Variability Model Research Articles

Modeling of variability of hydrological characteristics of the Tuzla group of lagoons under various variants of their water exchange with the sea

The article is devoted to solving the problem of stabilization of hydrological regime and hydro-ecological conditions of the Tuzla group of lagoons located in the central part of the Danube-Dniester interfluve by ensuring and regulating their water exchange with the open sea through artificially created channels in the barrier beach that separates the water body from the sea.
 The research presents the results of adaptation of the well-known modern numerical hydrodynamic model Delft3D Flow Flexible Mesh to the conditions of the Tuzla group of lagoons, as well as the results of its application to determine the effectiveness of various variants for ensuring water exchange with the sea based on the outcomes of scenario mathematical modeling.
 In particular, the following variants of channels functioning were studied: (1) a relatively narrow connecting channel at the 2nd km of the barrier beach (near the Burnas Lagoon) together with a wide and deep outlet that existed before 2015 at the 24th km of the barrier beach (near the Shagany Lagoon); (2) a wide controllable connecting channel in the Shagany Lagoon placed on the outlet, alongside with the above-mentioned channel in the Burnas Lagoon; (3) an additional (third) narrow channel at the strip of the barrier beach near the Alibey Lagoon.
 The characteristics of the annual cycle of water salinity spatio-temporal variability were used as indicators of the effectiveness of management decisions for ensuring the stability of the lagoons' hydro-ecological conditions.
 Based on the analysis of the scenario modeling results it was concluded that the optimum solution of the problem related to stabilizing the hydro-ecological regime of the water body consists in ensuring the constant multidirectional water exchange with the sea through two artificial connecting channels in the barrier beach (both with bottom marks of minus 1.0 m (Baltic Height System) and 60 m and 15 m wide for the Shagany Lagoon and the Burnas Lagoon respectively) throughout the year. This will deny any salinization of the water body over the long term.
 For the specified variant the time of complete water renewal with sea water for the main lagoons of the Tuzla group would constitute 8 and 12 months for the Shagany Lagoon and the Alibey/Burnas Lagoons respectively.

Spatial variability and stochastic finite element model of unreinforced masonry veneer wall system under Out-of-plane loading

Inconsistency in the quality of workmanship, the weather during construction, and materials may result in the high unit-to-unit spatial variability of mechanical properties in the same masonry structure. This paper develops a numerical modelling strategy and focuses on the stochastic assessment of unreinforced masonry (URM) veneer walls with spatially variable wall constituent material properties subjected to out-of-plane loading. 3-D finite element modelling of experimentally tested veneer walls was conducted using a micro-modelling approach combined with the Monte Carlo Simulation technique. Spatial stochastic finite element analysis considered the spatial variability of the properties of the wall components (mortar flexural tensile strength) and compared them with non-spatial analysis. The non-spatial analysis overestimates the wall system failure compared to spatial analysis, and the spatial analysis is considered to more realistically represent the variabilities of the URM veneer wall system. Sensitivity analysis is conducted to check the sensitivity of the veneer system behaviour to variability in the various input parameters. The variability of experimental results obtained in a parallel study are quantified and compared with stochastic finite element analysis (SFEA) results. The SFEA model developed in this study can estimate the behaviour and system peak load reasonably and are considered to be from the similar population as test results. Model errors are also included to assess the efficiency of the SFEA model.

How much does sleep vary from night-to-night? A quantitative summary of intraindividual variability in sleep by age, gender, and racial/ethnic identity across eight-pooled datasets.

