Effects of spherical inclusions on scattering properties of small ice cloud particles

The single-scattering properties of small ice crystals containing four types of spherical inclusions, ammonium sulfate (NH4)2SO4, ammonium nitrate NH3NO3, air bubbles, and soot, are investigated at 0.65 and 2.13 μm. Small, randomly oriented hexagonal ice columns with spherical inclusions that are randomly distributed with standard gamma size distributions in the columns are considered in the present study. Ice crystals with inclusions of (NH4)2SO4 and NH3NO3 essentially have the same features due to their similar refractive indices. Nonzero scattering matrix elements are sensitive to inclusion type and amount, and show differences between 0.65 and 2.13 μm. The extinction efficiency Qe of small ice crystals at 0.65 μm is near 2.0 and essentially unaffected by variations in inclusion volume, in contrast to strong influences of inclusion amount on Qe at 2.13 μm. The single-scattering albedo ϖ 0 of ice crystals, nearly equal to 1.0, is not affected by inclusions of (NH4)2SO4, NH3NO3, and air bubbles. Soot inclusions strongly affect ϖ 0, which decreases to about 0.5 with increasing soot amounts. The asymmetry factor g is substantially affected by (NH4)2SO4, NH3NO3, and soot and the variations in their amounts. Full Stokes parameters of cirrus clouds consisting of uniform hexagonal ice columns with inclusions are computed using a polarized radiative transfer model. Sensitivities of light intensity and polarization of cirrus clouds to types and amounts of inclusions and cirrus cloud optical thicknesses are found to depend on wavelength. The present results suggest that different types of inclusions for small ice crystals should be considered when developing realistic ice crystal optical properties, and that light intensity and polarization of cirrus clouds and their angular distribution features, in the absence of other effects such as cavities and surface roughness, imply the potential for identifying pure ice crystals from those with aerosol inclusions.