Backscattering by Nonspherical Particles, Using the Coupled-Dipole Method: AnApplication in Radar Meteorology

摘要

In the coupled-dipole method, an arbitrary particle is modeled as an array of Npolarizable subunits each of which gives rise to only electric dipole radiation. Of all scattering angles, backscattering is the most sensitive to small changes in particle size and shape. The coupled-dipole method's ability and limitations for calculating backscattering are demonstrated. For particles with size parameter less than that associated with the first backscattering minimum, the coupled-dipole method agrees favorably with Mie theory. For particles with larger size parameters the agreement decreases, but accuracy generally improves by increasing the number of dipolar subunits in the array. Backscattering of 94 GHz Doppler radar by raindrops can be used to infer clear air velocity; backscattering by ice crystals may provide similar information. Backscattering at 94 GHz by randomly oriented ice plates or columns does not agree with backscattering by equal-volume ice spheres for size parameters greater than 0.8 Backscattering depends on zenith angle for ice crystals whose principle axes are confined to the horizontal plane. The relationship between first backscattering minimum and size parameter varies with particle shape and zenith angle. Backscattering of vertically polarized light is more sensitive to the presence of ice columns while horizontally polarized light is more sensitive to ice plates.