SCATTERING PHASE FUNCTION OF BULLET ROSETTE ICE CRYSTALS

Ice crystals in cirrus frequently exhibit the shape of a bullet rosett e composed of multiple bullets that radiate from a junction center. Th e scattering phase function of these ice crystals, pertinent to the ra diation budget of cirrus, may differ from the one obtained for ice cry stals with a simple geometrical shape. In this paper, the authors stud ied the sensitivity of the scattering phase function of a bullet roset te to its geometrical characteristics: the shape, aspect ratio, and sp atial orientation. In doing so, they defined first an idealized bullet rosette according to the current knowledge of the crystalline structu re and nucleation process of bullet rosettes. The scattering phase fun ction was computed with a ray-tracing method. The scattering phase fun ction of a bullet rosette varies with its shape, and the lateral and b ackward scattering tends to increase with the number of bullets/bullet rosettes. This is due to the interaction of light scattered by a bull et with its adjacent bullets. This feature qualitatively agrees with t he earlier experimental results reported for irregularly shaped partic les. However, for a bullet rosette with 3D random orientation, the eff ect of this interaction is much smaller than expected. The normalized scattering phase function locally differs only by about 20% from one s hape to another. Earlier studies were made for ice crystals randomly o riented in a 3D space, and in this case, the scattering properties hav e been represented by 1D phase functions (versus the scattering angle) . For a bullet rosette with preferred orientation, the scattering patt ern (which is not azimuthally symmetrical and so depends on scattering angles in the 3D space) varies significantly with the shape of the bu llet rosette and the direction of incident light. Although the shape o f a bullet rosette with 3D random orientation does not greatly affect the general feature of normalized scattering phase functions, its geom etrical shape still remains an important factor for scattering and mic rophysical properties of cirrus. This is due to the fact that the geom etrical cross section of a bullet rosette, perpendicular to the incide nt light, changes with its shape. Thus, optical properties such as the extinction coefficient of cirrus with a given ice water content may c hange significantly with the ice crystal shape.