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Frontal Structures (frontal + structure)

Distribution by Scientific Domains

Earth and Environmental Science	60%

Selected Abstracts

Baroclinic development within zonally-varying flows

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 626 2007
David M. Schultz
Abstract Previous idealized-modelling studies have shown the importance of across-jet barotropic shear to the resulting evolution of cyclones, anticyclones, surface-based fronts, and upper-level fronts. Meanwhile, many observational studies of cyclones have shown the importance of along-jet variations in the horizontal wind speed (i.e. confluence and diffluence). This study investigates the importance of these along-jet (zonal, for zonally-oriented jets) variations in the horizontal wind speed to the resulting structures and evolutions of baroclinic waves, using idealized models of growing baroclinic waves. An idealized primitive-equation channel model is configured with growing baroclinic perturbations embedded within confluent and diffluent background flows. When the baroclinic perturbations are placed in background confluence, the lower-tropospheric frontal structure and evolution initially resemble the Shapiro,Keyser cyclone model, with a zonally-oriented cyclone, strong warm front, and bent-back warm front. Later, as the baroclinic wave is amplified in the stronger downstream baroclinicity, the warm sector of the cyclone narrows, becoming more reminiscent of the Norwegian cyclone model. The upper-level frontal structure develops with a southwest,northeast orientation, and becomes strongest at the base of the trough, where geostrophic cold advection is occurring. In contrast, when the baroclinic perturbations are placed in background diffluence, the lower-tropospheric frontal structure and evolution resemble the Norwegian cyclone model, with a meridionally-oriented cyclone, strong cold front, and occluded front. The upper-level frontal structure is initially oriented northwest,southeast on the western side of the trough, before becoming zonally oriented. Weak geostrophic temperature advection occurs along its length. These results are compared to those from previous observational and idealized-modelling studies. Copyright © 2007 Royal Meteorological Society [source]

Learning-associated regulation of polysialylated neural cell adhesion molecule expression in the rat prefrontal cortex is region-, cell type- and paradigm-specific

EUROPEAN JOURNAL OF NEUROSCIENCE, Issue 3 2008
Judith P. F. Ter Horst
Abstract The prefrontal cortex (PFC) is an interconnected set of cortical areas that function in the synthesis of a diverse range of information and production of complex behaviour. It is now clear that these frontal structures, through bidirectional excitatory communication with the hippocampal formation, also play a substantial role in long-term memory consolidation. In the hippocampus, morphological synaptic plasticity, supported by regulation of neural cell adhesion molecule (NCAM) polysialylation status, is crucial to information storage. The recent description of polysialylated neurons in the various fields of the medial PFC suggests these structures to possess a similar capacity for synaptic plasticity. Here, using double-labelling immunohistochemistry with glutamic acid decarboxylase 67, we report that the nature of NCAM polysialic acid-positive neurons in the PFC is region-specific, with a high proportion (30,50%) of a ,-aminobutyric acid (GABA)ergic phenotype in the more ventral infralimbic, orbitofrontal and insular cortices compared with just 10% in the dorsal structures of the cingulate, prelimbic and frontal cortices. Moreover, spatial learning was accompanied by activations in polysialylation expression in ventral PFC structures, while avoidance conditioning involved downregulation of this plasticity marker that was restricted to the dorsomedial PFC , the cingulate and prelimbic cortices. Thus, in contrast to other structures integrated functionally with the hippocampus, memory-associated plasticity mobilized in the PFC is region-, cell type- and task-specific. [source]

BOLD Response During Spatial Working Memory in Youth With Heavy Prenatal Alcohol Exposure

ALCOHOLISM, Issue 12 2009
Andrea D. Spadoni
Background:, Prenatal alcohol exposure has been consistently linked to neurocognitive deficits and structural brain abnormalities in affected individuals. Structural brain abnormalities observed in regions supporting spatial working memory (SWM) may contribute to observed deficits in visuospatial functioning in youth with fetal alcohol spectrum disorders (FASDs). Methods:, We used functional magnetic resonance imaging (fMRI) to assess the blood oxygen level dependent (BOLD) response in alcohol-exposed individuals during a SWM task. There were 22 young subjects (aged 10,18 years) with documented histories of heavy prenatal alcohol exposure (ALC, n = 10), and age- and sex-matched controls (CON, n = 12). Subjects performed a SWM task during fMRI that alternated between 2-back location matching (SWM) and simple attention (vigilance) conditions. Results:, Groups did not differ on task accuracy or reaction time to the SWM condition, although CON subjects had faster reaction times during the vigilance condition (617 millisecond vs. 684 millisecond, p = 0.03). Both groups showed similar overall patterns of activation to the SWM condition in expected regions encompassing bilateral dorsolateral prefrontal lobes and parietal areas. However, ALC subjects showed greater BOLD response to the demands of the SWM relative to the vigilance condition in frontal, insular, superior, and middle temporal, occipital, and subcortical regions. CON youth evidenced less increased brain activation to the SWM relative to the vigilance task in these areas (p < 0.05, clusters > 1,664 ,l). These differences remained significant after including Full Scale IQ as a covariate. Similar qualitative results were obtained after subjects taking stimulant medication were excluded from the analysis. Conclusions:, In the context of equivalent performance to a SWM task, the current results suggest that widespread increases in BOLD response in youth with FASDs could either indicate decreased efficiency of relevant brain networks, or serve as a compensatory mechanism for deficiency at neural and/or cognitive levels. In context of existing fMRI evidence of heightened prefrontal activation in response to verbal working memory and inhibition demands, the present findings may indicate that frontal structures are taxed to a greater degree during cognitive demands in individuals with FASDs. [source]

