A Discontinuous Galerkin Time Domain method (DGTD), using a fourth order Runge-Kutta time-stepping of Maxwell's equations, was applied to the simulation of the optical properties of dielectric particles in two-dimensional (2-D) geometry. As examples of the numerical implementation of this method, the single-scattering properties of 2D circular and hexagonal particles are presented. In the case of circular particles, the scattering phase matrix was computed using the DGTD method and compared with the exact solution. For hexagonal particles, the DGTD method was used to compute single-scattering properties of randomly oriented 2-D hexagonal ice crystals, and results were compared with those calculated using a geometric optics method. Both shortwave (visible) and longwave (infrared) cases are considered, with particle size parameters 50 and 100. Ice in shortwave and longwave cases is absorptive and non-absorptive, respectively. The comparisons between DG solutions and the exact solutions in computing the optical properties of circular ice crystals reveal the applicability of the DG method to calculations of both absorptive and non-absorptive particles. In the hexagonal case scattering results are also presented as a function of both incident and scattering angles, revealing structure apparently not reported before. Using the geometric optics method we are able to interpret this structure in terms of contributions from varying numbers of internal reflections within the crystal.

The single scattering properties of atmospheric particles are fundamental to radiative simulations and remote sensing applications. In this study, an efficient technique, namely, the pseudo-spectral time-domain (PSTD) method which was first developed to study acoustic wave propagation, is applied to the scattering of light by nonspherical particles with small and moderate size. Five different methods are used to discretize Maxwell’s equations in the time domain. The perfectly matched layer (PML) absorbing boundary condition is employed in the present simulation for eliminating spurious wave propagations caused by the spectral method. A 3-D PSTD code has been developed on the basis of the five aforementioned discretization methods. These methods provide essentially the same solutions in both absorptive and nonabsorptive cases. In this study, the applicability of the PSTD method is investigated in comparison with the Mie theory and the T-matrix method. The effects of size parameter and refractive index on simulation accuracy are discussed. It is shown that the PSTD method is quite accurate when it is applied to the scattering of light by spherical and nonspherical particles, if the spatial resolution is properly selected. Accurate solutions can also be obtained from the PSTD method for size parameter of 80 or refractive index of 2.0+j0. Six ice crystal habits are defined for the PSTD computational code. The PSTD results are compared with the results acquired from the finite difference time domain (FDTD) method at size parameter 20. The PSTD method is about 8-10 times more efficient than the conventional FDTD method with similar accuracy. In this study, the PSTD is also applied to the computation of the phase functions of ice crystals with a size parameter of 50. Furthermore, the PSTD, the FDTD, and T-matrix methods are applied to the study of the optical properties of horizontally oriented ice crystals. Three numerical schemes for averaging horizontal orientations are developed in this study. The feasibility of using equivalent circular cylinders as surrogates of hexagonal prisms is discussed. The horizontally oriented hexagonal plates and the equivalent circular cylinders have similar optical properties when the size parameter is in the region about from 10 to 40. Otherwise, the results of the two geometries are substantially different.