Habitual sleep duration and efficiency vary widely by age, gender, and racial/ethnic identity. Despite growing research on the importance of night-to-night, intraindividual variability (IIV) in sleep, few studies have examined demographic differences in sleep IIV. The present study describes typical sleep IIV overall and by demographics among healthy sleepers. Eight datasets of healthy sleepers (N=2,404; 26,121 total days of sleep data) were synthesised to examine age, gender, and racial/ethnic identity differences in sleep IIV measured via diaries, actigraphy, and electroencephalography (EEG). Sleep IIV estimates included the intraindividual standard deviation (iSD), root mean square of successive differences (RMSSD), coefficient of variation (CV), and a validated Bayesian Variability Model (BVM). There was substantial IIV in sleep across measurement types (diary, actigraphy, EEG) for both sleep duration (iSD: 85.80 [diary], 77.41 [actigraphy], 67.04 [EEG] minutes; RMSSD: 118.91, 108.89, 91.93 minutes; CV: 19.19%, 19.11%, 18.57%; BVM: 60.60, 58.20, 48.60 minutes) and sleep efficiency (iSD: 5.18% [diary], 5.22% [actigraphy], 6.46% [EEG]; RMSSD: 7.01%, 7.08%, 8.44%; CV: 5.80%, 6.27%, 8.14%; BVM: 3.40%, 3.58%, 4.16%). Younger adults had more diary and actigraphy sleep duration IIV. Gender differences were inconsistent. White and non-Hispanic/Latinx adults had less IIV in sleep duration and efficiency compared to racial/ethnic minority groups. Even among healthy sleepers, sleep varies widely from night-to-night. Like mean sleep, there also may be disparities in IIV in sleep by demographic characteristics. Study results help characterise normative values of sleep IIV in healthy sleepers.

Uncertainty Quantification of a High-Throughput Profilometry-Based Indentation Plasticity Test of Al 7075 T6 Alloy

The quantification of spatially variable mechanical response in structural materials remains a challenge. Additive manufacturing methods result in increased spatial property variations—the effect of which on component performance is of key interest. To assist iterative design of additively manufactured prototypes, lower-cost benchtop test methods with high precision and accuracy will be necessary. Profilometry-based indentation plastometry (PIP) promises to improve upon the instrumented indentation test in terms of the measurement uncertainty. PIP uses an isotropic Voce hardening model and inverse numerical methods to identify plasticity parameters. The determination of the baseline uncertainty of PIP test is fundamental to its use in characterizing spatial material property variability in advanced manufacturing. To quantify the uncertainty of the PIP test, ninety-nine PIP tests are performed on prepared portions of a traditionally manufactured Al 7075 plate sample. The profilometry data and the Voce parameter predictions are examined to distinguish contributions of noise, individual measurement uncertainty, and additional set-wide variations. Individual measurement uncertainty is estimated using paired profilometry measurements that are taken from each indentation. Principal component analysis is used to analyze and model the measurement uncertainty. The fitting procedure used within the testing device software is employed to examine the effect of profile variations on plasticity predictions. The expected value of the error in the plasticity parameters is given as a function of the number of tests taken, to support rigorous use of the PIP method. The modeling of variability in the presence of measurement uncertainty is discussed.

Detection of Forced Change Within Combined Climate Fields Using Explainable Neural Networks

AbstractAssessing forced climate change requires the extraction of the forced signal from the background of climate noise. Traditionally, tools for extracting forced climate change signals have focused on one atmospheric variable at a time, however, using multiple variables can reduce noise and allow for easier detection of the forced response. Following previous work, we train artificial neural networks to predict the year of single‐ and multi‐variable maps from forced climate model simulations. To perform this task, the neural networks learn patterns that allow them to discriminate between maps from different years—that is, the neural networks learn the patterns of the forced signal amidst the shroud of internal variability and climate model disagreement. When presented with combined input fields (multiple seasons, variables, or both), the neural networks are able to detect the signal of forced change earlier than when given single fields alone by utilizing complex, nonlinear relationships between multiple variables and seasons. We use layer‐wise relevance propagation, a neural network explainability tool, to identify the multivariate patterns learned by the neural networks that serve as reliable indicators of the forced response. These “indicator patterns” vary in time and between climate models, providing a template for investigating inter‐model differences in the time evolution of the forced response. This work demonstrates how neural networks and their explainability tools can be harnessed to identify patterns of the forced signal within combined fields.