Detection and climatology of fronts in a high-resolution model reanalysis over the Alps

METEOROLOGICAL APPLICATIONS, Issue 1 2010
J. Jenkner
Abstract The identification of low-level thermal fronts is particularly challenging in high-resolution model fields over complex terrain. Firstly, direct model output often contains numerical noise which spuriously influences the high-frequency variability of thermal parameters. Secondly, the boundary layer interferes via convection and consequently leaves its thermal marks on low levels. Here, an automated objective method for the detection of frontal lines is introduced which is designed to be insusceptible to consequences of small grid spacings. To this end, existing algorithms are readopted and combined in a novel way. The overall technique subdivides into a basic detection of fronts and a supplemental division into local fronts and synoptic fronts. The fundamental parts of the detection are: (1) a smoothing of the initial fields, (2) a definition of the frontal strength, and, (3) a localisation with the thermal front parameter. The local fronts are identified by means of a classification of open and closed thermal contours. The resulting data comprise the spatial outline of the frontal structures in a binary field as well as their type and movement. The novel methodology is applied to a 3 year high-resolution reanalysis over central Europe computed with the COSMO model using a grid spacing of 7 km. Grid-point based climatologies are derived for the Alpine region. Frequencies of occurrence and characteristics of motion are analysed for different frontal types. The novel climatology also provides quantitative evidence of dynamical properties such as the retardation of cold fronts ahead of mountains and the dissolution of warm fronts over mountains. Copyright © 2009 Royal Meteorological Society [source]

Modern and ancient fluvial megafans in the foreland basin system of the central Andes, southern Bolivia: implications for drainage network evolution in fold-thrust belts

BASIN RESEARCH, Issue 1 2001
B. K. Horton
ABSTRACT Fluvial megafans chronicle the evolution of large mountainous drainage networks, providing a record of erosional denudation in adjacent mountain belts. An actualistic investigation of the development of fluvial megafans is presented here by comparing active fluvial megafans in the proximal foreland basin of the central Andes to Tertiary foreland-basin deposits exposed in the interior of the mountain belt. Modern fluvial megafans of the Chaco Plain of southern Bolivia are large (5800,22 600 km2), fan-shaped masses of dominantly sand and mud deposited by major transverse rivers (Rio Grande, Rio Parapeti, and Rio Pilcomayo) emanating from the central Andes. The rivers exit the mountain belt and debouch onto the low-relief Chaco Plain at fixed points along the mountain front. On each fluvial megafan, the presently active channel is straight in plan view and dominated by deposition of mid-channel and bank-attached sand bars. Overbank areas are characterized by crevasse-splay and paludal deposition with minor soil development. However, overbank areas also contain numerous relicts of recently abandoned divergent channels, suggesting a long-term distributary drainage pattern and frequent channel avulsions. The position of the primary channel on each megafan is highly unstable over short time scales. Fluvial megafans of the Chaco Plain provide a modern analogue for a coarsening-upward, > 2-km-thick succession of Tertiary strata exposed along the Camargo syncline in the Eastern Cordillera of the central Andean fold-thrust belt, about 200 km west of the modern megafans. Lithofacies of the mid-Tertiary Camargo Formation include: (1) large channel and small channel deposits interpreted, respectively, as the main river stem on the proximal megafan and distributary channels on the distal megafan; and (2) crevasse-splay, paludal and palaeosol deposits attributed to sedimentation in overbank areas. A reversal in palaeocurrents in the lowermost Camargo succession and an overall upward coarsening and thickening trend are best explained by progradation of a fluvial megafan during eastward advance of the fold-thrust belt. In addition, the present-day drainage network in this area of the Eastern Cordillera is focused into a single outlet point that coincides with the location of the coarsest and thickest strata of the Camargo succession. Thus, the modern drainage network may be inherited from an ancestral mid-Tertiary drainage network. Persistence and expansion of Andean drainage networks provides the basis for a geometric model of the evolution of drainage networks in advancing fold-thrust belts and the origin and development of fluvial megafans. The model suggests that fluvial megafans may only develop once a drainage network has reached a particular size, roughly 104 km2, a value based on a review of active fluvial megafans that would be affected by the tectonic, climatic and geomorphologic processes operating in a given mountain belt. Furthermore, once a drainage network has achieved this critical size, the river may have sufficient stream power to prove relatively insensitive to possible geometric changes imparted by growing frontal structures in the fold-thrust belt. [source]