A new look at the two stream multiple scattering approximation

The two stream approximation (TSA) is used to compute radiative flux transfer when analytical solutions are a priority, for example to reduce computational burden in climate modeling or enable inversion of optical parameters in remote sensing. The traditional TSA assumes the upwelling and downwelling radiance fields have the same spatial distributions. The TSA presented here does not require this assumption. The result is a more realistic representation of scattering in the atmosphere, for which the source of illumination from above is collimated and that from below is closer to isotropic. The gain in precision is significant, estimated here over a wide range of atmospheric conditions.