An atmospheric radiative transfer model, code, or simulator calculates

radiative transfer 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 tran ...

electromagnetic radiation In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic field, electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, inf ...

through a

planetary atmosphere Planetary means relating to a planet or planets. It can also refer to: ;Science * Planetary habitability, the measure of an astronomical body's potential to develop and sustain life * Planetary nebula, an astronomical object ;People * Planetary ...

Methods

At the core of a radiative transfer model lies the radiative transfer equation that is numerically solved using a

solver A solver is 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 calculates t ...

such as a discrete ordinate method or a

Monte Carlo method Monte Carlo methods, or Monte Carlo experiments, are a broad class of computational algorithms that rely on repeated random sampling to obtain numerical results. The underlying concept is to use randomness to solve problems that might be determi ...

. The radiative transfer equation is a

monochromatic A monochrome or monochromatic image, object or color scheme, palette is composed of one color (or lightness, values of one color). Images using only Tint, shade and tone, shades of grey are called grayscale (typically digital) or Black and wh ...

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 A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to iden ...

for all

molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...

s in the atmospheric layer; this is called a ''line-by-line'' calculation. For an instrument response, this is then

convolved In mathematics (in particular, functional analysis), convolution is a operation (mathematics), mathematical operation on two function (mathematics), functions ( and ) that produces a third function (f*g) that expresses how the shape of one is ...

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 is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...

and other parameters). In addition, models may consider

scattering Scattering is a term used in physics to describe a wide range of physical processes where moving particles or radiation of some form, such as light or sound, are forced to deviate from a straight trajectory by localized non-uniformities (including ...

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

humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. Humidity depe ...

). Radiative transfer models are also used to optimize

solar photovoltaic A photovoltaic system, also PV system or solar power system, is an electric power system designed to supply usable solar power by means of photovoltaics. It consists of an arrangement of several components, including solar panels to absorb and c ...

systems for

renewable energy Renewable energy is energy that is collected from renewable resources that are naturally replenished on a human timescale. It includes sources such as sunlight, wind, the movement of water, and geothermal heat. Although most renewable energy ...

generation. Another common field of application is in a weather or climate model, where the

radiative forcing Radiative forcing (or climate forcing) is the change in energy flux in the atmosphere caused by natural or anthropogenic factors of climate change as measured by watts / metre2. It is a scientific concept used to quantify and compare the external ...

is calculated for

greenhouse gas A greenhouse gas (GHG or GhG) is a gas that Absorption (electromagnetic radiation), absorbs and Emission (electromagnetic radiation), emits radiant energy within the thermal infrared range, causing the greenhouse effect. The primary greenhouse ...

es,

aerosols An aerosol is a suspension of fine solid particles or liquid droplets in air or another gas. Aerosols can be natural or anthropogenic. Examples of natural aerosols are fog or mist, dust, forest exudates, and geyser steam. Examples of anthrop ...

, or

clouds In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may com ...

. 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. Continual Intercomparison of Radiation Codes
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Table of models

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.

References

;Footnotes ;General *Bohren, Craig F. and Eugene E. Clothiaux, Fundamentals of atmospheric radiation: an introduction with 400 problems, Weinheim : Wiley-VCH, 2006, 472 p., . * Goody, R. M. and Y. L. Yung, Atmospheric Radiation: Theoretical Basis. Oxford University Press, 1996 (Second Edition), 534 pages, . *Liou, Kuo-Nan, An introduction to atmospheric radiation, Amsterdam ; Boston : Academic Press, 2002, 583 p., International geophysics series, v.84, . *Mobley, Curtis D., Light and water: radiative transfer in natural waters; based in part on collaborations with Rudolph W. Preisendorfer, San Diego, Academic Press, 1994, 592 p., *Petty, Grant W, A first course in atmospheric radiation (2nd Ed.), Madison, Wisconsin : Sundog Pub., 2006, 472 p., *Preisendorfer, Rudolph W., Hydrologic optics, Honolulu, Hawaii : U.S. Dept. of Commerce, National Oceanic & Atmospheric Administration, Environmental Research Laboratories, Pacific Marine Environmental Laboratory, 1976, 6 volumes. *Stephens, Graeme L., Remote sensing of the lower atmosphere : an introduction, New York, Oxford University Press, 1994, 523 p. . * Thomas, Gary E. and Knut Stamnes, Radiative transfer in the atmosphere and ocean, Cambridge, New York, Cambridge University Press, 1999, 517 p., . * Zdunkowski, W., T. Trautmann, A. Bott, Radiation in the Atmosphere. Cambridge University Press, 2007, 496 pages,

External links