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1
Aizhulov, Daniar, Madina Tungatarova und Aidarkhan Kaltayev. „Streamlines Based Stochastic Methods and Reactive Transport Simulation Applied to Resource Estimation of Roll-Front Uranium Deposits Exploited by In-Situ Leaching“. Minerals 12, Nr. 10 (25.09.2022): 1209. http://dx.doi.org/10.3390/min12101209.
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Roll-front uranium deposits are ore mineralizations that occur in sandstones or arkoses downstream from redox fronts or reduced/oxidized geochemical barriers. They are often bounded above and below by impermeable shaly/muddy layers making them ideal for in-situ leaching exploitation. Several stochastic simulations were previously investigated either to characterize the ore grade distribution within roll-front type deposits, or for describing geological processes involved in their formation. This work suggests some modifications/improvements of conventional geostatistical algorithms for honoring hydrodynamic constraints that govern fluid flows in ore bearing layers. In particular, instead of using the classical Euclidian or curvilinear (for Sgrid) distance for computing the variogram, it is proposed to calculate the variogram accounting for the time of flight (TOF) of water particles down the streamlines together with available well data. Non-deterministic streamline-based methods seem to provide more accurate interpolation results and resource estimation compared to a traditional geostatistical approach when applied to roll-front deposits.
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Walter, Fabian, Zhen Zhang, Jordan Aaron, Brian McArdell und Christoph Graf. „Seismic Measurements of Roll Waves in Debris Flows“. E3S Web of Conferences 415 (2023): 03031. http://dx.doi.org/10.1051/e3sconf/202341503031.
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Accurate debris-flow modelling depends on the ability to simulate surging and pulsing behaviour. However, our understanding of these phenomena is starved of observational constraints. Here we propose the use of seismic measurements, which resolve the arrival of coarse-grained roll wave fronts in debris flows at Illgraben, Switzerland. Roll waves likely play a key role in flow pulses but are typically only observed with point measurements like depth gauges. We compare in-torrent force plate measurements with near-torrent seismic records and discuss how these data can test existing roll wave theories.
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HIGGINS, P. „FOSSILS TO FERTILIZER: TAPHONOMIC IMPLICATIONS OF URANIUM ROLL FRONTS“. PALAIOS 22, Nr. 5 (01.09.2007): 577–82. http://dx.doi.org/10.2110/palo.2005.p05-140r.
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Ng, Chiu-On, und Chiang C. Mei. „Roll waves on a shallow layer of mud modelled as a power-law fluid“. Journal of Fluid Mechanics 263 (25.03.1994): 151–84. http://dx.doi.org/10.1017/s0022112094004064.
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We give a theory of permanent roll waves on a shallow layer of fluid mud which is modelled as a power-law fluid. Based on the long-wave approximation, Kármán's momentum integral method is applied to derive the averaged continuity and the momentum equations. Linearized instability analysis of a uniform flow shows that the growth rate of unstable disturbances increases monotonically with the wavenumber, and therefore is insufficient to suggest a preferred wavelength for the roll wave. Nonlinear roll waves are obtained next as periodic shocks connected by smooth profiles with depth increasing monotonically from the rear to the front. Among all wavelengths only those longer than a certain threshold correspond to positive energy loss across the shock, and are physically acceptable. This threshold also implies a minimum discharge, viewed in the moving system, for the roll wave to exist. These facts suggest that a roll wave developed spontaneously from infinitesimal disturbances should have the shortest wavelength corresponding to zero dissipation across the shock, though finite dissipation elsewhere. The discontinuity at the wave front is a mathematical shortcoming needing a local requirement. Predictions for the spontaneously developed roll waves in a Newtonian case are compared with available experimental data. Longer roll waves, with dissipation at the discontinuous fronts, cannot be maintained if the uniform flow is linearly stable, when the fluid is slightly non-Newtonian. However, when the fluid is highly non-Newtonian, very long roll waves may still exist even if the corresponding uniform flow is stable to infinitesimal disturbances. Numerical results are presented for the phase speed, wave height and wavenumber, and wave profiles for a representative value of the flow index of fluid mud.
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DOELMAN, ARJEN, BJÖRN SANDSTEDE, ARND SCHEEL und GUIDO SCHNEIDER. „Propagation of hexagonal patterns near onset“. European Journal of Applied Mathematics 14, Nr. 1 (Februar 2003): 85–110. http://dx.doi.org/10.1017/s095679250200503x.
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For a pattern-forming system with two unbounded spatial directions that is near the onset to instability, we prove the existence of modulated fronts that connect (i) stable hexagons with the unstable trivial pattern, (ii) stable hexagons with unstable roll solutions, (iii) stable hexagons with unstable hexagons, and (iv) stable roll solutions with unstable hexagons. Our approach is based on spatial dynamics, bifurcation theory, and geometric singular perturbation theory.
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Schmidt, Christoph W., und Robert A. Goler. „Nonlinear Waves ahead of Fronts in the Great Australian Bight“. Monthly Weather Review 138, Nr. 9 (01.09.2010): 3474–97. http://dx.doi.org/10.1175/2010mwr3232.1.
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Abstract This study investigates nonlinear waves ahead of cold fronts in the Great Australian Bight, south of the Australian continent. These waves often form a series of roll clouds on their crests analogous to the “morning glory,” which is observed around the Gulf of Carpentaria in northeastern Australia. High-resolution visible satellite imagery from NASA’s polar-orbiting Aqua and Terra satellites between 23 October 2004 and 29 February 2008 is used to determine how frequently these cloud lines occur ahead of cold fronts. A total of 14 cases are identified with the most cases occurring in summer and none occurring in winter. The authors hypothesize that the summer maximum is due to a combination of lower cloud amounts associated with summertime cold fronts, and a stronger maritime stable layer, which is produced as hot continental air, is advected offshore. Three cloud line events are modeled using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5, version 3.6). In each case the low-level divergence field reveals convergence lines, which coincide with the cloud lines as identified on the satellite images. In two cases vertical cross sections of virtual potential temperature and horizontal and vertical velocity through the disturbances show a cold front advancing into a stratified environment leading to wave production at the leading edge of the cold air mass. Modeled maximal upward velocities range between 0.8 and 2.5 m s−1. Surface pressure jumps of about 1 hPa associated with the propagating waves occur in each case, which coincides with that predicted by simple bore theory. In two cases the front moves at supercritical speed (i.e., the frontal speed is larger than the speed of the fastest mode of small-amplitude long waves). In the third case the front does not propagate and the nonlinear waves produced become stationary as well.
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Skyllingstad, Eric D., und Roger M. Samelson. „Instability Processes in Simulated Finite-Width Ocean Fronts“. Journal of Physical Oceanography 50, Nr. 9 (01.09.2020): 2781–96. http://dx.doi.org/10.1175/jpo-d-20-0030.1.
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AbstractA simple, isolated front is modeled using a turbulence resolving, large-eddy simulation (LES) to examine the generation of instabilities and inertial oscillations by surface fluxes. Both surface cooling and surface wind stress are considered. Coherent roll instabilities with 200–300-m horizontal scale form rapidly within the front after the onset of surface forcing. With weak surface cooling and no wind, the roll axis aligns with the front, yielding results that are equivalent to previous constant gradient symmetric instability cases. After ~1 day, the symmetric modes transform into baroclinic mixed modes with an off-axis orientation. Traditional baroclinic instability develops by day 2 and thereafter dominates the overall circulation. Addition of destabilizing wind forcing produces a similar behavior, but with off-axis symmetric-Ekman shear modes at the onset of instability. In all cases, imbalance of the geostrophic shear by vertical mixing leads to an inertial oscillation in the frontal currents. Analysis of the energy budget indicates an exchange between kinetic energy linked to the inertial currents and potential energy associated with restratification as the front oscillates in response to the vertically sheared inertial current. Inertial kinetic energy decreases from enhanced mixed layer turbulence dissipation and vertical propagation of inertial wave energy into the pycnocline.
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Carver, Steve, Dave Sear und Eric Valentine. „An Observation of roll waves in a supraglacial meltwater channel, Harlech Gletscher, East Greenland“. Journal of Glaciology 40, Nr. 134 (1994): 75–78. http://dx.doi.org/10.3189/s0022143000003816.
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AbstractObservations of pulsating flow conditions in a supraglacier meltwater channel on Harlech Gletscher, East Greenland, are reported. Waves of water with turbulent wave fronts and smooth recessive limbs were observed passing downstream at regular intervals of 6-7 s. Peak channel discharge was estimated at 0.5-1.0 m3 s−1, in between which discharge was zero. It is suggested that the phenomenon as observed was due to the formation of roll waves in response to channel morphology and prevalent discharge conditions. Measurements of channel morphology are given together with calculations of critical flow conditions, supporting the hypothesis of roll waves in a natural ice channel. A brief introduction to roll waves and the theory regarding their formation is included.