Empirical analysis of the tool support for software product lines

For the last ten years, software product line (SPL) tool developers have been facing the implementation of different variability requirements and the support of SPL engineering activities demanded by emergent domains. Despite systematic literature reviews identifying the main characteristics of existing tools and the SPL activities they support, these reviews do not always help to understand if such tools provide what complex variability projects demand. This paper presents an empirical research in which we evaluate the degree of maturity of existing SPL tools focusing on their support of variability modeling characteristics and SPL engineering activities required by current application domains. We first identify the characteristics and activities that are essential for the development of SPLs by analyzing a selected sample of case studies chosen from application domains with high variability. Second, we conduct an exploratory study to analyze whether the existing tools support those characteristics and activities. We conclude that, with the current tool support, it is possible to develop a basic SPL approach. But we have also found out that these tools present several limitations when dealing with complex variability requirements demanded by emergent application domains, such as non-Boolean features or large configuration spaces. Additionally, we identify the necessity for an integrated approach with appropriate tool support to completely cover all the activities and phases of SPL engineering. To mitigate this problem, we propose different road map using the existing tools to partially or entirely support SPL engineering activities, from variability modeling to product derivation.

Satellite-derived estimations of the clear-air turbulence in the upper troposphere

The paper presents the results of a study of turbulence zones in the upper troposphere of the Northern Hemisphere based on measurements from European geostationary meteorological satellites in 2007 - 2018. The essence of the method for determining the zones of turbulence is the use of inhomogeneity’s of concentration of water vapor as tracers and the use of correlation-extremal algorithms. Turbulence zones with a horizontal mesoscale turbulent diffusion coefficient Ked ≥ 104 m2 / s can occupy up to 50% of the Northern Hemisphere area visible from satellites. It is shown that in analyzed time interval there is a significant (by 60-120%) increase in areas occupied by relatively weak and moderate turbulence and a slight decrease (by 6-33%) in areas with strong and very strong turbulence. The temporal variability of these zones and their relationship with the characteristics of jet streams and some climatic parameters are analyzed. A statistical model of the temporal variability of the turbulence zone areas has been built. It is shown that the multiple linear regression model with jet stream characteristics as predictors describes up to 68% of variability of zones of strong turbulence. The use of climatic parameters as predictors makes it possible to describe up to 74% of the temporal variability of strong turbulence zones.

The Role of Seasonality and the ENSO Mode in Central and East Pacific ENSO Growth and Evolution

Abstract A cyclostationary linear inverse model (CSLIM) is used to investigate the seasonal growth of tropical Pacific Ocean El Niño–Southern Oscillation (ENSO) events with canonical, central Pacific (CP), or eastern Pacific (EP) sea surface temperature (SST) characteristics. Analysis shows that all types of ENSO events experience maximum growth toward final states occurring in November and December. ENSO events with EP characteristics also experience growth into May and June, but CP events do not. A single dominant “ENSO mode,” growing from an equatorial heat content anomaly into a characteristic ENSO-type SST pattern in about 9 months (consistent with the delayed/recharge oscillator model of ENSO), is essential for the predictable development of all ENSO events. Notably, its seasonality is responsible for the late-calendar-year maximum in ENSO amplification. However, this ENSO mode alone does not capture the observed growth and evolution of diverse ENSO events, which additionally involve the seasonal evolution of other nonorthogonal Floquet modes. EP event growth occurs when the ENSO mode is initially “covered up” in combination with other Floquet modes. The ENSO mode’s slow seasonal evolution allows it to emerge while the other modes rapidly evolve and/or decay, leading to strongly amplifying and more predictable EP events. CP events develop when the initial state has a substantial contribution from Floquet modes with meridional mode–like SST structures. Thus, while nearly all ENSO events involve the seasonally varying ENSO-mode dynamics, the diversity and predictability of ENSO events cannot be understood without identifying contributions from the remaining Floquet modes. Significance Statement The purpose of this study is to identify structures that lead to seasonal growth of diverse types of El Niño–Southern Oscillation (ENSO) events. An important contribution from this study is that it uses an observationally constrained, empirically derived seasonal model. We find that processes affecting the evolution of diverse ENSO events are strongly seasonally dependent. ENSO events with eastern equatorial Pacific sea surface temperature (SST) characteristics are closely related to a single “ENSO mode” that resembles theoretical models of ENSO variability. ENSO events that have central equatorial Pacific SST characteristics include contributions from additional “meridional mode” structures that evolve via different physical processes. These findings are an important step in evaluating the seasonal predictability of ENSO diversity.

