The geoid EDIN2000 and mean sea surface topography around the British Isles   总被引：2，自引：0，他引：2  

Roger Hipkin  Keith Haines  Ciaran Beggan  Richard Bingley  Fabrice Hernandez  Jason Holt  Trevor Baker 《Geophysical Journal International》2004,157(2):565-577

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Inviscid behaviour of fines-rich pyroclastic flows inferred from experiments on gas-particle mixtures     

O. Roche  M.A. Gilbertson  R.S.J. Sparks 《Earth and Planetary Science Letters》2005,240(2):401-414

Experiments were carried out on granular flows generated by instantaneous release of gas-fluidised, bidisperse mixtures and propagating into a horizontal channel. The mixture consists of fine (< 100 μm) and coarse (> 100 μm) particles of same density, with corresponding grain size ratios of ∼ 2 to 9. Initial fluidisation of the mixture destroys the interparticle frictional contacts, and the flow behaviour then depends on the initial bed packing and on the timescale required to re-establish strong frictional contacts. At a fines mass fraction (α) below that of optimal packing (∼ 40%), the initial mixtures consist of a continuous network of coarse particles with fines in interstitial voids. Strong frictional contacts between the coarse particles are probably rapidly re-established and the flows steadily decelerate. Some internal friction reduction appears to occur as α and the grain size ratio increases, possibly due to particle rolling and the lower roughness of internal shear surfaces. Segregation only occurs at large grain size ratio due to dynamical sieving with fines concentrated at the flow base. In contrast, at α above that for optimal packing, the initial mixtures consist of coarse particles embedded in a matrix of fines. Flow velocities and run-outs are similar to that of the monodisperse fine end-member, thus showing that the coarse particles are transported passively within the matrix whatever their amount and grain size are. These flows propagate at constant height and velocity as inviscid fluid gravity currents, thus suggesting negligible interparticle friction. We have determined a Froude number of 2.61 ± 0.08 consistent with the dam-break model for fluid flows, and with no significant variation as a function of α, the grain size ratio, and the initial bed expansion. Very little segregation occurs, which suggests low intensity particle interactions during flow propagation and that active fluidisation is not taking place. Strong frictional contacts are only re-established in the final stages of emplacement and stop the flow motion. We infer that fines-rich (i.e. matrix-supported) pyroclastic flows propagate as inviscid fluid gravity currents for most of their emplacement, and this is consistent with some field data.  相似文献   

Short‐term intertidal bar mobility on a ridge‐and‐runnel beach,Merlimont, northern France     

E. J. Anthony  F. Levoy  D. Monfort  C. Degryse‐Kulkarni 《地球表面变化过程与地形》2005,30(1):81-93

Digital elevation models and topographic pro?les of a beach with intertidal bar and trough (ridge‐and‐runnel) morphology in Merlimont, northern France, were analysed in order to assess patterns of cross‐shore and longshore intertidal bar mobility. The beach exhibited a pronounced dual bar–trough system that showed cross‐shore stationarity. The bars and troughs were, however, characterized by signi?cant longshore advection of sand under the in?uence of suspension by waves and transport by strong tide‐ and wind‐driven longshore currents. Pro?le changes were due in part to the longshore migration of medium‐sized bedforms. The potential for cross‐shore bar migration appears to be mitigated by the large size of the two bars relative to incident wave energy, which is modulated by high vertical tidal excursion rates on this beach due to the large tidal range (mean spring tidal range = 8·3 m). Cross‐shore bar migration is also probably hindered by the well‐entrenched troughs which are maintained by channelled high‐energy intertidal ?ows generated by swash bores and by tidal discharge and drainage. The longshore migration of intertidal bars affecting Merlimont beach is embedded in a regional coastal sand transport pathway involving tidal and wind‐forced northward residual ?ows affecting the rectilinear northern French coast in the eastern English Channel. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

A complex, Subplinian-type eruption from low-viscosity, phonolitic to tephri-phonolitic magma: the AD 472 (Pollena) eruption of Somma-Vesuvius, Italy     

Roberto Sulpizio  Daniela Mele  Pierfrancesco Dellino  Luigi La Volpe 《Bulletin of Volcanology》2005,67(8):743-767

