Fast and reliable Markov chain Monte Carlo technique for cosmological parameter estimation

Markov chain Monte Carlo (MCMC) techniques are now widely used for cosmological parameter estimation. Chains are generated to sample the posterior probability distribution obtained following the Bayesian approach. An important issue is how to optimize the efficiency of such sampling and how to diagnose whether a finite-length chain has adequately sampled the underlying posterior probability distribution. We show how the power spectrum of a single such finite chain may be used as a convergence diagnostic by means of a fitting function, and discuss strategies for optimizing the distribution for the proposed steps. The methods developed are applied to current cosmic microwave background and large-scale structure data interpreted using both a pure adiabatic cosmological model and a mixed adiabatic/isocurvature cosmological model including possible correlations between modes. For the latter application, because of the increased dimensionality and the presence of degeneracies, the need for tuning MCMC methods for maximum efficiency becomes particularly acute.     

Contaminant transport in a fracture with spatially variable aperture in the presence of monodisperse and polydisperse colloids

A quasi-three-dimensional particle tracking model is developed to characterize the spatial and temporal effects of advection, molecular diffusion, Taylor dispersion, fracture wall deposition, matrix diffusion, and co-transport processes on two discrete plumes (suspended monodisperse or polydisperse colloids and dissolved contaminants) flowing through a variable aperture fracture situated in a porous medium. Contaminants travel by advection and diffusion and may sorb onto fracture walls and colloid particles, as well as diffuse into and sorb onto the surrounding porous rock matrix. A kinetic isotherm describes contaminant sorption onto colloids and sorbed contaminants assume the unique transport properties of colloids. Sorption of the contaminants that have diffused into the matrix is governed by a first-order kinetic reaction. Colloids travel by advection and diffusion and may attach onto fracture walls; however, they do not penetrate the rock matrix. A probabilistic form of the Boltzmann law describes filtration of both colloids and contaminants on fracture walls. Ensemble-averaged breakthrough curves of many fracture realizations are used to compare arrival times of colloid and contaminant plumes at the fracture outlet. Results show that the presence of colloids enhances contaminant transport (decreased residence times) while matrix diffusion and sorption onto fracture walls retard the transport of contaminants. Model simulations with the polydisperse colloids show increased effects of co-transport processes.     

Maximum urban heat island intensity in a medium-sized coastal Mediterranean city

This paper studies the maximum intensity of the urban heat island (UHI) that develops in Volos urban area, a medium-sized coastal city in central Greece. The maximum temperature difference between the city center and a suburb is 3.4°C and 3.1°C during winter and summer, respectively, while during both seasons the average maximum UHI intensity is 2.0°C. The UHI usually starts developing after sunset during both seasons. It could be attributed to the different nocturnal radiative cooling rate and to the different anthropogenic heat emission rate that are observed at the city center and at the suburb, as well as to meteorological conditions. The analysis reveals that during both seasons the daily maximum hourly (DMH) UHI intensity is positively correlated with solar radiation and with previous day’s maximum hourly UHI intensity and negatively correlated with wind speed. It is also negatively correlated with relative humidity during winter but positively correlated with it during summer. This difference could be attributed to the different mechanisms that mainly drive humidity levels (i.e., evaporation in winter and sea breeze (SB) in summer). Moreover, it is found that SB development triggers a delay in UHI formation in summer. The impact of atmospheric pollution on maximum UHI intensity is also examined. An increase in PM10 concentration is associated with an increase in maximum UHI intensity during winter and with a decrease during summer. The impact of PM10 on UHI is caused by the attenuation of the incoming and the outgoing radiation. Additionally, this study shows that the weekly cycle of the city activities induces a weekly variation in maximum UHI intensity levels. The weekly range of DMH UHI intensity is not very large, being more pronounced during winter (0.4°C). Moreover, a first attempt is made to predict the DMH UHI intensity by applying regression models, whose success is rather promising.     

Spectral investigation of the diffuse-to-direct solar beam irradiances ratio (UV–VIS) in the urban Athens atmosphere

This study explores the influence of air gaseous pollutants–aerosols and solar zenith angle (SZA) on the spectral diffuse-to-direct beam E _dλ/E _bλ irradiances ratio. It does so using ground-based spectroradiometric measurements taken over the Athens atmosphere during May 1995. It was found that the spectral E _dλ/E _bλ ratio decreases rapidly with increasing wavelength and regression curves of the form E _dλ/E _bλ = aλ^?b fitted the experimental data. These curves are strongly modified by aerosols–air pollutants, aerosol optical properties, and SZA. The log–log plot of E _dλ/E _bλ versus λ reveals a significant departure from linearity, which is likely to be associated with aerosol physical properties and SZA effects. The effect of atmospheric turbidity, as expressed through the aerosol optical at 500 nm and SZA on the spectral E _dλ/E _bλ ratio, is investigated in detail for two discernible atmospheric conditions observed in the urban Athens atmosphere. The first case includes different atmospheric turbidity levels under the same SZA, while the second corresponds to different SZA values under the same turbidity levels. It was found that the correlation between E _dλ/E _bλ and spectral aerosol optical depth can be a useful tool in determining the aerosol optical properties and aerosol types composition.     