A checklist for the evaluation of software process line approaches

A Software Process Line (SPrL) can help organisations to construct bespoke software development processes for specific project situations by reusing core assets. However, as there are diverse approaches for SPrL Engineering (SPrLE), this necessitates proper assistance to organisations in selecting the SPrL approach best suited to their needs. This paper aims to identify an evaluation checklist that can be used for evaluating SPrLs. The checklist was constructed in five stages: first, relevant aspects for managing process variability in the context of SPrLs were identified; based on these, research questions were then formed in the second stage. In the third stage, to answer the research questions, a literature review was conducted that focused on analysing 39 primary studies. In the fourth stage, the checklist was built by synthesising the literature results. In the fifth stage, the checklist was applied to two SPrL approaches as a proof of concept. The checklist includes seven main aspects, including the modelling language used, the type of the approach based on the number of artefacts produced, the language constructs provided for variability modelling, the process perspectives covered, the tool used for supporting the SPrL approach, the variability-specific features provided to support process variability throughout the SPrL lifecycle, and the empirical evaluation conducted to evaluate the approach. The checklist can be used by organisations to compare SPrLs and then select the most suitable SPrL approach; furthermore, it can be used by researchers to propose novel SPrL approaches that consider important aspects for variability management throughout the SPrL lifecycle. Although we have provided an example of the use of the checklist to compare SPrLs, an empirical evaluation of the checklist is required to get feedback from the organisations regarding the strengths and weaknesses of the checklist.

Towards an understanding of YSO variability: a multiwavelength analysis of bursting, dipping, and symmetrically varying light curves of disc-bearing YSOs

ABSTRACT Using simultaneous optical and infrared light curves of disc-bearing young stars in NGC 2264, we perform the first multiwavelength structure function study of YSOs. We find that dippers have larger variability amplitudes than bursters and symmetric variables at all time-scales longer than a few hours. By analysing optical-infrared colour time-series, we also find that the variability in the bursters is systematically less chromatic at all time-scales than the other variability types. We propose a model of YSO variability in which symmetric, bursting, and dipping behaviour is observed in systems viewed at low, intermediate, and high inclinations, respectively. We argue that the relatively short thermal time-scale for the disc can explain the fact that the infrared light curves for bursters are more symmetric than their optical counterparts, as the disc reprocesses the light from all rotational phases. From this model, we find that the accretion variability on to these YSOs roughly follows a random-walk.

A conceptual model for unifying variability in space and time: Rationale, validation, and illustrative applications

With the increasing demand for customized systems and rapidly evolving technology, software engineering faces many challenges. A particular challenge is the development and maintenance of systems that are highly variable both in space (concurrent variations of the system at one point in time) and time (sequential variations of the system, due to its evolution). Recent research aims to address this challenge by managing variability in space and time simultaneously. However, this research originates from two different areas, software product line engineering and software configuration management, resulting in non-uniform terminologies and a varying understanding of concepts. These problems hamper the communication and understanding of involved concepts, as well as the development of techniques that unify variability in space and time. To tackle these problems, we performed an iterative, expert-driven analysis of existing tools from both research areas to derive a conceptual model that integrates and unifies concepts of both dimensions of variability. In this article, we first explain the construction process and present the resulting conceptual model. We validate the model and discuss its coverage and granularity with respect to established concepts of variability in space and time. Furthermore, we perform a formal concept analysis to discuss the commonalities and differences among the tools we considered. Finally, we show illustrative applications to explain how the conceptual model can be used in practice to derive conforming tools. The conceptual model unifies concepts and relations used in software product line engineering and software configuration management, provides a unified terminology and common ground for researchers and developers for comparing their works, clarifies communication, and prevents redundant developments.

