Modeling Ice Multiplication and Simulated Fully Polarimetric Radar Signals

Ice multiplication is a significant, and yet poorly understood process. Ice multiplication leads to particle number concentrations orders of magnitude greater than the available ice nuclei concentrations. Despite such substantial impacts on cloud properties, there is no consensus on the most significant mechanisms of ice multiplication or their associated particle production rates. The secondary ice process most frequently implemented in microphysics parameterization schemes is Hallett-Mossop rime splintering. Such schemes tie ice splinter production to the accretion of cloud droplets by graupel, with maximum production rates occurring at -5 ºC. Other processes are less frequently implemented, but include shattering of large, supercooled drops upon freezing, and ice-ice collisions that result in particle fragmentation.

These ice multiplication processes fundamentally change the sizes, distributions, and shapes of ice crystals in clouds. In principle, such hydrometeor changes are observable by polarimetric radar. Indeed, signals observed in several radar variables, including differential reflectivity (ZDR), specific differential phase (KDP), and linear depolarization ratio (LDR), often are associated with ice multiplication. In addition, fully polarimetric radars can measure the cross-polar correlation coefficient (RhoXH) and cross-polar phase shift (PhiXH), which can be used to observe ice and similarly may have utility in detecting ongoing ice multiplication.

This study seeks to improve our understanding of the radar signals associated with various ice multiplication processes and parameterizations. By pairing a fully polarimetric forward operator with a microphysical model incorporating ice multiplication, we can better associate the radar signals (particularly those in less-common variables such as RhoXH and PhiXH) with physical processes. By incorporating several mechanisms and parameterizations of ice multiplication, we can improve our understanding of how they produce differing radar signals, and whether these radar signals can be used for microphysical process fingerprinting. Ultimately this will help improve future remote sensing and radar studies of ice multiplication.