Advection Mechanism Research Articles

North Atlantic Deep Water and Antarctic Bottom Water: Their interaction and influence on the variability of the global ocean circulation

Interhemispheric signal transmission in the Atlantic Ocean connects the deep water production regions of both hemispheres. The nature of these interactions and large‐scale responses to perturbations on timescales of years to millennia have been investigated using a global three‐dimensional ocean general circulation model coupled to a dynamic‐thermodynamic sea ice model. The coupled model reproduces many aspects of today's oceanic circulation. A set of experiments shows the sensitivity to changes in different surface boundary conditions. Buoyancy changes in the Weddell and Labrador Seas exert a direct effect on the overturning cells of the respective hemisphere. They influence the density structure of the deep ocean and thereby lead to alterations in the strength of the ACC. Changing the wind stress south of 30°S influences the magnitude of the deep water production of both hemispheres. The interhemispheric effect in these experiments cannot be explained solely by advective mechanisms. Switching off the wind stress over the latitude band of the Drake Passage leads to a slow gradual decrease of the water mass transport in the ACC, resulting in an almost complete cessation. The model results prove the necessity to continue integrations over thousands of years until new equilibria are established.

VELOCITY MEASUREMENTS AROUND NON-SUBMERGED AND SUBMERGED SPUR DYKES BY MEANS OF LARGE-SCALE PARTICLE IMAGE VELOCIMETRY

Field observations on the surface flow were carried out in a straight reach of the Uji River. Velocity distributions on the water surface were obtained based on the captured video images by means of large-scale particle image velocimetry (LSPIV). Flow structures around spur dykes are studied for both non-submerged and submerged cases.It is clearly seen that, for both cases, the dykes are working as an additional roughness. Their effect on channel conveyance capacity is estimated in terms of the effective channel width calculated from the velocity distributions. It is also of interest that the maximum velocity filament appears at a different position as the water depth varies in case of the channel with single-side dykes such as studied here. In addition, large-scale boils shedding from the submerged dykes are clearly observed and their origin and advection mechanisms are also evaluated.

Some features of chaotization of a pulsating barotropic flow over a seamount with elliptic cross-section

We consider a barotropic inviscid model of chaotic advection in a unidirectional pulsating background flow over a seamount of an elliptic form. We numerically study the process of passive tracer transport from the vortex area to the flow-through area, and in particular we give the dependence of evolution of the corresponding Poincaré maps on the frequency and amplitude of incident flow pulsations. We propose an approach to the study of the mechanism and parameters of chaotic advection in open systems with finite trajectories lifetime. It is based on studying the time it takes for tracers to be carried out from the vortex area to the flow-through area. We show the essential impact of the seamount orientation with respect to an incident flow on the rate and scenario of tracers transport from the vortex area.

Mathematical Model of One-dimensional Miscible Fluid Injection in Fractured Porous Media

A mathematical model for miscible displacement in fractured porous media is developed. The model takes into account advective, gravitational and crossflow mechanisms of mass exchange between fracture and matrix. The model is normalized by using the dimensionless parameters which characterize the process, and the analytical solutions of the resulting system of equations are provided by utilizing the method of characteristics. The model developed has been compared with experimental results and with previous model, which includes only crossflow between fracture and matrix. There is very good agreement between experiments and this model prediction.

Mixing in the Stokes flow in a cylindrical container

Kinematic features of three–dimensional mixing by advection of passive particles in time–periodic flows are the primary subject of this study. A classification of periodic points, providing important information about the mixing properties of a flow, is presented, and the dynamics of the Poincare map in the vicinity of periodic points is analysed for all identified types. Three examples of Stokes flow in a finite cylindrical cavity with discontinuous periodic motion of its end walls are used to illustrate the determination of both periodic lines and isolated periodic points in the flow domain. The stable and unstable manifolds of points on the periodic lines create two surfaces in the flow. A numerical technique based on tracking of a material surface is presented to study the manifold surfaces and their intersections. It is illustrated with numerical examples that flows with periodic lines possess only quasi–two–dimensional mechanisms of chaotic advection.

