On the lognormality of the small-scale structure of turbulence

Higher-order moments of turbulent velocity gradients and their behavior with Reynolds number were measured in the nearly isotropic turbulent field generated by a square-mesh grid and in a turbulent boundary layer along a flat plate with zero pressure gradient. Hot-wire anemometry and instrumentation combining analog and digital methods were used to measure moments up to the fourteenth order. Measurements of such high-order moments required that particular attention be given to their validity. Involved herein was the evaluation of such effects as nonlinearity, averaging intervals, and the adequacy of the statistics for the tails of the probability density distributions. The results obtained are compared with those of other investigators for a variety of flow configurations in the laboratory as well as in the atmosphere. The concept of the intermittency of the small-scale structure and the theoretical approach involving lognormality of the probability density distribution of the dissipation rate are evaluated.     

Applications of innovative polygonal trend analyses to precipitation series of Eastern Black Sea Basin,Turkey

Theoretical and Applied Climatology - Examining historical variations of hydroclimatic variables can provide crucial information about changes of water resources in a water cycle. In this study,...     

The Flare-Energy Distributions Generated by Kink-Unstable Ensembles of Zero-Net-Current Coronal Loops

It has been proposed that the million-degree temperature of the corona is due to the combined effect of barely detectable energy releases, called nanoflares, that occur throughout the solar atmosphere. Unfortunately, the nanoflare density and brightness implied by this hypothesis means that conclusive verification is beyond present observational abilities. Nevertheless, we investigate the plausibility of the nanoflare hypothesis by constructing a magnetohydrodynamic (MHD) model that can derive the energy of a nanoflare from the nature of an ideal kink instability. The set of energy-releasing instabilities is captured by an instability threshold for linear kink modes. Each point on the threshold is associated with a unique energy release; thus we can predict a distribution of nanoflare energies. When the linear instability threshold is crossed, the instability enters a nonlinear phase as it is driven by current sheet reconnection. As the ensuing flare erupts and declines, the field transitions to a lower energy state, which is modelled by relaxation theory; i.e., helicity is conserved and the ratio of current to field becomes invariant within the loop. We apply the model so that all the loops within an ensemble achieve instability followed by energy-releasing relaxation. The result is a nanoflare energy distribution. Furthermore, we produce different distributions by varying the loop aspect ratio, the nature of the path to instability taken by each loop and also the level of radial expansion that may accompany loop relaxation. The heating rate obtained is just sufficient for coronal heating. In addition, we also show that kink instability cannot be associated with a critical magnetic twist value for every point along the instability threshold.     

Modeling flood dynamics in a temporary river draining to an eutrophic reservoir in southeast Portugal

Enxoé is a small temporary river with a flushy regime, which flash floods carry significant loads to the reservoir. As a result, the reservoir, which supplies 25,000 inhabitants, exhibits a high trophic state and cyanobacteria blooms since its construction in 1998, with water abstractions requiring extensive treatment. This study aimed to understand the contribution of flash floods to the Enxoé’s reservoir high trophic state using a modeling approach. This was the first time the river was monitored and that a modeling study was carried out. The MOHID-Land model was implemented to assess the water path in the catchment, and was integrated with field data to compute river loads. Results confirmed the importance of flash events. During flash floods, water properties were determined by soil surface and river bottom wash out, and depended mostly on the flush sequence and intensity. Model simulations showed that soil surface permeability reduction was an important factor regulating surface runoff while soil moisture was low. The first flood after the dry period contributed to 2% of the yearly discharge, 3% of yearly N load, and 7% of the yearly P loads. Winter floods contribution differed, producing 10% of both yearly discharge and loads. However, concentration of particulate matter and organic compounds in the first flood were one order of magnitude higher than in winter floods. This was due to river bottom resuspension and erosion of riparian areas, representative dynamics of a flushy regime. During subsequent winter floods, nutrient concentrations tended to remain constant as the watershed surface and respective soils were washed. Further work should link a watershed model to a reservoir model to depict the flood impact in the reservoir, and test management strategies to reduce the reservoir trophic state.     

Cross-scale: multi-scale coupling in space plasmas

Most of the visible universe is in the highly ionised plasma state, and most of that plasma is collision-free. Three physical phenomena are responsible for nearly all of the processes that accelerate particles, transport material and energy, and mediate flows in systems as diverse as radio galaxy jets and supernovae explosions through to solar flares and planetary magnetospheres. These processes in turn result from the coupling amongst phenomena at macroscopic fluid scales, smaller ion scales, and down to electron scales. Cross-Scale, in concert with its sister mission SCOPE (to be provided by the Japan Aerospace Exploration Agency—JAXA), is dedicated to quantifying that nonlinear, time-varying coupling via the simultaneous in-situ observations of space plasmas performed by a fleet of 12 spacecraft in near-Earth orbit. Cross-Scale has been selected for the Assessment Phase of Cosmic Vision by the European Space Agency.      