The combined use of field investigation and laboratory analyses allowed the detailed stratigraphic reconstruction of the Pollena eruption (472 AD) of Somma-Vesuvius. Three main eruptive phases were recognized, related either to changes in the eruptive processes and/or to relative changes of melt composition. The eruption shows a pulsating behavior with deposition of pyroclastic fall beds and generation of dilute and dense pyroclastic density currents (PDC). The eruptive mechanisms and transportation dynamics were reconstructed for the whole eruption. Column heights were between 12 and 20 km, corresponding to mass discharge rates (MDR) of 7×10⁶ kg/s and 3.4×10⁷ kg/s. Eruptive dynamics were driven by magmatic fragmentation of a phono-tephritic to tephri-phonolitic magma during Phases I and II, whereas phreatomagmatic fragmentation dominated Phase III. Magma composition varies between phonolitic and tephritic-phonolitic, with melt viscosity likely not in excess of 10³ Pa s. The volume of the pyroclastic fall deposits, calculated by using of proximal isopachs, is 0.44 km³. This increases to 1.38 km³ if ash volumes are extrapolated on a log thickness vs. square root area diagram using one distal isopach and column height.Editorial responsibility: R Cioni  相似文献   

Ocean current estimation using a Multi-Model Ensemble Kalman Filter during the Grand Lagrangian Deployment experiment (GLAD)     

《Ocean Modelling》2015

In the summer and fall of 2012, during the GLAD experiment in the Gulf of Mexico, the Consortium for Advanced Research on Transport of Hydrocarbon in the Environment (CARTHE) used several ocean models to assist the deployment of more than 300 surface drifters. The Navy Coastal Ocean Model (NCOM) at 1 km and 3 km resolutions, the US Navy operational NCOM at 3 km resolution (AMSEAS), and two versions of the Hybrid Coordinates Ocean Model (HYCOM) set at 4 km were running daily and delivering 72-h range forecasts. They all assimilated remote sensing and local profile data but they were not assimilating the drifter’s observations. This work presents a non-intrusive methodology named Multi-Model Ensemble Kalman Filter that allows assimilating the local drifter data into such a set of models, to produce improved ocean currents forecasts. The filter is to be used when several modeling systems or ensembles are available and/or observations are not entirely handled by the operational data assimilation process. It allows using generic in situ measurements over short time windows to improve the predictability of local ocean dynamics and associated high-resolution parameters of interest for which a forward model exists (e.g. oil spill plumes). Results can be used for operational applications or to derive enhanced background fields for other data assimilation systems, thus providing an expedite method to non-intrusively assimilate local observations of variables with complex operators. Results for the GLAD experiment show the method can improve water velocity predictions along the observed drifter trajectories, hence enhancing the skills of the models to predict individual trajectories.  相似文献   

Postglacial variability in near‐bottom current speed on the continental shelf off south‐west Spitsbergen          下载免费PDF全文

BEATA STERNAL  WITOLD SZCZUCISKI  MATTHIAS FORWICK  MAREK ZAJĄCZKOWSKI  STANISŁAW LORENC  JOANNA PRZYTARSKA 《第四纪科学杂志》2014,29(8):767-777

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Flow processes and sedimentation in unidirectionally migrating deep‐water channels: From a three‐dimensional seismic perspective          下载免费PDF全文

Chenglin Gong  Yingmin Wang  Ronald J. Steel  Jeff Peakall  Xiaoming Zhao  Qiliang Sun 《Sedimentology》2016,63(3):645-661

Three‐dimensional seismic data were used to infer how bottom currents control unidirectional channel migration. Bottom currents flowing towards the steep bank would deflect the upper part of sediment gravity flows at an orientation of 1° to 11° to the steep bank, yielding a helical flow circulation consisting of a faster near‐surface flow towards the steep bank and a slower basal return flow towards the gentle bank. This helical flow model is evidenced by the occurrence of bigger, muddier (suggested by low‐amplitude seismic reflections) lateral accretion deposits and gentle channel wall with downlap terminations on the gentle bank and by smaller, sandier (indicated by high‐amplitude seismic reflectors) channel fills and steep channel walls with truncation terminations on the steep bank. This helical flow circulation promotes asymmetrical depositional patterns with dipping accretion sets restricted to the gentle bank, which restricts the development of sinuosity and yields unidirectional channel migration. These results aid in obtaining a complete picture of flow processes and sedimentation in submarine channels.  相似文献   

Contained density currents with high volume of release   总被引：1，自引：0，他引：1       下载免费PDF全文