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Carver, Steve, Dave Sear und Eric Valentine. „An Observation of roll waves in a supraglacial meltwater channel, Harlech Gletscher, East Greenland“. Journal of Glaciology 40, Nr. 134 (1994): 75–78. http://dx.doi.org/10.1017/s0022143000003816.
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Abstract Observations of pulsating flow conditions in a supraglacier meltwater channel on Harlech Gletscher, East Greenland, are reported. Waves of water with turbulent wave fronts and smooth recessive limbs were observed passing downstream at regular intervals of 6-7 s. Peak channel discharge was estimated at 0.5-1.0 m3 s−1, in between which discharge was zero. It is suggested that the phenomenon as observed was due to the formation of roll waves in response to channel morphology and prevalent discharge conditions. Measurements of channel morphology are given together with calculations of critical flow conditions, supporting the hypothesis of roll waves in a natural ice channel. A brief introduction to roll waves and the theory regarding their formation is included.
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Hamayotsu, Kikue, und Ronnie Nataatmadja. „Indonesia in 2015“. Asian Survey 56, Nr. 1 (Januar 2016): 129–37. http://dx.doi.org/10.1525/as.2016.56.1.129.
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The new Indonesian president, Joko Widodo (“Jokowi”), has faced formidable challenges on a number of fronts in his first year in office. Although Jokowi has managed to roll out his pet projects for the poor, management of competing partisan and personal interests as well as the economy and religious conflict continues to be a challenge. Overall, the new government’s contribution to democratic reforms has been modest thus far.
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Robertson, Alexander. „The centroid of tree crowns as an indicator of abiotic processes in a balsam fir wave forest“. Canadian Journal of Forest Research 17, Nr. 7 (01.07.1987): 746–55. http://dx.doi.org/10.1139/x87-119.
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Wave forests are rare and were previously known only in high altitude Abiesbalsamea forests in northeastern U.S.A. and in subalpine A. veitchii – A. mariesii forests in Japan. Wave forests have been discovered at several locations in Newfoundland, including a very large and unique wave forest extending over 100–150 km2 on the coastal plain of northwestern Newfoundland. The wave fronts, characterized by dead tree strips, are mostly sinusoidal with an axis aligned in the direction of the prevailing wind, are spaced 100–150 m apart, and move in 55-year cycles. Crest-shaped wave fronts are concave on the windward side, occur as random events, and leave a trailing edge to the right of the prevailing wind that becomes a sinusoidal wave front. Two types of wave cycles, uniformcycle and brokencycle, have been identified as regular and random events, respectively. This paper reports the initial results of a detailed study on the impact of wind on the dynamics of a wave forest and concentrates on the centroid as a measure of crown asymmetry useful for distinguishing between purely biotic (mainly competition) and abiotic (mainly wind) processes as the primary cause of crown asymmetry. The development of wave forests at Spirity Cove is described based on the hypothesis that longitudinal helical roll vortices are primarily responsible for their formation.
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Seilacher, Adolf, Cynthia Marshall, H. Catherine W. Skinner und Takanobu Tsuihiji. „A fresh look at sideritic “coprolites”“. Paleobiology 27, Nr. 1 (2001): 7–13. http://dx.doi.org/10.1666/0094-8373(2001)027<0007:aflasc>2.0.co;2.
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Sideritic “coprolites” from the late Miocene of southwest Washington, the Upper Cretaceous of Saskatchewan and Madagascar, and the Permian of China have often been claimed to be pseudofossils. They are here interpreted as intestinal casts (cololites) prefossilized by bacterial activity and later transformed into siderite with no traces of original food particles left. All occurrences are found within fluvial overbank deposits that carry no other vertebrate remains. Their absence could be due to aquifer roll-fronts that destroyed phosphatic bones and teeth but favored siderite precipitation.
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Capet, X., J. C. McWilliams, M. J. Molemaker und A. F. Shchepetkin. „Mesoscale to Submesoscale Transition in the California Current System. Part I: Flow Structure, Eddy Flux, and Observational Tests“. Journal of Physical Oceanography 38, Nr. 1 (01.01.2008): 29–43. http://dx.doi.org/10.1175/2007jpo3671.1.
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Abstract In computational simulations of an idealized subtropical eastern boundary upwelling current system, similar to the California Current, a submesoscale transition occurs in the eddy variability as the horizontal grid scale is reduced to O(1) km. This first paper (in a series of three) describes the transition in terms of the emergent flow structure and the associated time-averaged eddy fluxes. In addition to the mesoscale eddies that arise from a primary instability of the alongshore, wind-driven currents, significant energy is transferred into submesoscale fronts and vortices in the upper ocean. The submesoscale arises through surface frontogenesis growing off upwelled cold filaments that are pulled offshore and strained in between the mesoscale eddy centers. In turn, some submesoscale fronts become unstable and develop submesoscale meanders and fragment into roll-up vortices. Associated with this phenomenon are a large vertical vorticity and Rossby number, a large vertical velocity, relatively flat horizontal spectra (contrary to the prevailing view of mesoscale dynamics), a large vertical buoyancy flux acting to restratify the upper ocean, a submesoscale energy conversion from potential to kinetic, a significant spatial and temporal intermittency in the upper ocean, and material exchanges between the surface boundary layer and pycnocline. Comparison with available observations indicates that submesoscale fronts and instabilities occur widely in the upper ocean, with characteristics similar to the simulations.
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KUSKE, R., und P. MILEWSKI. „Modulated two-dimensional patterns in reaction–diffusion systems“. European Journal of Applied Mathematics 10, Nr. 2 (April 1999): 157–84. http://dx.doi.org/10.1017/s095679259800360x.
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New modulation equations for hexagonal patterns in reaction–diffusion systems are derived for parameter régimes corresponding to the onset of patterns. These systems include additional nonlinearities which are not present in Rayleigh–Bénard convection or Swift–Hohenberg type models. The dynamics of hexagonal and roll patterns are studied using a combination of analytical and computational approaches which exploit the hexagonal structure of the modulation equations. The investigation demonstrates instabilities and new phenomena not found in other systems, and is applied to patterns of flame fronts in a certain model of burner stabilized flames.
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Schertel, Lukas, Sofia Magkiriadou, Pavel Yazhgur und Ahmet Demirörs. „Manufacturing Large-scale Materials with Structural Color“. CHIMIA 76, Nr. 10 (26.10.2022): 833. http://dx.doi.org/10.2533/chimia.2022.833.
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Living organisms frequently use structural color for coloration as an alternative mechanism to chemical pigmentation. Recently there has been a growing interest to translate structural color into synthetic materials as a more durable and less hazardous alternative to conventional pigments. Efforts to fabricate structurally colored materials take place in different fronts, from 3D printing to spray-coating and roll-to-roll casting. Stability, performance, and quality of the color, the environmental impact of the materials or their manufacturing methods are some of the heavily researched topics we discuss. First, we highlight recent examples of large-scale manufacturing technologies to fabricate structurally colored objects. Second, we discuss the current challenges to be tackled to create perfect appearances which aim at the full color gamut while caring for environmental concerns. Finally, we discuss possible scenarios that could be followed in order to involve other manufacturing methods for creating structurally colored objects.
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Viroulet, S., J. L. Baker, F. M. Rocha, C. G. Johnson, B. P. Kokelaar und J. M. N. T. Gray. „The kinematics of bidisperse granular roll waves“. Journal of Fluid Mechanics 848 (13.06.2018): 836–75. http://dx.doi.org/10.1017/jfm.2018.348.
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Small perturbations to a steady uniform granular chute flow can grow as the material moves downslope and develop into a series of surface waves that travel faster than the bulk flow. This roll wave instability has important implications for the mitigation of hazards due to geophysical mass flows, such as snow avalanches, debris flows and landslides, because the resulting waves tend to merge and become much deeper and more destructive than the uniform flow from which they form. Natural flows are usually highly polydisperse and their dynamics is significantly complicated by the particle size segregation that occurs within them. This study investigates the kinematics of such flows theoretically and through small-scale experiments that use a mixture of large and small glass spheres. It is shown that large particles, which segregate to the surface of the flow, are always concentrated near the crests of roll waves. There are different mechanisms for this depending on the relative speed of the waves, compared to the speed of particles at the free surface, as well as on the particle concentration. If all particles at the surface travel more slowly than the waves, the large particles become concentrated as the shock-like wavefronts pass them. This is due to a concertina-like effect in the frame of the moving wave, in which large particles move slowly backwards through the crest, but travel quickly in the troughs between the crests. If, instead, some particles on the surface travel more quickly than the wave and some move slower, then, at low concentrations, large particles can move towards the wave crest from both the forward and rearward sides. This results in isolated regions of large particles that are trapped at the crest of each wave, separated by regions where the flow is thinner and free of large particles. There is also a third regime arising when all surface particles travel faster than the waves, which has large particles present everywhere but with a sharp increase in their concentration towards the wave fronts. In all cases, the significantly enhanced large particle concentration at wave crests means that such flows in nature can be especially destructive and thus particularly hazardous.
