The Polarimetric Radar Scattering Properties of Oriented Aggregates

The mechanics of how individual, pristine ice crystals collide and aggregate together are poorly understood. In particular, the orientation of the aggregate as it collects pristine crystals and how the pristine crystals behave when colliding with the aggregate are important factors in the evolution of shape and size during aggregation. Furthermore, how the shape and size evolve during aggregation determine the resulting radar scattering properties, and thus these radar measurements can provide information about the aggregation process. To better use polarimetric radar measurements to study aggregation, we randomly generate aggregates with different assumptions about the aggregate orientation (either uniformly random or uniformly random with respect to azimuth angle) and the rotation of pristine crystals upon collision (pivoting). We find that aggregate density tends to increase when pivoting is assumed and when the orientation distribution of the aggregate is azimuthally uniform. Additionally, aggregates of branched planar crystals tend to have larger densities compared to aggregates of columns. To connect the physical and scattering properties, we perform Discrete Dipole Approximation calculations at Ku and Ka band for the particles at various orientation angles. For aggregates with preferred orientations in the horizontal plane, differential reflectivity and specific differential phase increase with particle density, and the correlation coefficient values decrease with density. These relations between the radar moments and the aggregate density are consistent across each set of aggregates, suggesting that polarimetric measurements can provide information about aggregate density over regions of precipitation sampled by these radars.