Fast, approximate methods for electromagnetic wave scattering by complex ice crystals and snowflakes

McCusker, K. ORCID: https://orcid.org/0000-0002-1886-5323 (2020) Fast, approximate methods for electromagnetic wave scattering by complex ice crystals and snowflakes. PhD thesis, University of Reading. doi: 10.48683/1926.00101018

Abstract/Summary

Improvements to scattering models are important for accurate retrievals of cloud ice. This thesis involves analysing the internal electric fields of ice particles and using the findings to develop and test new scattering approximations. The discrete dipole approximation (DDA) is used to explore the internal fields and farfield scattering properties of ice particles. We show that the field is relatively uniform for size parameter x = 2, but for monocrystals of x = 10 there is a complex internal structure, with focussing of the field towards the forward side. As particle complexity is increased due to aggregation, the field becomes smoother and less focussing is seen. For complex aggregates, the individual monomers act almost independently of one another, suggesting simplified methods of calculating scattering. We find that the Rayleigh-Gans approximation (RGA) and soft spheres provide a poor representation of the internal and far fields. A logical elaboration on RGA is a formulation permitting higher scattering orders. This technique is evaluated, however we find convergence is restricted to a limited subset of size parameter and shape. A new approximation for aggregates called the Independent Monomer Approximation (IMA) is presented, where interactions between different monomers of an aggregate are ignored. This enables time and memory reductions compared to using DDA. The IMA results are superior to RGA. A microwave closure experiment is performed. Aggregate models are generated to match measurements. The IMA method is used to perform scattering calculations that are input into a radiative transfer model. Simulations are compared to measurements from the ISMAR radiometer. Unlike RGA, the new method can reproduce the measured brightness temperature depressions and polarimetric signal, but results are sensitive to choice of particle shape. These findings are useful to guide preparations for the Ice Cloud Imager which will measure ice in clouds and snowfall from space following launch in 2023.

Item Type	Thesis (PhD)
URI	https://reading-clone.eprints-hosting.org/id/eprint/101018
Item Type	Thesis
Divisions	Science > School of Mathematical, Physical and Computational Sciences > Department of Meteorology
Date on Title Page	December 2019
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