Abstract
The GV-HSRL is a high spectral resolution lidar capable of measuring calibrated backscatter, extinction and circular depolarization from the ground or NSF Gulfstream V platform. In the spring of 2012, the instrument was modified to measure the full backscatter matrix of atmospheric scatterers. This modification enabled us to investigate the polarization properties of oriented particles and further understand where particles orient and how they may impact depolarization lidar data. Observations were performed from the ground at different times with the lidar’s tilt angle at 4°, 22° and 32° degrees off zenith. This instrument found oriented ice crystals only produced observable polarization effects at the 32° lidar tilt angle. By contrast, a significant fraction of rain observations have oriented scattering matrices at all three lidar tilt angles. Thus conventional depolarization lidar is generally well suited for characterizing ice crystals but not liquid precipitation.Oriented ice crystals are most commonly investigated by looking for specular scatter from horizontally oriented plates. While this method offers excellent sensitivity to small populations of oriented plats, it has very little capability to determine the fraction of oriented plates if the population is larger than about 1%. We show here that observing f 12 scattering matrix element at 32° off zenith may be more effective for estimating the oriented fraction of particles in a volume.
Highlights
A typical depolarization lidar system intrinsically assumes that the particles under investigation are randomly oriented, which, by symmetry and reciprocity must have a diagonal backscatter matrix with three degrees of freedom[1; 2] f14 F(π) = β 1−d 0 0 d−1 (1) f14 00 2d − 1 where β is the volume backscatter coefficient and d is the propensity of the medium to depolarize
Observations performed by the GV-HSRL of oriented scattering matrices showed that oriented polarization effects are quite rare in ice clouds
Even when oriented particle polarization effects were observed, they were only significant in the f12 parameter and diagonal terms seemed to conform relatively well to the functional form of randomly oriented particles
Summary
A typical depolarization lidar system intrinsically assumes that the particles under investigation are randomly oriented, which, by symmetry and reciprocity must have a diagonal backscatter matrix with three degrees of freedom (backscatter, depolarization and where asymmetric particles exist, circular diattenuation)[1; 2]. The scattering angle argument π indicates this is a backscattering phase matrix independent of incident direction When this assumption is valid, the standard depolarization ratio is sufficient to fully describe the backscatter properties of the volume. The resulting depolarization measurements will depend on several factors, such as the lidar’s specific pointing angle, and the polarization transmitted and measured by the lidar These potential errors are not well quantified due to the very limited number of oriented scattering matrix observations. Lidar observations typically take the approach of looking for strong specular reflections from plates by directing the instrument very close to zenith or nadir This technique has the benefit of being highly sensitive to the presence of even a small population of ice crystals, but comes at the expense of diluting the signatures of all other particles in the volume. Diattenuation maintains a more consistent sensitivity over a large range of FO, making it a more useful parameter for estimating the oriented particle fraction
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