Backscattering by nonspherical particles, using the coupled-dipole method: An application in radar meteorology

In the coupled-dipole method, an arbitrary particle is modeled as an array of N polarizable subunits each of which gives rise to only electric dipole radiation. The total scattering is calculated by summing the waves scattered by each dipolar subunit excited by the incident wave and the waves of all the other dipolar subunits. By accounting for the dipolar interactions, the accuracy of the scattering calculations improves, but the mathematics become more complicated. The matrix inversion and scattering-order techniques are used to solve for the dipolar interactions.; The Clausius-Mosotti relation has been the most widely used effective- medium theory to relate the polarizability of the dipolar subunits to the refractive index of the bulk particle that the array represents. The polarizability of the dipolar subunits has been calculated using the electric dipole coefficient from Mie theory. This alternative expression for polarizability leads to scattering calculations that better agree with Mie theory.; 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.