An application of the boundary element method (BEM) to the calculation of the single-scattering properties of very complex ice crystals in the microwave and sub-millimetre regions of the electromagnetic spectrum

To improve the prediction of weather and climate models there is a need for the accurate computation of the single-scattering properties of randomly oriented complex atmospheric ice crystals. Here, we apply BEM to calculate these properties in the microwave and sub-millimetre region of the electromagnetic spectrum for the purposes of all-sky data assimilation. The properties are calculated at the frequencies of 50, 183, 243 and 664 GHz for the temperatures of -83 degrees C, -43 degrees C, and -3 degrees C. The particles are assumed to be complex aggregates of bullet rosettes with maximum dimensions that vary between 10 and 10, 000 mu m. Moreover, the rosette-aggregates are constructed to follow an observed mass-dimension power law that is consistent with an ice microphysics scheme in a weather model. To solve efficiently the BEM linear matrix equation, random orientation is simulated by fixing the particle with respect to the incident plane wave with the latter rotated about the particle. This representation is shown to replicate T-matrix solutions found for hexagonal columns to within a few percent for size parameters between 0.05 and 10. We further show that we can simulate the single-scattering properties with errors less than a few percent, using only 14 and up to 302 incident waves for the smallest and largest size parameters respectively. The errors grow larger only for some of the largest size parameters considered. We find that BEM can be applied to compute accurately the scattering properties of complex ice aggregates of importance to weather and climate models.