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SHOR, BORIS, und NOLAN McCARTY. „The Ideological Mapping of American Legislatures“. American Political Science Review 105, Nr. 3 (26.07.2011): 530–51. http://dx.doi.org/10.1017/s0003055411000153.
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The development and elaboration of the spatial theory of voting has contributed greatly to the study of legislative decision making and elections. Statistical models that estimate the spatial locations of individual decision-makers have made a key contribution to this success. Spatial models have been estimated for the U.S. Congress, the Supreme Court, U.S. presidents, a large number of non-U.S. legislatures, and supranational organizations. Yet one potentially fruitful laboratory for testing spatial theories, the individual U.S. states, has remained relatively unexploited, for two reasons. First, state legislative roll call data have not yet been systematically collected for all states over time. Second, because ideal point models are based on latent scales, comparisons of ideal points across states or even between chambers within a state are difficult. This article reports substantial progress on both fronts. First, we have obtained the roll call voting data for all state legislatures from the mid-1990s onward. Second, we exploit a recurring survey of state legislative candidates to allow comparisons across time, chambers, and states as well as with the U.S. Congress. The resulting mapping of America's state legislatures has great potential to address numerous questions not only about state politics and policymaking, but also about legislative politics in general.
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Rallakis, Dimitrios, Raymond Michels, Marc Brouand, Olivier Parize und Michel Cathelineau. „The Role of Organic Matter on Uranium Precipitation in Zoovch Ovoo, Mongolia“. Minerals 9, Nr. 5 (18.05.2019): 310. http://dx.doi.org/10.3390/min9050310.
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The Zoovch Ovoo uranium deposit is located in East Gobi Basin in Mongolia. It is hosted in the Sainshand Formation, a Late Cretaceous siliciclastic reservoir, in the lower part of the post-rift infilling of the Mesozoic East Gobi Basin. The Sainshand Formation corresponds to poorly consolidated medium-grained sandy intervals and clay layers deposited in fluvial-lacustrine settings. The uranium deposit is confined within a 60- to 80-m-thick siliciclastic reservoir inside aquifer driven systems, assimilated to roll-fronts. As assessed by vitrinite reflectance (%Rr < 0.4) and molecular geochemistry, the formation has never experienced significant thermal maturation. Detrital organic matter (type III and IV kerogens) is abundant in the Zoovch Ovoo depocenter. In this framework, uranium occurs as: (i) U-rich macerals without any distinguishable U-phase under SEM observation, containing up to 40 wt % U; (ii) U expressed as UO2 at the rims of large (several millimeters) macerals and (iii) U oxides partially to entirely replacing macerals, while preserving the inherited plant texture. Thus, uranium is accumulated gradually in the macerals through an organic carbon–uranium epigenization process, in respect to the maceral’s chemistry and permeability. Most macerals are rich in S and, to a lesser extent, in Fe. Frequently, Fe and S contents do not fit the stoichiometry of pyrite, although pyrite also occurs as small inclusions within the macerals. The organic matter appears thus as a major redox trap for uranium in this kind of geological setting.
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Flageat, Manon, Bryan Lim und Antoine Cully. „Beyond Expected Return: Accounting for Policy Reproducibility When Evaluating Reinforcement Learning Algorithms“. Proceedings of the AAAI Conference on Artificial Intelligence 38, Nr. 11 (24.03.2024): 12024–32. http://dx.doi.org/10.1609/aaai.v38i11.29090.
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Many applications in Reinforcement Learning (RL) usually have noise or stochasticity present in the environment. Beyond their impact on learning, these uncertainties lead the exact same policy to perform differently, i.e. yield different return, from one roll-out to another. Common evaluation procedures in RL summarise the consequent return distributions using solely the expected return, which does not account for the spread of the distribution. Our work defines this spread as the policy reproducibility: the ability of a policy to obtain similar performance when rolled out many times, a crucial property in some real-world applications. We highlight that existing procedures that only use the expected return are limited on two fronts: first an infinite number of return distributions with a wide range of performance-reproducibility trade-offs can have the same expected return, limiting its effectiveness when used for comparing policies; second, the expected return metric does not leave any room for practitioners to choose the best trade-off value for considered applications. In this work, we address these limitations by recommending the use of Lower Confidence Bound, a metric taken from Bayesian optimisation that provides the user with a preference parameter to choose a desired performance-reproducibility trade-off. We also formalise and quantify policy reproducibility, and demonstrate the benefit of our metrics using extensive experiments of popular RL algorithms on common uncertain RL tasks.
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Shakespeare, Callum J. „Curved Density Fronts: Cyclogeostrophic Adjustment and Frontogenesis“. Journal of Physical Oceanography 46, Nr. 10 (Oktober 2016): 3193–207. http://dx.doi.org/10.1175/jpo-d-16-0137.1.
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AbstractCurvature can play a significant role in the dynamics of density fronts at small scales and in low-latitude regions of the ocean. Fronts can be displaced from balance by rapid forcing and undergo an adjustment toward a more stable state or be strained and sharpened by surrounding flow in a process known as frontogenesis. This study investigates the role of curvature in adjustment and frontogenesis using the idealized configuration of an axisymmetric eddy and associated circular front. As a result of the curvature, the balanced state of this system is not geostrophic balance, where pressure and Coriolis forces exactly balance, but cyclogeostrophic balance, where pressure and Coriolis forces combine to supply a net inwards centripetal force on fluid parcels. The parameter range for which cyclogeostrophically balanced states exist for a given unbalanced initial condition is determined. This parameter range is smaller for anticyclonic fronts (i.e., fronts curved around a warm core), which have larger angular velocities than comparable straight fronts, implying they are more likely to break down during adjustment. The reverse is true for cyclonic fronts. A model for the sharpening of a curved front in a background strain flow, analogous to the Hoskins and Bretherton (1972) model for a straight front, is developed. Relative to a straight front subject to the same strain rate, vertical velocities are weaker for an anticyclonic front and stronger for a cyclonic front. Anticyclonic fronts collapse to a near discontinuity during frontogenesis far more rapidly than cyclonic fronts for the same strain rate.
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Shabaga, Brandi M., Mostafa Fayek, David Quirt, Charlie W. Jefferson und Alfredo Camacho. „Mineralogy, geochronology, and genesis of the Andrew Lake uranium deposit, Thelon Basin, Nunavut, Canada“. Canadian Journal of Earth Sciences 54, Nr. 8 (August 2017): 850–68. http://dx.doi.org/10.1139/cjes-2017-0024.
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The Thelon Basin located in Nunavut, Canada, shares many similarities with the U-producing Athabasca Basin in Saskatchewan. The Kiggavik project area, located near the northeastern edge of the Thelon Basin, contains U deposits and showings along the ∼30 km long NE–SW Kiggavik – Andrew Lake structural trend. The Andrew Lake deposit is near the southern end of this trend. Pre-mineralization is characterized by quartz ± carbonate veins that occupy fault systems later reactivated as conduits for U-mineralizing fluids. A four-phase genetic model is proposed for the Andrew Lake deposit. Phase 1 comprises vein-style uraninite (U1; 1031 ± 23 Ma) that is associated with illite and hematite, and contains variable PbO contents (0.2–9.5 wt.%). Phase 2 is characterized by altered uraninite (U2; ∼530 Ma) that is associated with coffinite. Altered uraninite (U3; <1 Ma) characterizes phase 3 and occurs as centimetre-scale “roll-fronts”. In phase 4, all three uraninite stages, and coffinite, are altered to boltwoodite. Although the oldest uraninite U–Pb age is ∼1030 Ma, illite associated with the U mineralization gives 40Ar/39Ar ages of 941 ± 31 and 1330 ± 36 Ma. The younger age is similar to the age for U1, suggesting that there was a fluid event that either precipitated U1 or reset the U–Pb isotopic system at ∼1000 Ma. While the older age for illite (1330 Ma) does not correlate with Andrew Lake U–Pb uraninite ages, it does correlate with ages previously reported for uraninite and clay alteration minerals in the Kiggavik area.
