Using the Boundary Element Method (BEM) to compute the Single-Scattering Properties of Ice Crystals in the microwave and sub-millimetre range

The boundary element method (BEM) is a popular computational tool used for electromagnetic scattering that can handle complicated domains and is therefore appealing when one considers scattering by complex-shaped ice crystals. BEM has been used to compute single-scattering properties (SSPs) of simple hexagonal ice columns, plates, and six-branched rosettes (Groth, et al., 2015), but due to memory costs, size parameters were limited to 15. With recent accelerating techniques that reduce memory by 99% and computational time by 75% at high frequencies (Kleanthous, et al., 2019) - (Kleanthous, et al., -), we applied BEM to very complex rosette aggregates of much bigger size parameter. The aggregates were generated via a Monte Carlo simulation that follows observed mass and area dimension power laws (Westbrook, et al., 2004). We computed the SSPs of 65 different aggregates, of maximum dimension between 10 and 10,000μm, at frequencies 50, 183, 243 and 664GHz, and at temperatures 190, 210, 230, 250 and 270K for application to the simulation of observed microwave and sub-millimetre radiances. The open-source boundary element software Bempp (Śmigaj, et al., 2015) was used for the simulations. Our method required 14 waves to simulate random orientation for the smallest aggregates and up to 230 for the largest, compared to about 4,000 orientations required to simulate random orientation in three dimensions by methods such as DDA. By distributing the multiple incoming waves on different CPUs, we reduced computational time even further. We discuss the implementation and results of our work.

Groth, S. P. et al., 2015. The boundary element method for light scattering by ice crystals and its implementation in BEM++. J Quant Spectrosc Ra, Volume 167, pp. 40-52.

Kleanthous, A. et al., -. Accelerated Calderón preconditioning for Maxwell transmission problems. In preparation.

Kleanthous, A. et al., 2019. Calderón preconditioning of PMCHWT boundary integral equations for scattering by multiple absorbing dielectric particles. J Quant Spectrosc Ra, Volume 224, pp. 383-395.

Śmigaj, W. et al., 2015. Solving boundary integral problems with BEM++. ACM T Math Software, 41(2), pp. 1-40.