Edge Of Front Research Articles

Investigation of MoSi2 formation in the presence of copper by self-propagating high-temperature synthesis

Abstract Combustion reactions in the Cu–Mo–Si system were investigated. A sample was ignited using electric arc and after completion of the combustion reaction, XRD, SEM and EDS analysis were carried out. The resulting microstructure consisted of almost spherical grains of MoSi 2 . Also a little amount of Mo-rich silicides (Mo 5 Si 3 , Mo 3 Si) and copper silicides were present at the grain boundaries. For investigating the formation mechanism of this microstructure, another sample was ignited and when the combustion wave was almost in the middle of the sample, the compact was quenched into liquid nitrogen. The results indicated that at the leading edge of combustion front, liquid solution of Cu–Si forms. The Mo particles dissolve in this melt and after that MoSi 2 precipitates. By growing MoSi 2 particles, remaining melt becomes richer in Cu which finally solidifies in cooling.

Different Methods of Forming Cold Fronts in Nonmerging Clusters

Sharp edges in X-ray surface brightness with continuous gas pressure called cold fronts have been often found in relaxed galaxy clusters such as Abell 496. Models that explain cold fronts as surviving cores of head-on subcluster mergers do not work well for these clusters and competing models involving gas sloshing have been recently proposed. Here, we test some concrete predictions of these models in a combined analysis of density, temperature, metal abundances and abundance ratios in a deep Chandra exposure of Abell 496. We confirm that the chemical discontinuities found in this cluster are not consistent with a core merger remnant scenario. However, we find chemical gradients across a spiral discovered at 73 kpc north of the cluster center and coincident with the sharp edge of the main cold front in the cluster. Despite the overall SN Ia iron mass fraction dominance found within the cooling radius of this cluster, the metal enrichment along the arm, determined from silicon and iron abundances, is consistent with a lower SN Ia iron mass fraction (51% +- 14%) than that measured in the surrounding regions (85% +- 14%). The is also significantly colder than the surroundings by 0.5-1.6 keV. The arm extends from a boxy colder region surrounding the center of the cluster, where two other cold fronts are found. This cold arm is a prediction of current high resolution numerical simulations as a result of an off-center encounter with a less massive pure dark matter halo and we suggest that the cold fronts in A496 provide the first clear corroboration of such model, where the closest encounter happened ~ 0.5 Gyr ago. We also argue for a possible candidate dark matter halo responsible for the cold fronts in the outskirts of A496.

Numerical Study of Precipitation Core-Gap Structure along Cold Fronts

Abstract The mechanism responsible for the core-gap structure of precipitation along narrow cold-frontal rainbands (NCFRs) is investigated through analyses of idealized cloud-resolving simulations of cold fronts. The control simulation, in which the prefrontal thermal stratification is characterized by a weak convective instability at low levels with convective available potential energy (CAPE) of ∼60 J kg−1, reproduces the typical alongfront variability of observed NCFRs. The simulated NCFR is broken up into regularly spaced, ellipsoidal cores oriented at a clockwise angle to the cold front. While horizontal-shear instability (HSI) has frequently been proposed as a mechanism leading to the alongfront variability of NCFRs, no characteristic features of HSI are recognized in the simulated vertical vorticity field at the leading edge of the cold front. The alongfront variability in precipitation is attributed to the formation of a wavelike disturbance just above the leading edge of the cold front. The wave phase lines are oriented nearly perpendicular to the direction of mean vertical shear, with enhanced (suppressed) precipitation occurring at the wave updrafts (downdrafts). An analysis of the eddy kinetic energy budget indicates that the wavelike disturbance derives most of its energy from the mean vertical shear and the buoyancy. Sensitivity experiments reveal a systematic relationship between the alongfront variability of NCFRs and the stability of the prefrontal thermal stratification. Simulated precipitation cores remain essentially parallel to the cold front when the prefrontal environment is absolutely stable or almost neutral to surface parcel ascent. The typical alongfront variability of NCFRs is reproduced for weakly unstable environments with small amounts of CAPE (≤140 J kg−1). On the other hand, simulations with sufficiently unstable environments produce precipitation cores oriented counterclockwise to cold fronts.

