Scattering and attenuation by non-spherical atmospheric particles

Abstract

The approximate theory of Gans for electro-magnetic scattering and absorption by ellipsoids is applied to atmospheric particles. It is found that the general form and state of orientation of such particles may be determined, at least in a qualitative manner, from the back-scattered radiation. Spherical particles return radiation polarized like the incident radiation. Non-spherical particles (with some exceptions) give rise to a cross-polarized component. If the particles have preferred orientations, the back-scattered energy depends not only on particle shape but also on the polarization of the radiation and on its angle of incidence. This dependence is more pronounced for water (or water-coated ice) particles than for ice, while snow, on account of its low refractive index, scatters almost like spheres of equal volume, regardless of particle shape or orientation. Randomly oriented shaped particles always scatter and attenuate more strongly than equivolumic spheres; if the particles are oriented, there may be an increase or decrease of their scattering and attenuating powers, depending on their orientation relative to the incident beam. These results in conjunction with Kerker, langleben and Gunn's [1] calculations on scattering and attenuation by water-coated ice spheres are shown to be essential constituents of the theory of the bright band (the intense radar echo received from precipitation particles in the process of melting).