bluuebunny/crossref_metadata_2025 huggingface dataset:
Physiological oxygenation status is required for fully differentiated phenotype in kidney cortex proximal tubules
Authors: de Laplanche Elodie , Gouget Karine , Cléris Guilhem , Dragounoff Franck , Demont Jocelyne , Morales Anne , Bezin Laurent , Godinot Catherine , Perrière Guy , Mouchiroud Dominique , Simonnet Hélène
Published: April 2006

Hypoxia has been suspected to trigger transdifferentiation of renal tubular cells into myofibroblasts in an epithelial-to-mesenchymal transition (EMT) process. To determine the functional networks potentially altered by hypoxia, rat renal tubule suspensions were incubated under three conditions of oxygenation ranging from normoxia (lactate uptake) to severe hypoxia (lactate production). Transcriptome changes after 4 h were analyzed on a high scale by restriction fragment differential display. Among 1,533 transcripts found, 42% were maximally expressed under severe hypoxia and 8% under mild hypoxia (Po2 = 48 mmHg), suggesting two different levels of oxygen sensing. Normoxia was required for full expression of the proximal tubule-specific transcripts 25-hydroxyvitamin D 1-hydroxylase ( Cyp27b1) and l-pyruvate kinase ( Pklr), transcripts involved in tissue cohesion such as fibronectin ( Fn1) and N-cadherin ( Cdh2), and non-muscle-type myosin transcripts. Mild hypoxia increased myogenin transcript level. Conversely, severe hypoxia increased transcripts involved in extracellular matrix remodeling, those of muscle-type myosins, and others involved in creatine phosphate synthesis and lactate transport ( Slc16a7). Accordingly, microscopy showed loss of tubule aggregation under hypoxia, without tubular disruption. Hypoxia also increased the levels of kidney-specific transcripts normally restricted to the less oxygenated medullary zone and others specific for the distal part of the nephron. We conclude that extensive oxygen supply to the kidney tubule favors expression of its differentiated functions specifically in the proximal tubule, whose embryonic origin is mesenchymal. The phenotype changes could potentially permit transient adaptation to hypoxia but also favor pathological processes such as tissue invasion.
FGF-10 and its receptor exhibit bidirectional paracrine targeting to urothelial and smooth muscle cells in the lower urinary tract
Authors: Zhang Dianzhong , Kosman Jeffrey , Carmean Nicole , Grady Richard , Bassuk James A.
Published: April 2006

Control of the regenerative properties of urothelial tissue would greatly aid the clinician in the management of urinary tract disease and disorders. Fibroblast growth factor 10 (FGF-10) is a mitogen which is particularly promising as a protein therapy for urothelial injury. The spatial synthesis, transport, targeting, and mechanistic pathway of FGF-10 and its receptor were studied in a human urothelial cell culture model and in fixed sections of lower urinary tract tissue. Synthesis of FGF-10 was restricted to mesenchymal fibroblasts, and secreted FGF-10 exhibited paracrine transport to two proximal sites, transitional epithelium and smooth muscle cell bundles, both of which were also the exclusive sites of FGF-10 receptor synthesis. The addition of recombinant FGF-10 to quiescent urothelial cells in vitro was sufficient to stimulate DNA synthesis. This stimulation was through a pathway independent of the epidermal growth factor receptor pathway. Deconvolution, light and transmission electron microscopic studies captured FGF-10 and its receptor in association with the urothelial cell surface, in cytoplasm, and within nuclei, observations that describe the mechanism that transduces the mitogenic signal in these tissues. Localization of the FGF-10 receptor to the superficial urothelial layer is clinically significant because intravesical administration of FGF-10 may provide the clinician a means to control the turnover of transitional epithelium in bladder disorders such as interstitial cystitis.
Kinetics of urothelial ATP release
Authors: Lewis Simon A. , Lewis Jamie R.
Published: April 2006

Recent reports have proposed that the urothelium can sense mechanical stretch and communicate this information to sensory afferent neurons by the release of ATP into the vicinity of P2X-containing neurons. This report investigates the bidirectional release of ATP by in vitro rabbit urothelium. ATP was measured using the luciferin-luciferase assay. Immediately after washing of both sides of the epithelium, there was a linear increase in ATP content in the mucosal compartment with a rate of 23 ± 6.5 fmol·min−1·cm−2 ( n = 18). Serosal ATP content increased as a saturating exponential function, suggesting a constant rate of release and degradation of ATP by ectonucleotidases/exonucleotidases. The presence of a serosal ectonucleotidase/exonucleotidases was demonstrated by the time-dependent decrease in exogenously added ATP. The maximum rate of hydrolysis was 11 pmol·min−1·cm−2 with a Km of 0.49 μM. The time course of serosal ATP release was modeled as a constant rate of release ( d: mol·min−1·cm−2) and rate constant of hydrolysis ( kh: min−). In control conditions d was 18 fmol·min−1·cm−2 and kh of 0.056 ± 0.01 min− ( n = 18). Steady-state serosal chamber content is 370 ± 90 fmol/cm2, and concentration is 50 ± 1.2 × 10−12 M. Stretching the tissue resulted in a transient fivefold increase in the rate of mucosal ATP release and a transient sixfold increase in serosal ATP release. Half-osmotic strength solutions increased mucosal release by 10-fold and serosal release by 5-fold. Tissue damage resulted in a step-increase in mucosal chamber ATP content by 6.6 ± 1 pmol/cm2 and serosal chamber ATP by 0.1 ± 0.06 pmol/cm2 ( n = 5).
Novel cystogenic role of basic fibroblast growth factor in developing rodent kidneys
Authors: Li Zhuangwu , Jerebtsova Marina , Liu Xue-Hui , Tang Pingtao , Ray Patricio E.
Published: April 2006

