Full Polarization Observations of Oriented Ice Crystals and Rain by the NCAR GV High Spectral Resolution Lidar

The polarization effects resulting from light scattering by oriented particles are different from those of their randomly oriented counterparts. In cases where oriented particles are present, these additional polarization effects may be useful for discerning the physical properties of the scattering volume to greater detail and accuracy. Additionally, the polarization effects exhibited by oriented particles can generate erroneous and non-unique depolarization ratios in conventional polarization lidar which have no diagnostic for detecting this potential data quality issue.

At present, there is very little observational information about the polarization properties of oriented particles. Several parameters or oriented particles are not well established, including the magnitude of their polarization effects, their frequency of occurrence, and the environmental and observational conditions where their unique polarization properties appear in measurements. Prior to this work, the only polarimetrically complete observations of oriented particles, published by Kaul et al. in 2004, indicated that polarization effects from azimuthally oriented ice crystals are the rule rather than the exception. This suggests that most polarization lidar observations of ice clouds are corrupted by the presence of oriented crystals and conflicts with the general assumption that ice crystals only orient in the horizontal plane.

At the National Center for Atmospheric Research (NCAR), we have developed a technique for resolving all backscatter polarization properties of oriented particles that required minor modification of the Gulfstream V High Spectral Resolution Lidar (GV-HSRL). The GV-HSRL has recorded the polarization effects of oriented particles in the atmosphere at lidar tilt angles of 4°, 16°, 20°, and 30° off zenith over approximately 300 days.

We will present these observations, including information on conditions where oriented ice crystals produce significant polarization effects, the magnitude of those effects, and observational platforms that may have data quality issues with those effects.

We will also show the same polarization effects are produced by flattened raindrops, whose deviation from spherical is known to occur in the horizontal plane as drop sizes become large. These observations allow us to resolve size sorting and dynamic effects on raindrops sizes below convective storms.