This dissertation reports on three different yet related topics in light scattering computation, radiative transfer simulation, and remote sensing implementation, regarding the cloud properties and the retrieval of cloud properties from satellite-based infrared radiometric measurements. First, the errors associated with the use of circular cylinders as surrogates for hexagonal columns in computing the optical properties of pristine ice crystals at infrared (8-12 µm) wavelengths are investigated. It is found that the differences between the results for circular cylinders and hexagonal columns are on the order of a few percent at infrared wavelengths. Second, investigated in this dissertation are the outgoing broadband longwave and window channel radiances at the top-of-atmosphere under clear-sky conditions on the basis of the data acquired by the Cloud and the Earth's Radiant Energy System (CERES) instrument onboard the NASA Terra satellite platform. Based on the comparison of the observed broadband radiances with those obtained from rigorous radiative transfer simulations, it is found that the theoretical results tend to be larger than their measured counterparts. Extensive sensitivity studies regarding the uncertainties of various parameters were carried out. Within the considered uncertainties of various factors, the computed radiances are still larger than the observed radiances if thin cirrus clouds are excluded. Thus, a potential cause for the differences could be associated with the presence of thin cirrus clouds whose visible optical thickness is smaller than approximately 0.3. Third, presented in this dissertation is an illustration of the application of hyperspectral infrared channel observations to the retrieval of the cloud properties. Specifically, the hyperspectral measurements acquired from the Atmospheric Infrared Sounder (AIRS) aboard the NASA Aqua platform are used to infer cloud top pressure, effective cloud amount, cloud thermodynamic phase, cloud optical thickness, and the effective size of cloud particles. The AIRS-based retrievals are compared with the counterparts of the operational cloud products derived from the Moderate Resolution Imaging Spectroradiometer (MODIS). The two retrievals agree reasonably well except for the retrieved cloud effective particle size. Furthermore, the diurnal and seasonal contrasts of cloud properties are also investigated on the basis of the cloud properties retrieved from the AIRS data.

The Greenland ice sheet (GIS) is a key component of the warming Arctic climate, having the potential to dramatically influence sea level through melting. Light-absorbing aerosols are thought to be significant contributors to warming in the Arctic, because of their effect on the radiation balance through both aerosol absorption in the atmosphere as well as absorption in surface snow after particulate deposition. At this time it is not possible to estimate the impact of aerosol absorption on the radiation balance over Greenland due to the lack of in-situ measurements. Here, we present time series and estimates of key aerosol optical properties in order to better understand the seasonality and sources of aerosols over central Greenland, and compare their values with other Arctic sites. In-situ measurements made at Summit, Greenland from May 8, 2011 to December 31, 2014 include aerosol light absorption coefficient (σap) and light scattering coefficient (σsp); calculated parameters include absorption Ångström exponent (AAE), and single scattering albedo (ωo). The light absorption and scattering coefficients were found to be low in the winter and highest in the spring and summer. Spring-summer means of σap and σsp were 0.15 ± 0.15 Mm-1 and 2.35 ± 2.80 Mm-1, respectively. Mean AAE was 0.97 ± 0.29 in the spring and summer, indicating that black carbon (BC), and not dust and/or organic brown carbon (BrC), is the main aerosol light absorber. Mean ωo was 0.93 ± 0.03, which is similar to values measured at Barrow, Alaska, USA (0.94 ± 0.05) and Ny-Ålesund, Svalbard, Norway (0.95 ± 0.06). Summit exhibits ωo as low as Barrow and Ny-Ålesund although it is an isolated high-altitude site indicating the importance of aerosol light absorption over the most remote Arctic locations.

Diplomová práce se zabývá vytvořením modelu, který by umožnil lepší interpretaci hloubkových profilů měřených metodou LEIS. Obtížnost interpretace těchto profilů je dána vysokým podílem vícenásobně odražených projektilů v meřených spektrech. Tyto projektily nepřináší užitečnou informaci z dané hloubky. Naproti tomu jednonásobně odražené projektily nesou přesnější informaci o složení a tloušťce vrstev. V této práci vytvořený model se snaží určit příspěvek jednonásobně odražených částic k celkovému tvaru spektra a na základě něj i hloubkový profil vzorku.

No description available.

