George Tsegas

Theory of electronic and optical properties of 3C-SiC

We study the electronic and optical properties of cubic (3C) SiC, using a combination of first-principles and tight-binding electronic structure calculations. We employ pseudopotential density functional theory calculations, with appropriate corrections to the energy of conduction bands, to investigate the band structure of this material and obtain band gaps that are in agreement with experimental results. The optical properties are then studied within the framework of the empirical tight-binding model, which is fitted to reproduce the first-principles calculations. This approach allows for a thorough investigation of the dielectric functions, the reflectivity, and the refractive index. Critical points are identified and connected to the appropriate transitions in the band structure. The results are in good agreement with available experimental data. In addition, we investigate spin splitting effects.

A metamodelling implementation of a two-way coupled mesoscale–microscale flow model for urban area simulations

Systems of coupled prognostic mesoscale and microscale models have been suggested as a tool to accurately simulate flows around artificial structures and over densely built urban areas. Implementations of such two-way coupling are burdened by scale mismatches as well as the formidable computational cost of online microscale calculations. A simplifying approach is proposed, where microscale feedbacks are spatially and temporally upscaled and assimilated into the mesoscale calculation. A two-way coupled model system is developed, consisting of the mesoscale model MEMO and the microscale model MIMO, employing interpolating metamodels. As an illustrative application, multi-day simulations for Athens, Greece are presented.

On the estimation of parameters of the gravitational-wave signal from a coalescing binary by a network of detectors

Estimation of parameters of the gravitational-wave signal from a coalescing binary by a network of laser interferometers is considered. A solution of the inverse problem for the network of three detectors is generalized to the network of N detectors. This enables, from measurements at individual detectors of the network, optimal estimation of the astrophysically interesting parameters of the binary system: its distance from Earth, its position in the sky, and the chirp mass of the system. Maximum likelihood and least-squares methods are used to obtain the solution. The existence of the solution in view of the nonlinear nature of the problem and the noise in the detectors is discussed. An alternative solution of the inverse problem that circumvents the singularities present in the problem is proposed. Accuracy of the estimation of the parameters is assessed from the inverse of the Fisher information matrix. The variance of the maximum likelihood estimator of the distance is calculated for a simple model and compared with the approximate one obtained from the Fisher matrix. Extensive Monte Carlo simulations are performed to assess the accuracy of the estimation of the astrophysical parameters by networks of three and four detectors. Addition of the fourth detector to the network markedly improves the performance of the network. Adding the fourth node in Australia to the LIGO/VIRGO network increases the number of detectable events roughly twofold. For the four-detector network one can find among all detectable events again roughly twice the number of events for which accurate determination of the binary distance is possible. Moreover, the position of the binary in the sky can be typically determined three to four times more accurately for the enhanced LIGO/VIRGO network.

An Air Quality Management System for Policy Support in Cyprus

The recent air quality directive (2008/50/EC) encourages the introduction of modelling as a necessary tool for air quality assessment and management. Towards this aim, an air quality management system (AQMS) has been developed and installed in the Department of Labour Inspection (DLI) of the Republic of Cyprus. The AQMS comprises of two operational modules, providing hourly nowcasting and daily forecasting of the air quality status, implemented as an integrated model system that performs nested grid meteorological and photochemical simulations. A third operational module provides the capability of an interactive configuration of custom emission scenarios and corresponding model runs covering user-defined domains of interest. Statistical indicators are calculated at the end of each day for the measurement locations of DLIs air quality monitoring network. Besides, the system provides an advanced user interface, which is realised as a web-based application providing access to model results from any computer with an internet connection and a web browser.

An approach for determining urban concentration increments

A simple approach for accurately estimating an urban concentration increment on top of the regional background for urban areas in Europe is presented. The method operates by establishing a functional relationship between the concentration increment and the local meteorological situation, the city characteristics, the urban emissions and the background concentrations. In the current study, the above method was applied for PM 10 and NO 2 . Pollutant concentrations needed for the multiple regression process can be derived both from measurements and high resolution urban scale model results. The results demonstrate the capability of this simple approach to reproduce the urban increment with satisfactory accuracy, thus providing a tool for fast but still reliable quantitative assessments of urban air quality that can subsequently be used in calculations of exposure and health impact assessment. Moreover, scenario calculations for the urban increments can be based on scenario emissions and respective modelled regional background concentrations.