Variable structure in the PDS 70 disc and uncertainties in radio-interferometric image restoration

ABSTRACT The compact mm-wavelength signal in the central cavity of the PDS 70 disc, revealed by deep ALMA observations, is aligned with unresolved H α emission, and is thought to stem from a circumplanetary disc (CPD) around PDS 70c. We revisit the available ALMA data on PDS 70c with alternative imaging strategies, and with special attention to uncertainties and to the impact of the so-called ‘JvM correction’, which is thought to improve the dynamic range of restored images. We also propose a procedure for the alignment and joint imaging of multi-epoch visibility data. We find that the JvM correction exaggerates the peak signal-to-noise of the data, by up to a factor of 10. In the case of PDS 70, we recover the detection of PDS 70c from the 2019 July data, but only at 8 σ. However, its non-detection in 2017 Dec. suggests that PDS 70c is variable by at least $42{{\ \rm per\ cent}} \pm 13{{\ \rm per\ cent}}$ over a 1.75 yr time-span, so similar to models of the H α variability. We also pick up fine structure in the inner disc, such that its peak is offset by ∼0${_{.}^{\prime\prime}}$04 from the disc centre. The inner disc is variable too, which we tentatively ascribe to Keplerian rotation as well as intrinsic morphological changes.

Drag Coefficient Constraints for Space Weather Observations in the Upper Thermosphere

AbstractThe space weather research community relies heavily on thermospheric density data to understand long‐term thermospheric variability, construct assimilative, empirical, and semiempirical global atmospheric models and validate model performance. One of the challenges in resolving accurate thermospheric density data sets from satellite orbital drag measurements is modeling appropriate physical aerodynamic drag force coefficients. The drag coefficient may change throughout the thermosphere due to model dependencies on composition and altitude. As such, existing drag coefficient model errors and corresponding errors in orbit‐derived density data sets and models may be altitude and solar cycle dependent with greater errors at higher altitudes around 500 km near the oxygen‐to‐helium transition region. In this paper, inter‐satellite observed‐to‐modeled density comparisons at ∼500 km are evaluated to constrain drag coefficient modeling assumptions. Observed densities are derived from accelerometer data for the Gravity Recovery and Climate Experiment (GRACE) satellites and Two‐Line Element data for a set of compact satellites, while the NRLMSISE‐00 atmospheric model is used to obtain modeled densities and composition information. Density consistency results indicate that drag coefficient models with incomplete energy and momentum accommodation produce the most consistent densities, while the standard diffuse modeling approach may not be appropriate at these altitudes. Models with momentum accommodation between 0.5 and 0.9 and energy accommodation between 0.83 and 0.96 may be most appropriate at upper thermospheric altitudes. Modeling drag coefficients with diffuse gas‐surface interactions for the GRACE satellites could lead to errors in derived density of ∼25% and in‐track satellite orbit prediction uncertainty during solar maximum conditions on the order of kilometers.

Enantiospecific pharmacokinetics of intravenous dexmedetomidine in beagles.

The goal of this study was to investigate the pharmacokinetic (PK) behaviour of dexmedetomidine in dogs administered as a pure enantiomer versus as part of a racemic mixture. Eight unmedicated intact purpose‐bread beagles were included. Two intravenous treatments of either medetomidine or dexmedetomidine were administered at 10‐ to 14‐day intervals. Atipamezole or saline solution was administered intramuscularly 45 min later. Venous blood samples were collected into EDTA collection tubes, and the quantification of dexmedetomidine and levomedetomidine was performed by chiral LC–MS/MS. All dogs appeared sedated after each treatment without complication. Plasma concentrations of levomedetomidine were measured only in the racemic group and were 51.4% (51.4%–56.1%) lower than dexmedetomidine. Non‐compartmental analysis (NCA) was performed for both drugs, while dexmedetomidine data were further described using a population pharmacokinetic approach. A standard two‐compartment mammillary model with linear elimination with combined additive and multiplicative error model for residual unexplained variability was established for dexmedetomidine. An exponential model was finally retained to describe inter‐individual variability on parameters of clearance (Cl1) and central and peripheral volumes of distribution (V1, V2). No effect of occurrence, levomedetomidine or atipamezole could be observed on dexmedetomidine PK parameters. Dexmedetomidine did not undergo significantly different PK when administered alone or as part of the racemic mixture in otherwise unmedicated dogs.