Gneiss domes and orogeny

Many gneiss domes record positive feedback between decompression and partial melting of orogenic middle crust. Exhumed orogens are riddled with gneiss domes cored by migmatites that underwent dehydration melting during decompression. The decreasing buoyancy associated with increasing melt fraction drives further decompression at near-isothermal conditions as the partially molten crust rises diapirically. This combination of processes may explain the generation and retention of large volumes of crustally derived melt recorded in many deep-seated migmatite terranes and inferred for active orogens. In exhumed orogens, the signature of the rapid ascent of partially molten crust is a gneiss dome cored by migmatite ± granite. The large volume of material involved in the vertical transfer of partially molten crust indicates that the formation of gneiss domes is an efficient mechanism for heat advection during orogenesis.

Potential artifacts in interpretation of differential breakthrough of colloids and dissolved tracers in the context of transport in a zero-valent iron permeable reactive barrier.

Many published studies have used visual comparison of the timing of peak breakthrough of colloids versus conservative dissolved tracers (hereafter referred to as dissolved tracers or tracers) in subsurface media to determine whether they are advected differently, and to elucidate the mechanisms of differential advection. This purely visual approach of determining differential advection may have artifacts, however, due to the attachment of colloids to subsurface media. The attachment of colloids to subsurface media may shift the colloidal peak breakthrough to earlier times, causing an apparent "faster" peak breakthrough of colloids relative to dissolve tracers even though the transport velocities for the colloids and the dissolved tracers may actually be equivalent. In this paper, a peak shift analysis was presented to illustrate the artifacts associated with the purely visual approach in determining differential advection, and to quantify the peak shift due to colloid attachment. This peak shift analysis was described within the context of microsphere and bromide transport within a zero-valent iron (ZVI) permeable reactive barrier (PRB) located in Fry Canyon, Utah. Application of the peak shift analysis to the field microsphere and bromide breakthrough data indicated that differential advection of the microspheres relative to the bromide occurred in the monitoring wells closest to the injection well in the PRB. It was hypothesized that the physical heterogeneity at the grain scale, presumably arising from differences in inter- versus intra-particle porosity, contributed to the differential advection of the microspheres versus the bromide in the PRB. The relative breakthrough (RB) of microspheres at different wells was inversely related to the ionic strength of ground water at these wells, in agreement with numerous studies showing that colloid attachment is directly related to solution ionic strength.

A Further Study of the Mechanisms of Cell Regeneration, Propagation, and Development within Two-Dimensional Multicell Storms

The mechanisms of cell regeneration, development, and propagation within a two-dimensional multicell storm proposed by Lin, Deal, and Kulie (hereafter LDK) were further investigated by conducting a series of sensitivity tests. LDK’s advection mechanism was reexamined by performing simulations utilizing a plateau with five additional wind profiles having a wider range of shear. All five cases gave results that show that the cell regeneration period decreases with the storm-relative midlevel inflow, similar to that proposed by LDK. It was also found that a rigid lid is not an appropriate upper boundary condition for multicell storm simulations. In order to test whether the advection mechanism is responsible for cell regeneration with a different sounding, an idealized sounding was used. A multicell storm was produced along with a strong density current and gust front updraft. Investigation of this storm supports the advection mechanism within the growing mode and a gravity wave mechanism in the propagation mode, as proposed by LDK. From further investigation, the relaxation mechanism proposed by Fovell and Tan was shown to exist within these simulations yet found to be dependent on the advection mechanism to cause cell regeneration. To avoid some problems that occurred when using a plateau, a prescribed heat sink was used to produce a more realistic density current. This experiment demonstrates that the advection mechanism is responsible for cell regeneration and the gravity wave mechanism is responsible for cell propagation within the storm. It was found that without precipitation loading, an individual cell is still able to split. In this case, the compensating downdraft produced by vertical differential advection is responsible for cell splitting and merging.

On the hydro-dispersive equivalence between multi-layered mineral barriers

In the context of municipal solid waste and hazardous waste disposal, the notion of “equivalence” between different barrier designs appears in regulatory documents from several industrialized countries. While in the past, equivalence has been thought of mainly in terms of contaminant travel times, in recent years it has been defined more in terms of the magnitude of a disposal site's potential impact on groundwater resources. This paper presents some original analytical solutions to the problem of contaminant migration through a multi-layered mineral barrier. The solutions account for the two major mechanisms of subsurface contaminant migration, namely, advection and diffusion–dispersion. An example application using the proposed solutions and a numerical model illustrates how one multi-layered mineral barrier can be considered superior to another from a strictly hydro-dispersive viewpoint. The influence of partial saturation of the mineral barrier is investigated using a numerical solution to the Richards equation for unsaturated flow. It is emphasized that conclusions relative to the superiority of one multi-layered barrier, with respect to another, should not only consider hydro-dispersive aspects, but also other processes such as the mechanical and chemical evolutions of the different barrier components. Although such phenomena are poorly addressed by existing models, failure to take them into account, at least in a qualitative fashion, may lead to unconservative conclusions with respect to barrier equivalence.