The IMPROVE-1 Storm of 1–2 February 2001. Part II: Cloud Structures and the Growth of Precipitation

Abstract On 1–2 February 2001, a strong cyclonic storm system developed over the northeastern Pacific Ocean and moved onto the Washington coast. This storm was one of several that were documented during the first field phase of the Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE). In the 1–2 February case, soundings and wind profiler measurements showed that a wide cold-frontal rainband was coincident with the leading edge of an upper-level cold front in a classical warm occlusion. Ground-based radar observations revealed the presence of subbands within the wide cold-frontal rainband and two layers of precipitation generating cells within this rainband: one at 5–7 km MSL and the other at 9–10 km MSL. The lower layer of generating cells produced fallstreaks that were traced from the cells down to the radar bright band at 2 km MSL. Observations suggest a connection between the subbands and the lower layer of generating cells. A research aircraft, equipped for cloud microphysical measurements, passed through at least two generating cells in the 5–7-km region. These cells were in their formative stage, with elevated liquid water contents and low ice particle concentrations. The microphysical structure of the wide cold-frontal rainband was elucidated by particle imagery from a Cloud Particle Imaging (CPI) probe aboard the research aircraft. These images provide detailed information on crystal habits and degrees of riming throughout the depth of the rainband. The crystal habits are used to deduce the temperature and saturation conditions under which the crystals grew and, along with in situ measurements of particle size spectra, they are used to estimate particle terminal fall velocities, precipitation rates, radar reflectivities, and vertical air motions. The radar reflectivity derived in this way generally compared well with direct measurement. Both the derived and directly measured parameters are used to determine the primary particle growth processes in the wide cold-frontal rainband. Above the melting layer, vapor deposition was the dominant growth process in the rainband; growth of ice particles by riming was small. Significant aggregation of ice particles occurred in the region just above the melting layer. A doubling in the air-relative vertical precipitation mass flux occurred between the region where sheath ice crystals formed (−3° ≤ T ≤ −8°C) and the surface. Substantial amounts of liquid water were found within the melting layer where growth occurred by the accretion of cloud droplets and also by condensation. Growth by the collision and coalescence of raindrops was not significant below the melting layer.

Sedimentary processes on a Gilbert-type delta in Lake Llanquihue, southern Chile

A small Gilbert-type delta on the southern shore of Lake Llanquihue, southern Chile, was studied over a period of two months. Bottom samples were taken by scuba diving along a 10 x 10 m grid to determine the distribution of sedimentary facies. The wind direction, wave conditions, underwater current directions and the orientation of sedimentary structures were recorded on a daily basis. Observations made during fair-weather conditions indicate that neither waves nor currents have any significant effect on the bottom sediments, and that the only transport is produced by bivalves causing small avalanches on the steep delta slope. This probably prevents the oversteepening and large-scale slope collapse typical of many Gilbert deltas. Observations recorded from shore during one major storm, show an excellent agreement with wave theory and empirical predictions, indicating that storm waves can affect the whole delta front and slope. These waves break about 35 m from shore where there is a clear transition from clast-supported gravel on the inner delta front to matrix-supported gravel and gravelly sand on the outer front. The storm waves are able to transport cobbles up to 40 cm in diameter. During these events, strong lakeward-directed bottom currents enhanced by the effluent plume of the river, transport some of these clasts to the edge of the delta front, where they avalanche down to the foot of the delta. During the waning stages of the storm and shortly thereafter, dense, sediment-laden bottom currents discharged from the river mouth carry plant material down the delta slope and over the pro-delta, burying the cobbles just deposited. A single cycle of delta progradation should produce two coarsening-upward cycles, which might be confused in the rock record with two distinct phases of delta progradation.

Gas Cycling and the Development of Miscibility in Condensate Reservoirs

Summary This paper presents a detailed analysis of the development of miscibility during gas cycling in condensate displacements and the formation of condensate banks at the leading edge of the displacement front. Dispersion-free, analytical 1D calculations are presented for enhanced condensate recovery by gas injection. The analytical approach allows investigation of the possible formation of condensate banks (often at saturations that exceed the residual liquid saturation) and allows fast screening of optimal injection-gas compositions. We describe construction of the analytical solutions, a process that differs in some ways from related displacements for oil systems. All analytical solutions are verified by numerical calculations. We use an analysis of key equilibrium tie lines that are part of the displacement composition path to demonstrate that the mechanism controlling the development of miscibility in gas condensates may vary from first-contact miscible drives to pure vaporizing and combined vaporizing/condensing drives. Depending on the compositions of the condensate and the injected gas, multicontact miscibility can develop at or below the dewpoint pressure of the reservoir-fluid mixture. Finally, we discuss the possible impact on performance prediction of the formation of a mobile condensate bank at the displacement front in near-miscible gas-cycling/injection schemes.

