Effect of Cavities on the Optical Properties of Bullet Rosettes: Implications for Active and Passive Remote Sensing of Ice Cloud Properties

Abstract

Bullet rosette particles are common in ice clouds, and the bullets may often be hollow. Here the singlescattering properties of randomly oriented hollow bullet rosette ice particles are investigated. A bullet, which is an individual branch of a rosette, is defined as a hexagonal column attached to a hexagonal pyramidal tip. For this study, a hollow structure is included at the end of the columnar part of each bullet branch and the shape of the hollow structure is defined as a hexagonal pyramid. A hollow bullet rosette may have between 2 and 12 branches. An improved geometric optics method is used to solve for the scattering of light in the particle. The primary optical effect of incorporating a hollow end in each of the bullets is to decrease the magnitude of backscattering. In terms of the angular distribution of scattered energy, the hollow bullets increase the scattering phase function values within the forward scattering angle region from 1°to 20°but decrease the phase function values at side-and backscattering angles of 60°-180°. As a result, the presence of hollow bullets tends to increase the asymmetry factor. In addition to the scattering phase function, the other elements of the phase matrix are also discussed. The backscattering depolarization ratios for hollow and solid bullet rosettes are found to be very different. This may have an implication for active remote sensing of ice clouds, such as from polarimetric lidar measurements. In a comparison of solid and hollow bullet rosettes, the effect of the differences on the retrieval of both the ice cloud effective particle size and optical thickness is also discussed. It is found that the presence of hollow bullet rosettes acts to decrease the inferred effective particle size and to increase the optical thickness in comparison with the use of solid bullet rosettes.

Remote sensing of ice crystal asymmetry parameter using multi-directional polarization measurements – Part 1: Methodology and evaluation with simulated measurements

“…Next we apply the proposed method to simulated measurements assuming a selection of hexagonal and complex ice crystal habits: droxtals (Zhang et al, 2004), columns, plates, bullet rosettes, aggregates of columns, hollow columns (Yang and Liou, 1998), hollow bullet rosettes (Yang et al, 2008c) and aggregates of plates . Details about the geometries of these crystals are given in the cited references.…”

Scanning electron microscopy and molecular dynamics of surfaces of growing and ablating hexagonal ice crystals

“…Radiative transfer methods, moreover, allow one to predict how these varying shapes affect the optical properties of cloud particles (Neshyba et al, 2003;Yang et al, 2008a).…”

“…Optical micrographs of falling atmospheric ice crystals originating under cirrus-like conditions exhibit mesoscopic texturing of various kinds, including step formation on the inside of hollow hexagonal prisms, and corrugations on the outside (Walden et al, 2003;Yang et al, 2008a); micrographs of laboratory-grown ice crystals exhibit a similarly large array of surface structures (e.g., Hallett and Mason, 1958). Limitations inherent to optical microscopy, however, place definite limits on one's ability to discern details of such structures, and relatively little attention has been paid to how those structures evolve over the time scale pertinent to cirrus cloud formation and dissipation.…”

On the relationship between the scattering phase function of cirrus and the atmospheric state

“…Theoretical light-scattering studies by (Schmitt and Heymsfield 2010;Macke et al, 1996a;Macke et al, 1996b;Yang and Liou, 1998;Yang et al, 2008;van Diedenhoven, 2014b) have shown that the processes of surface roughness and air inclusions within ice crystals can profoundly alter their scattering phase functions. As surface roughness increases, the 22 and 46 • halos are reduced or completely removed, resulting in featureless phase functions with a high degree of side scattering.…”