This thesis investigates analysis and simulation of the backscattering enhancement phenomenon from randomly distributed point scatterers. These point scatterers are randomly distributed within a cube or a sphere and then the backscattering enhancement phenomenon from both cubical and spherical distributions are examined throughout the thesis. The general characteristic differences between cubical and spherical distribution about the scattering phenomenon are observed. T-matrix method is used for analytic investigations of the backscattering enhancement and also a certain number of approximate formulas are obtained. As for Monte Carlo simulation method, it is used for simulated investigations of the backscattering enhancement. Some Monte Carlo simulations are prepared by using MATLAB programming language and verified by showing their confidence intervals. Both analytic and simulated investigations of the backscattering enhancement due to single and double scattering are analyzed / however, only simulated investigation of the backscattering enhancement due to multiple scattering are analyzed because of its computational complexity. The thesis traces differences between single scattering and multiple scattering from randomly distributed point scatterers. Effects of both incident field frequency and point scatterer density on the backscattering enhancement are indicated. The thesis seeks answers to questions such as which conditions cause the backscattering enhancement phenomenon from randomly distributed point scatterers, why we need to consider multiple scattering to examine the backscattering phenomenon and how we can discriminate the backscattering enhancement from the specular enhancement.

No description available.

This thesis is based on measurements of aerosol optical and microphysical properties made at inland, coastal and island locations in India. Aerosol vertical distribution measurements have also been made both using surface based and aircraft borne instruments. In addition to these, satellite based measurements (MODIS and OMI) have also been used to estimate regional aerosol radiative forcing over the oceanic regions around India. The measurements at an inland, continental, urban location reveals the large effect of anthropogenic activities on aerosol characteristics at surface and the atmospheric vertical column. A clear seasonality is observed in aerosol optical and microphysical properties as a consequence of modulation by anthropogenic activities and the effect of meteorological parameters like rainfall, winds and boundary layer dynamics. The variability observed at different time scales (from diurnal, weekly, monthly to annual) reveals the importance of anthropogenic and natural processes in modulating the aerosol loading. The estimates of aerosol radiative forcing at surface were as high as ~ 40W m-2. A large discrepancy was observed between the observed and modeled aerosol forcing efficiency (forcing per unit optical depth) at surface. These discrepancies are due to the inadequate representation of aerosol mixing state in models. In addition, the large difference found in the observed forcing between winter and summer could also be influenced due to the presence of elevated aerosols during the summer. Measurements made over coastal and central India shows that a large fraction (75-85%) of aerosol column optical depth was contributed by aerosols located above 1 km. The horizontal gradients were sharp with e-1 scaling distance as small as ~250 km in the well-mixed regions mostly under the influence of local source effects. However, above the atmospheric boundary layer, the gradients were much shallower (~800 to 1200 km). In addition, a large fraction (60-75%) of aerosol was found located above clouds leading to enhanced aerosol absorption. Large spatial gradient in aerosol optical depth and hence radiative impacts between the coastal landmass and the adjacent oceans within a short distance of <300 km (even at an altitude of 3 km) during summer and pre-monsoon is of importance to regional climate. Observations at Minicoy, a remote island in southern Arabian Sea to study the characteristics of transported aerosols reveals variability at daily, weekly, monthly and seasonal time scales associated with changes in precipitation and air mass characteristics. The daily mean Black Carbon (BC) mass mixing ratio varied between as low as ~ 0.2 to 9.0%. The resultant average aerosol atmospheric forcing for the observation period was ~15 W m-2. Trajectory based cluster analysis has shown six distinct advection/transport pathways influencing aerosol characteristics over southern Arabian Sea. The Indo-Gangetic Plain, northern Arabian Sea and west Asia are identified to be the most important source regions having a major impact on aerosols loading over the southern Arabian Sea. The cluster analysis, concentration weighted trajectory (CWT) analysis and the MODIS retrievals show an asymmetry in aerosol characteristics between the Arabian Sea and the Bay of Bengal, with the Arabian Sea characterized by large loading by natural aerosols (eg., dust and sea salt) and the Bay of Bengal characterized by anthropogenic loading (eg., BC). The low value of the BC mass mixing ratio measured at the island (mostly ~ 1 to 1.6%), has major implications for regional radiative forcing. The annually averaged net aerosol atmospheric forcing was as low as ~1.7 W m-2 with highest forcing corresponding to IGP cluster. The single scattering albedo (SSA) which is an important parameter in the estimation of aerosol radiative forcing was retrieved by utilizing a joint OMI-MODIS retrieval methodology. The SSA over the oceanic regions around India shows that the largest absorption (SSA < 0.9) occurs during winter. The largest gradients in AOD and SSA were observed over Arabian Sea during the summer as a result of large dust emissions. The largest forcing observed also was confined to the northern Arabian Sea (~ 37 W m-2) as a result of high aerosol column loading and dust transport. The observed annual mean forcing at Minicoy were comparable to that estimated using satellite measurements, but were much lower than those observed during INDOEX.

