Comparison with Other Models

A variety of radiative transfer models exist, both for the calculation of radiative fluxes and the simulation of radiances measured by satellite sensors. However, those used for the calculation of radiative fluxes are generally components of climate models (cf., Ellingson et al., 1991) and are not well documented or easy to use. For this reason they will not be discussed further in this paper. Radiative transfer models that are well documented, reliable, and available to the scientific community include LOWTRAN (Kneizys et al., 1988), MODTRAN (Snell et al., 1995), and 6S (Vermote et al., 1994). While other general-purpose models exist, these three have been widely used in remote sensing applications. They are all medium or high spectral resolution band models and incorporate thorough treatments of gas absorption. However, the cloud models in LOWTRAN/MODTRAN are not very easy to modify, and the two-stream approximation for multiple scattering can result in significant errors under certain conditions. The 6S model does not include clouds but is very flexible for clear sky satellite simulations. None of these models computes fluxes directly, and the user interfaces are somewhat crude. Some of the major similarities and differences between these models are listed in the table below.

Table 1. Comparison of a few common radiative transfer models.




Numerical approximation method(s)

Discrete ordinates and two-stream

Two-stream, including atmospheric refraction; discrete ordinates also in MODTRAN-3

Successive orders of scattering

Spectral resolution

24 shortwave bands;
20 cm-1 bandwidth in longwave

20 cm-1 (LOWTRAN);
2 cm-1 (MODTRAN)

10 cm-1, shortwave only


Flexible specification of cloud physical properties; multiple ice cloud particle types; user-specified optical properties

Eight cloud models; user-specified optical properties

No clouds


Six optical models, some user control

Four optical models

Six optical models plus user-defined

Gas absorption*

Principle gases only

Principle and trace gases

Principle and trace gases

Atmospheric profiles

Standard and user-specified

Standard and user-specified

Standard and user-specified

Surface characteristics

Lambertian and BRDF, built-in spectral albedo models and user-specified BRDF

Lambertian, no built-in models

Lambertian spectral albedo models built-in; bidirectionally reflecting surface possible

Primary output


Radiance/reflectance/brightness temperature or flux



User interface

Input file with command language; interactive mode; web interface

Formatted input file

Input file

*In this table, principle gases are H2 O, O3, CO2, and O2. Trace gases include, among others, CH4, N2O, and CO.