Discussion

Encyclopedia
Radiative transfer is the physical phenomenon of energy transfer in the form of electromagnetic radiation. The propagation of radiation through a medium is affected by absorption, emission and scattering processes. The equation of radiative transfer describes these interactions mathematically...

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
Solver (computer science)
A solver is a generic term indicating a piece of mathematical software, possibly in the form of a stand-alone computer program or as a software library, that 'solves' a mathematical problem. A solver takes problem descriptions in some sort of generic form and calculate their solution...

such as a discrete ordinate method or a Monte Carlo method
Monte Carlo method
Monte Carlo methods are a class of computational algorithms that rely on repeated random sampling to compute their results. Monte Carlo methods are often used in computer simulations of physical and mathematical systems...

. 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
Numerical integration
In numerical analysis, numerical integration constitutes a broad family of algorithms for calculating the numerical value of a definite integral, and by extension, the term is also sometimes used to describe the numerical solution of differential equations. This article focuses on calculation of...

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
Spectral line
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from a deficiency or excess of photons in a narrow frequency range, compared with the nearby frequencies.- Types of line spectra :...

for all molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

s in the atmospheric layer; this is called a line-by-line calculation.
For an instrument response, this is then convolved
Convolution
In mathematics and, in particular, functional analysis, convolution is a mathematical operation on two functions f and g, producing a third function that is typically viewed as a modified version of one of the original functions. Convolution is similar to cross-correlation...

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
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

and other parameters). In addition, models may consider scattering
Scattering
Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...

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
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

or humidity
Humidity
Humidity is a term for the amount of water vapor in the air, and can refer to any one of several measurements of humidity. Formally, humid air is not "moist air" but a mixture of water vapor and other constituents of air, and humidity is defined in terms of the water content of this mixture,...

).
Another common field of application is in a weather or climate model, where the radiative forcing
In climate science, radiative forcing is generally defined as the change in net irradiance between different layers of the atmosphere. Typically, radiative forcing is quantified at the tropopause in units of watts per square meter. A positive forcing tends to warm the system, while a negative...

is calculated for greenhouse gas
Greenhouse gas
A greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone...

es, aerosols or clouds. In such applications radiative transfer codes are often called radiation parameterization
Parametrization (climate)
Parameterization in a weather or climate model within numerical weather prediction refers to the method of replacing processes that are too small-scale or complex to be physically represented in the model by a simplified process. This can be contrasted with other processes—e.g., large-scale flow of...

. 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 effeorts for intercomparison of radiation codes. One such project was ICRCCM (Intercomparison of Radiation Codes in Climate Models) effort that spanned the late 80's - early 00's. Current (2011) project Continual Intercomparison of Radiation Codes emphasises also using observations to define intercomparison cases.

## Table of models

Name
Website
References
UV
Visible
mm/sub-mm
Microwave
Scattering
Polarised
Geometry
Notes
4A/OP
4A/OP
4A/OP or, Automatized Atmospheric Absorption Atlas,is an operational fast and accurate radiative transfer model for the infrared.4A/OP is a user-friendly software for various scientific applications , co-developed by LMD and NOVELTIS with the support of CNES .NOVELTIS...

http://www.noveltis.fr/4AOP/ Scott and Chédin (1981) line-by-line freeware
6S/6SV1
6SV1 is an advanced radiative transfer code designed to simulate the reflection of solar radiation by a coupled atmosphere-surface system for a wide range of atmospheric, spectral and geometrical conditions.It belongs to the group of procedures called...

http://6s.ltdri.org/ Kotchenova et al. (1997) band non-Lambertian surface
ARTS http://www.sat.ltu.se/arts/ Buehler et al. (2005) line-by-line spherical 1D, 2D, 3D GPL
CRM http://www.cgd.ucar.edu/cms/crm/ freely available Part of NCAR Community Climate Model
CRTM

http://www.ssec.wisc.edu/~paulv/Fortran90/CRTM/Developmental/ band
DISORT
DISORT
DISORT - general and versatile plane-parallel radiative transfer program applicable to problems from the ultraviolet to the radar regions of the electromagnetic spectrum....

[ftp://climate1.gsfc.nasa.gov/wiscombe/Multiple_Scatt/] Stamnes et al. (1988) plane-parallel free with restrictions discrete ordinate, used by others
Fu-Liou http://snowdog.larc.nasa.gov/rose/fu200503/flp200503_web.htm Fu and Liou (1993) plane-parallel usage online, source code available web interface online at http://snowdog.larc.nasa.gov/cgi-bin/rose/flp200503/flp200503.cgi
FUTBOLIN
FUTBOLIN
FUTBOLIN : Multi-level multiple scattering radiative transfer model for the calculation of line-by-line atmospheric emission/transmission spectra in planetary atmospheres. It has been developed by Javier Martin-Torres FUTBOLIN (FUll Transfer By Optimized LINe-by-line methods): Multi-level multiple...