Basic fibroblast growth factor (bFGF) is a heparin-binding growth factor that is accumulated in human dysplastic and cystic renal diseases. Previous studies have shown that bFGF can modulate the growth of developing renal tubules; however, its role in the pathogenesis of renal cyst formation is not clearly understood. Here, we tested the hypothesis that overexpression of bFGF in developing rodent kidneys induces cyst formation in vivo. We used two different adenoviral-mediated gene-transferring approaches to overexpress bFGF in developing rodent kidneys. Initially, metanephric kidney (MK) explants harvested from embryonic day 15 Sprague-Dawley rats were infected with adenoviral vectors (rAd) encoding human bFGF or LacZ genes and transplanted under the renal capsule of adult female rats. Subsequently, to determine whether bFGF could induce renal cysts in developing kidneys with an intact renal collecting system, we injected rAd-bFGF or LacZ vectors in the retroorbital plexus of newborn mice. Basic FGF induced a more efficient integration of the MK explants into the host kidneys and increased the vascularization and proliferation of developing tubules, leading to tubular dilatation and rapid formation of renal cysts. In addition, we successfully expressed human bFGF in the kidney of newborn mice in vivo and induced tubular dilatation and renal cysts. In contrast, mice injected with rAd- lacZ did not develop tubular dilatation or renal cysts. To the best of our knowledge, these experiments show for the first time that overexpression of bFGF in developing rodent kidneys can induce the formation of renal cysts in vivo.
Effects of expression of p53 and Gadd45 on osmotic tolerance of renal inner medullary cells
Authors: Cai Qi , Dmitrieva Natalia I. , Ferraris Joan D. , Michea Luis F. , Salvador Jesus M. , Hollander M. Christine , Fornace Albert J. , Fenton Robert A. , Burg Maurice B.
Published: April 2006

The response of renal inner medullary (IM) collecting duct cells (mIMCD3) to high NaCl involves increased expression of Gadd45 and p53, both of which have important effects on growth and survival of the cells. However, mIMCD3 cells, being immortalized by SV40, proliferate rapidly, which is known to sensitize cells to high NaCl, whereas IM cells in situ proliferate very slowly and survive much higher levels of NaCl. In the present studies, we have examined the importance of Gadd45 and p53 for survival of normal IM cells in their usual high-NaCl environment by using more slowly proliferating second-passage mouse inner medullary epithelial (p2mIME) cells and comparing cells from wild-type and gene knockout mice. Acutely elevating NaCl (and/or urea) reduces Gadd45a, but increases Gadd45b and Gadd45g mRNA, depending on the mix of NaCl and urea and the rate of increase of osmolality. Nevertheless, p2mIME cells from Gadd45b−/−, Gadd45g−/−, and Gadd45bg−/− mice survive elevation of NaCl (or urea) essentially the same as do wild-type cells. p53−/− Cells do not tolerate as high a concentration of NaCl (or urea) as p53+/+ cells, but urinary concentrating ability of p53−/− mice is normal, as is the histology of inner medullas from p53−/− and Gadd45abg−/− mice. Thus although Gadd45 and p53 may play roles in osmotically stressed mIMCD3 cells, we do not find that their expression makes an important difference, either for Gadd45 in slower proliferating p2mIME cells or for Gadd45 or p53 in normal inner medullary epithelial cells in situ.
Direct corticospinal pathways contribute to neuromuscular control of perturbed stance
Authors: Taube Wolfgang , Schubert Martin , Gruber Markus , Beck Sandra , Faist Michael , Gollhofer Albert
Published: April 2006

The antigravity soleus muscle (Sol) is crucial for compensation of stance perturbation. A corticospinal contribution to the compensatory response of the Sol is under debate. The present study assessed spinal, corticospinal, and cortical excitability at the peaks of short- (SLR), medium- (MLR), and long-latency responses (LLR) after posterior translation of the feet. Transcranial magnetic stimulation (TMS) and peripheral nerve stimulation were individually adjusted so that the peaks of either motor evoked potential (MEP) or H reflex coincided with peaks of SLR, MLR, and LLR, respectively. The influence of specific, presumably direct, corticospinal pathways was investigated by H-reflex conditioning. When TMS was triggered so that the MEP arrived in the Sol at the same time as the peaks of SLR and MLR, EMG remained unaffected. Enhanced EMG was observed when the MEP coincided with the LLR peak ( P < 0.001). Similarly, conditioning of the H reflex by subthreshold TMS facilitated H reflexes only at LLR ( P < 0.001). The earliest facilitation after perturbation occurred after 86 ms. The TMS-induced H-reflex facilitation at LLR suggests that increased cortical excitability contributes to the augmentation of the LLR peaks. This provides evidence that the LLR in the Sol muscle is at least partly transcortical, involving direct corticospinal pathways. Additionally, these results demonstrate that ∼86 ms after perturbation, postural compensatory responses are cortically mediated.
Midbrain Periaqueductal Gray and Vocal Patterning in a Teleost Fish
Authors: Kittelberger J. Matthew , Land Bruce R. , Bass Andrew H.
Published: April 2006

