Radar scattering by realistic ice aggregates

Aggregation plays an important role in the evolution of ice clouds, and an understanding of the scattering from ice aggregates is crucial if microphysical properties of the cloud such as ice water content and average particle size are to be accurately inferred from radar measurements.

We have applied both the Rayleigh-Gans and discrete-dipole approximations to calculate the scattering from realistic ice aggregates produced by computer simulation of the aggregation process, in both Rayleigh and non-Rayleigh regimes. The results of these calculations allow us significant insight into how dual wavelength radar data may be inverted to estimate the microphysical properties of the cloud. We find that, depending on the pristine crystal size and type, multiple scattering between the crystals in the aggregate can play a significant role in determining the overall radar cross section in the non-Rayleigh regime. We also analyse the errors associated with modelling an aggregate as an 'equivalent ice-air sphere' and suggest a prescription for an appropriate effective dielectric constant and sphere diameter, so as to minimise those errors.