McSCIA: application of the Equivalence Theorem in a Monte Carlo radiative transfer model for spherical shell atmospheres F. Spada1, M. C. Krol1,2,3, and P. Stammes4 1Institute for Marine and Atmospheric research Utrecht (IMAU), Utrecht, The Netherlands 2Netherlands Institute for Space Research (SRON), Utrecht, The Netherlands 3Meteorology and Air Quality, Wageningen University, The Netherlands 4Koninklijk Nederlands Meteorologisch Instituut (KNMI), De Bilt, The Netherlands
Abstract. A new multiple-scattering Monte Carlo 3-D radiative transfer model named
McSCIA (Monte Carlo for SCIAmachy) is presented. The backward technique is
used to efficiently simulate narrow field of view instruments. The McSCIA
algorithm has been formulated as a function of the Earth's radius, and can
thus perform simulations for both plane-parallel and spherical atmospheres.
The latter geometry is essential for the interpretation of limb satellite
measurements, as performed by SCIAMACHY on board of ESA's Envisat. The model
can simulate UV-vis-NIR radiation.
First the ray-tracing algorithm is presented in detail, and then successfully
validated against literature references, both in plane-parallel and in
spherical geometry. A simple 1-D model is used to explain two different ways
of treating absorption. One method uses the single scattering albedo while
the other uses the equivalence theorem. The equivalence theorem is based on a
separation of absorption and scattering. It is shown that both methods give,
in a statistical way, identical results for a wide variety of scenarios. Both
absorption methods are included in McSCIA, and it is shown that also for a 3-D
case both formulations give identical results. McSCIA limb profiles for
atmospheres with and without absorption compare well with the one of the
state of the art Monte Carlo radiative transfer model MCC++.
A simplification of the photon statistics may lead to very fast calculations
of absorption features in the atmosphere. However, these simplifications
potentially introduce biases in the results. McSCIA does not use
simplifications and is therefore a relatively slow implementation of the
Citation: Spada, F., Krol, M. C., and Stammes, P.: McSCIA: application of the Equivalence Theorem in a Monte Carlo radiative transfer model for spherical shell atmospheres, Atmos. Chem. Phys., 6, 4823-4842, doi:10.5194/acp-6-4823-2006, 2006.