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Biard, James C., und Kenneth E. Kunkel. „Automated detection of weather fronts using a deep learning neural network“. Advances in Statistical Climatology, Meteorology and Oceanography 5, Nr. 2 (21.11.2019): 147–60. http://dx.doi.org/10.5194/ascmo-5-147-2019.
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Abstract. Deep learning (DL) methods were used to develop an algorithm to automatically detect weather fronts in fields of atmospheric surface variables. An algorithm (DL-FRONT) for the automatic detection of fronts was developed by training a two-dimensional convolutional neural network (2-D CNN) with 5 years (2003–2007) of manually analyzed fronts and surface fields of five atmospheric variables: temperature, specific humidity, mean sea level pressure, and the two components of the wind vector. An analysis of the period 2008–2015 indicates that DL-FRONT detects nearly 90 % of the manually analyzed fronts over North America and adjacent coastal ocean areas. An analysis of fronts associated with extreme precipitation events shows that the detection rate may be substantially higher for important weather-producing fronts. Since DL-FRONT was trained on a North American dataset, its extensibility to other parts of the globe has not been tested, but the basic frontal structure of extratropical cyclones has been applied to global daily weather maps for decades. On that basis, we expect that DL-FRONT will detect most fronts, and certainly most fronts with significant weather. However, where complex terrain plays a role in frontal orientation or other characteristics, it might be less successful.
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Schemm, Sebastian, Michael Sprenger und Heini Wernli. „When during Their Life Cycle Are Extratropical Cyclones Attended by Fronts?“ Bulletin of the American Meteorological Society 99, Nr. 1 (01.01.2018): 149–65. http://dx.doi.org/10.1175/bams-d-16-0261.1.
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Abstract For nearly a century, the study of atmospheric dynamics in the midlatitudes has presented dichotomic perspectives on one of its focal points: the birth and life cycle of cyclones. In particular, the role of fronts has driven much of the historical discourse on cyclogenesis. In the 1910s–20s, the Bergen School of Meteorology postulated that cyclogenesis occurs on a preexisting front. This concept was later replaced by the baroclinic instability paradigm, which describes the development of a surface front as a consequence of the growing cyclone rather than its cause. However, there is ample observational evidence for cyclogenesis on well-marked fronts (frontal-wave cyclones) as well as for cyclogenesis in the absence of fronts in broader baroclinic zones. Thus, after a century of research on the link between extratropical cyclones and fronts, this study has the objective of climatologically quantifying their relationship. By combining identification schemes for cyclones and fronts, the fraction of cyclones with attendant fronts is quantified at all times during the cyclones’ life cycle. The storm-track regions over the North Atlantic are dominated by cyclones that form on preexisting fronts. Over the North Pacific, the result more strongly depends on the front definition. Cyclones that acquire their fronts during the life cycle dominate over the continents and in the Mediterranean. Further, cyclones that develop attendant fronts during their life cycle typically do so around the time they attain maximum intensity. At the time of cyclolysis, at least 40% of all cyclones are still associated with a front. The number of occluded fronts at lysis has not been considered.
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Cao, Yuhan, Chunyan Li und Changming Dong. „Atmospheric Cold Front-Generated Waves in the Coastal Louisiana“. Journal of Marine Science and Engineering 8, Nr. 11 (11.11.2020): 900. http://dx.doi.org/10.3390/jmse8110900.
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Atmospheric cold front-generated waves play an important role in the air–sea interaction and coastal water and sediment transports. In-situ observations from two offshore stations are used to investigate variations of directional waves in the coastal Louisiana. Hourly time series of significant wave height and peak wave period are examined for data from 2004, except for the summer time between May and August, when cold fronts are infrequent and weak. The intra-seasonal scale variations in the wavefield are significantly affected by the atmospheric cold frontal events. The wave fields and directional wave spectra induced by four selected cold front passages over the coastal Louisiana are discussed. It is found that significant wave height generated by cold fronts coming from the west change more quickly than that by other passing cold fronts. The peak wave direction rotates clockwise during the cold front events. The variability of the directional wave spectrum shows that the largest spectral density is distributed at low frequency in the postfrontal phase associated with migrating cyclones (MC storms) and arctic surges (AS storms).
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Marchais, T., B. Pérot, C. Carasco, J.-L. Ma, P.-G. Allinei, H. Toubon, R. Goupillou und J. Collot. „The use of self-induced X-ray fluorescence in gamma-ray spectroscopy of uranium ore samples“. EPJ Web of Conferences 225 (2020): 05003. http://dx.doi.org/10.1051/epjconf/202022505003.
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Gamma logging for uranium exploration are currently based on total counting with Geiger Müller gas detectors or NaI (TI) scintillators. However, the total count rate interpretation in terms of uranium concentration may be impaired in case of roll fronts, when the radioactive equilibrium of the natural 238U radioactive chain is modified by differential leaching of uranium and its daughter radioisotopes of thorium, radium, radon, etc. Indeed, in case of secular equilibrium, more than 95 % of gamma rays emitted by uranium ores come from 214Pb and 214Bi isotopes, which are in the back-end of 238U chain. Consequently, these last might produce an intense gamma signal even when uranium is not present, or with a much smaller activity, in the ore. Therefore, gamma spectroscopy measurements of core samples are performed in surface with high-resolution hyper-pure germanium HPGe detectors to directly characterize uranium activity from the 1001 keV gamma ray of 234mPa, which is in the beginning of 238U chain. However, due to the low intensity of this gamma ray, i.e. 0.84 %, acquisitions of several hours are needed. In view to characterize uranium concentration within a few minutes, we propose here a method using both the 92 keV gamma ray of 234Th and the 98.4 keV uranium X-ray. This last is due to uranium self-induced fluorescence caused by gamma radiations of 214Pb and 214Bi, which create a significant Compton scattering continuum acting as a fluorescence source and resulting in the emission of uranium fluorescence X-rays. The comparison of the uranium activity obtained with the 92 keV and 98.4 keV lines allows detecting a uranium heterogeneity in the ore. Indeed, in case of uranium nugget, the 92 keV line leads to underestimated uranium concentration due to gamma self-absorption, but on the contrary the 98.4 keV line leads to an overestimation because of increased fluorescence. In order to test this new approach, several tens of uranium ore samples have been measured with a handheld HPGe FALCON 5000 detector.
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Wang, Xiong Liang, und Chun Ling Wang. „Extraction of Ocean Fronts Based on Empirical Mode Decomposition“. Applied Mechanics and Materials 701-702 (Dezember 2014): 303–7. http://dx.doi.org/10.4028/www.scientific.net/amm.701-702.303.
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Ocean front is a narrow transitional zone that the penetration of sea is obviously different between two or more waters there. It is an important feature of geophysical turbulence which plays an important role in ocean dynamics. Ocean fronts become visible on radar images because they are associated with a variable surface current which modulates the sea surface roughness and thus the backscattered radar power. This paper propose a new integrated method to extract ocean fronts based on two-dimensional Empirical Mode Decomposition (EMD), image edge detection and mathematical morphology processing. Experimental results show that this integrated method can be effective in ocean front feature extraction.
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Curtarelli, M. P., E. H. Alcântara, C. D. Rennó und J. L. Stech. „Modeling the effects of cold front passages on the heat fluxes and thermal structure of a tropical hydroelectric reservoir“. Hydrology and Earth System Sciences Discussions 10, Nr. 7 (02.07.2013): 8467–502. http://dx.doi.org/10.5194/hessd-10-8467-2013.
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Abstract. We study the influence of cold fronts on the heat fluxes and thermal structure of a tropical reservoir located in Brazil. The period chosen for this study consisted of 49 days between 28 April 2010 and 15 July 2010 and was defined based on information from the Brazilian Centre for Weather Forecasting and Climate Studies (CPTEC), data collected in situ and the interpretation of remotely sensed images. During the selected time period, five cold front passages were identified, allowing us to analyze the cumulative effect of cold fronts and the reservoir's resilience on the days that elapsed between the passages. To better understand the physical processes that drive changes in heat fluxes and thermal structure, a simulation was performed that utilized a three-dimensional hydrodynamic model. The results showed that during the cold front days, the sensible and latent heat fluxes were enhanced by approximately 24% and 19%, respectively. The daily average heat loss was up to 167% higher on the cold front days than on the non-cold front days. The high heat loss and the increased wind intensity that occurred during the cold front passages destabilized the water column and provided partial or complete mixing. The colder waters of the Paranaíba River contributed to reestablish the thermal stratification following the passages of the cold fronts. These results suggest that cold front passages play an important role in the stratification and mixing regimes of Brazilian reservoirs located in southern and southeastern regions.
