730 research outputs found
New spectral functions of the near-ground albedo derived from aircraft diffraction spectrometer observations
No full textThe airborne spectral observations of the upward and downward irradiances are
revisited to investigate the dependence of the near-ground albedo as a
function of wavelength in the entire solar spectrum for different surfaces
(sand, water, snow) and under different conditions (clear or cloudy sky). The
radiative upward and downward fluxes were determined by a diffraction
spectrometer flown on a research aircraft that was performing multiple flight
paths near the ground. The results obtained show that the near-ground albedo
does not generally increase with increasing wavelengths for all kinds of
surfaces as is widely believed today. Particularly, in the case of water
surfaces it was found that the albedo in the ultraviolet region is more or
less independent of the wavelength on a long-term basis. Interestingly, in
the visible and near-infrared spectra the water albedo obeys an almost
constant power-law relationship with wavelength. In the case of sand surfaces
it was found that the sand albedo is a quadratic function of wavelength,
which becomes more accurate if the ultraviolet wavelengths are neglected.
Finally, it was found that the spectral dependence of snow albedo behaves
similarly to that of water, i.e. both decrease from the ultraviolet to the
near-infrared wavelengths by 20–50%, despite the fact that their values
differ by one order of magnitude (water albedo being lower). In addition, the
snow albedo vs. ultraviolet wavelength is almost constant, while in the
visible near-infrared spectrum the best simulation is achieved by a
second-order polynomial, as in the case of sand, but with opposite slopes
Peeling-ballooning stability of tokamak plasmas with applied 3D magnetic fields
No full textThe poloidal harmonics of the toroidal normal modes of an unstable axisymmetric tokamak plasma are employed as basis functions for the minimisation of the 3D energy functional. This approach presents a natural extension of the perturbative method considered in Anastopoulos Tzanis et al (2019 Nucl. Fusion 59 126028). This variational formulation is applied to the stability of tokamak plasmas subject to external non-axisymmetric magnetic fields. A comparison of the variational and perturbative methods shows that for D-shaped, high β N plasmas, the coupling of normal modes becomes strong at experimentally relevant applied 3D fields, leading to violation of the assumptions that justify a perturbative analysis. The variational analysis employed here addresses strong coupling, minimising energy with respect to both toroidal and poloidal Fourier coefficients. In general, it is observed that ballooning unstable modes are further destabilised by the applied 3D fields and field-aligned localisation of the perturbation takes place, as local ballooning theory suggests. For D-shaped high β N plasmas, relevant to experimental cases, it is observed that the existence of intermediate n unstable peeling-ballooning modes, where a maximum in the growth rate spectrum typically occurs, leads to a destabilising synergistic coupling that strongly degrades the stability of the 3D system
Total ozone observations at Athens, Greece by satellite-borne and ground-based instrumentation
No full textA comparison is made of total ozone (TOZ) content observations conducted by the Dobson spectrophotometer No. 118, the SCanning Imaging Absorption SpectroMeter for Atmospheric CHartographY (SCIAMACHY), the Total Ozone Mapping Spectrometer (TOMS) and the Ozone Monitoring Instrument (OMI) over Athens, Greece, during 1991-2008. Spearman'S and Wilcoxon's tests were used to determine the measure of the agreement between the ground-based and satellite column ozone data. The correlation coefficient between Dobson and Nimbus-7, ADEOS, Earth Probe, OMI and SCIAMACHY observations was found to be 0.95, 0.96, 0.94, 0.93 and 0.87, respectively, while the correlation coefficient between total ozone observations of SCIAMACHY and Earth Probe-TOMS and OMI is 0.85 and 0.93, respectively. SCIAMACHY overestimates the column ozone with respect to Dobson, Earth Probe-TOMS and OMI by 10, 15 and 3 DU, respectively, while Dobson underestimates the column ozone with respect to Nimbus-7, ADEOS and OMI by 5, 10 and 8 DU. The results obtained confirm that the Athens Dobson station may continue to be considered as a ground-truth total ozone station for the validation of the satellite column ozone observations. In addition, linear regression analysis of the deseasonalized monthly mean column ozone, as derived from Dobson measurements, gives an increase of +0.33 ± 0.07% per year during 1991-2000 and a decrease of -0.33 ± 0.07% per year for the period 2001-2008. © 2009 Taylor & Francis
On the relationship between total ozone and temperature in the troposphere and the lower stratosphere
No full textTotal ozone and atmospheric temperature are two variables that are closely connected with two important environmental problems: ozone depletion and global warming. Vertical temperature profiles and column ozone data are analysed to explore the relationship between these two variables at various levels in the troposphere and lower stratosphere over Athens, Greece during the period 1992-2004. The results obtained show an out-of-phase relationship between total ozone and temperature in the troposphere and an in-phase relationship in the lower stratosphere. Such a phase change is characteristic of all seasons. © 2009 Taylor & Francis
Ground-based observations of ozone at Athens, Greece during the solar eclipse of 1999
No full textMeasurements of column ozone were carried out at Athens, Greece, using a Dobson spectrophotometer during the solar eclipse of 11 August 1999. Taking into account the performance of the Dobson spectrophotometer during an eclipse event, the observation analysis showed positive and negative fluctuations of the column ozone of ca 2% within a period of two hours, approximately. Furthermore, observations of the surface ozone concentration and solar ultraviolet radiation reaching the ground during the eclipse event are presented. © 2005 Taylor & Francis
Maximum inspiratory pressure, a surrogate parameter for the assessment of ICU-acquired weakness
