Advective Motion Research Articles

Convection and Diffusion in Patterns in Oscillated Granular Media

Motions of individual particles within the stripe and square patterns formed in oscillated granular media are studied using numerical simulations. Our event-driven molecular dynamics simulations yield standing wave patterns in good accord with those observed in experiments at the same frequency and acceleration amplitude. The patterns are subharmonic and so return to their initial macroscopic state after two external cycles. However, simulations reveal that individual particles do not return to their initial position. In addition to diffusive motion, an organized flow of particles within the patterns is found; associated with each peak and each valley of the pattern is a pair of counterrotating convection rolls. The diffusion is anisotropic: transport perpendicular to stripes is enhanced over that parallel to stripes. This enhancement is computed as a function of the layer depth, acceleration amplitude, frequency, and coefficient of restitution of the particles, and is attributed to the effect of the advective motion. Velocity distributions, granular temperature, and the dependence of the diffusion coefficient parallel to the stripes on the average granular temperature are studied.

An Exact Solution for Steady State Magnetic Reconnection in Three Dimensions

An exact three-dimensional solution is derived for the steady state magnetic reconnection of incompressible, resistive plasmas. The analysis provides a natural extension of the analytic, two-dimensional reconnection solution of Craig & Henton. The solution shows how advective motions through the separatrix spine-curve lead to global current sheets aligned to the separatrix fan.

Comparison of satellite and drifter surface flow estimates in the northwestern Gulf of Mexico

A comparison of flow observations has been made with drifting buoys and infrared satellite images for northwestern Gulf of Mexico surface waters during the interval from 10 March to 23 April, 1989. Each drifter was drogued to a depth of 2.7 m and reported four-nine positions along its trajectory every 24 h through the NOAA satellite Argos data collection system. Corresponding flow observations were computed from sequential infrared imagery obtained by Advanced Very-High Resolution Radiometer (AVHRR) on the same satellites. Vector estimates of advective motion were extracted from surface temperature pattern displacements between images paired at 24 and 12 h intervals without knowledge of the drifter trajectories. Twenty-two blind test cases at 24 h intervals formed the data base for examining the relative accuracy between trajectories simulated with satellite flow estimates and those obtained with the drifters. The comparison of actual and computed velocities for 24 h displacements resulted in a counterclockwise regression angle of simulated to actual velocities of −1.7°, a standard error in estimates of simulated speeds from actual speeds of 5.5 cm s −1 for actual speeds in the range from 0 to 60 cm s −1, and a regression complex coefficient of determination ( ρ 2) of 0.936. The study indicates a convergence of results for the satellite image and Argos drifter methods of surface flow estimation and a combined capacity to investigate inertial range circulation variability.

Improvements to models and methods for evaluating the land-surface energy balance and ‘effective’ roughness using radiosonde reports and satellite-measured ‘skin’ temperature data

In this paper, we expand on a technique developed previously which uses radiosonde measurements of the daytime change of the height of the planetary boundary layer in combination with geostationary satellite measurements of surface skin temperature to evaluate the energy balance and an ‘effective’ roughness of the land surface. The improvements described here are primarily designed to increase the amount of usable data produced by the system by expanding the range of circumstances under which measurements can be taken. Previously, to make surface evaluations it was necessary that the local time-change of the surface temperature and PBL structure be dominated by the diurnal surface flux exchanges. In this paper, we test several techniques to compensate for the effects of horizontal and vertical temperature advection and vertical motions above the planetary boundary layer. The improved techniques and the sensitivities of the determination of the surface energy balance and effective roughness to advection and vertical motions are investigated in a case study including 3 days of data for locations in the Midwest and Great Plains areas of continental USA in the summers of 1987 and 1988.

