Extinction efficiency and single‐scattering albedo for laboratory and natural cirrus clouds

Abstract

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 cirrus clouds. The two methods are applicable to small and large size parameters, respectively. The results obtained by the two methods converge when effective size parameters are larger than about 6. For laboratory ice crystals the overall features of the computed extinction efficiency are in general agreement with those determined from measurements. In particular, significant extinction windows at 2.85 and 10.5 μm, associated with the Christiansen effect, are observed in both theoretical and experimental results. These extinction minima appear because the real part of the refractive index approaches unity, so that absorption dominates light attenuation. The single‐scattering albedos at the two Christiansen spectral regions are found to be smaller than 0.5 for 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 significant because the extinction efficiency converges to its asymptotic value of 2, regardless of size parameters. For a number of size distributions observed during FIRE II IFO, the Christiansen effect is small. However, 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 obtain a reliable optical depth and emissivity for cirrus clouds at infrared wavelengths. Finally, we show that using spherical particles with Mie theory is inadequate to explain the extinction measurements.