Analysis of the Internal Fields of Snowflakes using the Discrete Dipole Approximation, and the Significance of this for the Development of New Approximate Scattering Methods

In order to obtain accurate radar and radiometer retrievals, we require good scattering models for ice particles at millimetre and sub-millimetre wavelengths. Many studies have shown that large errors ensue from the traditional method of assuming that ice particles are spherical or spheroidal in order to solve Maxwell’s solutions exactly. This is because ice crystals generally have complex habits, and the scattering properties are sensitive to that structure.

In this work, we use the discrete dipole approximation to examine the internal electric field within the scattering ice particle, and show how this structure varies with geometry. We find that the field exhibits a strong focussing behaviour within simple particles like hexagonal plates and prisms. The total field is a combination of two distinct waves; a forward propagating wave through the middle of the particle, and a standing wave close to the perimeter which influences side-scattering. As the complexity and irregularity of the particles is increased, we find that the focussing decreases and the total field weakens. Fluffy aggregates have a weak internal field, and the monomers within these aggregates seem to behave independently of one another.

Addressing the problem at this fundamental level we aim to acquire greater physical insight into how scattering at radar and radiometer frequencies works, and what clues it offers towards development of new approximate scattering methods for ice particles.