Rigorous Testing of the Rapid Radiative Transfer Model Across the Infrared Spectrum
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Global circulation models (GCMs) and climate simulations often use radiative fluxes and heating rates from radiative transfer models. However, the calculations that are used are those where scattering of a cloudy atmosphere is neglected. In this study, computed fluxes and heating rates are compared when absorption is the only process, and when scattering is included. Computations for the absorption only process were performed using the Rapid Radiative Transfer Model (RRTM), and the Discrete Ordinates Radiative Transfer Model (DISORT) is used when scattering is included. Over 8,000 model runs were conducted across various cloud layers, cloud water paths, cloud particle sizes, cloud particle shapes, and atmospheric profiles to deduce the effects of scattering in the infrared (IR) portion of the electromagnetic spectrum due to clouds. On average, the difference in upward flux at the top of the atmosphere (TOA) was roughly 4-12 W/m^2 and difference in downward flux at the surface (SFC) was roughly 1- 4 W/m^2. These differences were found mainly in the middle portion of the IR spectrum, although some instances were found to be close to the far IR portion of the spectrum as well. As mentioned in other similar studies, these numbers are significant when compared to average longwave radiation budget values. Neglecting them could lead to inaccurate calculations in GCMs and climate simulations. Similar tests were also computed when carbon dioxide was doubled in the atmosphere. Results show that the differences in fluxes compared to an atmosphere with current carbon dioxide values was less than 0.5 W/m^2.
Henning, William Lark (2016). Rigorous Testing of the Rapid Radiative Transfer Model Across the Infrared Spectrum. Master's thesis, Texas A & M University. Available electronically from