A fast hybrid (3‐D/1‐D) model for thermal radiative transfer in cirrus via successive orders of scattering. Issue 1 (6th January 2017)

• Record Type: Journal Article
• Authors: Fauchez, Thomas; Davis, Anthony B.; Cornet, Céline; Szczap, Fredéric; Platnick, Steven; Dubuisson, Philippe; Thieuleux, François
• Is Part Of: Journal of geophysical research. Volume 122:Issue 1(2017)
• Page Start: 344
• Page End: 366
• Publication Date: 2017-01-06
• Subjects: cirrus -- 3‐D radiative transfer -- thermal infrared -- scattering by ice crystals; Atmospheric physics -- Periodicals; Geophysics -- Periodicals; 551.5
• DOI: 10.1002/2016JD025607
• Languages: English
• ISSNs: 2169-897X

Abstract

Abstract: We investigate the impact of cirrus cloud heterogeneity on the direct emission by cloud or surface and on the scattering by ice particles in the thermal infrared (TIR). Realistic 3‐D cirri are modeled with the 3DCLOUD code, and top‐of‐atmosphere radiances are simulated by the 3‐D Monte Carlo radiative transfer (RT) algorithm 3DMCPOL for two (8.65 μm and 12.05 μm) channels of the Imaging Infrared Radiometer on CALIPSO. At nadir, comparisons of 1‐D and 3‐D RT show that 3‐D radiances are larger than their 1‐D counterparts for direct emission but smaller for scattered radiation. For our cirrus cases, 99% of the 3‐D total radiance is computed by the third scattering order, which corresponds to 90% of the total computational effort, but larger optical thicknesses need more scattering orders. To radically accelerate the 3‐D RT computations (using only few percent of 3‐D RT time with a Monte Carlo code), even in the presence of large optical depths, we develop a hybrid model based on exact 3‐D direct emission, the first scattering order from 1‐D in each homogenized column, and an empirical adjustment linearly dependent on the optical thickness to account for higher scattering orders. Good agreement is found between the hybrid model and the exact 3‐D radiances for two very different cirrus models without changing the empirical parameters. We anticipate that a future deterministic implementation of the hybrid model will be fast enough to process multiangle thermal imagery in a practical tomographic reconstruction of 3‐D cirrus fields. Key Points: Three‐dimensional heterogeneous pixels lead to larger direct emission but smaller scattering contribution than 1‐D homogeneous pixels On average, for thermal infrared wavelengths, 99% of the total radiance is computed after the third scattering Three‐dimensional radiances can be approximated using an efficient hybrid model combining 3‐D direct emission and 1‐D scatterings