4,106 research outputs found
Using dissolved oxygen concentrations to determine mixed layer depths in the Bellingshausen Sea
Concentrations of oxygen (O<sub>2</sub>) and other dissolved gases in the oceanic mixed layer are often used to calculate air-sea gas exchange fluxes. The mixed layer depth (<i>z</i><sub>mix</sub>) may be defined using criteria based on temperature or density differences to a reference depth near the ocean surface. However, temperature criteria fail in regions with strong haloclines such as the Southern Ocean where heat, freshwater and momentum fluxes interact to establish mixed layers. Moreover, the time scales of air-sea exchange differ for gases and heat, so that <i>z</i><sub>mix</sub> defined using oxygen may be different than <i>z</i><sub>mix</sub> defined using temperature or density. Here, we propose to define an O<sub>2</sub>-based mixed layer depth, <i>z</i><sub>mix</sub>(O<sub>2</sub>), as the depth where the relative difference between the O<sub>2</sub> concentration and a reference value at a depth equivalent to 10 dbar equals 0.5 %. This definition was established by analysis of O<sub>2</sub> profiles from the Bellingshausen Sea (west of the Antarctic Peninsula) and corroborated by visual inspection. Comparisons of <i>z</i><sub>mix</sub>(O<sub>2</sub>) with <i>z</i><sub>mix</sub> based on potential temperature differences, i.e., <i>z</i><sub>mix</sub>(0.2 °C) and <i>z</i><sub>mix</sub>(0.5 °C), and potential density differences, i.e., <i>z</i><sub>mix</sub>(0.03 kg m<sup>&minus;3</sup>) and <i>z</i><sub>mix</sub>(0.125 kg m<sup>&minus;3</sup>), showed that <i>z</i><sub>mix</sub>(O<sub>2</sub>) closely follows <i>z</i><sub>mix</sub>(0.03 kg m<sup>&minus;3</sup>). Further comparisons with published <i>z</i><sub>mix</sub> climatologies and <i>z</i><sub>mix</sub> derived from World Ocean Atlas 2005 data were also performed. To establish <i>z</i><sub>mix</sub> for use with biological production estimates in the absence of O<sub>2</sub> profiles, we suggest using <i>z</i><sub>mix</sub>(0.03 kg m<sup>&minus;3</sup>), which is also the basis for the climatology by de Boyer Montégut et al. (2004)
Derivation of a multilayer approach to model suspended sediment transport: application to hyperpycnal and hypopycnal plumes
We propose a multi-layer approach to simulate hyperpycnal and hypopycnal
plumes in flows with free surface. The model allows to compute the vertical
profile of the horizontal and the vertical components of the velocity of the
fluid flow. The model can describe as well the vertical profile of the sediment
concentration and the velocity components of each one of the sediment species
that form the turbidity current. To do so, it takes into account the settling
velocity of the particles and their interaction with the fluid. This allows to
better describe the phenomena than a single layer approach. It is in better
agreement with the physics of the problem and gives promising results. The
numerical simulation is carried out by rewriting the multi-layer approach in a
compact formulation, which corresponds to a system with non-conservative
products, and using path-conservative numerical scheme. Numerical results are
presented in order to show the potential of the model
On the Distribution of Stellar Masses in Gamma-ray Burst Host Galaxies
We analyze Spitzer images of 30 long-duration gamma-ray burst (GRB) host galaxies. We estimate their total stellar masses (M_*) based on the rest-frame K-band luminosities (L_K_(rest)) and constrain their star formation rates (SFRs; not corrected for dust extinction) based on the rest-frame UV continua. Further, we compute a mean M_*/ L_K_(rest) = 0.45 M_☉/L_☉. We find that the hosts are low M_*, star-forming systems. The median M_* in our sample ( = 10^(9.7) M_☉) is lower than that of "field" galaxies (e.g., Gemini Deep Deep Survey). The range spanned by M_* is 10^7 M_☉ < M_* < 10^(11) M_☉, while the range spanned by the dust-uncorrected UV SFR is 10^(–2) M_☉ yr^(–1) < SFR < 10 M_☉ yr^(–1). There is no evidence for intrinsic evolution in the distribution of M_* with redshift. We show that extinction by dust must be present in at least 25% of the GRB hosts in our sample and suggest that this is a way to reconcile our finding of a relatively lower UV-based, specific SFR (φ ≡ SFR/M_*) with previous claims that GRBs have some of the highest φ values. We also examine the effect that the inability to resolve the star-forming regions in the hosts has on φ
Multiscale Modelling Approach for a Fungal Biofilter Unit for the Hydrophobic Abatement of Volatile Organic Compounds
QoSatAr: a cross-layer architecture for E2E QoS provisioning over DVB-S2 broadband satellite systems
