EXTINCTION EFFICIENCY AND SINGLE-SCATTERING ALBEDO FOR LABORATORY ANDNATURAL CIRRUS CLOUDS

A combination of the finite-difference time domain technique and a ray -by-ray integration method has been applied to compute the extinction efficiency and single-scattering albedo for various size distributions associated with nonspherical ice crystals in laboratory and natural c irrus clouds. The two methods are applicable to small and large size p arameters, respectively. The results obtained by the two methods conve rge when effective size parameters are larger than about 6. For labora tory ice crystals the overall features of the computed extinction effi ciency are in general agreement with those determined from measurement s. In particular, significant extinction windows at 2.85 and 10.5 mu m , associated with the Christiansen effect, are observed in both theore tical and experimental results. These extinction minima appear because the real part of the refractive index approaches unity, so that absor ption dominates light attenuation. The single-scattering albedos at th e two Christiansen spectral regions are found to be smaller than 0.5 f or the laboratory ice crystals. The contours of extinction efficiency and single-scattering albedo versus wavelength and particle size show that the magnitude of the Christiansen effect is dependent on particle size. For large ice crystals, the extinction windows are not signific ant because the extinction efficiency converges to its asymptotic valu e of 2, regardless of size parameters. For a number of size distributi ons observed during FIRE II IFO, the Christiansen effect is small. How ever, for cold cirrus, the extinction efficiencies in the Christiansen bands are approximately one half of the values at nearby wavelengths due to a significant number of small ice crystals that are present in cold cirrus clouds. It is concluded that the Christiansen effect must be accounted for in the determination of the extinction efficiency and the single-scattering albedo for small ice particles in order to obta in a reliable optical depth and emissivity for cirrus clouds at infrar ed wavelengths. Finally, we show that using spherical particles with M ie theory is inadequate to explain the extinction measurements.