Scattering of millimeter radiowaves by dry snowflakes

The Purcell-Pennypacker coupled dipole method is used to calculate scattering of millimeter waves by the dry snowflakes modelled by a collection of intersected dendrite-like crystals, whose maximum length is comparable to the wavelength of the incident radiation. Averaged over sixteen orientations the extinction, scattering and absorption cross sections of the model snowflake in the frequency range from 10 to 130 GHz are computed. For comparison the corresponding cross sections of the equivalent sphere, whose radius is taken to be the gyroradius of aggregate and effective permittivity is taken to be that of the medium consisting of ice spheres and randomly oriented ice needles and disks immersed in air are also determined using Mie theory. As a result the effective medium model utilizing ice needles turns out to be the best matching of the flake extinction properties. No one of the effective medium models used adequately describes flake absorption in the hole above the mentioned frequency range. The authors also compare flake extinction and absorption properties with those for the equal volume ice sphere. In this case the computation shows that both the extinction and absorption properties of the equal volume ice sphere are in drastic contradiction with those for aggregated snowflakes.<>