Les métamatériaux sont des milieux prometteurs pour l'imagerie acoustique. Grâce à ces milieux, il est possible de concevoir des lentilles à faces parallèles pouvant dépasser la limite conventionnelle de résolution d'une lentille et par conséquent améliorer les systèmes d'imagerie. Malgré l'intérêt grandissant des chercheurs pour les métamatériaux, le comportement des ondes acoustiques dans ces milieux n'est pas totalement connu. Nous proposons de développer la problématique de la propagation des ondes acoustiques dans un milieu de type métamatériau en détail dans ce manuscrit. Cette étude a permis d'extraire un critère discriminant un métamatériau d'un matériau classique et d'apporter un regard nouveau sur l'amplification des ondes évanescentes dans les métamatériaux.Nous explorons une piste peu empruntée en vue du dimensionnement de métamatériaux : les milieux aléatoires. Nous nous focalisons sur les milieux à deux dimensions dont les phases sont fluides. Dans cette optique, une phase de validation de techniques d'homogénéisation dynamique existantes est réalisée \textit{via} la comparaison des réponses acoustiques d'un écran de diffuseurs répartis aléatoirement obtenues par des simulations numériques FDTD avec celles prédites par des modèles analytiques. L'étude de ces modèles, utiles au dimensionnement de structures aléatoires présentant des réponses acoustiques ciblées, nous a amené à examiner avec attention leur comportement quasi-statique. Une technique d'homogénéisation permettant de prendre en compte explicitement les interactions entre diffuseurs est proposée dans ce contexte. Développée dans le cadre de la diffusion simple et multiple, elle relie les propriétés mécaniques effectives aux moyennes des champs acoustiques dans un volume représentatif.Finalement, l'analyse du comportement d'un milieu aléatoire \og réaliste \fg~possédant théoriquement des bandes fréquentielles à réfraction négative, grâce à des diffuseurs résonants à basses fréquences, a été menée. Différents régimes de fonctionnement atypiques sont identifiés à l'aide de simulations numériques. La confrontation des réponses de ce milieu avec celles d'un cristal phononique est ensuite présentée et révèle une étonnante similitude entre les deux arrangements. / Metamaterials are promising media for acoustic imaging. For example, such media give the possibility to build flat lenses exhibiting sub-diffraction-limit resolution, thereby improving imaging setup. Despite the growing interest of the researcher for metamaterials, acoustic wave propagation is still not widely known. This work addresses the topic of wave propagation in metamaterials. In this work, we have defined a criterion which differentiate metamaterial from classical material and provide a new insight in the amplification of evanescent waves.We explore how to design metamaterials with random media. We focus on two dimensional media with fluid components. A validation process of existing dynamic homogenization techniques is done via the comparison between the responses of a screen of scatterers obtained by numerical simulations from FDTD with those predict by the analytical models. The study of those models, useful for designing random media with atypical responses, lead us to consider their quasi-static limit. In this context, we propose a homogenization technique which includes explicitly the interactions between scatterers. It is developed for multiple and simple scattering and link the effective properties to the averages of the acoustic fields in a representative volume.Finally, the analysis of the acoustic responses of a realistic random medium having theoretical negative refraction frequency bandwidth, thanks to low frequency resonant scatterers is done. Different atypical responses are identified from the numerical simulations. The comparison between the responses of this medium and those of phononic crystals is presented and shows a surprising similarity of the two arrangements.