Natural convection in a confined saturated porous medium with horizontal temperature and vertical solutal gradients

Double-diffusive natural convection in a horizontal enclosure filled with saturated porous medium is investigated numerically. Brinkman extension of Darcy model is adopted in the analysis. The enclosure is heated and cooled along vertical walls and solutal gradient is imposed vertically. The objective of the work is to understand the physics of the flow and to identify the flow regimes for thermal and solutal dominated flows. Since the number of parameters is large, the results are reported for an aspect ratio of two, Prandtl number of 0.71 (air) and Lewis number of 10 (hydrocarbon fuel). Grashof and Darcy numbers may vary between 10 6–10 8 and 10 −4–10 −6, respectively. It is found that the flow becomes unstable for finite range of solutal-to-thermal buoyancy ratio and it is possible to obtain different solutions in this region, depending on the initial condition. Funnel-type channelling flow is predicted along the vertical boundaries for thermally driven flow. Also, for thermally driven flow, concentration reversal is possible due to the thermal advection mechanism. Strong stratified fluid may suppress the thermal convection and heat transfer becomes conductive.

Marangoni Instability in a Finite Container-Transition Between Short and Long Wavelengths Modes

Marangoni instability in a finite container with a deformable interface in the absence of gravity has been investigated. It is shown that the critical Marangoni number Macr is a non-monotonic function of the length of the container. Two different physical mechanisms driving convection are indicated. The advection of heat is essential for the first, advective (“classical”) mechanism that gives rise to short wavelength modes. The interface deformation is essential for the second mechanism that gives rise to long wavelength modes. If the container is sufficiently long, the second mechanism leads to an unconditional instability. The available results suggest that the unconditional instability leads to segmentation of the interface.

On the Mechanism of the Antarctic Circumpolar Wave

Abstract The variability in the subpolar Southern Hemisphere is studied with a coupled atmosphere–ocean–sea-ice model (the ECBilt). After having reached an approximate statistical equilibrium in coupled mode without flux corrections, a subsequent 1000-yr integration is performed and analyzed. A singular value decomposition of austral winter SST anomalies and 800-hPa geopotential height in the Antarctic Circumpolar Current region reveals a mode of covariability that resembles the observed Antarctic circumpolar wave. Subsequent analysis of this mode shows that it is basically an oscillation in the subsurface of the ocean. Additional experiments suggest that it is generated by the advective resonance mechanism: the oscillation is excited by the dominant modes of variability in the atmosphere, whereas the timescale is set by the ratio of the horizontal scale of these atmospheric modes and the advection velocity of the mean oceanic currents. The atmospheric response mainly consists of a local temperature adjus...

Serpuloidea (Annelida: Polychaeta) from Milos, an island in the Aegean Sea with submarine hydrothermalism

Serpuloidea were collected by diving within 45 m depth, by indirect sampling in deeper waters (50 and 81 m), and among the fouling settled on oceanographic instruments at various depths (10, 50, 80 and 83 m depth). A total of 33 species or subspecific taxa was found: 25 Serpulidae and eight Spirorbidae. All the species collected are already known for the western Mediterranean and have Atlantic–Mediterranean or worldwide distribution. Several of these, however, may be species-complexes hiding species with restricted geographic ranges. No Lessepsian migrants were found. The number of species found at vent sites was significantly higher than that found at non-vent sites, although no vent-obligate species were recognized. Hydrothermal vents might influence serpuloidean richness through four main mechanisms: (i) increasing food sources to these filter-feeders, due to the chemiosynthetic production by vent microbiota; (ii) enhancing the development of biogenic carbonate mounds, which provide habitats for encrusting and cryptic species; (iii) inducing advective mechanisms that concentrate larval stages in the vicinity of vents and thus favouring recruitment; (iv) creating, through the periodic emission of toxic fluids, a regime of `intermediate disturbance' that allows a larger number of species to coexist.