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Cordone, Angelina, Matteo Selci, Bernardo Barosa, Alessia Bastianoni, Deborah Bastoni, Francesco Bolinesi, Rosaria Capuozzo et al. „Surface Bacterioplankton Community Structure Crossing the Antarctic Circumpolar Current Fronts“. Microorganisms 11, Nr. 3 (09.03.2023): 702. http://dx.doi.org/10.3390/microorganisms11030702.
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The Antarctic Circumpolar Current (ACC) is the major current in the Southern Ocean, isolating the warm stratified subtropical waters from the more homogeneous cold polar waters. The ACC flows from west to east around Antarctica and generates an overturning circulation by fostering deep-cold water upwelling and the formation of new water masses, thus affecting the Earth’s heat balance and the global distribution of carbon. The ACC is characterized by several water mass boundaries or fronts, known as the Subtropical Front (STF), Subantarctic Front (SAF), Polar Front (PF), and South Antarctic Circumpolar Current Front (SACCF), identified by typical physical and chemical properties. While the physical characteristics of these fronts have been characterized, there is still poor information regarding the microbial diversity of this area. Here we present the surface water bacterioplankton community structure based on 16S rRNA sequencing from 13 stations sampled in 2017 between New Zealand to the Ross Sea crossing the ACC Fronts. Our results show a distinct succession in the dominant bacterial phylotypes present in the different water masses and suggest a strong role of sea surface temperatures and the availability of Carbon and Nitrogen in controlling community composition. This work represents an important baseline for future studies on the response of Southern Ocean epipelagic microbial communities to climate change.
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Possanza, D. Mark. „Cornua and Frontes in [Tibullus] 3.1.13“. Classical Quarterly 44, Nr. 1 (Mai 1994): 281–82. http://dx.doi.org/10.1017/s0009838800017407.
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The transmitted text of line 13, ‘inter geminas…frontes’, has long presented an anomaly in the description of the decorated papyrus roll. If, in the context of book production, frons means the flat, round cross section located at either end of the rolled up book (TLL 6.1.1362, 84) and if cornu means an ornamental projection attached to the ends of the umbilicus and extending beyond the plane of the frons, then the transmitted text is a physical impossibility. For it is the frontes that lie between the cornua and not the other way round. In the words of Heyne's paraphrase: ‘geminae frontes inter duo cornua, non duo cornua inter geminas frontes.’ Emendation is required not only because an author is unlikely to be inaccurate or imprecise about the physical details of his book but also because the transmitted text can be salvaged only by recourse to tortuous theories about the meaning of cornu and frons, about their locations with respect to one another, and about the interpretation of inter with geminas.
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Zhang, Hao-Ran, Yi Yu, Zhibin Gao, Yanwei Zhang, Wentao Ma, Dezhou Yang, Baoshu Yin und Yuntao Wang. „Seasonal and Interannual Variability of Fronts and Their Impact on Chlorophyll-a in the Indonesian Seas“. Journal of Physical Oceanography 53, Nr. 12 (Dezember 2023): 2847–59. http://dx.doi.org/10.1175/jpo-d-23-0041.1.
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Abstract The spatiotemporal variability of oceanic fronts in the Indonesian seas was investigated using high-resolution satellite observations. The study aimed to understand the underlying mechanism driving these fronts and their impact on chlorophyll-a variability. A high value of frontal probability was found near the coasts of major islands, exhibiting a distinct seasonal cycle with peaks occurrences during austral winter. The distribution variability of chlorophyll-a was generally consistent with the presence of active frontal zones, although a significantly positive relationship between fronts and chlorophyll-a was limited to only some specific areas, e.g., south Java Island and the Celebes Sea. Wind-driven upwelling played a major role in front generation in the Java upwelling region and enhanced frontal activity can promote the growth of phytoplankton, leading to higher chlorophyll-a. Furthermore, the study demonstrated that wind patterns preceded variations in front probability and chlorophyll-a by approximately two months. This lag suggests that the spatiotemporal variability of fronts and chlorophyll-a in this region is primarily influenced by the monsoon system. In addition, the sea surface temperature (SST) simultaneously modulated the chlorophyll-a variability. Negative SST anomalies were typically associated with positive anomalies in front probability the chlorophyll-a in most areas. Notably, the interannual variability of fronts and chlorophyll-a are prominent in the Java upwelling region. During El Niño years, this region experienced an enhanced monsoon, resulting in a negative SST anomaly alongside positive anomalies in front probability and chlorophyll-a. A comprehensive description and underlying dynamics of frontal activity in the Indonesian seas are provided by this study. The findings are helpful to delineate the variability in chlorophyll-a, thereby facilitating the future understanding of local primary production and the carbon cycle. Significance Statement As typical mesoscale processes, oceanic fronts have significant impacts on biological processes and fisheries in marginal seas. The complex spatiotemporal variability of fronts and their effects on biological processes in the Indonesian seas remain poorly understood. This study aimed to address this knowledge gap by investigating the seasonal and interannual variability of fronts and their influence on chlorophyll-a, a key indicator of phytoplankton biomass and primary productivity. The study identified a high frontal probability in south Java Island during austral winter and El Niño years. Wind-driven upwelling was found to be a major factor in front generation and promoting phytoplankton growth. The findings of this study will improve the theoretical knowledge of regional dynamics, local primary production, and the carbon cycle in the Indonesian seas, benefiting fisheries management and ecosystem conservation efforts.
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Schultz, David M. „Comments on “Unusually Long Duration, Multiple-Doppler Radar Observations of a Front in a Convective Boundary Layer”“. Monthly Weather Review 135, Nr. 12 (01.12.2007): 4237–39. http://dx.doi.org/10.1175/2007mwr2189.1.
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Abstract Stonitsch and Markowski perform multiple-Doppler radar analyses of a cold front over Oklahoma and Kansas. Despite their interesting results, their explanations include a number of misconceptions about cold fronts. These misconceptions include the proper interpretation of the frontogenesis function, the role of entrainment versus differential surface sensible heat flux toward weakening the virtual potential temperature gradient across a cold front, a separation of the wind shift from the virtual potential temperature gradient, and the factors that affect the motion of the cold front.
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Li, Ke, Zhexuan Zhang, Greg Chini und Glenn Flierl. „Langmuir Circulation: An Agent for Vertical Restratification?“ Journal of Physical Oceanography 42, Nr. 11 (01.11.2012): 1945–58. http://dx.doi.org/10.1175/jpo-d-11-0225.1.
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Abstract Comparably little is known about the impact of down-front-propagating surface waves on the stability of submesoscale lateral fronts in the ocean surface mixed layer. In this investigation, the stability of lateral fronts in gradient–wind balance to two-dimensional (down-front invariant) disturbances is analyzed using the stratified, rotating Craik–Leibovich (CL) equations. Through the action of the CL vortex force, the surface waves fundamentally alter the superinertial, two-dimensional linear stability of these fronts, with the classical symmetric instability mode being replaced by a hybrid Langmuir circulation/symmetric mode. The hybrid mode is shown to exhibit much larger growth rates than the pure symmetric mode, to exist in a regime in which the vertical Richardson number is greater than 1, and to accomplish significant cross-isopycnal transport. Nonhydrostatic numerical simulations reveal that the nonlinear evolution of this hybrid instability mode can lead to rapid, that is, superinertial, vertical restratification of the mixed layer. Paradoxically, Langmuir circulation—generally viewed as a prominent vertical mixing mechanism in the upper ocean—may thus play a role in mixed layer restratification.
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Deremble, Bruno, Guillaume Lapeyre und Michael Ghil. „Atmospheric Dynamics Triggered by an Oceanic SST Front in a Moist Quasigeostrophic Model“. Journal of the Atmospheric Sciences 69, Nr. 5 (01.05.2012): 1617–32. http://dx.doi.org/10.1175/jas-d-11-0288.1.
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Abstract To understand the atmospheric response to a midlatitude oceanic front, this paper uses a quasigeostrophic (QG) model with moist processes. A well-known, three-level QG model on the sphere has been modified to include such processes in an aquaplanet setting. Its response is analyzed in terms of the upper-level atmospheric jet for sea surface temperature (SST) fronts of different profiles and located at different latitudes. When the SST front is sufficiently strong, it tends to anchor the mean atmospheric jet, suggesting that the jet’s spatial location and pattern are mainly affected by the latitude of the SST front. Changes in the jet’s pattern are studied, focusing on surface sensible heat flux and on moisture effects through latent heat release. It is found that latent heat release due to moist processes is modified when the SST front is changed, and this is responsible for the meridional displacement of the jet. Moreover, both latent heat release and surface sensible heat flux contribute to the jet’s strengthening. These results highlight the role of SST fronts and moist processes in affecting the characteristics of the midlatitude jet stream and of its associated storm track, particularly their positions.