Local magnetohydrodynamic instabilities and the wave-driven dynamo in accretion disks

view Abstract Citations (49) References (30) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Local Magnetohydrodynamic Instabilities and the Wave-driven Dynamo in Accretion Disks Vishniac, Ethan T. ; Diamond, Patrick Abstract We consider the consequences of magnetic buoyancy and the magnetic shearing instability (MSI) on the strength and organization of the magnetic field in a thin accretion disk. We discuss a model in which the wave-driven dynamo growth rate is balanced by the dissipative effects of the MSI. As in earlier work, the net helicity is due to small advective motions driven by nonlinear interactions between internal waves. Assuming a simple model of the internal wave spectrum generated from the primary m = 1 internal waves, we find that the magnetic energy density saturates at about (H/r) exp 4/3 times the local pressure (where H is the disk thickness and r is its radius). On very small scales the shearing instability will produce an isotropic fluctuating field. For a stationary disk this is equivalent to a dimensionless 'viscosity' of about (H/r) exp 4/3. The vertical and radial diffusion coefficients will be comparable to each other. Magnetic buoyancy will be largely suppressed by the turbulence due to the MSI. We present a rough estimate of its effects and find that it removes magnetic flux from the disk at a rate comparable to that caused by turbulent diffusion. Publication: The Astrophysical Journal Pub Date: October 1992 DOI: 10.1086/171881 Bibcode: 1992ApJ...398..561V Keywords: Accretion Disks; Dynamo Theory; Magnetohydrodynamic Stability; Stellar Physics; Buoyancy; Computational Astrophysics; Interstellar Magnetic Fields; Magnetohydrodynamic Turbulence; Propagation Modes; Viscosity; Astrophysics; ACCRETION; ACCRETION DISKS; INSTABILITIES; MAGNETOHYDRODYNAMICS: MHD full text sources ADS |

Rotational and gravitational influences on the degree of balance in the shallow-water equations

Abstract Numerical solutions of the Shallow-Water Equations with random, balanced initial conditions are analyzed for their persistence of balance. The standard for assessing balance is provided by the pressure and velocity potential fields determined diagnostically from the instantaneous streamfunction through the momentum divergence (i.e., gradient-wind balance) and omega relations of the Balance Equations. The amplitude, wavenumber and frequency content of both the balanced and unbalanced components are examined for a wide range of Rossby R, and Froude F, numbers. These properties are found to be generally consistent with the estimates from a formal scaling analysis for small R and F, and the scaling estimates are apt even when these parameters are not particularly small. The unbalanced component is dominated by higher order advective motion for R ≪ 1 and by nearly linear inertia-gravity waves, whose frequencies are larger than those of the balanced component, for R ≫ l. These waves are forced by nonlinear interaction with advective terms formally of O(F 2) which are neglected in the Balance Equations. These results support the hypotheses that the Balance Equations are an accurate model for the advective component of the flow (also called the slow manifold), broadly throughout the regimes of significant environmental rotation and/or stable stratification, and that the dynamical coupling is weak with the small-amplitude, fast wave component, especially so for R ≪ l.

Increased mass transfer to microorganisms with fluid motion

The effect of fluid flow and laminar shear on bacterial uptake was examined under conditions representative of the fluid environment of unattached and attached cells in wastewater treatment bioreactors. Laminar shear rates below 50 s(-1) did not increase leucine uptake by suspended cultures of Zoogloea ramigera. However, leucine uptake by cells fixed in a flow field of approximately 1 mm s(-1) was 55-65% greater than uptake by suspended cells. Enhanced microbial uptake with advective motion is consistent with mass transfer rates calculated using Sherwood number correlations. Advective flow increases microbial uptake by increasing collisions between substrate molecules and cells through compression of the concentration boundary layer surrounding a cell. The rate of leucine uptake suggests that binding proteins used to transport leucine into the cell can occupy approximately 1% of the cell surface area.

