In models of radiative transfer, the two-stream approximation is a discrete ordinate approximation in which radiation propagating along only two discrete directions is considered. It was first used by
Arthur Schuster in 1905. The two ordinates are chosen such that the model captures the essence of
radiative transport in light scattering atmospheres.
[W.E. Meador and W.R. Weaver, 1980, Two-Stream Approximations to Radiative Transfer in Planetary Atmospheres: A Unified Description of Existing Methods and a New Improvement, 37, Journal of the Atmospheric Sciences, 630–643
http://journals.ametsoc.org/doi/pdf/10.1175/1520-0469%281980%29037%3C0630%3ATSATRT%3E2.0.CO%3B2] A practical benefit of the approach is that it reduces the computational cost of integrating the radiative transfer equation. The two-stream approximation is commonly used in parameterizations of radiative transport in
global circulation models and in
weather forecasting models, such as the
WRF. There is a large number of applications of the two-stream approximation, including variants such as the
Kubelka-Munk approximation. It is the simplest approximation that can be used to explain common observations inexplicable by single-scattering arguments, such as the brightness and color of the clear sky, the brightness of clouds, the whiteness of a glass of milk, and the darkening of sand upon wetting. The two-stream approximation comes in many variants, including the
Eddington approximation, as well as the modified Eddington, Quadrature, and Hemispheric constant models.
Mathematical descriptions of the two-stream approximation are given in several books.
See also
*
List of atmospheric radiative transfer codes
Notes and references
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Atmospheric radiation