Variability and task-responsiveness of electrophysiological dynamics: Scale-free stability and oscillatory flexibility

Cortical oscillations and scale-free neural activity are thought to influence a variety of cognitive functions, but their differential relationships to neural stability and flexibility has never been investigated. Based on the existing literature, we hypothesize that scale-free and oscillatory processes in the brain exhibit different trade-offs between stability and flexibility; specifically, cortical oscillations may reflect variable, task-responsive aspects of brain activity, while scale-free activity is proposed to reflect a more stable and task-unresponsive aspect. We test this hypothesis using data from two large-scale MEG studies (HCP: n = 89; CamCAN: n = 195), operationalizing stability and flexibility by task-responsiveness and spontaneous intra-subject variability in resting state. We demonstrate that the power-law exponent of scale-free activity is a highly stable parameter, which responds little to external cognitive demands and shows minimal spontaneous fluctuations over time. In contrast, oscillatory power, particularly in the alpha range (8–13 Hz), responds strongly to tasks and exhibits comparatively large spontaneous fluctuations over time. In sum, our data support differential roles for oscillatory and scale-free activity in the brain with respect to neural stability and flexibility. This result carries implications for criticality-based theories of scale-free activity, state-trait models of variability, and homeostatic views of the brain with regulated variables vs. effectors.

Using AGN light curves to map accretion disc temperature fluctuations

ABSTRACT We introduce a new model for understanding AGN continuum variability. We start from a Shakura–Sunyaev thin accretion disc with a steady-state radial temperature profile T(R) and assume that the variable flux is due to axisymmetric temperature perturbations δT(R, t). After linearizing the equations, we fit UV–optical AGN light curves to determine δT(R, t) for a sample of seven AGNs. We see a diversity of |δT/T| ∼ 0.1 fluctuation patterns which are not dominated by outgoing waves travelling at the speed of light as expected for the ‘lamppost’ model used to interpret disc reverberation mapping studies. Rather, the most common pattern resembles slow (v ≪ c) ingoing waves. An explanation for our findings is that these ingoing waves trigger central temperature fluctuations that act as a lamppost, producing lower amplitude temperature fluctuations moving outwards at the speed of light. The light curves are dominated by the lamppost signal – even though the temperature fluctuations are dominated by other structures with similar variability time-scales – because the discs exponentially smooth the contributions from the slower moving (v ≪ c) fluctuations to the observed light curves. This leads to light curves that closely resemble the expectations for a lamppost model but with the slow variability time-scales of the ingoing waves. This also implies that longer time-scale variability signals will increasingly diverge from lamppost models because the smoothing of slower moving waves steadily decreases as their period or spatial wavelength increases.

Periodic stellar variability from almost a million NGTS light curves

ABSTRACT We analyse 829 481 stars from the Next Generation Transit Survey (NGTS) to extract variability periods. We utilize a generalization of the autocorrelation function (the G-ACF), which applies to irregularly sampled time series data. We extract variability periods for 16 880 stars from late-A through to mid-M spectral types and periods between ∼0.1 and 130 d with no assumed variability model. We find variable signals associated with a number of astrophysical phenomena, including stellar rotation, pulsations, and multiple-star systems. The extracted variability periods are compared with stellar parameters taken from Gaia DR2, which allows us to identify distinct regions of variability in the Hertzsprung–Russell Diagram. We explore a sample of rotational main-sequence objects in period-colour space, in which we observe a dearth of rotation periods between 15 and 25 d. This ‘bi-modality’ was previously only seen in space-based data. We demonstrate that stars in sub-samples above and below the period gap appear to arise from a stellar population not significantly contaminated by excess multiple systems. We also observe a small population of long-period variable M-dwarfs, which highlight a departure from the predictions made by rotational evolution models fitted to solar-type main-sequence objects. The NGTS data spans a period and spectral type range that links previous rotation studies such as those using data from Kepler, K2, and MEarth.