Midbrain structures, including the periaqueductal gray (PAG), are essential nodes in vertebrate motor circuits controlling a broad range of behaviors, from locomotion to complex social behaviors such as vocalization. Few single-unit recording studies, so far all in mammals, have investigated the PAG's role in the temporal patterning of these behaviors. Midshipman fish use vocalization to signal social intent in territorial and courtship interactions. Evidence has implicated a region of their midbrain, located in a similar position as the mammalian PAG, in call production. Here, extracellular single-unit recordings of PAG neuronal activity were made during forebrain-evoked fictive vocalizations that mimic natural call types and reflect the rhythmic output of a known hindbrain–spinal pattern generator. The activity patterns of vocally active PAG neurons were mostly correlated with features related to fictive call initiation. However, spike trains in a subset of neurons predicted the duration of vocal output. Duration is the primary feature distinguishing call types used in different social contexts and these cells may play a role in directly establishing this temporal dimension of vocalization. Reversible, lidocaine inactivation experiments demonstrated the necessity of the midshipman PAG for fictive vocalization, whereas tract-tracing studies revealed the PAG's connectivity to vocal motor centers in the fore- and hindbrain comparable to that in mammals. Together, these data support the hypotheses that the midbrain PAG of teleosts plays an essential role in vocalization and is convergent in both its functional and structural organization to the PAG of mammals.
Symmetric Interactions Within a Homogeneous Starburst Cell Network Can Lead to Robust Asymmetries in Dendrites of Starburst Amacrine Cells
Authors: Münch Thomas A. , Werblin Frank S.
Published: April 2006

Starburst amacrine cells in the mammalian retina respond asymmetrically to movement along their dendrites; centrifugal movement elicits stronger responses in each dendrite than centripetal movement. It has been suggested that the asymmetrical response can be attributed to intrinsic properties of the processes themselves. But starburst cells are known to release and have receptors for both GABA and acetylcholine. We tested whether interactions within the starburst cell network can contribute to their directional response properties. In a computational model of interacting starburst amacrine cells, we simulated the response of individual dendrites to moving light stimuli. By setting the model parameters for “synaptic connection strength” ( cs) to positive or negative values, overlapping starburst dendrites could either excite or inhibit each other. For some values of cs, we observed a very robust inward/outward asymmetry of the starburst dendrites consistent with the reported physiological findings. This is the case, for example, if a starburst cell receives inhibition from other starburst cells located in its surround. For other values of cs, individual dendrites can respond best either to inward movement or respond symmetrically. A properly wired network of starburst cells can therefore account for the experimentally observed asymmetry of their response to movement, independent of any internal biophysical or biochemical properties of starburst cell dendrites.
Voltage-Gated Sodium Channels in Cerebellar Purkinje Cells of Mormyrid Fish
Authors: de Ruiter Martijn M. , De Zeeuw Chris I. , Hansel Christian
Published: April 2006

Cerebellar Purkinje cells of mormyrid fish differ in some morphological as well as physiological parameters from their counterparts in mammals. Morphologically, Purkinje cells of mormyrids have larger dendrites that are characterized by a lower degree of branching in the molecular layer. Physiologically, there are differences in electrophysiological response patterns that are related to sodium channel activity: first, sodium spikes in mormyrid Purkinje cells have low amplitudes, typically not exceeding 30 mV. Second, the response to climbing fiber stimulation in mormyrid Purkinje cells does not consist of a complex spike (with an initial fast sodium spike) as in mammals, but instead it consists of an all-or-none excitatory postsynaptic potential, the so-called climbing fiber response. Because of these unique properties, we have begun to characterize mormyrid Purkinje cells electrophysiologically. In this study, we provide a description of voltage-gated Na+ channels and conductances in Purkinje cells of the mormyrid fish Gnathonemus petersii. Various types of Na+ channel α-subunits, i.e., Nav1.1, Nav1.2, and Nav1.6, have been described in rodent Purkinje cells. Using immunohistochemical techniques, we found that these subunits are present in Purkinje cells of mormyrids. To test whether these Na+ channel subunits can mediate fast inactivating and resurgent Na+ currents in Gnathonemus Purkinje cells, we conducted patch-clamp recordings in acutely dissociated cells and in cerebellar slices. Both types of Na+ currents could be measured in rat and fish Purkinje cells. These data show that, despite prominent differences in electrophysiological response characteristics, Purkinje cells of rats and mormyrids share the same voltage-gated Na+ conductances.
Multivesicular Release at Developing Schaffer Collateral–CA1 Synapses: An Analytic Approach to Describe Experimental Data
Authors: Ricci-Tersenghi F. , Minneci F. , Sola E. , Cherubini E. , Maggi L.
Published: April 2006