This article presents QoSatAr, a cross-layer architecture developed to provide end-to-end quality of service (QoS) guarantees for Internet protocol (IP) traffic over the Digital Video Broadcasting-Second generation (DVB-S2) satellite systems. The architecture design is based on a cross-layer optimization between the physical layer and the network layer to provide QoS provisioning based on the bandwidth availability present in the DVB-S2 satellite channel. Our design is developed at the satellite-independent layers, being in compliance with the ETSI-BSM-QoS standards. The architecture is set up inside the gateway, it includes a Re-Queuing Mechanism (RQM) to enhance the goodput of the EF and AF traffic classes and an adaptive IP scheduler to guarantee the high-priority traffic classes taking into account the channel conditions affected by rain events. One of the most important aspect of the architecture design is that QoSatAr is able to guarantee the QoS requirements for specific traffic flows considering a single parameter: the bandwidth availability which is set at the physical layer (considering adaptive code and modulation adaptation) and sent to the network layer by means of a cross-layer optimization. The architecture has been evaluated using the NS-2 simulator. In this article, we present evaluation metrics, extensive simulations results and conclusions about the performance of the proposed QoSatAr when it is evaluated over a DVB-S2 satellite scenario. The key results show that the implementation of this architecture enables to keep control of the satellite system load while guaranteeing the QoS levels for the high-priority traffic classes even when bandwidth variations due to rain events are experienced. Moreover, using the RQM mechanism the user’s quality of experience is improved while keeping lower delay and jitter values for the high-priority traffic classes. In particular, the AF goodput is enhanced around 33% over the drop tail scheme (on average)
Shallow Water Moment models for bedload transport problems
In this work a simple but accurate shallow model for bedload sediment
transport is proposed. The model is based on applying the moment approach to
the Shallow Water Exner model, making it possible to recover the vertical
structure of the flow. This approach allows us to obtain a better approximation
of the fluid velocity close to the bottom, which is the relevant velocity for
the sediment transport. A general Shallow Water Exner moment model allowing for
polynomial velocity profiles of arbitrary order is obtained. A regularization
ensures hyperbolicity and easy computation of the eigenvalues. The system is
solved by means of an adapted IFCP scheme proposed here. The improvement of
this IFCP type scheme is based on the approximation of the eigenvalue
associated to the sediment transport. Numerical tests are presented which deal
with large and short time scales. The proposed model allows to obtain the
vertical structure of the fluid, which results in a better description on the
bedload transport of the sediment layer
Draft genome sequence of the UV-Resistant antarctic bacterium Sphingomonas sp. strain UV9
We report the draft genome sequence of the Antarctic UV-resistant bacterium Sphingomonas sp. strain UV9. The strain has a genome size of 4.25 Mb, a 65.62% GC content, and 3,879 protein-coding sequences. Among others, genes encoding the resolving of the DNA damage produced by the UV irradiation were identified
Early elimination of cyclosporine in kidney transplant recipients receiving sirolimus prevents progression of chronic pathologic allograft lesions
Cyclosporine elimination in a regimen including sirolimus has been shown to be a safe and effective approach to improve graft function. Nevertheless, it is still unknown whether the functional benefit of CyA withdrawal coincides with a subsequent reduction in histologic lesions of chronic damage or development of chronic allograft nephropathy. This consideration would forecast a reduction in the rate of long-term graft loss. We analyzed 114 graft biopsies from a subgroup of 57 patients that had been included in a randomized study to eliminate CyA at 3 months posttransplant from a regimen including sirolimus either in group A CyA + SRL vs group B of SRL with CyA elimination at 3 months. Every patient had two biopsies, one at transplantation and another at 1 year. The biopsy reading was performed in a blinded manner by a central pathologist using the Banff 1997 and the CADI classifications. A significantly lower rate of progression of tubular and interstitial chronic lesions between basal and 1-year biopsies was observed for group B patients. In addition, the incidence of new cases of chronic allograft nephropathy during the first year was significantly lower in the group in which CyA had been eliminated at 3 months posttransplant. We conclude that early elimination of CyA in the first months posttransplant, when SRL is used as the main immunosuppressant, reduces the appearance or worsening of chronic histologic lesions, probably as a consequence of long-term CyA toxicity prevention
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