Effect of the inhomogeneity of ice crystals on retrieving ice cloud optical thickness and effective particle size

Spherical or spheroidal air bubbles are often trapped in rapidly growing ice crystals. In this study, the single-scattering properties of inhomogeneous ice crystals containing air bubbles are investigated. Specifically, a combination of the ray-tracing technique and the Monte Carlo method is used to simulate the scattering of light by randomly oriented large hexagonal ice crystals containing spherical or spheroidal air bubbles. The effect of the air bubbles within ice crystals is to smooth the phase functions, diminish the 22 degrees and 46 degrees halo peaks, and reduce the backscatter in comparison with the case of bubble-free ice crystals. These features vary with the number, sizes, locations, and shapes of the air bubbles within the ice crystals. Moreover, the asymmetry factors of inhomogeneous ice crystals decrease as the ratio of air-bubble volume to ice crystal volume increases. Cloud reflectance look-up tables were generated at the wavelengths of 0.65 mu m and 2.13 mu m to examine the impact of accounting for air bubbles in ice crystal morphology on the retrieval of ice cloud optical thickness and effective particle size. The reflectances simulated for inhomogeneous ice crystals are larger than those computed for homogeneous ice crystals at a wavelength of 0.65 mu m. Thus the retrieved cloud optical thickness is reduced by employing inhomogeneous ice cloud models. At a wavelength of 2.13 mu m, including air bubbles in ice crystal morphology may also increase the reflectance. This effect implies, particularly in the case of large air bubbles, that the retrieved effective particle size for inhomogeneous ice crystals is larger than that retrieved for homogeneous ice crystals.