We developed and analytically solved a simple and general stochastic model to distinguish the univesicular from the multivesicular mode of glutamate release. The model solution gives analytical mathematical expressions for average values of quantities that can be measured experimentally. Comparison of these quantities with the experimental measures allows one to discriminate the release mode and to determine the most probable values of model parameters. The model has been validated at glutamatergic CA3–CA1 synapses in the hippocampus from newborn (P1–P5 old) rats. Our results strongly support a multivesicular type of release process requiring a variable pool of immediately releasable vesicles. Moreover, computing quantities that are functions of the model parameters, the mean amplitude of the synaptic response to the release of a single vesicle ( q) was estimated to be 5–10 pA, in very good agreement with experimental findings. In addition a multivesicular type of release was supported by the following experimental evidences: 1) a high variability of the amplitude of successes, with a coefficient of variation ranging from 0.12 to 0.73; 2) an average potency ratio a2/a1 between the second and first response to a pair of stimuli >1; and 3) changes in the potency of the synaptic response to the first stimulus when the release probability was modified by increasing or decreasing the extracellular calcium concentration. Our results indicate that at Schaffer collateral–CA1 synapses of the neonatal rat hippocampus a single action potential may induce the release of more than one vesicle from the same release site.
Realistic Modeling of Entorhinal Cortex Field Potentials and Interpretation of Epileptic Activity in the Guinea Pig Isolated Brain Preparation
Authors: Labyt E. , Uva L. , de Curtis M. , Wendling F.
Published: April 2006

Mechanisms underlying epileptic activities recorded from entorhinal cortex (EC) were studied through a computational model based on review of cytoarchitectonic and neurobiological data about this structure. The purpose of this study is to describe and use this model to interpret epileptiform discharge patterns recorded in an experimental model of ictogenesis (guinea pig isolated brain perfused with bicuculline). A macroscopic modeling approach representing synaptic interactions between cells subpopulations in the EC was chosen for its adequacy to mimic field potentials reflecting overall dynamics rising from interconnected cells populations. Therefore intrinsic properties of neurons were not included in the modeling design. Model parameters were adjusted from an identification procedure based on quantitative comparison between real and simulated signals. For both EC deep and superficial layers, results show that the model generates very realistic signals regarding temporal dynamics, spectral features, and cross-correlation values. These simulations allowed us to infer information about the evolution of synaptic transmission between principal cell and interneuronal populations and about connectivity between deep and superficial layers during the transition from background to ictal activity. In the model, this transition was obtained for increased excitation in deep versus superficial layers. Transitions between epileptiform activities [interictal spikes, fast onset activity (25 Hz), ictal bursting activity] were explained by changes of parameters mainly related to GABAergic interactions. Notably, the model predicted an important role of GABAa,fast- and GABAb-receptor–mediated inhibition in the generation of ictal fast onset and burst activities, respectively. These findings are discussed with respect to experimental data.
Heme Oxygenase-1/Carbon Monoxide: From Basic Science to Therapeutic Applications
Authors: Ryter Stefan W. , Alam Jawed , Choi Augustine M. K.
Published: April 2006

The heme oxygenases, which consist of constitutive and inducible isozymes (HO-1, HO-2), catalyze the rate-limiting step in the metabolic conversion of heme to the bile pigments (i.e., biliverdin and bilirubin) and thus constitute a major intracellular source of iron and carbon monoxide (CO). In recent years, endogenously produced CO has been shown to possess intriguing signaling properties affecting numerous critical cellular functions including but not limited to inflammation, cellular proliferation, and apoptotic cell death. The era of gaseous molecules in biomedical research and human diseases initiated with the discovery that the endothelial cell-derived relaxing factor was identical to the gaseous molecule nitric oxide (NO). The discovery that endogenously produced gaseous molecules such as NO and now CO can impart potent physiological and biological effector functions truly represented a paradigm shift and unraveled new avenues of intense investigations. This review covers the molecular and biochemical characterization of HOs, with a discussion on the mechanisms of signal transduction and gene regulation that mediate the induction of HO-1 by environmental stress. Furthermore, the current understanding of the functional significance of HO shall be discussed from the perspective of each of the metabolic by-products, with a special emphasis on CO. Finally, this presentation aspires to lay a foundation for potential future clinical applications of these systems.
Cloud-Resolving Satellite Data Assimilation: Information Content of IR Window Observations and Uncertainties in Estimation
Authors: Vukicevic T. , Sengupta M. , Jones A. S. , Haar T. Vonder
Published: April 2006

This study addresses the problem of four-dimensional (4D) estimation of a cloudy atmosphere on cloud-resolving scales using satellite remote sensing measurements. The motivation is to develop a methodology for accurate estimation of cloud properties and the associated atmospheric environment on small spatial scales but over large regions to aid in better understanding of the clouds and their role in the atmospheric system. The problem is initially approached by the study of the assimilation of the Geostationary Operational Environmental Satellite (GOES) imager observations into a cloud-resolving model with explicit bulk cloud microphysical parameterization. A new 4D variational data assimilation (4DVAR) research system with the cloud-resolving capability is applied to a case of a multilayered cloud evolution without convection. In the experiments the information content of the IR window channels is addressed as well as the sensitivity of estimation to lateral boundary condition errors, model first guess, decorrelation length in the background statistical error model, and the use of a generic linear model error. The assimilation results are compared with independent observations from the Atmospheric Radiation Measurement (ARM) central facility archive.

