Consistent simulations of the growth and radar scattering of branched planar ice crystals

Description

Polarimetric radar measurements provide information about ice microphysical processes; vapor deposition, in particular, is a key process governing the longevity of Arctic mixed-phase clouds. To interpret these radar measurements, assumptions about the scattering properties of ice crystals are needed. In the most detailed models of vapor depositional growth, the particles are represented by homogeneous, bulk-density spheroids, with aspect ratios, densities, and sizes that evolve according to the environmental temperature and supersaturation. However, these physical properties do not provide information about the detailed structure of the simulated ice crystals; this detailed structure can have large impacts on the radar scattering properties of natural branched planar ice crystals. Additionally, there is uncertainty in the evolution of density during vapor growth. To include information about branched planar crystal structure during vapor deposition, a parameterization for the density, corresponding to realistic branched planar crystal shapes, is developed. This parameterization is also consistent with physical structures that can be used to directly calculate the radar scattering properties with the Discrecte Dipole Approximation (DDA) code. It is shown that different assumptions about the detailed shape evolution of the ice crystals have large impacts on both the physical and radar scattering properties. Narrowing the range of probable shapes that occur during vapor growth using radar observations will likely improve the representation of this process in microphysical models.