambertian surface |
| ARTS | | Eriksson et al. (2011) Buehler et al. (2018) | No | No | No | Yes | Yes | Yes | line-by-line | Yes | Yes | spherical 1D, 2D, 3D | GPL | |
| BTRAM | | Chapman et al. (2009) | No | Yes | Yes | Yes | Yes | Yes | line-by-line | No | No | 1D,plane-parallel | proprietary commercial | |
| COART | | Jin et al. (2006) | Yes | Yes | Yes | Yes | No | No | | Yes | No | plane-parallel | free | |
| CMFGEN | | Hillier (2020) | Yes | Yes | Yes | Yes | Yes | Yes | line-by-line | Yes | Yes | 1D | | |
| CRM | | | No | Yes | Yes | Yes | No | No | band | Yes | No | | freely available | Part of NCAR Community Climate Model |
| CRTM | | Johnson et al. (2023) | v3.0 | Yes | Yes | Yes | Yes | passive, active | band | Yes | v3.0, UV/VIS | 1D, Plane-Parallel | Public Domain | Fresnel ocean surfaces, Lambertian non-ocean surface |
| DART radiative transfer model | | Gastellu-Etchegorry et al. (1996) | 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) Lin et al. (2015) | Yes | Yes | Yes | Yes | Yes | radar | | Yes | No | plane-parallel or pseudo-spherical (v4.0) | free with restrictions | discrete ordinate, used by others |
| Eradiate | | | No | Yes | Yes | No | No | No | band or line-by-line | Yes | No | plane-parallel, spherical | LGPL | 3D surface simulation |
| FARMS | | Xie et al. (2016) | 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) | No | Yes | Yes | ? | No | No | | Yes | ? | plane-parallel | usage online, source code available | web interface online at |
| FUTBOLIN | | Martin-Torres (2005) | 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) | ? | ? | ? | Yes | ? | ? | line-by-line | ? | ? | | | |
| KARINE | | Eymet (2005) | No | No | Yes | 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) | Yes | Yes | Yes | Yes | Yes | Yes | line-by-line | ? | ? | | | |
| LEEDR | | Fiorino et al. (2014) | 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) | Yes | Yes | Yes | Yes | Yes | Yes | line-by-line | No | No | spherical (Earth and Mars), plane-parallel | freely available with restrictions | web interface, SpectralCalc |
| libRadtran | | Mayer and Kylling (2005) | Yes | Yes | Yes | Yes | No | No | band or line-by-line | Yes | Yes | plane-parallel or pseudo-spherical | GPL | |
| MATISSE | | Caillault et al. (2007) | No | Yes | Yes | Yes | No | No | band | Yes | ? | | proprietary freeware | |
| MCARaTS | | | | | | | | | | | | | GPL | 3-D Monte Carlo |
| MODTRAN | | Berk et al. (1998) | ṽ<50,000 cm$^{−1}$ (eq. to λ>0.2 µm) | Yes | Yes | Yes | Yes | Yes | band or line-by-line | Yes | ? | | proprietary commercial | solar and lunar source, uses DISORT |
| MOSART | | Cornette (2006) | λ>0.2 µm | Yes | Yes | Yes | Yes | Yes | band | Yes | No | | freely available | |
| MSCART | | Wang et al. (2017) Wang et al. (2019) | Yes | Yes | Yes | No | No | No | | Yes | Yes | 1D, 2D, 3D | available on request | |
| PICASO | link | Batalha et al. (2019) Mukherjee et al. (2022) | λ>0.3 μm | Yes | Yes | Yes | No | No | band or correlated-k | Yes | No | plane-parallel, 1D, 3D | GPL Github | exoplanet, brown dwarf, climate modeling, phase-dependence |
| PUMAS | | | Yes | Yes | Yes | Yes | Yes | Yes | Line-by-line and correlated-k | Yes | Yes | plane-parallel and pseudo-spherical | Free/online tool | |
| RADIS | | Pannier (2018) | No | No | Yes | Yes | No | No | | No | No | 1D | 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) | ṽ<50,000 cm$^{−1}$ (eq. to λ>0.2 µm) | Yes | Yes | Yes | Yes | ṽ>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) | λ>0.4 µm | Yes | Yes | Yes | Yes | Yes | band | Yes | ? | | available on request | |
| SASKTRAN | | Bourassa et al. (2008) Zawada et al. (2015) | 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) | Yes | Yes | Yes | ? | No | No | | Yes | ? | plane-parallel | | uses DISORT |
| SCIATRAN | | Rozanov et al. (2005) , Rozanov et al. (2014) | Yes | Yes | Yes | No | No | No | band or line-by-line | Yes | Yes | plane-parallel or pseudo-spherical or spherical | | |
| SHARM | | Lyapustin (2002) | No | Yes | Yes | No | No | No | | Yes | ? | | | |
| SHDOM | | Evans (2006) | ? | ? | Yes | Yes | ? | ? | | Yes | ? | | | |
| σ-IASI | | Amato et al. (2002) Liuzzi et al. (2017) | No | No | Yes | Yes | Yes | No | band | Yes | No | plane-parallel | Available on request | Semi-analytical Jacobians. |
| SMART-G | | Ramon et al. (2019) | 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 | | Key and Schweiger (1998) | 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 | |
| VLIDORT/LIDORT | | Spurr and Christi (2019) | Yes | Yes | Yes | Yes | ? | ? | line-by-line | Yes | Yes VLIDORT only | plane-parallel | | Used in SMART and VSTAR radiative transfer |
| 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.
| Name | Author | Description |
| HITRAN | 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 | 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. |