The 2013 meeting of the Intergovernmental Panel on Climate Change concluded that the coupling of clouds to the Earth's climate is one of the biggest uncertainties faced in predicting climate change today. Cirrus clouds are of particular interest because the extensive variability in particle size, shape and complexity poses a number of challenges in the accurate modelling of optical properties. The dependence of the single-scattering properties on particle shape demand accurate representation of the crystal geometries in scattering models, geometries which can vary from simple hexagonal prisms to complex multi-branched aggregates. This work, presented as a series of papers, uses laboratory studies to investigate the single scattering properties of ice crystals. Of particular focus here are columns with internal cavities. The first paper investigates the phase function, P11 and asymmetry parameter, g of varying crystal habits, whilst the second paper focuses on polarised scattering. One of the main findings is the difference in internal structure between hollow columns grown at warmer temperatures (-7 degrees C) and those grown at colder temperatures (-30 degrees C). Measurements were used to create new particle geometries for use in scattering models. The third paper makes use of findings from papers 1 & 2 to create an optical parametrization for cirrus, utilizing the new particle geometry.

An investigation was carried out into light scattering by Gaussian rough ice crystals. Gaussian rough crystal geometries were generated using roughness parameters derived from mineral dust grains, which have been reported to be suitable proxies for rough ice crystals. Light scattering data for these geometries was computed using the discrete dipole approximation (DDA) method. Phase functions, 2D scattering patterns, degree of linear polarisation patterns and asymmetry parameters were computed for smooth, moderately rough and highly rough crystals with a variety of orientations and size parameters. A sodium fluorosilicate ice analogue crystal with three partially roughened prism facets was created using focused ion beam (FIB) milling and 2D scattering patterns were collected from it using the small ice detector (SID) 3 cloud probe. It was found that roughness reduces features in the phase function compared to scattering by smooth hexagonal prisms, particularly when the roughness features were horizontally much larger than the wavelength. However, the most effective roughness model also takes account of horizontal features whose size is closer to that of the wavelength. Horizontal features smaller than the wavelength have very little effect.

Orientador: Luiz Guimarães Ferreira / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-10T00:36:47Z (GMT). No. of bitstreams: 1 Pinho_MariaGloriadeOliveira_M.pdf: 832379 bytes, checksum: 05244cdafb054c3e2fffdc0bd34cdc3a (MD5) Previous issue date: 2007 / Resumo: Apresentamos neste trabalho o estudo da fotoionização, incluindo os efeitos relativísticos, de elétrons das camadas-s mais externas dos átomos de Sódio, Potássio, Rubídio e Césio. Tratamos do processo onde um átomo não polarizado é fotoionizado por luz incidente linearmente polarizada, utilizando o Estado de Transição de meia ocupação, sob a ótica da aproximação de dipolo elétrico e da Aproximação da Densidade Local (LDA). Em nosso procedimento fazemos uso das equações de Dirac para o cálculo das funções de onda relativísticas, utilizadas para a obtenção das seções de choque totais de fotoionização e dos parâmetros de assimetria. Mostramos as vantagens da utilização do Estado de Transção de meia ocupação e sua precisão numérica nos cálculos de grandezas físicas como o Potencial de Ionização dos átomos. Através dos resultados obtidos, observamos a influência que os mínimos das seções de choque, chamados Mínimos de Cooper, exercem sobre os parâmetros de assimetria. Estes dados sâo comparados com cálculos efetuados através de outras formulações por outros autores e também com resultados experimentais encontrados na literatura / Abstract: In this work we present photoionization results for the valence s-electrons of Sodium, Potassium, Rubidium and Cesium atoms including relativistic effects. In our calculations we consider the photoionization of a non-polarized atom by linearly polarized light, using half occupation transition states, within the electric dipole approximation and the Local Density Approximation. Our total photoionization cross sections and asymmetry parameters were calculated using relativistic wave functions obtained from Dirac¿s equations. We show that the use of half occupation transition states improves the results, producing many advantages, such as numeric precision in the evaluation of the Ionization Potentials. We have also studied the in uence of the cross section minimum (Cooper Minimum) on the asymmetry parameters. We compare our results with experimental data and with other calculations from the literature / Mestrado / Física Atômica e Molecular / Mestre em Física