Atmospheric radiative transfer codes
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, one has to integrate this over a band of frequencies. 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. 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. 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 effort that spanned the late 1980s - early 2000s. The more current project, Continual Intercomparison of Radiation Codes, emphasises also using observations to define intercomparison cases.
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.| Name | Author | Description |
| HITRAN | Rothman et al. | 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. The database is maintained and developed at the Harvard-Smithsonian Center for Astrophysics in Cambridge MA, USA. |
| GEISA | Jacquinet-Husson et al. | 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. It was started in 1974 at Laboratoire de Météorologie Dynamique in France. GEISA is maintained by the ARA group at LMD for its scientific part and by the ETHER group at IPSL for its technical part. Currently, GEISA is involved in activities related to the assessment of the capabilities of IASI through the GEISA/IASI database derived from GEISA. |