Benthic foraminifera and the late Quaternary (last 150 ka) paleoceanographic and sedimentary history of the Bounty Trough, east of New Zealand

The Bounty Trough, east of New Zealand, lies along the southeastern edge of the present-day Subtropical Front (STF), and is a major conduit via the Bounty Channel, for terrigenous sediment supply from the uplifted Southern Alps to the abyssal Bounty Fan. Census data on 65 benthic foraminiferal faunas (>63 μm) from upper bathyal (ODP 1119), lower bathyal (DSDP 594) and abyssal (ODP 1122) sequences, test and refine existing models for the paleoceanographic and sedimentary history of the trough through the last 150 ka (marine isotope stages, MIS 6-1). Cluster analysis allows recognition of six species groups, whose distribution patterns coincide with bathymetry, the climate cycles and displaced turbidite beds. Detrended canonical correspondence analysis and comparisons with modern faunal patterns suggest that the groups are most strongly influenced by food supply (organic carbon flux), and to a lesser extent by bottom water oxygen and factors relating to sediment type. Major faunal changes at upper bathyal depths (1119) probably resulted from cycles of counter-intuitive seaward-landward migrations of the Southland Front (SF) (north–south sector of the STF). Benthic foraminiferal changes suggest that lower nutrient, cool Subantarctic Surface Water (SAW) was overhead in warm intervals, and higher nutrient-bearing, warm neritic Subtropical Surface Water (STW) was overhead in cold intervals. At lower bathyal depths (594), foraminiferal changes indicate increased glacial productivity and lowered bottom oxygen, attributed to increased upwelling and inflow of cold, nutrient-rich, Antarctic Intermediate Water (AAIW) and shallowing of the oxygen-minimum zone (upper Circum Polar Deep Water, CPDW). The observed cyclical benthic foraminiferal changes are not a result of associations migrating up and down the slope, as glacial faunas (dominated by Globocassidulina canalisuturata and Eilohedra levicula at upper and lower bathyal depths, respectively) are markedly different from those currently living in the Bounty Trough. On the abyssal Bounty Fan (1122), faunal changes correlate most strongly with grain size, and are attributed to varying amounts of mixing of displaced and in-situ faunas. Most of the displaced foraminifera in turbiditic sand beds are sourced from mid-outer shelf depths at the head of the Bounty Channel. Turbidity currents were more prevalent during, but not restricted to, glacial intervals.

Spreading of Perfluoropolyethers on FDTS-Coated Amorphous Carbon Surfaces

The spreading properties of mobile perfluoropolyethers (PFPEs) on amorphous carbon surfaces coated with 1 H, 1 H, 2 H, 2 H-perfluorodecyltrichlorosilane (FDTS) islands were investigated using a scanning ellipsometer. The average thicknesses of the FDTS layers varied from 0.5 to 1.4 nm and the film thickness of the mobile PFPEs was about 1.2 nm. We find that a sharp on the spreading profile occurs and the height of the spike decreases with the spreading time. After partial dip-coating, the initial film boundary of the PFPE on the FDTS islands having a low surface energy cannot spread and the moving film boundary on the bare carbon surface is obstructed by the FDTS island obstacles, which results in the spike formation (i.e., the lubricant buildup) and the retardation of molecular migration. This tendency is remarkable with the increasing average film thickness of the FDTS layer. The spreading is observed at the leading edge of the lubricant front originating from the migration of the lubricant molecules between the FDTS islands by diffusive processes.

Crossing the elliptic region in a hyperbolic system with change-of-type behavior arisingin flow between two parallel plates.

Change-of-type behavior from hyperbolic to elliptic is common to quasilinear hyperbolic systems. This issue is addressed here for the particular case of miscible flow of three fluids between two parallel plates. Change of type occurs at the leading edge of the displacement front and reflects the failing of the equilibrium assumption, necessary for the quasilinear hyperbolic formalism, at the front. To cross the elliptic region requires the solution of the full, higher-dimensionality problem, obtained here using lattice gas simulations. For the specific example, it is found that the system self-selects a front structure independent of injection conditions.