Contrasting oxygen fugacity of I- and S-type granites from the Araçuaí orogen,SE Brazil: an approach based on opaque mineral assemblages

Mineralogy and Petrology - This study presents mineralogical characterization of opaque assemblages from I- and S-type granites from the Araçuaí orogen, southeastern Brazil that belong...     

Fate and transport of volatile organic compounds in glacial till and groundwater at an industrial site in Northern Ireland

Volatile organic compound (VOC) contamination of subsurface geological material and groundwater was discovered on the Nortel Monkstown industrial site, Belfast, Northern Ireland. The objectives of this study were to (1) investigate the characteristics of the geological material and its influences on contaminated groundwater flow across the site using borehole logs and hydrological evaluations, and (2) identify the contaminants and examine their distribution in the subsurface geological material and groundwater using chemical analysis. This report focuses on the eastern car park (ECP) which was a former storage area associated with trichloroethene (TCE) degreasing operations. This is where the greatest amount of volatile organic compounds (VOCs), particularly TCE, were detected. The study site is on a complex deposit of clayey glacial till with discontinuous coarser grained lenses, mainly silts, sands and gravel, which occur at 0.45–7.82 m below ground level (bgl). The lenses overall form an elongated formation that acts as a small unconfined shallow aquifer. There is a continuous low permeable stiff clayey till layer beneath the lenses that performs as an aquitard to the groundwater. Highest concentrations of VOCs, mainly TCE, in the geological material and groundwater are in these coarser lenses at ∼4.5–7 m bgl. Highest TCE measurements at 390,000 μg L⁻¹ for groundwater and at 39,000 μg kg⁻¹ at 5.7 m for geological material were in borehole GA19 in the coarse lens zone. It is assumed that TCE gained entrance to the subsurface near this borehole where the clayey till was thin to absent above coarse lenses which provided little retardation to the vertical migration of this dense non-aqueous phase liquid (DNAPL) into the groundwater. However, TCE is present in low concentrations in the geological material overlying the coarse lens zone. Additionally, VOCs appear to be associated with poorly drained layers and in peat <3.0 m bgl in the ECP. Some indication of natural attenuation as VOCs degradation products vinyl chloride (VC) and dichloromethane (DCM) also occur on the site.     

Metals from star-forming dwarfs: retention or ejection?

We show evidences that gas outflows occur in starburst galaxies as superbubbles evolve. We then question whether hot gas will be expelled and enrich the IGM with metals or be retained within the host galaxy. For this purpose we construct three extreme scenarios of the star formation histories for a sample of dwarf galaxies using either their present metallicity or their luminosity. The three scenarios imply different mechanical energy input rates, those are compared with theoretical lower limits for the ejection of processed matter out of host galaxies. The comparison strongly points at the existence of extended gaseous haloes acting as a barrier that allows these galaxies to retain their metals and enhance their abundance. Our findings strongly point that continuous star-forming processes, rather than coeval bursts, must contribute to the overall metallicity in our galaxy sample. This revised version was published online in September 2006 with corrections to the Cover Date.     

Chemical composition of falling snow at Dumont d'Urville,Antarctica

Fourteen samples of fresh falling snow were collected at Antarctic coastal base Dumont d'Urville in 1984. The samples have been analysed for major ions (including MSA) by ion chromatography and acid titration. The results are relevant to the chemical composition of background precipitation in polar marine conditions. The seasalt aerosol contribution is dominant. All samples are found to be acidic in the range 3–16 eq/l. The calculated non-seasalt sulfate (nssSO₄ ^2-) concentration is significantly negative for 3 of the 14 samples. NssSO₄ ^2- is found to be relatively high in summer and fall. MSA also exhibits the same pattern probably linked to local marine biogenic activity and/or atmospheric photochemical processes. The MSA to nssSO₄ ^2- ratio is in good agreement with values reported for coastal Antarctic ice cores and subantarctic acrosol. The background mean value for nitrate concentration is 1.1 eq/l but two very strong spikes (up to 16 eq/l) are observed. The first seems to be linked with long range transport of continental air masses while the second (in winter) is clearly due to a sudden input of nitric acid, possibly from the stratosphere.This paper represents a preliminary approach to a larger air and snow monitoring to be developped at this site.