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CASSARO, P., F. SCHILLIRÓ, V. COSTA, G. BELVEDERE, R. A. ZAPPALÁ und G. LANZAFAME. „THE ENGINE OF OUTFLOWS IN AGN: THE ROLE OF PHYSICAL TURBULENT VISCOSITY“. International Journal of Modern Physics D 17, Nr. 09 (September 2008): 1635–40. http://dx.doi.org/10.1142/s0218271808013248.
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Adopting the smoothed particle hydrodynamics (SPH) numerical method, we performed a grid of evolving models of a 3D, axially symmetric, physically viscous accretion disc around a black hole (BH) in an AGN. In such disc models, the role of the specific angular momentum λ and of the physical turbulent viscosity parameter α, according to the Shakura and Sunyaev prescription, are examined. One or two shock fronts develop in the radial inviscid flow, according to the assigned initial kinematic and thermodynamic conditions. Couples of (α, λ) values determine radial periodical oscillations in the shock front. An outflow can develop from the subsonic post shock region, close to the black hole, in some cases. This provides evidence for a link between the accretion disc and the fueling of a jet, through the presence of shock fronts in an accretion disc close to the centrifugal barrier.
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Messager, C., S. Speich und E. Key. „Marine atmospheric boundary layer over some Southern Ocean fronts during the IPY BGH 2008 cruise“. Ocean Science 8, Nr. 6 (23.11.2012): 1001–23. http://dx.doi.org/10.5194/os-8-1001-2012.
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Abstract. A set of meteorological instruments was added to an oceanographic cruise crossing the Southern Ocean from Cape Town to 57°33' S during the summer of 2008. The Cape Cauldron, the Subtropical, Subantarctic, Polar and southern Antarctic Circumpolar current fronts were successively crossed. The recorded data permitted to derive the exchange of momentum, heat and water vapour at the ocean-atmosphere interface. A set of 38 radiosonde releases complemented the dataset. The marine atmospheric boundary layer characteristics and air-sea interaction when the ship crossed the fronts and eddies are discussed. The specific role of the atmospheric synoptic systems advection on the air-sea interaction over these regions is highlighted. Additionally, the Subantarctic front mesoscale variability induced an anticyclonic eddy considered as part of the Subantarctic front. The specific influence of this Agulhas ring on the aloft atmosphere is also presented.
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Ostroumova, S. A., I. D. Drozd und D. I. Frey. „Structure and Temporal Variability of the Northern Branch of the Antarctic Circumpolar Current in the Drake Passage“. Океанология 63, Nr. 4 (01.07.2023): 511–25. http://dx.doi.org/10.31857/s0030157423040160.
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The Antarctic Circumpolar Current plays the key role in the circulation of the Southern Ocean and affects the distribution of heat by the ocean on the global scale. The study of the dynamics and structure of this current becomes especially important in a changing climate. The current is well revealed by satellite altimetry data, which makes possible to study temporal and spatial variability of its structure at different scales. In these studies, the methods for determining the position of individual fronts of the Antarctic Circumpolar Current based on satellite altimetry data become especially important. In this work, we compare various approaches for detection of front locations. The structure of the northernmost branch of the Antarctic Circumpolar Current, the Subantarctic Front, and its spatial and temporal variability was studied based on satellite altimetry data from 1993 to 2020 and the results of a hydrographical section occupied by the R/V “Akademik Mstislav Keldysh” in the northern part of the Drake Passage in February 2020. We selected a 350 km long part of the TOPEX/Poseidon and Jason-1,2,3 satellite altimeter track from Tierra del Fuego to the south for the analysis of the dynamics of the front. Criteria for determining the position of the northern and main branches of the Subantarctic Front are presented based on satellite altimetry data. A long-term shift of the position of the fronts relative to the previously accepted levels of absolute dynamic topography has been found. It was found that the accuracy of determining the position of fronts using fixed values of dynamic topography may decrease with time, in particular due to changes in the mean sea level. A statistically significant long-term trend of sea level rise in the region of the Subantarctic Front was found. This trend is 4 mm/yr for the northern branch and 2.5 mm/yr for the main branch.
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Wang, Y. L., J. Raeder und C. T. Russell. „Plasma depletion layer: the role of the slow mode waves“. Annales Geophysicae 22, Nr. 12 (22.12.2004): 4259–72. http://dx.doi.org/10.5194/angeo-22-4259-2004.
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Abstract. The plasma depletion layer (PDL) is a layer on the sunward side of the magnetopause with lower plasma density and higher magnetic field compared to their corresponding values in the upstream magnetosheath. The depletion layer usually occurs during northward (IMF) conditions with low magnetic shear across the magnetopause. We have previously validated the Raeder global model by comparing the computed formation of a magnetosheath density depletion with in-situ observations. We also have performed a detailed force analysis and found the varying roles that different MHD forces play along the path of a plasma parcel flowing around the magnetopause. That study resulted in a new description of the behavior of magnetosheath magnetic flux tubes which better explains the plasma depletion along a flux tube. The slow mode waves have been observed in the magnetosheath and have been used to explain the formation of the PDL in some of the important PDL models. In this study, we extend our former work by investigating the possible role of the slow mode waves for the formation of the PDL, using global MHD model simulations. We propose a new technique to test where a possible slow mode front may occur in the magnetosheath by comparing the slow mode group velocity with the local flow velocity. We find that the slow mode fronts can exist in certain regions in the magnetosheath under certain solar wind conditions. The existence and location of such fronts clearly depend on the IMF. We do not see from our global simulation results either the sharpening of the slow mode front into a slow mode shock or noticeable changes of the flow and field in the magnetosheath across the slow mode front, which implies that the slow mode front is not likely responsible for the formation of the PDL, at least for the stable solar wind conditions used in these simulations. Also, we do not see the two-layered slow mode structures shown in some observations and proposed in certain PDL models. Instead, we see only a one-layered spatial PDL structure under the stable solar wind conditions used in this study.
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Wenegrat, Jacob O., Leif N. Thomas, Miles A. Sundermeyer, John R. Taylor, Eric A. D’Asaro, Jody M. Klymak, R. Kipp Shearman und Craig M. Lee. „Enhanced mixing across the gyre boundary at the Gulf Stream front“. Proceedings of the National Academy of Sciences 117, Nr. 30 (10.07.2020): 17607–14. http://dx.doi.org/10.1073/pnas.2005558117.
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The Gulf Stream front separates the North Atlantic subtropical and subpolar ocean gyres, water masses with distinct physical and biogeochemical properties. Exchange across the front is believed to be necessary to balance the freshwater budget of the subtropical gyre and to support the biological productivity of the region; however, the physical mechanisms responsible have been the subject of long-standing debate. Here, the evolution of a passive dye released within the north wall of the Gulf Stream provides direct observational evidence of enhanced mixing across the Gulf Stream front. Numerical simulations indicate that the observed rapid cross-frontal mixing occurs via shear dispersion, generated by frontal instabilities and episodic vertical mixing. This provides unique direct evidence for the role of submesoscale fronts in generating lateral mixing, a mechanism which has been hypothesized to be of general importance for setting the horizontal structure of the ocean mixed layer. Along the Gulf Stream front in the North Atlantic, these observations further suggest that shear dispersion at sharp fronts may provide a source of freshwater flux large enough to explain much of the freshwater deficit in the subtropical-mode water budget and a flux of nutrients comparable to other mechanisms believed to control primary productivity in the subtropical gyre.
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Cremaschini, Claudio, und Massimo Tessarotto. „The Wave-Front Equation of Gravitational Signals in Classical General Relativity“. Symmetry 12, Nr. 2 (02.02.2020): 216. http://dx.doi.org/10.3390/sym12020216.
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In this paper the dynamical equation for propagating wave-fronts of gravitational signals in classical general relativity (GR) is determined. The work relies on the manifestly-covariant Hamilton and Hamilton–Jacobi theories underlying the Einstein field equations recently discovered (Cremaschini and Tessarotto, 2015–2019). The Hamilton–Jacobi equation obtained in this way yields a wave-front description of gravitational field dynamics. It is shown that on a suitable subset of configuration space the latter equation reduces to a Klein–Gordon type equation associated with a 4-scalar field which identifies the wave-front surface of a gravitational signal. Its physical role and mathematical interpretation are discussed. Radiation-field wave-front solutions are pointed out, proving that according to this description, gravitational wave-fronts propagate in a given background space-time as waves characterized by the invariant speed-of-light c. The outcome is independent of the actual shape of the same wave-fronts and includes the case of gravitational waves which are characterized by an eikonal representation and propagate in a generic curved space-time along a null geodetics. The same waves are shown: (a) to correspond to the geometric-optics limit of the same curved space-time solutions; (b) to propagate in a flat space-time as plane waves with constant amplitude; (c) to display also the corresponding form of the wave-front in curved space-time. The result is consistent with the theory of the linearized Einstein field equations and the existence of gravitational waves achieved in such an asymptotic regime. Consistency with the non-linear Trautman boundary-value theory is also displayed.
