McSCIA: application of the Equivalence Theorem in a Monte Carlo radiative transfer model for spherical shell atmospheres

[摘要] A new multiple-scattering Monte Carlo 3-D radiative transfer model namedMcSCIA (Monte Carlo for SCIAmachy) is presented. The backward technique isused to efficiently simulate narrow field of view instruments. The McSCIAalgorithm has been formulated as a function of the Earth's radius, and canthus perform simulations for both plane-parallel and spherical atmospheres.The latter geometry is essential for the interpretation of limb satellitemeasurements, as performed by SCIAMACHY on board of ESA's Envisat. The modelcan simulate UV-vis-NIR radiation.

First the ray-tracing algorithm is presented in detail, and then successfullyvalidated against literature references, both in plane-parallel and inspherical geometry. A simple 1-D model is used to explain two different waysof treating absorption. One method uses the single scattering albedo whilethe other uses the equivalence theorem. The equivalence theorem is based on aseparation of absorption and scattering. It is shown that both methods give,in a statistical way, identical results for a wide variety of scenarios. Bothabsorption methods are included in McSCIA, and it is shown that also for a 3-Dcase both formulations give identical results. McSCIA limb profiles foratmospheres with and without absorption compare well with the one of thestate of the art Monte Carlo radiative transfer model MCC++.

A simplification of the photon statistics may lead to very fast calculationsof absorption features in the atmosphere. However, these simplificationspotentially introduce biases in the results. McSCIA does not usesimplifications and is therefore a relatively slow implementation of theequivalence theorem.

[学科分类] 大气科学