Radiative properties of cirrus clouds in the infrared (8– [formula omitted]) spectral region

• Published in: Journal of quantitative spectroscopy & radiative transfer 2001-08, Vol.70 (4), p.473-504
• Main Authors: Yang, Ping, Gao, Bo-Cai, Baum, Bryan A., Hu, Yong X., Wiscombe, Warren J., Tsay, Si-Chee, Winker, Dave M., Nasiri, Shaima L.
• Format: Article
• Language: English
• Subjects: Cirrus; Crystal; Infrared
• ISSN: 0022-4073; 1879-1352
• DOI: 10.1016/S0022-4073(01)00024-3

Atmospheric radiation in the infrared (IR) 8– 13 μm spectral region contains a wealth of information that is very useful for the retrieval of ice cloud properties from aircraft or space-borne measurements. To provide the scattering and absorption properties of nonspherical ice crystals that are fundamental to the IR retrieval implementation, we use the finite-difference time-domain (FDTD) method to solve for the extinction efficiency, single-scattering albedo, and the asymmetry parameter of the phase function for ice crystals smaller than 40 μm . For particles larger than this size, the improved geometric optics method (IGOM) can be employed to calculate the asymmetry parameter with an acceptable accuracy, provided that we properly account for the inhomogeneity of the refracted wave due to strong absorption inside the ice particle. A combination of the results computed from the two methods provides the asymmetry parameter for the entire practical range of particle sizes between 1 and 10,000 μm over the wavelengths ranging from 8 to 13 μm . For the extinction and absorption efficiency calculations, several methods including the IGOM, Mie solution for equivalent spheres (MSFES), and the anomalous diffraction theory (ADT) can lead to a substantial discontinuity in comparison with the FDTD solutions for particle sizes on the order of 40 μm . To overcome this difficulty, we have developed a novel approach called the stretched scattering potential method (SSPM). For the IR 8– 13 μm spectral region, we show that SSPM is a more accurate approximation than ADT, MSFES, and IGOM. The SSPM solution can be further refined numerically. Through a combination of the FDTD and SSPM, the extinction and absorption efficiencies are computed for hexagonal ice crystals with sizes ranging from 1 to 10,000 μm at 12 wavelengths between 8 and 13 μm . Calculations of the cirrus bulk scattering and absorption properties are performed for 30 size distributions obtained from various field campaigns for midlatitude and tropical cirrus cloud systems. Ice crystals are assumed to be hexagonal columns randomly oriented in space. The bulk scattering properties are parameterized through the use of second-order polynomial functions for the extinction efficiency and the single-scattering albedo and a power-law expression for the asymmetry parameter. We note that the volume-normalized extinction coefficient can be separated into two parts: one is inversely proportional to effective size and is in