Impacts of small-scale surface irregularities, or surface roughness, of
atmospheric ice crystals on lidar backscattering properties are
quantified. Geometric ice crystal models with various degrees of surface
roughness and state-of-the-science light-scattering computational
capabilities are used to simulate single-scattering properties across
the entire practical size parameter range. The simulated bulk lidar and
depolarization ratios of polydisperse ice crystals at 532 nm are
strongly sensitive to the degree of surface roughness. Comparisons of
these quantities between the theoretical simulations and counterparts
inferred from spaceborne lidar observations for cold cirrus clouds
suggest a typical surface roughness range of 0.03--0.15, which is most
consistent with direct measurements of scanning electron microscopic
images. The degree of surface roughness needs to be accounted for to
properly interpret lidar backscattering observations of ice clouds.