On the influence of bottom topography and the Deep Western Boundary Current on Gulf Stream separation

The Gulf Stream separates abruptly from the North American coastline at Cape Hatteras. The absence of significant seasonal and interannual variability in the separation point, compared with that of other separating boundary currents, suggests that Gulf Stream separation is locally controlled. In this paper we consider the possible influence of bottom topography and the Deep Western Boundary Current (DWBC), which descends underneath the Gulf Stream at Cape Hatteras. The path of the DWBC is strongly constrained by bottom topography. At Cape Hatteras, the continental shelf widens and the DWBC is forced to swing offshore and pass beneath the Gulf Stream. Three possible mechanisms by which bottom topography and the DWBC can affect the separation of the Gulf Stream are proposed and investigated: (i) topography modifies the background potential vorticity contours; (ii) the DWBC advects the Gulf Stream separation point southward; (iii) intense downwelling as the DWBC passes beneath the Gulf Stream induces an adverse pressure gradient in the Gulf Stream, leading to its separation. Results from a series of idealized numerical experiments with a geostrophic vorticity model are presented to investigate these mechanisms. Topography alone does have an impact on the separation point, broadly consistent with modification of the background potential vorticity. We also show that the presence of a DWBC does, indeed, push the time average separation of the Gulf Stream farther southward, consistent with both the advection and adverse pressure gradient mechanisms. However, the time-dependent boundary current separation is more nonlinear than suggested by each of the above mechanisms, undergoing a series of abrupt transitions between northern and southern separation states. As the DWBC transport is increased, the southern separation state is occupied more and more frequently.

Modelling Sand Resuspension by Waves over a Rippled Bed

With a two-dimensional k-ϵ model and equations to describe sediment entrainment, diffusion, convection and settling, wave-induced flow velocities and sediment concentration fields over rippled beds have been calculated. Results from the simulations are compared with time-averaged vertical suspended sediment concentration profiles, C-profiles, measured in a large wave flume using the autonomous multi-sensor instrument STABLE. The simulations make it possible to examine in detail the mechanisms of sediment entrainment, vortex formation and advection. The resulting animations of model output allow a quasi-3-D visualization of sediment resuspension processes and aid quantification of the resulting vertical suspended sediment concentration field due to regular waves, asymmetric waves and wave groups. The work contributes therefore, to knowledge about wave-sediment interactions and to understanding of sediment transport mechanics.

Climatic controls on interannual variability of precipitation δ18O: Simulated influence of temperature, precipitation amount, and vapor source region

We use an atmospheric GCM that incorporates stable isotopes and regional vapor source tracers in the hydrologic cycle to explore the relationship between interannual variability in climate and precipitation δ18O globally. On the basis of a 12‐year simulation forced by observed sea surface temperatures (SSTs), we identify changes in the amount of precipitation and in the contributions of local and nearby vapor sources as the most important determinants of simulated interannual isotopic changes. The model simulates weak positive correlation between temperature and isotopic variability only in certain continental regions, mostly in the extratropics. Comparison with long observed records of isotopes and climate indicates that the model simulates realistic patterns of temperature‐isotope correlation but may overestimate the isotopic influence of precipitation amount. Perturbations in circulation patterns that alter the transport and mixing of air masses at a site also change the relative contributions of vapor from different source regions. Simulated changes in vapor source regions are large, reaching ±10–15% of the total precipitation, and cause significant isotopic variability in nearly all grid cells. Our results suggest that shifts among vapor sources may provide an important control on the interannual isotopic variability observed in modern precipitation and paleoclimatic records. The isotopic variability simulated in this experiment results from the interaction of several aspects of climate. Interannual temperature variability generally involves circulation changes that alter air mass transport, vapor source regions, and condensation history; this advective mechanism may explain the relative weakness of temperature‐isotope correlations in both the model and the observations.