Convection in the atmospheric boundary layer

Recent advances in the study of clear and partly cloudy convective boundary layers are reviewed. The techniques and results of observational studies of the structure and dynamics of convective elements are discussed. These findings have important consequences for pollution dispersion. They also form the basis for a test of LES dispersion models which is more discriminating than those based on replication of observed turbulence statistics. Several new approaches to CBL similarity theory and parameterization have appeared. They avoid some of the problems associated with the over-simplification of previous techniques. Particularly noteworthy is the ability to handle counter-gradient fluxes with relatively simple models. Progress towards parameterization of CBL turbulence in the presence of horizontal inhomogeneity of the atmosphere or surface has been slower. Efforts to locate the limits of validity of the assumption of horizontal homogeneity have been more successful than the efforts to develop parameterizations which do not rely upon it. Mesoscale models need CBL parameterizations which are valid over complex terrain and in the presence of strong differential advection and mean vertical motion. LES models may prove useful in producing the data sets on which such parameterizations could be based.

Isotopic fractionation during ion filtration: I. Theory

Fractionation of the isotopes of H, O, and Cl during flow of aqueous solutions through semipermeable clay discs has been observed in the laboratory. The observed isotopic abundance trends of these elements in deep sedimentary basins have been attributed to similar phenomena. However, the mechanism responsible for such isotopic fractionation has not been identified. We propose that the fractionation of O and H isotopes in the water molecule results from increased activity of the heavy isotopic species in the membrane solution because high cation concentrations in the membrane pore fluid increase hydrationsphere fractionation effects. The heavy isotopic species thus diffuse away from the membrane and therefore are concentrated behind it. Predictions based on this theory satisfactorily model both laboratory and field observations. We propose that the fractionation of chlorine isotopes is a result of differences in the ionic mobility of 35Cl − and 37Cl −. Negative charges on the membrane repel the isotopic species at slightly different velocities against the uniform advective motion of the bulk water and thus the more mobile 35Cl − is enriched behind the membrane. Predictions calculated from this theory compare well with laboratory and field data.

Modeling of Ion-Exchange Column Operation. I. Equilibrium Model for Univalent-Divalent Exchange

Abstract A mathematical model is presented for the operation of an ion-exchange column exchanging divalent cations for univalent cations. Local equilibrium is assumed to be maintained between the ion-exchange resin and the aqueous phase. The cubic equations arising from the assumption of local equilibrium are solved by Tartaglia's method. The two partial differential equations describing the advective motion of the ions are approximated by two sets of coupled ordinary differential equations. The advective terms in these are represented by an asymmetrical “upwind” algorithm which greatly reduces spurious numerical dispersion.

On the use and significance of isentropic potential vorticity maps

AbstractThe two main principles underlying the use of isentropic maps of potential vorticity to represent dynamical processes in the atmosphere are reviewed, including the extension of those principles to take the lower boundary condition into account. the first is the familiar Lagrangian conservation principle, for potential vorticity (PV) and potential temperature, which holds approximately when advective processes dominate frictional and diabatic ones. the second is the principle of ‘invertibility’ of the PV distribution, which holds whether or not diabatic and frictional processes are important. the invertibility principle states that if the total mass under each isentropic surface is specified, then a knowledge of the global distribution of PV on each isentropic surface and of potential temperature at the lower boundary (which within certain limitations can be considered to be part of the PV distribution) is sufficient to deduce, diagnostically, all the other dynamical fields, such as winds, temperatures, geopotential heights, static stabilities, and vertical velocities, under a suitable balance condition. the statement that vertical velocities can be deduced is related to the well‐known omega equation principle, and depends on having sufficient information about diabatic and frictional processes. Quasi‐geostrophic, semigeostrophic, and ‘nonlinear normal mode initialization’ realizations of the balance condition are discussed. an important constraint on the mass‐weighted integral of PV over a material volume and on its possible diabatic and frictional change is noted.Some basic examples are given, both from operational weather analyses and from idealized theoretical models, to illustrate the insights that can be gained from this approach and to indicate its relation to classical synoptic and air‐mass concepts. Included are discussions of (a) the structure, origin and persistence of cutoff cyclones and blocking anticyclones, (b) the physical mechanisms of Rossby wave propagation, baroclinic instability, and barotropic instability, and (c) the spatially and temporally nonuniform way in which such waves and instabilities may become strongly nonlinear, as in an occluding cyclone or in the formation of an upper air shear line. Connections with principles derived from synoptic experience are indicated, such as the ‘PVA rule’ concerning positive vorticity advection on upper air charts, and the role of disturbances of upper air origin, in combination with low‐level warm advection, in triggering latent heat release to produce explosive cyclonic development. In all cases it is found that time sequences of isentropic potential vorticity and surface potential temperature charts—which succinctly summarize the combined effects of vorticity advection, thermal advection, and vertical motion without requiring explicit knowledge of the vertical motion field—lead to a very clear and complete picture of the dynamics. This picture is remarkably simple in many cases of real meteorological interest. It involves, in principle, no sacrifices in quantitative accuracy beyond what is inherent in theconceptof balance, as used for instance in the initialization of numerical weather forecasts.

