Spectral dependence of backscattering coefficient of mixed phase clouds over West Africa measured with two-wavelength Raman polarization lidar: Features attributed to ice-crystals corner reflection

- Corner reflection of randomly oriented ice crystals of simple shape may lead to spectral dependence of backscattering coefficient.
- Backscattering, extinction coefficients and particle depolarization ratios of mixed phased clouds over West Africa were measured by Mie-Raman lidar at 355 nm and 532 nm.
- Measurements were performed at slant trajectory (43 degree off zenith), so specular reflections of oriented crystals are excluded.
- In most of the cases no spectral dependence of backscattering was observed, meaning that corner reflection of randomly oriented crystals can be suppressed by crystals deformation or by surface roughness.
- In two measurement sessions we observed spectral dependence of backscattering and depolarization in the height regions containing precipitating crystals. It can be related to the corner reflection by oriented plates.

The existing models predict that corner reflection (CR) of laser radiation by simple ice crystals of perfect shape, such as hexagonal columns or plates, can provide a significant contribution to the ice cloud backscattering. However in real clouds the CR effect may be suppressed due to crystal deformation and surface roughness. In contrast to the extinction coefficient, which is spectrally independent, consideration of diffraction associated with CR results in a spectral dependence of the backscattering coefficient. Thus measuring the spectral dependence of the cloud backscattering coefficient, the contribution of CR can be identified. The paper presents the results of profiling of backscattering coefficient (β) and particle depolarization ratio (δ) of ice and mixed-phase clouds over West Africa by means of a two-wavelength polarization Mie-Raman lidar operated at 355 nm and 532 nm during the SHADOW field campaign. The lidar observations were performed at a slant angle of 43 degree off zenith, thus CR from both randomly oriented crystals and oriented plates could be analyzed. For the most of the observations the cloud backscatter color ratio β355/β532 was close to 1.0, and no spectral features that might indicate the presence of CR of randomly oriented crystals were revealed. Still, in two measurement sessions we observed an increase of backscatter color ratio to a value of nearly 1.3 simultaneously with a decrease of the spectral depolarization ratio δ355/δ532 ratio from 1.0 to 0.8 inside the layers containing precipitating ice crystals. We attribute these changes in optical properties to corner reflections by horizontally oriented ice plates.