Effective parameters for flow in saturated heterogeneous porous media

Effective parameters for flow in saturated porous media are obtained via Taylor-Aris-Brenner moment analysis considering both periodic as well as stationary porous medium properties. It is assumed that a slug is instantaneously introduced into an unbounded, anisotropic porous medium having a compressible matrix, and that the correlation length of the local hydraulic conductivity and specific storage fluctuations is smaller than the correlation length of hydraulic head fluctuations (gradually varying flow). It is shown that the effective specific storage is equal to its volume average. The effective hydraulic conductivity is derived by a small-perturbation analysis and it is shown to consist of its volume average and of a second term which accounts for the ‘small’ local conductivity fluctuations.     

Trends of urban surface temperature and heat island characteristics in the Mediterranean

Urban air temperature studies usually focus on the urban canopy heat island phenomenon, whereby the city center experiences higher near surface air temperatures compared to its surrounding non-urban areas. The Land Surface Temperature (LST) is used instead of urban air temperature to identify the Surface Urban Heat Island (SUHI). In this study, the nighttime LST and SUHI characteristics and trends in the seventeen largest Mediterranean cities were investigated, by analyzing satellite observations for the period 2001–2012. SUHI averages and trends were based on an innovative approach of comparing urban pixels to randomly selected non-urban pixels, which carries the potential to better standardize satellite-derived SUHI estimations. A positive trend for both LST and SUHI for the majority of the examined cities was documented. Furthermore, a 0.1 °C decade^?1 increase in urban LST corresponded to an increase in SUHI by about 0.04 °C decade^?1. A longitudinal differentiation was found in the urban LST trends, with higher positive values appearing in the eastern Mediterranean. Examination of urban infrastructure and development factors during the same period revealed correlations with SUHI trends, which can be used to explain differences among cities. However, the majority of the cities examined show considerably increased trends in terms of the enhancement of SUHI. These findings are considered important so as to promote sustainable urbanization, as well as to support the development of heat island adaptation and mitigation plans in the Mediterranean.     

Simultaneous Design Characteristics Optimization in an Earth Retaining Structure

The aim of this paper is to present a method for simultaneous optimization of the design characteristics of an earth retaining structure design using quality tools. The design characteristics examined in this paper are safety factor, total displacements and cost. The methodology for the multi-response optimization used is the desirability analysis which gives the appropriate combinations for the design variables. Through standard experimental runs, we process the results for this optimization. The experimental measurements are calculated via finite elements analysis. The designs used are taken from two real-life case studies. This methodology is intended as a guide tool for civil and geotechnical engineers to predict the values of the design variables as long as they can be named and take discrete values. The uniqueness in this study is that approved experimental methodology can simultaneously optimize the design characteristics of an earth retaining structural design, which were until now calculated empirically.     

A numerical study on deblending of land simultaneous shooting acquisition data via rank-reduction filtering and signal enhancement applications

We propose a workflow of deblending methodology comprised of rank-reduction filtering followed by a signal enhancing process. This methodology can be used to preserve coherent subsurface reflections and at the same time to remove incoherent and interference noise. In pseudo-deblended data, the blending noise exhibits coherent events, whereas in any other data domain (i.e. common receiver, common midpoint and common offset), it appears incoherent and is regarded as an outlier. In order to perform signal deblending, a robust implementation of rank-reduction filtering is employed to eliminate the blending noise and is referred to as a joint sparse and low-rank approximation. Deblending via rank-reduction filtering gives a reasonable result with a sufficient signal-to-noise ratio. However, for land data acquired using unconstrained simultaneous shooting, rank-reduction–based deblending applications alone do not completely attenuate the interference noise. A considerable amount of signal leakage is observed in the residual component, which can affect further data processing and analyses. In this study, we propose a deblending workflow via a rank-reduction filter followed by post-processing steps comprising a nonlinear masking filter and a local orthogonalization weight application. Although each application shows a few footprints of leaked signal energy, the proposed combined workflow restores the signal energy from the residual component achieving significantly signal-to-noise ratio enhancement. These hierarchical schemes are applied on land simultaneous shooting acquisition data sets and produced cleaner and reliable deblended data ready for further data processing.     

Anisotropic radiation modelling of macroseismic intensities for estimation of the attenuation structure of the upper crust in Greece

A method is suggested for the analysis of macroseismic intensity data in order to accurately determine an average attenuation structure of the upper part of the crust in an area. The method is based on a model which assumes that the observed intensities depend on source properties (radiation pattern, size, focal depth), geometrical spreading and anelastic attenuation. The method is applied to 13,008 intensity values, observed in corresponding sites of Greece and grouped (in 4228 groups), according to their spatial clustering in order to diminish observational errors and site effects. An average intensity attenuation coefficient,c=–0.0039±0.0016, corresponding to a quality factor, Q=350±140, is determined for the upper 20 km of the crust in this area. This value is relatively low, in good agreement with the relatively high heat flow and high seismic activity of this area. A byproduct of the present study is the determination, for each earthquake, of a macroseismic focal depth and of a macroseismic size, which is strongly correlatted with both the earthquake's magnitude and its seismic moment determined by independent methods.