The modeled 3D spatial distribution and short-term evolution of the ice cloud mass is significantly improved by the assimilation of IR window channels when the model already contains conditions for the ice cloud formation. The assimilated ice cloud in this case is in good agreement with the independent cloud radar measurements. The simulation of liquid clouds below thick ice clouds is not influenced by the IR window observations. The assimilation results clearly demonstrate that increasing the observational constraint from individual to combined channel measurements and from less to more frequent observation times systematically improves the assimilation results. The experiments with the model error indicate that the current specification of this error in the form of a generic linear forcing, which was adopted from other data assimilation studies, is not suitable for the cloud-resolving data assimilation. Instead, a parameter estimation approach may need to be explored in the future. The experiments with varying decorrelation lengths suggest the need to use the model horizontal grid spacing that is several times smaller than the GOES imager native resolution to achieve equivalent spatial variability in the assimilation.
Trends in Stratospheric Ozone: Lessons Learned from a 3D Chemical Transport Model
Authors: Stolarski Richard S. , Douglass Anne R. , Steenrod Stephen , Pawson Steven
Published: April 2006

Stratospheric ozone is affected by external factors such as chlorofluorcarbons (CFCs), volcanoes, and the 11-yr solar cycle variation of ultraviolet radiation. Dynamical variability due to the quasi-biennial oscillation and other factors also contribute to stratospheric ozone variability. A research focus during the past two decades has been to quantify the downward trend in ozone due to the increase in industrially produced CFCs. During the coming decades research will focus on detection and attribution of the expected recovery of ozone as the CFCs are slowly removed from the atmosphere. A chemical transport model (CTM) has been used to simulate stratospheric composition for the past 30 yr and the next 20 yr using 50 yr of winds and temperatures from a general circulation model (GCM). The simulation includes the solar cycle in ultraviolet radiation, a representation of aerosol surface areas based on observations including volcanic perturbations from El Chichon in 1982 and Pinatubo in 1991, and time-dependent mixing ratio boundary conditions for CFCs, halons, and other source gases such as N2O and CH4. A second CTM simulation was carried out for identical solar flux and boundary conditions but with constant “background” aerosol conditions. The GCM integration included an online ozonelike tracer with specified production and loss that was used to evaluate the effects of interannual variability in dynamics. Statistical time series analysis was applied to both observed and simulated ozone to examine the capability of the analyses for the determination of trends in ozone due to CFCs and to separate these trends from the solar cycle and volcanic effects in the atmosphere. The results point out several difficulties associated with the interpretation of time series analyses of atmospheric ozone data. In particular, it is shown that lengthening the dataset reduces the uncertainty in derived trend due to interannual dynamic variability. It is further shown that interannual variability can make it difficult to accurately assess the impact of a volcanic eruption, such as Pinatubo, on ozone. Such uncertainties make it difficult to obtain an early proof of ozone recovery in response to decreasing chlorine.
Probability Distributions of Angle of Approach and Relative Velocity for Colliding Droplets in a Turbulent Flow
Authors: Wang Lian-Ping , Franklin Charmaine N. , Ayala Orlando , Grabowski Wojciech W.
Published: April 2006

Prediction of the effect of air turbulence on statistics relevant to a collision–coalescence process represents a key challenge in the modeling of cloud microphysics. In this paper, collision-related statistics for gravity-driven motion of droplets are considered and various probability distributions associated with geometric configuration and relative motion of colliding droplets are theoretically derived. The theoretical results agree well with numerical results obtained from direct numerical simulations (DNSs). In the absence of air turbulence, the probability distributions, calculated at the beginning of the time steps used for collision detection, nontrivially depend on the time step size. Next, a novel theory is developed to quantify the effect of turbulence on the angle-of-approach θ and radial relative velocity |wr,c| for colliding pairs. A logical decomposition is used to construct extended collision volumes for a specific level of radial motion caused by air turbulence. It is shown that the inward relative motion due to turbulent fluctuations dominates the effect of turbulence in modifying the probability distributions of θ and |wr,c|. Two key dimensionless parameters are identified in the theory: one measures the effect of finite time step size in numerical collision detection and the second measures the relative magnitude of air turbulence. The theory is compared with 11 numerical experiments from DNS. It is shown that the theory captures the essential physics of the effect of air turbulence and provides a quantitatively good representation of the statistics for θ. For most numerical experiments, the theory predicts 〈θ〉 to within 5%. The probability distribution of |wr,c| is more sensitive to the influence of air turbulence and shows larger intermittency at large |wr,c| than what is assumed in the theory. The theoretical framework developed here may be of value to other problems involving gravitational settling and weak turbulence, such as parameterization of collision kernel and hydrodynamic interactions of droplets in warm rain processes.
Influence of the Equatorial QBO on the Extratropical Stratosphere
Authors: Hampson John , Haynes Peter
Published: April 2006

