Millimeter wave scattering from spatial and planar bullet rosettes

The electromagnetic scattering characteristics of several bullet-rosette ice crystal forms are computationally evaluated at 35-, 94-, and 220-GHz frequencies and compared with those of stellar crystals, hexagonal plates, and columns. One of the bullet rosettes is a planar crystal with four branches, the other two are spatial rosettes with six and eight branches. Two orientation models are used, one represents highly oriented crystals for which side and vertical incidence directions are considered, and the other represents randomly oriented crystals (the incidence direction does not affect this case). It is observed that the linear depolarization ratio (LDR). as well as the copolarized correlation coefficient (rho(h upsilon)), can be used to differentiate columns from planar (including plates and stellar crystals) and spatial crystals based on their values at vertical incidence or their trends as a function of the elevation angle. For the random orientation case, LDR and rho(h upsilon) can differentiate columns from spatial crystals (except for sizes larger than 1.2 mm at 220 GHz) but not from planar crystals. Furthermore, the elevation angle dependence of LDR and Z(DR) (differential reflectivity) has the potential for differentiating columnar, planar, and spatial crystals for sizes from a few tenths of a millimeter to 2 mm at 220 GHz, and from about 1 to 2 mm at 94 GHz, At 35 GHz, spatial crystals smaller than 2-mm resemble spherical particles in terms of their Z(DR) and LDR signatures. The results for high-density (0.9 g cm(-3)) and low-density (representing hollow crystals) crystal models show significant differences in the values of LDR, Z(DR), rho(h upsilon), and the backscattering cross sections. However, their general trends as a function of size and elevation angle are very similar, Fitted curves for the orientation-averaged backscattering cross sections for vertical incidence and the random orientation model are also presented.