Scattering Phase Function of Bullet Rosette Ice Crystals

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Abstract
Ice crystals in cirrus frequently exhibit the shape of a bullet rosette composed of multiple bullets that radiate from a junction center. The scattering phase function of these ice crystals, pertinent to the radiation budget of cirrus, may differ from the one obtained for ice crystals with a simple geometrical shape. In this paper, the authors studied the sensitivity of the scattering phase function of a bullet rosette to its geometrical characteristics: the shape, aspect ratio, and spatial orientation. In doing so, they defined first an idealized bullet rosette according to the current knowledge of the crystalline structure and nucleation process of bullet rosettes. The scattering phase function was computed with a ray-tracing method. The scattering phase function of a bullet rosette varies with its shape, and the lateral and backward scattering tends to increase with the number of bullets/bullet rosettes. This is due to the interaction of light scattered by a bullet with its adjacent bullets. This feature qualitatively agrees with the earlier experimental results reported for irregularly shaped particles. However, for a bullet rosette with 3D random orientation, the effect of this interaction is much smaller than expected. The normalized scattering phase function locally differs only by about 20% from one shape to another. Earlier studies were made for ice crystals randomly oriented in a 3D space, and in this case, the scattering properties have been represented by ID phase functions (versus the scattering angle). For a bullet rosette with preferred orientation, the scattering pattern (which is not azimuthally symmetrical and so depends on scattering angles in the 3D space) varies significantly with the shape of the bullet rosette and the direction of incident light. Although the shape of a bullet rosette with 3D random orientation does not greatly affect the general feature of normalized scattering phase functions, its geometrical shape still remains an important factor for scattering and microphysical properties of cirrus. This is due to the fact that the geometrical cross section of a bullet rosette, perpendicular to the incident light, changes with its shape. Thus, optical properties such as the extinction coefficient of cirrus with a given ice water content may change significantly with the ice crystal shape.

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