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Wang, Tao, Roy Barkan, James C. McWilliams und M. Jeroen Molemaker. „Structure of Submesoscale Fronts of the Mississippi River Plume“. Journal of Physical Oceanography 51, Nr. 4 (April 2021): 1113–31. http://dx.doi.org/10.1175/jpo-d-20-0191.1.
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AbstractSubmesoscale currents (SMCs), in the forms of fronts, filaments, and vortices, are studied using a high-resolution (~150 m) Regional Oceanic Modeling System (ROMS) simulation in the Mississippi River plume system. Fronts and filaments are identified by large horizontal velocity and buoyancy gradients, surface convergence, and cyclonic vertical vorticity with along-coast fronts and along-plume-edge filaments notably evident. Frontogenesis and arrest/destruction are two fundamental phases in the life cycle of fronts and filaments. In the Mississippi River plume region, the horizontal advective tendency induced by confluence and convergence plays a primary role in frontogenesis. Confluent currents sharpen preexisting horizontal buoyancy gradients and initiate frontogenesis. Once the fronts and filaments are formed and the Rossby number reaches O(1), they further evolve frontogenetically mainly by convergent secondary circulations, which can be maintained by different cross-front momentum balance regimes. Confluent motions and preexisting horizontal buoyancy gradients depend on the interaction between wind-induced Ekman transport and the spreading plume water. Consequently, the direction of wind has a significant effect on the temporal variability of SMCs, with more active SMCs generated during a coastally downwelling-favorable wind and fewer SMCs during an upwelling-favorable wind. Submesoscale instabilities (~1–3 km) play a primary role in the arrest and fragmentation of most fronts and filaments. These instabilities propagate along the fronts and filaments, and their energy conversion is a mixed barotropic–baroclinic type with horizontal-shear instabilities dominating.
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Papritz, Lukas, Stephan Pfahl, Irina Rudeva, Ian Simmonds, Harald Sodemann und Heini Wernli. „The Role of Extratropical Cyclones and Fronts for Southern Ocean Freshwater Fluxes“. Journal of Climate 27, Nr. 16 (07.08.2014): 6205–24. http://dx.doi.org/10.1175/jcli-d-13-00409.1.
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Abstract In this study, the important role of extratropical cyclones and fronts for the atmospheric freshwater flux over the Southern Ocean is analyzed. Based on the Interim ECMWF Re-Analysis (ERA-Interim), the freshwater flux associated with cyclones is quantified and it is revealed that the structure of the Southern Hemispheric storm track is strongly imprinted on the climatological freshwater flux. In particular, during austral winter the spiraliform shape of the storm track leads to a band of negative freshwater flux bending toward and around Antarctica, complemented by a strong freshwater input into the midlatitude Pacific, associated with the split storm track. The interannual variability of the wintertime high-latitude freshwater flux is shown to be largely determined by the variability of strong precipitation (>75th percentile). Using a novel and comprehensive method to attribute strong precipitation uniquely to cyclones and fronts, it is demonstrated that over the Southern Ocean between 60% and 90% of the strong precipitation events are due to these synoptic systems. Cyclones are the dominant cause of strong precipitation around Antarctica and in the midlatitudes of the Atlantic and the Pacific, while in the south Indian Ocean and the eastern Atlantic fronts bring most of the strong precipitation. A detailed analysis of the spatial variations of intense front and cyclone precipitation associated with the interannual variability of the wintertime frequency of cyclones in the midlatitude and high-latitude branches of the Pacific storm track underpins the importance of considering both fronts and cyclones in the analysis of the interannual variability of freshwater fluxes.
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Hopkins, Jo, Peter Challenor und Andrew G. P. Shaw. „A New Statistical Modeling Approach to Ocean Front Detection from SST Satellite Images“. Journal of Atmospheric and Oceanic Technology 27, Nr. 1 (01.01.2010): 173–91. http://dx.doi.org/10.1175/2009jtecho684.1.
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Abstract Ocean fronts are narrow zones of intense dynamic activity that play an important role in global ocean–atmosphere interactions. Owing to their highly variable nature, both in space and time, they are notoriously difficult features to adequately sample using traditional in situ techniques. In this paper, the authors propose a new statistical modeling approach for detecting and monitoring ocean fronts from Advanced Very High Resolution Radiometer (AVHRR) SST satellite images that builds on a previous “front following” algorithm. Weighted local likelihood is used to provide a smooth, nonparametric description of spatial variations in the position, mean temperature, width, and temperature change of an individual front within an image. Weightings are provided by a Gaussian kernel function whose width is automatically determined by likelihood cross-validation. The statistical model fitting approach allows estimation of the uncertainty of each parameter to be quantified, a capability not possessed by other techniques. The algorithm is shown to be robust to noise and missing data in an image, problems that hamper many of the existing front-detection schemes. The approach is general and could be used with other remotely sensed datasets, model output, or data assimilation products.
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Han, Wenhu, Wenjun Kong, Yang Gao und Chung K. Law. „The role of global curvature on the structure and propagation of weakly unstable cylindrical detonations“. Journal of Fluid Mechanics 813 (19.01.2017): 458–81. http://dx.doi.org/10.1017/jfm.2016.873.
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The role of the global curvature on the structure and propagation of cylindrical detonations is studied allowing and without allowing the development of cellular structures through two-dimensional (2-D) and 1-D simulations, respectively. It is shown that as the detonation transitions from being overdriven to the Chapman–Jouguet (CJ) state, its structure evolves from no cell, to growing cells and then to diverging cells. Furthermore, the increased dimension of the average structure of the cellular cylindrical detonation, coupled with the curved transverse wave, leads to bulk un-reacted pockets as the cells grow, and consequently lower average propagation velocities as compared to those associated with smooth fronts. As the global detonation front expands and its curvature decreases, the extent of the un-reacted pockets diminishes and the average velocity of the cellular cylindrical detonation eventually degenerates to that of the smooth fronts. Consequently, the presence of cellular instability renders detonation more difficult to initiate for weakly unstable detonations.
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Rohe, K., und J. Cisternas. „Propagation failure in discrete reaction–diffusion system based on the butterfly bifurcation“. Chaos: An Interdisciplinary Journal of Nonlinear Science 32, Nr. 5 (Mai 2022): 053124. http://dx.doi.org/10.1063/5.0086239.
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Reaction–diffusion systems are used in biology, chemistry, and physics to model the interaction of spatially distributed species. Particularly of interest is the spatial replacement of one equilibrium state by another, depicted as traveling waves or fronts. Their profiles and traveling velocity depend on the nonlinearities in the reaction term and on spatial diffusion. If the reaction occurs at regularly spaced points, the velocities also depend on lattice structures and the orientation of the traveling front. Interestingly, there is a wide region of parameters where the speeds become zero and the fronts do not propagate. In this paper, we focus on systems with three stable coexisting equilibrium states that are described by the butterfly bifurcation and study to what extent the three possible 1D traveling fronts suffer from propagation failure. We demonstrate that discreteness of space affects the three fronts differently. Regions of propagation failure add a new layer of complexity to the butterfly diagram. The analysis is extended to planar fronts traveling through different orientations in regular 2D lattices. Both propagation failure and the existence of preferred orientations play a role in the transient and long-time evolution of 2D patterns.
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Heilman, Warren E., Kenneth L. Clark, Xindi Bian, Joseph J. Charney, Shiyuan Zhong, Nicholas S. Skowronski, Michael R. Gallagher und Matthew Patterson. „Turbulent Momentum Flux Behavior above a Fire Front in an Open-Canopied Forest“. Atmosphere 12, Nr. 8 (24.07.2021): 956. http://dx.doi.org/10.3390/atmos12080956.