Numerical study of the diapycnal flow through a tidal front with passive tracers

A two‐dimensional numerical model is used to study the diapycnal flow through a tidal front with passive tracers. In a basic numerical experiment a passive tracer is released into the bottom water at the offshore edge of a tidal front, and it subsequently moves on‐bank with a velocity that decreases with time. This qualitatively agrees with a recent field experiment using a dye tracer on Georges Bank. Additional experiments are performed to investigate the sensitivity of the tracer dispersion to the tidal phase and the location of tracer release within the front. As the release point is moved on‐bank across the front, the tracer velocity decreases until it weakly reverses on the on‐bank edge of the front. This trend can be understood by considering the structure of the Lagrangian velocity field in the tidal front, the degree of vertical mixing of the tracer, and the concentration‐weighted mean patch velocity. The tidal phase at the time of tracer release does not significantly affect the tracer dispersion.

Quaternary palaeogeomorphologic evolution of the Wadi Faynan area, southern Jordan

The Faynan area comprises a complex assemblage of deposits and landforms of varying nature, age and position in the landscape. The dominant facies are fluvial, slope and aeolian. At least eight palaeochannels are present at different heights within the Wadis Dana and Ghuwayr. Fewer terraces are preserved in the Wadi Faynan. Planated surfaces leaving low-gradient straths, rock-cut channels and fluvial deposits are all evident. The continuing drop in regional base level as a result of subsidence of the Dead Sea trough has driven incision and aided in the retention of many of the terrace deposits as they have been abandoned as the wadi systems have eroded downwards. Calcretisation has enhanced the preservation potential of many of the fluvial deposits. A large proportion of these deposits are comparable to those found in the modern wadi systems, and it is thought that they formed under arid conditions similar to those that exist today. Two of the terrace deposits appear to reflect slightly wetter conditions as they consist of finer grained, better sorted materials and display evidence of biological activity. There are at least four distinct alluvial fan and slope deposits that have formed at the edge of various mountain fronts. The fans have been entrenched to varying degrees and are likely to have formed at different times in the Quaternary, possibly when conditions were more humid or during periods of increased tectonic activity. Aeolian deposits indicate a marginally drier phase in the Late Glacial maximum and similar sediments of mid-Holocene age could either reflect increasing aridity at this time as has been suggested by Frumkin et al. (1991) and/or be a result of the increasing impact of humans on the landscape. Many of the deposits are beyond the age range of optically stimulated luminescence (OSL) and radiocarbon dating techniques, but a proposed stratigraphy is supported by some numerical dates on the younger deposits and associated archaeological artefacts.

The outbreak of severe storms along convergence lines northeast of the Alps. Case study of the 3 August 2001 mesoscale convective system with a pronounced bow echo

Thunderstorm development in Austria is highly influenced by orographical forcing especially in summer. However, the most severe thunderstorms occur in the lower areas north and southeast of the Alps where mountains are not high. The reason is the undisturbed initiation and intensification of thunderstorms due to the low-level wind field which is modified near the edge of the Alps. Typically, severe convective weather events in Austria develop when a slow-moving cold front approaches from west (e.g., Switzerland) and when unstable air is advected from the Mediterranean by southwesterly flow. During such conditions, a convergence zone is frequently formed north of the Alps by a low-level airflow as a result of diabatic heating and evaporative cooling along the cloud edge of the cold front. This convergence line moves faster to the east than the cold front itself and triggers new cells or intensifies existing cells. The case study presented in this work is based on analyses of data from the Austrocontrol Weather Radar Network and objective surface pressure analyses. Two supercells initiated along a convergence line due to enhanced low level wind shear on the lee side of the Alps. Both severe storms, linked to the northern slopes of the Alps, approached Austria from Bavaria which evolved into a Mesoscale Convective System with a pronounced bow echo within the orographic gap between the mountain range of Bohemian Forest and the Alps. The case study shows a typical storm configuration and how to improve nowcasting of mesoscale modified thunderstorms north of the Alps.

