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.^[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	License	Notes
4A/OP	Archived 2011-07-21 at the Wayback Machine	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] Buehler et al. (2018)^[6]	No	No	No	Yes	Yes	Yes	line-by-line	Yes	Yes	spherical 1D, 2D, 3D	GPL
BTRAM		Chapman et al. (2009)^[7]	No	Yes	Yes	Yes	Yes	Yes	line-by-line	No	No	1D, plane-parallel	proprietary commercial
COART		Jin et al. (2006)^[8]	Yes	Yes	Yes	Yes	No	No		Yes	No	plane-parallel	free
CMFGEN		Hillier (2020)^[9]	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)^[10]	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)^[11]	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)^[12] Lin et al. (2015)^[13]	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)^[14]	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)^[15]	No	Yes	Yes	?	No	No		Yes	?	plane-parallel	usage online, source code available	web interface online:^[16]
FUTBOLIN		Martin-Torres (2005)^[17]	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)^[18]	?	?	?	Yes	?	?	line-by-line	?	?
KARINE	^{[link removed]}	Eymet (2005)^[19]	No	No	Yes		No	No		?

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