670 research outputs found
Language, nationhood, and systems: insights from language policy at the University of Makeni, Sierra Leone
Thresholds, switches and hysteresis in hydrology from the pedon to the catchment scale: a non-linear systems theory
International audienceHysteresis is a rate-independent non-linearity that is expressed through thresholds, switches, and branches. Exceedance of a threshold, or the occurrence of a turning point in the input, switches the output onto a particular output branch. Rate-independent branching on a very large set of switches with non-local memory is the central concept in the new definition of hysteresis. Hysteretic loops are a special case. A self-consistent mathematical description of hydrological systems with hysteresis demands a new non-linear systems theory of adequate generality. The goal of this paper is to establish this and to show how this may be done. Two results are presented: a conceptual model for the hysteretic soil-moisture characteristic at the pedon scale and a hysteretic linear reservoir at the catchment scale. Both are based on the Preisach model. A result of particular significance is the demonstration that the independent domain model of the soil moisture characteristic due to Childs, Poulavassilis, Mualem and others, is equivalent to the Preisach hysteresis model of non-linear systems theory, a result reminiscent of the reduction of the theory of the unit hydrograph to linear systems theory in the 1950s. A significant reduction in the number of model parameters is also achieved. The new theory implies a change in modelling paradigm
Systematic attribution of observed Southern Hemisphere circulation trends to external forcing and internal variability
A critical question in the global warming debate concerns the causes of the
observed trends of the Southern Hemisphere (SH) atmospheric circulation over
recent decades. Secular trends have been identified in the frequency of
occurrence of circulation regimes, namely the positive phase of the Southern
Annular Mode (SAM) and the hemispheric wave-3 pattern which is associated
with blocking. Previous studies into the causes of these secular trends have
either been purely model based, have not included observational forcing data
or have mixed external forcing with indices of internal climate variability
impeding a systematic and unbiased attribution of the causes of the secular
trends. Most model studies also focused mainly on the austral summer season.
However, the changes to the storm tracks have occurred in all seasons and
particularly in the austral winter and early spring when midlatitude
blocking is most active and stratospheric ozone should not play a role. Here
we systematically attribute the secular trends over the recent decades using
a non-stationary clustering method applied to both reanalysis and
observational forcing data from all seasons. While most previous studies
emphasized the importance of stratospheric ozone depletion in causing austral
summer SH circulation trends, we show observational evidence that
anthropogenic greenhouse gas concentrations have been the major driver of
these secular trends in the SAM and blocking when all seasons are considered.
Our results suggest that the recovery of the ozone hole might delay the
signal of global warming less strongly than previously thought and that
effects from all seasons are likely crucial in understanding the causes of
the secular trends
Modular organization enhances the robustness of attractor network dynamics
Modular organization characterizes many complex networks occurring in nature,
including the brain. In this paper we show that modular structure may be
responsible for increasing the robustness of certain dynamical states of such
systems. In a neural network model with threshold-activated binary elements, we
observe that the basins of attractors, corresponding to patterns that have been
embedded using a learning rule, occupy maximum volume in phase space at an
optimal modularity. Simultaneously, the convergence time to these attractors
decreases as a result of cooperative dynamics between the modules. The role of
modularity in increasing global stability of certain desirable attractors of a
system may provide a clue to its evolution and ubiquity in natural systems.Comment: 6 pages, 4 figure
Evidence for chemokine synergy during neutrophil migration in ARDS.
BACKGROUND: Acute respiratory distress syndrome (ARDS) is a life-threatening condition characterised by pulmonary oedema, respiratory failure and severe inflammation. ARDS is further characterised by the recruitment of neutrophils into the lung interstitium and alveolar space. OBJECTIVES: The factors that regulate neutrophil infiltration into the inflamed lung and our understanding of the pathomechanisms in ARDS remain incomplete. This study aimed at determining the role of the chemokine (C-C motif) ligand (CCL)2 and CCL7 in ARDS. METHODS: CCL2 and CCL7 protein levels were measured in bronchoalveolar lavage (BAL) fluid obtained from lipopolysaccharide(LPS)-challenged human volunteers and two separate cohorts of patients with ARDS. Neutrophil chemotaxis to ARDS BAL fluid was evaluated and the contribution of each was assessed and compared with chemokine (C-X-C motif) ligand 8 (CXCL8). Chemokine receptor expression on neutrophils from blood or BAL fluid of patients with ARDS was analysed by flow cytometry. RESULTS: CCL2 and CCL7 were significantly elevated in BAL fluid recovered from LPS-challenged volunteers and patients with ARDS. BAL fluid from patients with ARDS was highly chemotactic for human neutrophils and neutralising either CCL2 or CCL7 attenuated the neutrophil chemotactic response. Moreover, CCL2 and CCL7 synergised with CXCL8 to promote neutrophil migration. Furthermore, neutrophils isolated from the blood or BAL fluid differentially regulated the cell surface expression of chemokine (C-X-C motif) receptor 1 and C-C chemokine receptor type 2 during ARDS. CONCLUSION: This study highlights important inflammatory chemokines involved in regulating neutrophil migration, which may have potential value as therapeutic targets for the treatment of ARDS
Very low density lipoprotein subfractions in type II diabetes mellitus: alterations in composition and susceptibility to oxidation.
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