The work described here examines the influence of the equatorial quasi-biennial oscillation (QBO) on the extratropics in a zonally truncated 3D mechanistic stratospheric model. Model results show that the extratropical response to the QBO depends critically on the phase alignment of the QBO with the annual cycle: the signal of extratropical response varies by a factor of 8 between the phase alignment that gives minimum response and that which gives maximum response. Model simulations in which the time and height structure of the QBO are varied suggest that, in this zonally truncated model, the equatorial height of 21–23 km is most influential for the extratropical response and that late autumn/early winter is the time at which the QBO has the most influence over the extratropical circulation. The correlation coefficient between the QBO (measured by zonal wind) and the extratropics (measured by zonal wind or potential temperature) is as high as 0.95. The correlation coefficient is largest for simulations with lower boundary wave forcing weaker than that which gives largest extratropical interannual variability. For stronger extratropical wave forcing, the correlation coefficient is slightly smaller, but the regression coefficient of the linear term in a least squares fit is significantly larger.
Rayleigh Wave–Internal Wave Coupling and Internal Wave Generation above a Model Jet Stream
Authors: Sutherland B. R.
Published: April 2006

Linear theory for modes in a nonuniformly stratified, semi-infinite shear flow demonstrates that Rayleigh waves (stable waves propagating in fluid with spatially varying shear) couple with evanescent internal waves. If the bulk Richardson number (the squared ratio of the buoyancy frequency and shear) lies between 1/4 and 1, the waves have infinite e-folding depth for waves with critical relative horizontal wavenumbers. Fully nonlinear numerical simulations examine the effect of Rayleigh wave–internal wave coupling when the shear layer is localized and is thus Kelvin–Helmholtz unstable. Diagnostics examining profiles of the wave-induced mean flow show that if the bulk Richardson number is of order unity, significant momentum is extracted from a shear layer as a consequence of transport by waves. The work is extended to the study of unstable jet flows and applications of this work for internal wave generation by dynamic instability of the upper flank of the jet stream are discussed.
The Seasonal Cycles in the Distribution of Precipitation around Cyclones in the Western North Pacific and Atlantic
Authors: Chang Edmund K. M. , Song Siwon
Published: April 2006

The seasonal cycles in the distribution of precipitation around the western North Pacific and Atlantic cyclones have been examined by compositing quantitative estimates of the precipitation rate relative to cyclone centers. The precipitation data sources considered include estimates produced by the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) project, the satellite-based daily precipitation estimates produced by the Global Precipitation Climatology Project, and estimates derived based on observed weather reports contained in the Comprehensive Ocean–Atmosphere Data Set (COADS).

Results from all three datasets suggest that for Pacific cyclones, substantially more precipitation is found in the warm sector in fall than in winter and less precipitation is found behind the cold front in spring and summer than in winter. The seasonal cycle for Atlantic cyclones is found to be distinctly different. The distribution in precipitation around cyclones in fall and winter are not very different, while in spring and summer less precipitation is found over much of the cyclone.

The implications for the observed seasonal cycles are discussed. The seasonal cycle for Pacific cyclones suggests that diabatic contributions to the generation of eddy available potential energy (APE) due to latent heat release should be maximal in fall with a relative minimum in midwinter, while for Atlantic cyclones diabatic generation of eddy APE in fall and winter is nearly the same. This is suggested to be one of the factors that can contribute to the observed midwinter minimum in the Pacific storm track, and the absence of such a minimum in the Atlantic.

Possible reasons contributing to the differences in the seasonal cycle between the two basins are discussed. Preliminary analyses suggest that differences in static stability, availability of moisture, as well as dynamical forcing may all be contributing factors.

Issues on estimating rates of precipitation based on ship reports are addressed in appendix A. It is argued that it may be a good time to recalibrate existing schemes.
Modeling the Impact of Convective Entrainment on the Tropical Tropopause
Authors: Robinson F. J. , Sherwood S. C.
Published: April 2006

Simulations with the Weather Research and Forecasting (WRF) cloud-resolving model of deep moist convective events reveal net cooling near the tropopause (∼15–18 km above ground), caused by a combination of large-scale ascent and small-scale cooling by the irreversible mixing of turbulent eddies overshooting their level of neutral buoyancy. The turbulent cooling occurred at all CAPE values investigated (local peak values ranging from 1900 to 3500 J kg−1) and was robust to grid resolution, subgrid-scale turbulence parameterization, horizontal domain size, model dimension, and treatment of ice microphysics. The ratio of the maximum downward heat flux in the tropopause to the maximum tropospheric upward heat flux was close to 0.1. This value was independent of CAPE but was affected by changes in microphysics or subgrid-scale turbulence parameterization.