Mobile Doppler Radar Observations of a Tornado in a Supercell near Bassett, Nebraska, on 5 June 1999. Part I: Tornadogenesis

Abstract This two-part paper details an analysis of high-resolution wind and reflectivity data collected by a mobile, W-band Doppler radar: The dataset captures the near-surface life history of a tornado in a supercell in north-central Nebraska on 5 June 1999. The formation of the tornado vortex near the ground is described from a sequence of sector scans ranging from 30-s intervals prior to tornadogenesis to 10–15-s intervals during much of the lifetime of the tornado. Cyclonic vortices of 100–200 m width were found along a bow-shaped line of enhanced radar reflectivity, at what appears to have been the leading edge of a rear-flank gust front. At the time of tornadogenesis, one of these vortices was located just ahead of the nose of the bow-shaped radar echo and a jet, which were embedded within a larger-scale cyclone. At other times, small-scale cyclonic vortices coexisted with the tornado along an arc-shaped line extending to its north and northeast but did not appear to interact with the tornado. The ...

Near-Surface Frontal Zone Trapping and Deep Upward Propagation of Internal Wave Energy in the Japan/East Sea

The full-depth current structure in the Japan/East Sea was investigated using direct velocity measurements performed with lowered and shipboard acoustic current Doppler profilers. Rotary spectral analysis was used to investigate the three-dimensional energy distribution as well as wave polarization with respect to vertical wavenumbers, yielding information about the net energy propagation direction. Highly energetic near-inertial downward-propagating waves were found in localized patches along the southern edge of the subpolar front. Between 500- and 2500-m depth, the basin average energy propagation was found to be upward, with the maximum of relative difference between upward- and downward-propagating energy lying at about 1500-m depth. This difference was most pronounced in the southeastern part of the basin.

On the physics of unstable infiltration, seepage, and gravity drainage in partially saturated tuffs

To improve understanding of the physics of dynamic instabilities in unsaturated flow processes within the Paintbrush nonwelded unit (PTn) and the middle nonlithophysal portion of the Topopah Spring welded tuff unit (TSw) of Yucca Mountain, we analyzed data from a series of infiltration tests carried out at two sites (Alcove 4 and Alcove 6) in the Exploratory Studies Facility (ESF), using analytical and empirical functions. The analysis of infiltration rates measured at both sites showed three temporal scales of infiltration rate: (1) a macro-scale trend of overall decreasing flow, (2) a meso-scale trend of fast and slow motion exhibiting three-stage variations of the flow rate (decreasing, increasing, and [again] decreasing flow rate, as observed in soils in the presence of entrapped air), and (3) micro-scale (high frequency) fluctuations. Infiltration tests in the nonwelded unit at Alcove 4 indicate that this unit may effectively dampen episodic fast infiltration events; however, well-known Kostyakov, Horton, and Philip equations do not satisfactorily describe the observed trends of the infiltration rate. Instead, a Weibull distribution model can most accurately describe experimentally determined time trends of the infiltration rate. Infiltration tests in highly permeable, fractured, welded tuff at Alcove 6 indicate that the infiltration rate exhibits pulsation, which may have been caused by multiple threshold effects and water–air redistribution between fractures and matrix. The empirical relationships between the extrinsic seepage from fractures, matrix imbibition, and gravity drainage versus the infiltration rate, as well as scaling and self-similarity for the leading edge of the water front are the hallmark of the nonlinear dynamic processes in water flow under episodic infiltration through fractured tuff. Based on the analysis of experimental data, we propose a conceptual model of a dynamic fracture flow and fracture–matrix interaction in fractured tuff, incorporating the time-dependent processes of water redistribution in the fracture–matrix system.