Martin-Torres (2005) line-by-line spherical or plane-parallel handles line-mixing, continuum absorption and NLTE
GENLN2
GENLN2
-See also:*List of atmospheric radiative transfer codes*Atmospheric radiative transfer codes*Absorption spectrum*MODTRAN*HITRAN-References:*Edwards, D. P. , GENLN2: A general line-by-line atmospheric transmittance and radiance model, Version 3.0 description and users guide, NCAR/TN-367-STR,...

http://acd.ucar.edu/~edwards/ Edwards (1992) line-by-line
KARINE
Karine
Karine may refer to:* Karine Bakhoum, an Egyptian American chef* Karine Costa , a French singer* Karine Haaland , a Norwegian comic strip creator* Karine Icher , a French professional golfer...

http://web.lmd.jussieu.fr/~eymet/karine.html Eymet (2005) colspan=2 plane-parallel GPL
KCARTA http://asl.umbc.edu/pub/packages/kcarta.html line-by-line plane-parallel freely available AIRS
Atmospheric Infrared Sounder
The Atmospheric Infrared Sounder is one of six instruments flying on board NASA’s Aqua satellite, launched on May 4, 2002. The instrument is designed to support climate research and improve weather forecasting....

reference model
KOPRA http://www-imk.fzk.de/asf/ame/publications/kopra_docu/kopra_docu.html
LBLRTM
LBLRTM

http://rtweb.aer.com/lblrtm.html Clough et al. (2005) line-by-line
libRadtran http://www.libradtran.org/doku.php Mayer and Kylling (2005) band or line-by-line plane-parallel or pseudo-spherical GPL
MATISSE http://matisse.onera.fr/index_english.htm Caillault et al. (2007) band propriety freeware
MODTRAN
MODTRAN
MODTRAN is a computer program designed to model atmospheric propagation of electromagnetic radiation for the 100-50,000 cm-1 spectral range....

http://www.modtran.org/ Berk et al. (1998) band propriety commercial solar and lunar source, uses DISORT
RFM http://www.atm.ox.ac.uk/RFM/ line-by-line available on request MIPAS reference model based on GENLN2
RRTM/RRTMG http://rtweb.aer.com/ Mlawer, et al. (1997) free of charge uses DISORT
RTMOM [ftp://ftp.eumetsat.int/pub/MET/out/govaerts/RTMOM-BETA/index.htm] line-by-line plane-parallel freeware
RTTOV

http://research.metoffice.gov.uk/research/interproj/nwpsaf/rtm/ Saunders et al. (1999) band available on request
SBDART http://arm.mrcsb.com/sbdart/ Ricchiazzi et al. (1998) plane-parallel uses DISORT
SCIATRAN http://www.iup.uni-bremen.de/sciatran/ Rozanov et al. (2005) plane-parallel
SHARM Lyapustin (2002)
Streamer, Fluxnet
FluxNet
FluxNet is a global network of micrometeorological tower sites that use eddy covariance methods to measure the exchanges of carbon dioxide, water vapor, and energy between the biosphere and atmosphere. Fluxnet is a global 'network of regional networks' that serves to provide an infrastructure to...

http://stratus.ssec.wisc.edu/streamer/streamer.htmlhttp://stratus.ssec.wisc.edu/fluxnet/ Key and Schweiger (1998) band plane-parallel Fluxnet is fast version of STREAMER using neural nets
Artificial neural network
An artificial neural network , usually called neural network , is a mathematical model or computational model that is inspired by the structure and/or functional aspects of biological neural networks. A neural network consists of an interconnected group of artificial neurons, and it processes...

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 line width and the shape.
Name Author Description
HITRAN
HITRAN
HITRAN - HITRAN is a compilation of 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 compiled by the Air Force Cambridge Research Laboratories...

Rothman et al. (1987, 1992, 1998, 2003, 2005, 2009) 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
GEISA
GEISA - GEISA is a computer-accessible spectroscopic database, designed to facilitate accurate forward radiative transfer calculations using a line-by-line and layer-by-layer approach...

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 over three decades 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.

• Discrete dipole approximation codes
Discrete dipole approximation codes
This article contains list of discrete dipole approximation codes and their applications.The discrete dipole approximation is a flexible technique for computing scattering and absorption by targets of arbitrary geometry. Given a target of arbitrary geometry, one seeks to calculate its scattering...

• Codes for electromagnetic scattering by cylinders
Codes for electromagnetic scattering by cylinders
Codes for electromagnetic scattering by cylinders - this article list codes for electromagnetic scattering by a cylinder. Some of the source codes may be available on ....

• Codes for electromagnetic scattering by spheres
Codes for electromagnetic scattering by spheres
Codes for electromagnetic scattering by spheres - this article list codes for electromagnetic scattering by a homogeneous sphere, layered sphere, and cluster of spheres...

### Light scattering

• Discrete dipole approximation codes
Discrete dipole approximation codes
This article contains list of discrete dipole approximation codes and their applications.The discrete dipole approximation is a flexible technique for computing scattering and absorption by targets of arbitrary geometry. Given a target of arbitrary geometry, one seeks to calculate its scattering...

• Codes for electromagnetic scattering by cylinders
Codes for electromagnetic scattering by cylinders
Codes for electromagnetic scattering by cylinders - this article list codes for electromagnetic scattering by a cylinder. Some of the source codes may be available on ....

• Codes for electromagnetic scattering by spheres
Codes for electromagnetic scattering by spheres
Codes for electromagnetic scattering by spheres - this article list codes for electromagnetic scattering by a homogeneous sphere, layered sphere, and cluster of spheres...