Quentin Theiler  Mário J. Franca 《Sedimentology》2016,63(6):1820-1842

Contained density currents with high volume of release reflect against the boundaries of the reception environment commonly leading to oscillatory flow. These flows exist in sediment clarifiers, compromising their operations, and deposited signatures of contained turbidity currents are found as part of the infill of sedimentary basins; for operation of the former and interpretation of the latter it is essential to understand the dynamic processes of these flows. Six lock‐exchange experiments with different initial densities were made in a horizontal flume, where the volume of the saline mixture in the lock was equivalent to the volume of the ambient fluid. A further two tests, with a repeated initial density, were made: one with high volume of release and very long duration; and another with low volume of release. Firstly, the movement of the current is discussed, including the oscillations within the experimental tank involving the density current and an upper layer counter‐current. It is shown that the cyclic behaviour is self‐similar with the reduced gravity of the initial density in the lock. Secondly, entrainment and water mixing processes are characterized. The time evolution of mixing is characterized qualitatively by analysing the background potential energy of the density distributions to show that mixing occurs even in the earlier stages of the current, and mainly within the first cycle of the oscillation. Quantified analysis reveals that, in currents with high volume of release, entrainment discharge is one order of magnitude higher, mainly due to the larger interface between the ambient fluid and the current. A model for the evolution of the mixing process is proposed for density currents with high volume of release. Finally, the dynamics of the head of the current is analysed. The entrainment in the head, when compared to the entrainment in the remainder body of the flows, is less important for the configuration with a larger lock.  相似文献   

Tide,Wind, and River Forcing of the Surface Currents in the Fraser River Plume     

M. Halverson  R. Pawlowicz 《大气与海洋》2016,54(2):131-152

A long-term record of surface currents from a high-frequency radar system, along with near-surface hydrographic transects, moored current meter records, and satellite imagery, are analyzed to determine the relative importance of river discharge, wind, and tides in driving the surface flow in the Fraser River plume. The observations show a great deal of oceanographic and instrumental variability. However, averaged quantities yielded robust results. The effect of river flow, which determines buoyancy and inertia near the river mouth, was found by taking a long-term average. The resulting flow field was dominated by a jet with two asymmetric gyres; the anticyclonic gyre to the north had flow speeds consistent with geostrophy. The mean flow speed near the river mouth was 14.3?cm?s^–1, while the flow further afield was 5?cm?s^–1 or less. Wind stress and surface currents were highly coherent in the subtidal frequency band. Northwesterly winds drive a surface flow to the southeast at speeds of nearly 30?cm?s^–1. Southeasterly winds drive a surface flow to the northwest at speeds reaching 20?cm?s^–1; however, there is more spatial variability in speed and direction relative to the northwesterly wind case. A harmonic analysis was used to extract the tidally driven flows. Ellipse parameters for the major tidal constituents varied considerably in both alignment and aspect ratio over the radar domain, in direct contrast to a barotropic model which predicted rectilinear flow along the Strait of Georgia. This is a result of water filling and draining the shallow mud flats north of the Fraser's main channel. The M₂ velocities at the surface were also weaker than their barotropic counterparts. However, the shallow water constituent MK₃ was enhanced at the surface and nearly as strong as the mean flow, implying that non-linear interactions are important to surface dynamics.  相似文献   

A Loop Current experiment: Field and remote measurements     

《Dynamics of Atmospheres and Oceans》2016

An overview of a new comprehensive observational study of the Loop Current (LC) in the eastern Gulf of Mexico that encompassed full-depth and near-bottom moorings, pressure-equipped inverted echo sounders (PIES) and remote sensing is presented. The study array was designed to encompass the LC from the Campeche Bank to the west Florida escarpment. This overview centers about principal findings as they pertain to mesoscale dynamics. Two companion papers provide in-depth analyses. Three LC anticyclonic eddy separation events were observed with good 3D spatial coverage over the 2½ year extent of the field study; the three separations exhibited similar processes after the LC had extended into the eastern Gulf. Large scale (∼300 km wavelength, 40–60 day periods) southward propagating meanders developed on the eastern side of the LC over deep (∼3000 m) water that were the result of baroclinic instability between the upper layer meandering jet and lower layer cyclones and anticyclones. The lower layer was only highly energetic during relatively short (∼2–3 months) intervals just prior to or during eddy detachments because of baroclinic instability. The steepening of the meanders lead to a pinch-off of LC eddies. The deep lower-layer eddies, constrained by the closed topography of the southeastern Gulf, propagated westward across the detachment zone and appear to assist in achieving separation. Small scale (∼50–100 km, periods ∼10 days) frontal eddies, observed on the western side of the LC along the Campeche Bank slope, decay over the deep water of the northern part of an extended LC, and have little influence on lower layer eddies, the east side meanders and the eddy detachment processes.  相似文献