Using a Coupled Meteorological and Chemical Transport Modelling Scheme to Evaluate the Impact of the Aerosol Direct Effect on Pollutant Concentration Fields in Paris

Aerosols have a significant impact on the Earth’s radiation budget by scattering and absorbing solar radiation thus decreasing the radiation absorbed at the surface and increasing low-level static stability. Recent developments in urban air pollution modelling have focused on the introduction of two-way coupling between aerosol modules and the driving meteorological models, aiming to a more accurate description of aerosol-induced radiative forcings in the atmosphere. In this study, the feedback mechanism due to the aerosol direct effect is examined with the aid of a mesoscale Eulerian meteorological model and a chemical transport model, as part of an on-line coupled scheme. For the assessment of the performance of the coupled model system, pollutant dispersion over the Paris area is simulated over a summer period in 2005 identified in the framework of the MEGAPOLI project. A comparison between the standalone and coupled calculations reveals that the radiative forcing due to the direct effect has a substantial impact on the calculated meteorological fields and leads to an improved representation of the pollutant concentration patterns.

Statistical analysis of the estimators of the parameters of the gravitational-wave signal from a coalescing binary

Matched filtering is proposed as a way to detect the gravitational-wave signal from a coalescing binary and to estimate its parameters. One of the authors (AK) has investigated the theoretical performance of this method by calculating the signal-to-noise ratio and the covariance matrix for the parameters of the signal. In this work we try to verify how the above method will work in practice. We generate a Gaussian, approximately white noise and add the signal, and then, using the algorithm derived from the maximum likelihood principle, we find the maximum likelihood estimators of the parameters. The procedure amounts essentially to the maximization of the correlation of the data with the filter matched to the signal. The size of the maximum of the correlation determines the probability of the detection of the signal. We repeat the procedure a thousand times to obtain suitable statistics for the estimators. We find that it agrees well with the theoretical predictions. We also investigate the post-Newtonian effects. It was recently shown by the Caltech group that the matched filtering technique is sensitive to the post-Newtonian corrections. We demonstrate this by inputing the signal with the first post-Newtonian term and correlating the data with the Newtonian filter. We find that we can still detect the post-Newtonian signal with a Newtonian filter but the maximum of the correlation falls by 40% and consequently the probability of the detection decreases. The estimates of the mass parameter of the post-Newtonian signal and its time-of-arrival are shifted by a certain amount from the true values. We also address the problem of the estimation of the individual masses of the binary.

A Multi-model Case Study on Aerosol Feedbacks in Online Coupled Chemistry-Meteorology Models Within the COST Action ES1004 EuMetChem

The importance of different processes and feedbacks in online coupled chemistry-meteorology models for air quality simulations and weather prediction was investigated in COST Action ES1004 (EuMetChem). Case studies for Europe were performed with different models as a coordinated exercise for two episodes in 2010 in order to analyse the aerosol direct and indirect radiative effect and the response of different models to aerosol-radiation interactions.

An Integrated Numerical Methodology for the Study of Transport of Air Pollution in the Coupled Indoor and Outdoor Environment

Exposure to indoor air pollution, as a result of transport from outdoor emission sources in the vicinity of buildings, has been receiving an increasing attention over the past few years. Towards the aim of quantifying the contribution of inter-canopy transport of air pollution on the quality of the indoor environment, a holistic numerical modelling approach based on transient, three-dimensional computational fluid dynamics models has been developed and validated. Numerical issues, such as temporal and spatial discretisation are addressed and parametric studies are performed to assess the impact of the external flow on the inter-canopy transport of air pollution in the coupled indoor and outdoor environment. The contributions of the main emission sources in the vicinity of buildings are considered. The main architectural features of the built environment near the buildings of interest are explicitly resolved in order to account for their contribution on the generation of atmospheric turbulence on the lower part of the approaching atmospheric boundary layer, which dominates the transport of the external flow. The methodology is validated based on comparisons with field measurements for the concentration levels of NOx, PM2.5 and PM10 in selected locations, both indoors and outdoors in close proximity to a real building.

Application of inverse dispersion modelling for the determination of PM emission factors from fugitive dust sources in open-pit lignite mines

The operation of large open-pit lignite mines represents a significant source of fugitive dust emissions connected to energy production. Emission inventories can be used to provide operational estimates of the total dust burden in the surrounding areas attributable to mining operations. In this work, a methodology based on the inverse dispersion modelling approach, combining two different dispersion models, is used for preparing a dust emissions inventory for several activity types in the lignite mines of Western Macedonia, Greece. A three-year campaign of field measurement experiments provides the necessary meteorological data and upwind-downwind concentration levels in the area of each activity. A comparison of calculated emission rates provided by the two dispersion models indicates a very good agreement, while the normalised downwind concentration timeseries are accurately reproduced. Emission factors are calculated for each experiment and per-activity, leading to the formulation of empirical relations for the total fugitive dust emissions.