Shelf-margin deltas: their stratigraphic significance and relation to deepwater sands

Abstract Shelf-margin or shelf-edge deltas are a common constituent of Quaternary shelves, where their genetic link to falling and lowstand sea level has been well established, but they are rarely reported from older successions. Recognition of this delta type in the ancient record is nevertheless important, because (1) it helps to position the fossil shelf edge, (2) it provides insight into how sand budgets were partitioned between shelf edge, slope and basin-floor settings, and (3) it can contribute to the debate concerning three versus four systems-tract systematics and the positioning of a sequence boundary. Shelf-margin deltas create wide (tens of kilometers), high (hundreds of meters) and steep (3–6°) clinoforms, because they build across the relatively deepwater shelf margin. Such bodies tend to form strike-elongate wedges that initially thicken basinwards, attaining a maximum thickness of 50–200 m just outboard of the shelf edge, then thin on the middle/lower slope, whereas they commonly pinch out landwards by lapping back onto shelf shales. Prior to reaching the shelf edge, small-scale (tens of meters), tangential foresets (usually The key facies association in shelf-edge deltas is the mouth bar-to-delta front association. Mouth-bar facies, landwards of the shelf edge, consist mainly of thick, clean, flat to low-angle and ripple-laminated medium to fine sands. Basinwards from the shelf edge, such sands commonly alternate with heterolithic slumped unit, creating characteristic slumped to laminated couplets. Because the delta front is superimposed on a preexisting, steep and extended shelf margin, it commonly contains (beyond the mouth bars) thick successions (up to many tens of meters) of sandy, slope turbidites. Shelf-margin deltas differ from inner shelf deltas in showing: (1) an order of magnitude higher clinoforms (hundreds instead of tens of meters), and strike-elongated, locally pod-like sand bodies commonly affected and augmented by growth faulting; (2) paleoecological evidence of abrupt shallowing (foreshortened stratigraphy); (3) turbidite-prone delta-fronts; (4) larger scale and greater abundance of slope-controlled soft-sediment deformation and (5) the general absence of a paralic ‘tail’ along the trailing edge of the delta front. Shelf-margin deltas can be classified into two types. Stable shelf-margin deltas are usually tens of meters thick and are not associated with shelf-edge incision or major slope collapse/disruption features. Slope turbidites are common and occur as unconfined sheets and lobes. Such deltas form when relative sea level falls no lower than the level of the shelf platform or, after longer sea-level falls, when rivers reestablish at the shelf edge on the rise. Unstable shelf-margin deltas are associated with large-scale slope collapse, listric growth faults and sometimes with salt diapirs. Slides are common on the upper slope, whereas imbricated sediment packages and compressional ridges affect the slope toe. Slope turbidites can be ponded within structurally controlled mini-basins on the slope. Such deltas are often related to unusually great sand influx to the shelf edge, or simply to prolonged or large fall of relative sea level below the shelf edge. The diachronous erosional unconformity atop the shelf-to-shelf-edge deltaic wedge is viewed by some researchers as the most easily recognizable and persistent surface on the shelf and slope. This surface contrasts with the downlap erosional surface developed from the beginning of sea-level fall, which is viewed by other researchers as the key boundary surface within this complex. It should be noted that sea level can fall for a long period of time (tens of thousands of years) before deltas even reach the shelf margin and, therefore, before significant volumes of sand are delivered across the shelf break. Hence, the time of incision of the shelf edge and emplacement of deepwater sand is commonly long after the initial fall so the time of (maximum) relative lowstand of sea level may be a more practical choice for the timing of the sequence boundary. Recognition of shelf-margin deltas and analysis of their architecture help in the prediction of presence or absence of basin-floor fans. Deltaic complexes that are aggradational to backstepping, downlap onto disrupted or complex slopes, and overlie an incised shelf and shelf edge, predict that there should be basin-floor fans present. In contrast, delta complexes that show a prolonged and preserved progradational to downstepping architecture implies the presence of turbidite accumulations on the slope but not on the basin floor.

Appearance of neurons and glia with respect to the wavefront during colonization of the avian gut by neural crest cells.

The enteric nervous system is formed by neural crest cells that migrate, proliferate, and differentiate into neurons and glia distributed in ganglia along the gastrointestinal tract. In the developing embryo some enteric crest cells cease their caudal movements, whereas others continue to migrate. Subsequently, the enteric neurons form a reticular network of ganglia interconnected by axonal projections. We studied the developing avian gut to characterize the pattern of migration of the crest cells, and the relationship between migration and differentiation. Crest cells at the leading edge of the migratory front appear as strands of cells; isolated individual crest cells are rarely seen. In the foregut and midgut, these strands are located immediately beneath the serosa. In contrast, crest cells entering the colon appear first in the deeper submucosal mesenchyme and later beneath the serosa. As the neural crest wavefront passes caudally, the crest cell cords become highly branched, forming a reticular lattice that presages the mature organization of the enteric nervous system. Neurons and glia first appear within the strands at the advancing wavefront. Later neurons are consistently located at the nodes where branches of the lattice intersect. In the most rostral foregut and in the colon, some neurons initially appear in close association with extrinsic nerve fibers from the vagus and Remak's nerve, respectively. We conclude that crest cells colonize the gut as chains of cells and that, within these chains, both neurons and glia appear close to the wavefront.