Thermal evolution of comet nuclei

The properties of cometary surfaces which influence the thermal behavior of the nucleus such as the albedo, the emissivity and the surface temperature are critically reviewed. It is shown that these properties are less firmly established than generally believed. A brief presentation is given of the state of the art in comet modelling. It is concluded that models which are supposed to give an adequate desription of cometary activity must take into account short term variations in local temperature due to the nucleus rotation and due to the orientation of the rotation axis. The activity can also be strongly influenced by the nucleus topography. If comets are porous it is further necessary to include all type of transport mechanisms of heat conduction, advection, local deposition and extraction of latent heat and all mechanisms of redistribution of volatile matter (advection, diffusion, sublimation, recondensation). For porous nuclei, no direct relationship between the coma composition and the abundance ratio in the nucleus can be established. An adequate description of the formation and growth of dust mantles can only be obtained from multdimensional models running over several revolutions with time steps much smaller than the rotation period.

Drift Velocity Concept for Sediment-Laden Flows

The vertical distribution of suspended sediment concentration in open channel flows is usually described by the advection-diffusion equation. Hence, only the mechanisms of advection and diffusion affect the movement of sediment particles. However, recent publications show that the equation could not describe fully the sediment concentration distribution in open channels. A new model for particle migration in sediment-laden turbulent flow is presented. The model assumes there is an additional mechanism influencing the particle movement in sediment-laden flows. A new mechanism, which is called the drift, acts independently of advection and turbulent diffusion. Based on all three mechanisms, a new equation for the concentration distribution is derived. The equation describes the vertical distribution of sediment concentration more accurately than the advection-diffusion equation.

Chlorpyrifos Release Rate from Clay Granules: Experimental Observations and Simple Algorithm Development for Use in Computer-Based Exposure Assessments

Environmental models can approximate pesticide mass leaving a treated field in runoff water and sediment but do not typically account for controlled-release formulations such as granules. Mathematical algorithms are developed from experimental observations to predict the environmental release rate of chlorpyrifos from a commercial formulation [Lorsban (trademark of Dow AgroSciences) 15G insecticide] into the surrounding soil environment. Chlorpyrifos release rates into flowing water were measured by HPLC analysis of water pumped through a column containing 15G granules. Analysis of these observations provides a basis for predicting water-induced release rates during natural precipitation events. Chlorpyrifos diffusion and volatilization rates from the granule in the absence of precipitation were determined from experimental observations using first-order kinetic modeling. Multiyear simulations and distribution analysis were performed using the GLEAMS model for the Midwestern corn belt region to determine release mechanism sensitivity and edge-of-field runoff potential for chlorpyrifos in 15G granules when the release mechanisms of advection, volatilization, and diffusion are accounted for. It is found that advection release was the dominate mechanism for chlorpyrifos release from clay granules, followed by diffusion and volatilization for Midwestern U.S. scenarios. Predicted losses of chlorpyrifos in runoff from soil incorporated applications are <1% of applied.

Mechanisms of Cell Regeneration, Development, and Propagation within a Two-Dimensional Multicell Storm

In this study, mechanisms of cell regeneration, development, and propagation within a two-dimensional multicell storm are investigated using a numerical cloud model. The cell regeneration is explained by the advection mechanism. The following processes occur periodically during cell regeneration: (i) Near the edge of the gust front, the gust front updraft is formed by low-level convergence ahead of the gust front near the surface. (ii) The upper portion of the gust front updraft grows by midlevel inflow since the gust front propagates faster than the basic wind. (iii) The growing cell tends to produce and is flanked by strong compensating downdrafts. The upstream downdraft tends to cut off the growing cell from the gust front updraft. It is found that the period of cell regeneration is inversely proportional to the midlevel, strong relative wind speed. This advection mechanism is different from that proposed by Yang and Houze, which views the rearward propagating cell as gravity waves generated by the quasi-steady updraft moving through the ambient flow. Cell development and propagation within a two-dimensional multicell storm may be described in terms of two distinctive modes: (i) a growing mode and (ii) a propagating mode. When a growing cell reaches its maximum intensity, it splits and then propagates downstream without amplification. The dynamics of cell development and propagation is explained here by critical level argument. For the growing mode there is growth because of a conditionally unstable environment leading to steering level propagation, while for the propagating mode there is no growth because of a more stable environment leading to propagation relative to the flow (i.e., absence of critical level). It is found that the phase relationship between w′ and θ′ (w′ and u′) in the growing mode is different from that in the propagating mode and can be explained by the dominance of latent heating in the thermodynamic equation. The propagating mode is dominated by horizontal advection. The propagating mode exhibits gravity wave properties and propagates faster than the growing mode.