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Atmospheric turbulent circulations in the vicinity of wildland fire fronts play an important role in the transfer of momentum into and out of combustion zones, which in turn can potentially affect the behavior and spread of wildland fires. The vertical turbulent transfer of momentum is accomplished via individual sweep, ejection, outward interaction, and inward interaction events, collectively known as sweep-ejection dynamics. This study examined the sweep-ejection dynamics that occurred before, during, and after the passage of a surface fire front during a prescribed fire experiment conducted in an open-canopied forest in the New Jersey Pine Barrens. High-frequency (10 Hz), tower-based, sonic anemometer measurements of horizontal and vertical wind velocity components in the vicinity of the fire front were used to assess the relative frequencies of occurrence of the different types of momentum-flux events, their contributions to the overall momentum fluxes, and their periodicity patterns. The observational results suggest that the presence of surface fire fronts in open-canopied forests can substantially change the sweep-ejection dynamics that typically occur when fires are not present. In particular, sweep events resulting in the downward transport of high horizontal momentum air from above were found to be more prominent during fire-front-passage periods.
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Commins, Morgan L., Isabelle Ansorge und Peter G. Ryan. „Multi-scale factors influencing seabird assemblages in the African sector of the Southern Ocean“. Antarctic Science 26, Nr. 1 (31.05.2013): 38–48. http://dx.doi.org/10.1017/s0954102013000138.
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AbstractOceanic fronts are important foraging areas for many top predators, but they also define biogeographical boundaries to animals in the Southern Ocean and play a role in structuring seabird assemblages. Understanding the factors driving patterns in the spatial and temporal distribution of seabirds is important to infer the likely impact of a changing climate. Latitudinal transects south of Africa in two summers indicate that fronts and sea ice extent play key roles in determining seabird assemblages. We observed 37 seabird taxa and found five seabird assemblages. The Subtropical Convergence and pack ice-edge form the strongest biogeographical boundaries, whereas the Sub-Antarctic Front and Antarctic Polar Front are less well defined. As summer progresses, the Southern Antarctic Circumpolar Current Front (the Antarctic Divergence or southern boundary of the Antarctic Circumpolar Current) becomes important, when a distinct seabird assemblage forms north of the retreating sea ice following an influx of great shearwatersPuffinus gravis(O'Reilly), blue petrelsHalobaena caerulea(Gmelin), Kerguelen petrelsLugensa brevirostris(Lesson) and southern fulmarsFulmarus glacialoides(Smith). Seabird assemblages show strong seasonality and are predictable between years. They are structured primarily by latitudinal gradients and secondarily by seasonal variation in sea-surface temperature and ice cover within their latitudinal habitat zones.
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Morel, Yves G., David S. Darr und Claude Talandier. „Possible Sources Driving the Potential Vorticity Structure and Long-Wave Instability of Coastal Upwelling and Downwelling Currents“. Journal of Physical Oceanography 36, Nr. 5 (01.05.2006): 875–96. http://dx.doi.org/10.1175/jpo2899.1.
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Abstract It is well known that upwelling and downwelling currents are unstable to perturbations. Less is, however, known about the physical mechanism responsible for the observed and modeled instabilities. It is shown that the origin of the long-wave barotropic/baroclinic instability observed on upwelling currents has to be sought among diabatic or thermobaric mechanisms. In particular, the role of mixing associated with Kelvin–Helmholtz instability and of wind forcing is investigated. Low Richardson numbers occur when the pycnocline outcrops at the sea surface. The criterion for instability (Ri ≤ 1/4) can be reached in a narrow region close to the upwelling front, permitting Kelvin–Helmholtz instability and mixing. This can precondition the current for long-wave instability by transforming the current's potential vorticity. A constant wind can likewise modify the potential vorticity. The resulting potential vorticity anomaly is always negative for both upwelling and downwelling currents, and this anomaly interacts with the outcropped front, destabilizing it. Examples are provided via numerical calculations using an idealized front. A wind stress is an effective means of inducing the negative PV necessary for instability; with wind, Kelvin–Helmholz instability, when present, merely modifies the instability characteristics. In addition, upwelling fronts are always less stable than comparable downwelling fronts.
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Qiu, Bo, Shuiming Chen und Niklas Schneider. „Dynamical Links between the Decadal Variability of the Oyashio and Kuroshio Extensions“. Journal of Climate 30, Nr. 23 (Dezember 2017): 9591–605. http://dx.doi.org/10.1175/jcli-d-17-0397.1.
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Rather than a single and continuous boundary current outflow, long-term satellite observations reveal that the Oyashio Extension (OE) in the North Pacific Subarctic Gyre comprises two independent, northeast–southwest-slanted front systems. With a mean latitude along 40°N, the western OE front exists primarily west of 153°E and is a continuation of the subarctic gyre western boundary current. The eastern OE front, also appearing along 40°N, is located between 153° and 170°E, whose entity is disconnected from its western counterpart. During 1982–2016, both of the OE fronts exhibit prominent decadal fluctuations, although their signals show little contemporaneous correlation. An upper-ocean temperature budget analysis based on the Estimating the Circulation and Climate of the Ocean, phase II (ECCO2), state estimate reveals that the advective temperature flux convergence plays a critical role in determining the low-frequency temperature changes relating to the OE fronts. Specifically, the western OE front variability is controlled by the decadal mesoscale eddy modulations in the upstream Kuroshio Extension (KE). An enhanced eddy activity increases the poleward heat transport and works to strengthen the western OE front. The eastern OE front variability, on the other hand, is dictated by both the meridional shift of the KE position and the circulation intensity change immediately north of the eastern OE. Different baroclinic adjustment speeds for the KE and OE are found to cause the in-phase changes between these latter two processes. Lack of contemporaneous correlation between the decadal western and eastern OE variability is found to be related to the interaction of the meridionally migrating KE jet with the Shatsky Rise near 159°E.
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Thomas, Leif N., und Callum J. Shakespeare. „A New Mechanism for Mode Water Formation involving Cabbeling and Frontogenetic Strain at Thermohaline Fronts“. Journal of Physical Oceanography 45, Nr. 9 (September 2015): 2444–56. http://dx.doi.org/10.1175/jpo-d-15-0007.1.
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AbstractA simple analytical model is used to elucidate a potential mechanism for steady-state mode water formation at a thermohaline front that involves frontogenesis, submesoscale lateral mixing, and cabbeling. This mechanism is motivated in part by recent observations of an extremely sharp, density-compensated front at the North Wall of the Gulf Stream. Here, the intergyre, along-isopycnal, salinity–temperature difference is compressed into a span of a few kilometers, making the flow susceptible to cabbeling. The sharpness of the front is caused by frontogenetic strain, which is presumably balanced by submesoscale lateral mixing processes. The balance is studied with the simple model, and a scaling is derived for the amount of water mass transformation resulting from the ensuing cabbeling. The transformation scales with the strain rate, equilibrated width of the front, and the square of the isopycnal temperature contrast across the front. At the major ocean fronts where mode waters are found, this isopycnal temperature contrast decreases with increasing density near the isopycnal layers where mode waters reside. This implies that cabbeling should result in a convergent diapycnal mass flux into mode water density classes. The scaling for the transformation suggests that at these fronts the process could generate 0.01–1 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1) of mode water. These formation rates, while smaller than mode water formation by air–sea fluxes, should be independent of season and thus could fill select isopycnal layers continuously and play an important role in the dynamics of mode waters on interannual time scales.
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Bosse, Anthony, Pierre Testor, Pierre Damien, Claude Estournel, Patrick Marsaleix, Laurent Mortier, Louis Prieur und Vincent Taillandier. „Wind-Forced Submesoscale Symmetric Instability around Deep Convection in the Northwestern Mediterranean Sea“. Fluids 6, Nr. 3 (17.03.2021): 123. http://dx.doi.org/10.3390/fluids6030123.
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During the winter from 2009 to 2013, the mixed layer reached the seafloor at about 2500 m in the northwestern Mediterranean Sea. Intense fronts around the deep convection area were repeatedly sampled by autonomous gliders. Subduction down to 200–300 m, sometimes deeper, below the mixed layer was regularly observed testifying of important frontal vertical movements. Potential Vorticity dynamics was diagnosed using glider observations and a high resolution realistic model at 1-km resolution. During down-front wind events in winter, remarkable layers of negative PV were observed in the upper 100 m on the dense side of fronts surrounding the deep convection area and successfully reproduced by the numerical model. Under such conditions, symmetric instability can grow and overturn water along isopycnals within typically 1–5 km cross-frontal slanted cells. Two important hotpspots for the destruction of PV along the topographically-steered Northern Current undergoing frequent down-front winds have been identified in the western part of Gulf of Lion and Ligurian Sea. Fronts were there symmetrically unstable for up to 30 days per winter in the model, whereas localized instability events were found in the open sea, mostly influenced by mesoscale variability. The associated vertical circulations also had an important signature on oxygen and fluorescence, highlighting their under important role for the ventilation of intermediate layers, phytoplankton growth and carbon export.