Multiband Emission Properties of 1ES 1959+650

We have systemically studied the properties of 1ES 1959+650, specifically its multiband variability and emission mechanisms. We carefully analyze multiband variability properties by four different kinds of trusted approaches. In contrast with previous research and thanks to our more detailed data, we find that an unprecedented significant quasi-periodic component with timescale in the optical R band indeed exists. We suggest that this light curve may result from accretion flow instabilities. Further, among the multiband spectra, we find no significant correlation between different bands, which implies that the physical origin of the dominant radiation for different bands is not the same. Motivated by these results, we go on further to study the radiation mechanisms based on quasi-simultaneous spectra. Here, a model that self-consistently involves protons, electrons, and photons is used to study the light curve and interpret the spectral energy distribution of 1ES 1959+650. Considering the variability properties and model calculation, we suggest that the variation of the cutoff energy of electrons and protons may dominate the variability of X-ray and γ-ray bands in 1ES 1959+650.

Uncertainty Quantification for Aircraft Noise Emission Simulation: Methods and Limitations

Implementing measures for the reduction of aircraft noise impact, such as optimization of flight paths and aircraft, requires sophisticated simulation capabilities. These tools have to incorporate simulation of aircraft noise generation at the source (i.e., emission) and account for prevailing sound propagation effects to ultimately predict noise levels as received on the ground. Obviously, understanding the associated uncertainties is crucial when aiming at a reliable and meaningful assessment. It also becomes essential when comparing different technologies, mixing experimental and numerical data, or using simulation methods of different fidelity (e.g., semi-empirical and first-order methods). This research focuses on quantifying uncertainties in the first step in aircraft noise simulation (i.e., the prediction of the emission situation). The quantified uncertainties reflect imperfections of models for different aircraft noise sources and variability of model input parameters at different operating conditions. A general framework is presented, which also accounts for limited knowledge of the underlying data distributions, and quantitative comparisons of different uncertainty methods are provided. In particular, the first-order second-moment analysis is compared to higher-order polynomial chaos methods, and the advantages and disadvantages of the different methods are discussed.

The relation between X-ray and ultraviolet variability of quasars

ABSTRACT The relation between X-ray and UV/optical variability in AGNs has been explored in many individual sources, however a large sample study is yet absent. Through matching the XMM-Newton serendipitous X-ray and UV source catalogs with SDSS quasars, we build a sample of 802 epoch-pairs of 525 quasars showing clear variability in logFX − logFUV space. After correcting for the effect of photometric noise, we find 35.6 ± 2.1 per cent of the epoch-pairs show asynchronous variability between X-ray and UV (brightening in one band but dimming in the other). This indicates only in 28.8 ± 4.2 per cent of the epoch-pairs the X-ray and UV variability are intrinsically coordinated. The variability synchronicity exhibits no dependence on physical parameters of quasars or the time lag of the epoch-pairs, except for stronger variability tends to have stronger synchronicity. Switches between synchronous and asynchronous variability are also seen in individual sources. The poor coordination clearly contradicts both the X-ray reprocessing model and the accretion rate variation model for AGN variability. The ratios of the observed X-ray variability amplitude to that in UV span a broad range and peak at ∼2. The dominant fraction of the ratios appear too small to be attributed to X-ray reprocessing, and too large for accretion rate variation. The inhomogeneous disc model which incorporates both X-ray and UV/optical variability in AGNs is favoured by the observed stochastic relation between X-ray and UV variations.