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.