The convective cooling peaked roughly 1 km above the cold point in the background input sounding and the mean cloud- and (turbulent kinetic energy) TKE-top heights, which were all near 16.5 km above ground. It was associated with turbulent entrainment of stratospheric air from as high as 18.25 km into the troposphere. Typical cooling in the experiments was of order 1 K during convective events that produced order 10 mm of precipitation, which implied a significant contribution to the tropopause energy budget. Given the sharp concentration gradients and long residence times near the cold point, even such a small entrainment rate is likely consequential for the transport and ambient distribution of trace gases such as water vapor and ozone, and probably helps to explain the gradual increase of ozone typically observed below the tropical tropopause.
Investigation of ammonium perchlorate by nanoindentation
Authors: Lucca D. A. , Klopfstein M. J. , Mejia O. R. , Rossettini L. , DeLuca L. T.
Published: April 2006

The near surface of cleaved {210} planes of ammonium perchlorate (AP) was studied by nanoindentation. The crystals were etched to determine the {210} surface and the resulting etch pits were investigated with atomic force microscopy (AFM). The sides and bottom of the etch pits had a terraced structure. Nanoindentation was used to measure hardness and elastic modulus over the contact depth range of 30–130 nm. The hardness exhibited the indentation size effect, that is, the hardness increased with decreasing depth. The measured hardness of the near surface was higher than that previously reported for deeper Vickers indentations where cracking was observed. The elastic modulus of the near surface was consistent with the low end of values reported in the literature. Multiple pop-in events were observed for all indentations. Indenting with high loads (>1 mN) using either a Berkovich indenter or spherical indenter resulted in significant asymmetric pile-up, slip traces on the surface and cracks. Cracking was observed in the 〈120〉 direction and a step on the surface was observed in then 〈001〉 direction.
Preparation and reactivity analysis of novel perfluoroalkyl coated aluminium nanocomposites
Authors: Jouet R. J. , Carney J. R. , Granholm R. H. , Sandusky H. W. , Warren A. D.
Published: April 2006

Passivation of unpassivated aluminium nanoparticles using C13F27COOH is reported with materials containing as much as 32·95%Al. Characterisation data, including scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and attenuated total reflectance Fourier transform infrared (ATR FTIR) spectrometry, indicate that the C13F27COOH molecule binds to the surface of the Al particle, protecting the surface from oxidation in ambient air. The fast reaction capability of the Al–C13F27COOH material was investigated using two different means. Response of the material to strong shock was analysed using the small scale shock reactivity test (SSRT). SSRT results show that the Al–C13F27COOH material formulated with cyclotetramethylenetetranitramine (HMX) and hydroxyl terminated polybutadiene (HTPB) binder produced a significantly larger dent than comparable conventional Al specimen. Additionally, laser ablation/ignition studies of the prepared Al–C13F27COOH material demonstrate the fast reaction capability of the material.
Materials science and technology aspects of energetic (explosive) materials
Authors: Armstrong R. W. , Elban W. L.
Published: April 2006

Crystallographic, phase diagram, X-ray diffraction and mechanical property aspects of energetic materials are described in relation, first, to the properties of individual crystals as employed in composite explosive material formulations and then, second, to initiation of chemical decomposition by imposed mechanical forces and deformations. The mechanically induced decomposition properties are tied to the special character of dislocations in the molecularly bonded energetic crystal structures formed by individual covalently bonded molecules. Microindentation fracture mechanics, hardness stress–strain and drop weight impact measurements on energetic crystals are assessed. Of particular interest are the material granular compaction properties associated with a mechanically induced deflagration to detonation transition (DDT) behaviour. Shock induced initiations of detonation involve experimental and theoretical model considerations at nanometric dimensions. The total results point to improved mechanical insensitivities and higher energy release rates at smaller particle dimensions.
During detonation chemistry may precede heat
Authors: Gilman J. J.
Published: April 2006

Thermal activation of exothermic chemical reactions produces heat which may activate more reaction thereby creating a self-sustained event. However, reactions can also be activated athermally. Photo activation is one example; and mechanical activation is another. Then heating follows the chemical event, rather than leading it. Which sequence prevails depends on the circumstances, particularly the time available for the process. The mechanical sequence, being electronic, is much faster than the thermal sequence. Thus deflagrations may be thermally activated, while detonations may be mechanically activated. To introduce mechanical activation, a brief review of mechanochemistry and of chemical reactivity is given. Examples of reactions that demonstrate the importance of purely shear strains (elastic) are: (1) ‘hammer chemistry'; (2) oxidation of silicon at stressed crack tips; and, (3) increase of catalytic rates by surface acoustic waves. A computational example is the bending of TATB molecules. It is argued that the shear strains in the leading edges of detonation fronts initiate the detonation process. Because the process is confined, pressures in the megabar range are generated. The resulting hot plasma appears within several femtoseconds. Observable ‘hot spots’ follow this chemical process. An interpretation of the ‘size effect’ associated with initiation is given in terms of cage recombination. The behaviour at compressed cavities is also discussed.
Material dynamics under extreme conditions of pressure and strain rate
Authors: Remington B. A. , Allen P. , Bringa E. M. , Hawreliak J. , Ho D. , Lorenz K. T. , Lorenzana H. , McNaney J. M. , Meyers M. A. , Pollaine S. W. , Rosolankova K. , Sadik B. , Schneider M. S. , Swift D. , Wark J. , Yaakobi B.
Published: April 2006

