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UP: Fachverband Umweltphysik

UP 19: Atmosphäre und Aerosole: Datenauswertung und Modellierung

UP 19.1: Talk

Wednesday, March 28, 2007, 16:15–16:30, Phy 5.0.20

Modelling of light scattering by ice crystals using the Ray Tracing with Diffraction on Facets method — •Evelyn Hesse, Adrian Clarke, Zbigniew Ulanowski, and Paul Kaye — University of Hertfordshire, UK

One of the greatest uncertainties in climate modelling is the influence of cirrus clouds on the radiation balance. Exact electromagnetic theory of light scattering can be applied only to particles with size parameters up to about 50. For moderate size parameters the Finite Difference Time Domain method is applicable, however it is computationally expensive. Scattering data calculated using Yang's Improved Geometric Optics (IGO)method are only available for a restricted range of particle shapes, sizes and wavelengths.

We have developed a new method combining ray tracing with diffraction on facets, which is suitable for rapid computation of scattering by facetted particles of intermediate size. Modelling of diffraction is based on the concept of energy flow lines. Each facet is treated as an aperture: When passing, a ray is bent towards the nearest edge by the far field deflection angle of an energy flow line through the same point. For small and large size parameters the model has been tested against the T-Matrix method, which is an exact method, the Separation of Variables method, and IGO, respectively. Comparisons with scattering data measured at single ice analog crystals have been made. The model has been applied for interpretation of aircraft cirrus radiance data and of linear depolarization measurements. Modelled 2D scattering patterns will be used for interpretation of nephelometric data.