No full textBackground: Physical examination has been advocated as a primary determinant of ICU-acquired weakness (ICU-AW). The purpose of the study is to investigate ICU-AW development by using Maximum Inspiratory Pressure (MIP) as a surrogate parameter of the standardized method to evaluate patients' peripheral muscle strength.Methods: Seventy-four patients were recruited in the study and prospectively evaluated in a multidisciplinary university ICU towards the appearance of ICU-AW. APACHE II admission score was 16 ± 6 and ICU stay 26 ± 18 days. ICU-AW was diagnosed with the Medical Research Council (MRC) scale for the clinical evaluation of muscle strength. MIP was measured using the unidirectional valve method, independently of the patients' ability to cooperate.Results: A significant correlation was found between MIP and MRC (r = 0.68, p < 0.001). Patients that developed ICU-AW (MRC<48) had a longer weaning period compared to non ICU-AW patients (12 ± 14 versus 2 ± 3 days, p < 0.01). A cut-off point of 36 cmH2O for MIP was defined by ROC curve analysis for ICU-AW diagnosis (88% sensitivity,76% specificity). Patients with MIP below the cut-off point of 36 cmH2O had a significant greater weaning period (10 ± 14 versus 3 ± 3 days, p = 0.004) also shown by Kaplan-Meier analysis (log-rank:8.2;p = 0.004).Conclusions: MIP estimated using the unidirectional valve method may be a potential surrogate parameter for the assessment of muscle strength compromise, useful for the early detection of ICU-AW. © 2011 Tzanis et al; licensee BioMed Central Ltd
Non-destructive evaluation of cement-based materials from pressure-stimulated electrical emission - Preliminary results
No full textThis is the post-print version of the final paper published in Construction and Building Materials. The published article is available from the link below. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. Copyright @ 2010 Elsevier B.V.This paper introduces the possibility of in situ assessment of loading and remaining strength in concrete structures by means of measuring discharge of electric current from loaded specimens.
The paper demonstrates that the techniques have been applied to other rock-like materials, but that for the first time they are applied to cement-based materials and a theoretical model is proposed in relation to the appearance of electrical signals during sample loading and up to fracture.
A series of laboratory experiments on cement mortar specimens in simple uniaxial compression, and subsequently in bending – hence displaying both tension and compression – are described and show clear correlations between resulting strains and currents measured. Under uniaxial loading there is a well-defined relationship between the pressure-stimulated current (PSC) as a result of a monotonic mechanical loading regime. Similar results are observed in the three-point bending tests where a range of loading regimes is studied, including stepped changes in loading.
While currents can be measured at low strains, best results seem to be obtained when strains approach and exceed yield stress values.
This technique clearly has immense potential for structural health monitoring of cement-based structures. Both intermittent and continuous monitoring becomes possible, and given an ongoing campaign of monitoring, remaining strength can be estimated
A new tool for the study of the ozone hole dynamics over Antarctica
No full textAn analysis of the time series of the maximum daily ozone hole area over Antarctica for each year during the period 1979-2009 is presented, based on the entropy S defined in a new time domain termed natural time domain, that captures characteristics of the dynamics of the ozone hole complex system. The results obtained show that the entropy in natural time for scales 3-7 years and its value under time reversal for all scales (3-15 years) almost stabilizes during the last several years. On the other hand, characteristic features of this entropy are clearly found before the unprecedented event of the major, sudden stratospheric warming and the subsequent break-up of the Antarctic ozone hole into two holes in September 2002. In particular, the following precursory changes have been identified: First, for scales larger than 8 years, the entropy in natural time exhibits a gradual increase after around 1999. Second, from 2000 to 2001, the entropy in natural time under time reversal shows an increase for all scales (3-15 years) except for the scale of 13 years. Third, the values of the entropy change in natural time almost coincide at 2000 for the short scales 3-7 years and then decrease. The analysis in the natural time domain is also applied on the eddy heat flux, which is proportional to the vertically propagating wave activity affecting the ozone hole over Antarctica. The results drawn confirm those deduced from the ozone hole area diagnostics. © 2011 Elsevier Ltd
Did the 7/9/1999 M5.9 Athens earthquake come with a warning?
No full textPrior to the 7/9/1999 MS = 5.9 Athens earthquake, regional seismicity has exhibited a power-law increase, of the form ΣΩ = K + A (tc - t)n, where Ω is estimated using an expression log Ω = cM + d and tc is the time of the culminating event. Such changes appeared after the 17/8/1999 M7.4 Izmit event. We quantified the performance of the power law vs. the null hypothesis of constant seismic release rates. by defining the curvature C as the ratio of the power law fit RMS/linear fit RMS, so that the smaller C is, the better the power law behaviour. By mapping C, we have established a critical radius of 110 km and observed that the region of correlated accelerating seismic release extended from the N. Aegean, through Euboea and Attica to the SW Peloponnese. A few days prior to the Athens event, min(C) was centred at the epicentral area and numerical simulation yielded tc = 1999.676 and predicted MS = 5.77. Seismicity rates returned to normal (quasi-constant) after the Athens event. We interpret this effect as critical point behaviour, following remote excitation of a broad area by stress redistribution due to the Izmit event which, at Athens, has triggered 'premature' failure of a fault nearing its load bearing capacity. If this is correct, we have documented a case of remote earthquake triggering by another earthquake, as well as insight into the mechanisms producing it. As a corollary, we note that a large event may beget another large event in its broader region of interaction, which may be preceded by characteristic precursory seismicity changes
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