Solid state experiments at extreme pressures (10–100 GPa) and strain rates (106–108s−1) are being developed on high energy laser facilities, and offer the possibility for exploring new regimes of materials science. These extreme solid state conditions can be accessed with either shock loading or with a quasi-isentropic ramped pressure drive. Velocity interferometer measurements establish the high pressure conditions. Constitutive models for solid state strength under these conditions are tested by comparing 2D continuum simulations with experiments measuring perturbation growth from the Rayleigh–Taylor instability in solid state samples. Lattice compression, phase and temperature are deduced from extended X-ray absorption fine structure (EXAFS) measurements, from which the shock induced α– ω phase transition in Ti and the α– ϵ phase transition in Fe, are inferred to occur on subnanosec time scales. Time resolved lattice response and phase can also be measured with dynamic X-ray diffraction measurements, where the elastic–plastic (1D–3D) lattice relaxation in shocked Cu is shown to occur promptly (<1 ns). Subsequent large scale molecular dynamics (MD) simulations elucidate the microscopic dislocation dynamics that underlies this 1D–3D lattice relaxation. Deformation mechanisms are identified by examining the residual microstructure in recovered samples. The slip-twinning threshold in single crystal Cu shocked along the [001] direction is shown to occur at shock strengths of ∼20 GPa, whereas the corresponding transition for Cu shocked along the [134] direction occurs at higher shock strengths. This slip twinning threshold also depends on the stacking fault energy (SFE), being lower for low SFE materials. Designs have been developed for achieving much higher pressures, P>1000 GPa, in the solid state on the National Ignition Facility (NIF) laser.
Deformation effects in shocked metals and alloys
Authors: Esquivel E. V. , Murr L. E.
Published: April 2006

Plane wave shock loading produces twins or twin faults in many metals and alloys, and, especially for fcc materials with decreasing stacking fault energy (SFE), a critical twinning pressure (CTP), and crystallographic orientations which control the onset and extent of twinning. The CTP increases with increasing SFE, and is lowest in [001] orientations for fcc. The deformation associated with plane shock is generally within the realm of the plastic regime of the stress–strain diagram. Correspondingly, spherical shock, characteristic of impact cratering, produces large strains through solid state flow and sliding of overlapping shear bands composed of dynamically recrystallised grains, and is therefore only partly encompassed in the realm of the stress–strain diagram. Below this zone for impact craters, and similar to plane shock loading, there is a slip–twinning or slip–microbanding transition in fcc materials, which depends on the SFE; twinning persists for low SFE while microbands dominate in high SFE fcc materials such as nickel or copper. A simple model is used to explain differences in residual microstructures in plane shock loading and spherical shock or impact cratering as they relate to SFE and shock wave geometry. Simple schematics are used to explain the connection between plane shock and spherical shock loading in relation to the stress–strain diagram as a paradigm.
Nugget growth characteristic for AZ31B magnesium alloy during resistance spot welding
Authors: Feng J. C. , Wang Y. R. , Zhang Z. D.
Published: April 2006

An axisymmetrical finite element model for studying the distribution of temperature for resistance spot welding (RSW) to predict weld nugget growth of AZ31B Mg alloy was developed by employing a contact resistance model based on the microcontact theory. The RSW of a Mg alloy, with regard to nugget formation, consists of the initiation of a nugget in the first cycle, a rapid growth of the nugget in the following 2–3 cycles and a plateau of nugget growth after ∼4 cycles. Because of its high thermal conductivity, low melting point and low volumetric heat capacity, Mg alloy has many characteristics during nugget formation, compared with Al alloy and mild steel. In the RSW of a Mg alloy, the contact resistance in the interface has an important effect on the nugget formation; the welding time is similar to that in Al alloy but smaller than that in low carbon steel; and the welding current lever is required slightly lower than that in Al alloy but higher than that in low carbon steel. The computational simulations based on this model agree well with the experimental data.
Energy utilisation and generation during friction stir spot welding
Authors: Su P. , Gerlich A. , North T. H. , Bendzsak G. J.
Published: April 2006

Energy utilisation during spot welding is investigated using a combination of calorimetry, peak temperature measurement and plunge testing. When a steel tool, clamp and anvil support is used, only 12·6% of the energy generated during the spot welding is transferred into the welded Al 6111 sheets. In contrast, when a mica clamp and anvil support are used, 50% of the energy generated during spot welding transfers into the welded Al 6111 sheets. Only a small percentage of the energy generated during the friction stir spot welding operation is required for stir zone formation. During plunge testing of 6·3 mm thick Al 6061-T6 material, less than 4·03% of the energy which is generated during friction stir spot welding is required for stir zone formation. The remainder of the energy generated dissipates into the tool assembly, clamp, anvil support and the aluminium sheets which are being welded. The rotating pin produces around 70% of the energy generated during spot welding of 6·3 mm thick Al-6061 material, with the remainder being contributed by the tool shoulder.
Parametric studies of friction stir welding by commercial fluid dynamics simulation
Authors: Long T. , Reynolds A. P.
Published: April 2006

In previous work, some of the authors have demonstrated that a 2-D model using a commercial fluid dynamics (CFD) code and an appropriate user defined viscosity law can accurately capture many of the trends observed in friction stir welding (FSW). The fully thermo-mechanically coupled simulations may be run in a few hours on a PC based system. The low computational cost for this model enables rapid assessment of the effects of various material properties and the criticality of inclusion or exclusion of some property details. In the present study, the effects of varying material properties and process parameters, on the trends in x-axis forces and potential weld defect formation (via material flow pattern) are studied. The model trends are compared to experimental observation wherever possible.