The Inclusion of Thermal Emissions Within the SASKTRAN Framework

The current capabilities of SASKTRAN – a radiative transfer model at the University of Saskatchewan in Saskatoon, Canada – are to accurately model the scattering of solar radiation within the earth’s atmosphere for the ultraviolet-visible (UV-Vis) and near infra-red (NIR) regions of the electromagnetic spectrum. However, the current model does not account for the radiation emitted by the ground and atmosphere, approximated by the blackbody spectrum. In the UV-Vis, this contribution is unimportant, but when transitioning to wavelengths longer than 2.5 μm, the solar spectrum decreases in intensity while radiation of terrestrial and atmospheric origin increases along the blackbody curve. At wavelengths longer than 5 μm in the far infra-red (FIR), the blackbody radiation is the dominant source in the atmosphere. A modification to the source code of SASKTRAN was made in order to include the additional effect of this “thermal” radiation – with the help of the spectral line database HITRAN – while still maintaining scattering capabilities of solar radiation. This would make SASKTRAN one of the first radiative transfer models with the ability to model radiation in the difficult region between 3 and 5 μm – the mid infra-red (MIR) region – where the solar and thermal radiation sources are equally diminished and are the same order of magnitude. An introduction is given to atmospheric physics with a focus on the science of infra-red active molecules like H2O, CO2, CH4, N2O, O3, and CO – the so-called “greenhouse gases” – and the measurement techniques used to determine their atmospheric distribution. A theoretical basis is then provided for general radiative transfer, and the physics of molecular absorption and emission is examined in detail. A summary of the implementation of thermal radiation within the SASKTRAN framework is given, followed by verification studies where the model’s radiative transfer calculations in the infra-red are compared against measurements, including those made by the ground-based instrument E-AERI, the space-borne instruments IASI and GOSAT, and against model results from the LBLRTM, another well-verified radiative transfer model