Laurence Taylor, Evelyn Hesse, Z. Ulanowski, Paul H. Kaye, Antti Penttila, Timo Nousiainen, ‘Beam tracing with diffraction’, poster presented at the 15th Electromagnetic & Light Scattering Conference, Leipzig, Germany, 21-26 June, 2015.Non peer reviewe

Hesse, Evelyn

Kaye, Paul H.

Nousiainen, Timo

Penttila, Antti

Taylor, Laurence

Ulanowski, Z.

University of Hertfordshire Research Archive

ELS-XV-2015 AbstractsELS-XV-2015-50-1Electromagnetic & Light Scattering XV 2015, Leipzig© Author(s) 2015. CC Attribution 3.0 License.Beam Tracing with DiffractionL. Taylor, E. Hesse, Z. Ulanowski, P.H. Kaye, A. Penttilä, and T. NousiainenCentre for Atmospheric and Instrumentation Research, University of Hertfordshire, Hatfield, UK (l.taylor3@herts.ac.uk)It is well accepted that uncertainties still persist regarding the quantitative contribution of ice clouds to theradiative balance of the Earth [1]. Techniques for modelling the interaction of light with ice crystals found in theseclouds have helped reduce this uncertainty; however techniques such as the discrete dipole approximation (DDA)[2] and T-matrix [3] are computationally expensive for particles with large size parameters. Despite the successof geometric optics (GO) and its variants for modelling electromagnetic scattering by such ice crystals, theiraccuracy is still limited by their use of approximations of phenomena associated with physical optics, for example,diffraction. Advances in GO methods have been proposed resulting in hybrid physical - geometric optics models,for example [4,5] which consider diffraction on facets and on equivalent circular areas respectively; alternativehybrid methods have been discussed in [6,7].The Beam Tracing with Diffraction (BTD) code developed at the University of Hertfordshire is a new hybridmodel for numerical determination of the phase matrix elements. The model considers the propagation ofelectromagnetic beams as they propagate through smooth polyhedral particles with complex refractive indexesby way of Snell’s law and Fresnel’s equations, in a manner similar to geometric optics. The beams are split asthey propagate through the crystal according to the regions that they illuminate on the crystal’s surface. Phasetracing allows for consideration of interference between scattered beams, the intensity of which is redistributedthrough the use of Kirchhoff’s approximation for far-field diffraction. The diffraction integral is evaluated over thearea illuminated by an exiting beam, that is, one which has either undergone external reflection or transmissionfollowing a number of internal reflections. External diffraction is evaluated by considering diffraction on upwardfacing facets, which act as diffracting apertures. We present results acquired from using this method in the formof 2-dimensional scattering patterns along with 1-dimensional, azimuthally averaged phase functions. Results arecompared to exact methods.[1] IPCC (2013), IPCC, 2013: Climate Change 2013: The physical science basis. Contribution of workinggroup I to the fifth assessment report of the Intergovernmental Panel on Climate Change.[2] MA. Yurkin, AG. Hoekstra (2007), The discrete dipole approximation: An overview and recent developments.JQRST, 106, 558-589.[3] MI. Mishchenko, et al (2004), T-matrix theory of electromagnetic scattering by particles and its applications: acomprehensive reference database. JQRST, 88, 357-406.[4] E. Hesse, A. Macke, et al (2012), Modelling diffraction by facetted particles. JQRST, 113, 342-347.[5] K. Muinonen (1989), Scattering of light by crystals: a modified Kirchhoff approximation. Applied Optics, 28,3044-3050.[6] AG. Borovoi, IA. Grishin (2003), Scattering matrices for large ice crystal particles. Journal of the OpticalSociety of America, A, 20, 2071-2080.[7] L. Bi, P. Yang, et al (2011), Scattering and absorption of light by ice particles: Solution by a new physical-geometric optics hybrid method. JQRST, 112, 1492-1508.

Beam tracing with diffraction

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Beam tracing with diffraction

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