Diapycnal flow through a tidal front: a dye tracer study on Georges Bank

A fluorescent dye tracer, fluorescein, injected into the bottom mixed layer (BML) at the off-bank edge of the tidal front on Georges Bank in late May and early June 1999 has provided the first quantitative measurement of an on-bank diapycnal Lagrangian flow through the front. From the warming of the dye patch, 1.9×10 −6 and 7.6×10 −6 °C/s on the south flank and northeast peak, respectively, as it passed through the frontal temperature gradient, we infer a mean on-bank flow of 1.6 cm/s on the south flank and 3.1 cm/s on the northeast peak. The heat flux required for this warming is predominantly due to vertical mixing within the tidal front. From the dispersion of the dye patch, we estimate a vertical diffusivity of 1.6 and 3.1×10 −3 m 2/s and a cross-front diffusivity of 18 and 30 m 2/s on the south flank and northeast peak, respectively. The evolution of a second dye injection on the south flank was distinctly different from the first because of the proximity of the foot of shelfbreak front now displaced onto the bank by a transient meander to within 6 km of the tidal front. Although the dispersion of this patch was the same as for the first injection, its cross-bank displacement was much less reflecting the divergence of the cross-bank flow in this region.

Norwegian-Type and Cold Front Aloft–Type Cyclones East of the Rocky Mountains

Abstract Conventional data and mesoscale model simulations are used to analyze two cyclones that developed east of the Rocky Mountains in June and November 1998. Both cyclones formed when a Pacific cold front overtook a lee trough/dryline east of the Rockies. In one case the leading edge of the Pacific cold front was on the surface, as depicted in the classic Norwegian model of a cyclone. In the other case, which is referred to as a cold front aloft (CFA) cyclone, the leading edge of the Pacific cold front was aloft and in advance of the lee trough. The lifting and severe weather associated with the Pacific cold front was along the leading edge of this front in both the Norwegian-type and CFA-type cyclones. To obtain an estimate of how often CFA cyclones, with a coincident CFA rainband, occurred in the central United States during the period 15 September 1994 through 15 September 1995, 70 cyclones that maintained closed surface low pressure centers for at least 24 h, and produced precipitation in the Unit...

Interactions of the TGB1 protein during cell-to-cell movement of Barley stripe mosaic virus.

We have recently used a green fluorescent protein (GFP) fusion to the gammab protein of Barley stripe mosaic virus (BSMV) to monitor cell-to-cell and systemic virus movement. The gammab protein is involved in expression of the triple gene block (TGB) proteins encoded by RNAbeta but is not essential for cell-to-cell movement. The GFP fusion appears not to compromise replication or movement substantially, and mutagenesis experiments demonstrated that the three most abundant TGB-encoded proteins, betab (TGB1), betac (TGB3), and betad (TGB2), are each required for cell-to-cell movement (D. M. Lawrence and A. O. Jackson, Mol. Plant Pathol. 2:65-75, 2001). We have now extended these analyses by engineering a fusion of GFP to TGB1 to examine the expression and interactions of this protein during infection. BSMV derivatives containing the TGB1 fusion were able to move from cell to cell and establish local lesions in Chenopodium amaranticolor and systemic infections of Nicotiana benthamiana and barley. In these hosts, the GFP-TGB1 fusion protein exhibited a temporal pattern of expression along the advancing edge of the infection front. Microscopic examination of the subcellular localization of the GFP-TGB1 protein indicated an association with the endoplasmic reticulum and with plasmodesmata. The subcellular localization of the TGB1 protein was altered in infections in which site-specific mutations were introduced into the six conserved regions of the helicase domain and in mutants unable to express the TGB2 and/or TGB3 proteins. These results are compatible with a model suggesting that movement requires associations of the TGB1 protein with cytoplasmic membranes that are facilitated by the TGB2 and TGB3 proteins.