Radiative properties of cirrus clouds in the infrared (8-13 mu m) spectralregion

Atmospheric radiation in the infrared (IR) 8-13 mum spectral region contain s a wealth of information that is very useful for the retrieval of ice clou d properties from aircraft or space-borne measurements. To provide the scat tering and absorption properties of nonspherical ice crystals that are fund amental to the IR retrieval implementation, we use the finite-difference ti me-domain (FDTD) method to solve for the extinction efficiency, single-scat tering albedo, and the asymmetry parameter of the phase function for ice cr ystals smaller than 40 mum. For particles larger than this size, the improv ed geometric optics method (IGOM) can be employed to calculate the asymmetr y 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 metho ds provides the asymmetry parameter for the entire practical range of parti cle sizes between 1 and 10, 000 mum over the wavelengths ranging from 8 to 13 mum. For the extinction and absorption efficiency calculations, several methods including the IGOM, Mie solution for equivalent spheres (MSFES), an d the anomalous diffraction theory (ADT) can lead to a substantial disconti nuity in comparison with the FDTD solutions for particle sizes on the order of 40 mum. To overcome this difficulty, we have developed a novel approach called the stretched scattering potential method (SSPM), For the IR 8-13 m um 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 fr om 1 to 10,000 mum at 12 wavelengths between 8 and 13 mum. Calculations of the cirrus bulk scattering and absorption properties are pe rformed 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 prope rties are parameterized through the use of second-order polynomial function s for the extinction efficiency and the single-scattering albedo and a powe r-law expression for the asymmetry parameter. We note that the volume-norma lized extinction coefficient can be separated into two parts: one is invers ely proportional to effective size and is independent of wavelength, and th e other is the wavelength-dependent effective extinction efficiency. Unlike conventional parameterization efforts, the present parameterization scheme is more accurate because only the latter part of the volume-normalized ext inction coefficient is approximated in terms of an analytical expression. A fter averaging over size distribution, the single-scattering albedo is show n to decrease with an increase in effective size for wavelengths shorter th an 10.0 mum whereas the opposite behavior is observed for longer wavelength s. The variation of the asymmetry parameter as a function of effective size is substantial when the effective size is smaller than 50 mum For effectiv e sizes larger than 100 mum, the asymmetry parameter approaches its asympto tic value. The results derived in this study can be useful to remote sensin g studies of ice clouds involving IR window bands. (C) 2001 Elsevier Scienc e Ltd. All rights reserved.