Effects of Diabatic Heating on the Ageostrophic Circulation of an Upper Tropospheric Jet Streak

The interaction between the mass circulations within a mesoscale convective complex (MCC) and the entrance region of an upper tropospheric polar jet streak is examined to investigate mechanisms responsible for linking these two scales of motion. During NASA's fourth Atmospheric Variability Experiment (AVE IV), maximum wind speeds within a jet streak increased nearly 15 m s−1 over three to six hours as the jet streak propagated eastward over the Great Lakes region. Severe convection located on the rear anticyclonic flank of the jet streak within the direct circulation of the entrance region also intensified and increased in areal extent. The results analyzed within isentropic coordinates establish that latent heating in the MCC modified the direct mass circulation in the jet streak entrance region through the forcing of diabatic components of ageostrophic motion. The net isallobaric ageostrophic component in the entrance region, determined through the gradients of differential heating and mass flux, exceeded 4 m s−1. The mass divergence in the upper troposphere was due to the slight excess of the diabatic isallobaric mode over the opposing adiabatic mode, while mass convergence in the lower troposphere was due to the slight excess of the adiabatic isallobaric mode over the diabatic mode. The intensity of the other diabatically forced ageostrophic component, induced through vertical advection of momentum in a sheared environment, ranged from 5 to 10 m s−1 in the middle and upper troposphere of the jet's entrance region. Over much of the convective region, both the total isallobaric and the inertial diabatic ageostrophic components were directed from the anticyclonic to the cyclonic side of the jet streak at jet streak level in the same sense as pre-existing ageostrophic motion in the upper branch of the jet's direct mass circulation. This diabatically forced ageostrophic motion directed along the pressure gradient of the larger scale resulted in additional generation of kinetic energy which ultimately produced stronger winds in the jet streak downstream. A comparison between actual and geostrophic momentum forms for ageostrophic motion revealed discrepancies of 20 m s−1 that were mainly due to differences in the horizontal fields of inertial advective ageostrophic motion. This expected result points out that the rapid evolution of ageostrophic motion within the shorter time scales of MCCs limits the applicability of geostrophic momentum theory in prescribing the structure of ageostrophic motion.

Current, temperature, and salinity beneath George VI Ice Shelf, Antarctica

Speed, direction, temperature, and conductivity were recorded from February to July 1980 within the thermocline near the northern ice front of George VI Ice Shelf. There were no significant changes in temperature or salinity from summer to winter. Fluctuations of around 10 and 40 days periodicity were observed in the current and temperature, and similar variations are evident in meteorological observations. Temperature oscillations were observed at tidal frequencies and may be caused by horizontal advection or internal wave motion. The horizontal kinetic energy is dominated by low-frequency periods (46%), semi-diurnal tides (40%), and diurnal tides (10%). Tidal ellipses have their major axes aligned along George VI Sound and are described anticlockwise. Terdiurnal constituents, which may be a particular effect in the response of a floating ice shelf to tide generating forces, were observed. The M 2 constituent was highly suppressed. Both the amplitude of M 2 current and the speed of the mean flow decreased sharply in mid-April. These changes may be related to increasing sea-ice cover with the onset of winter. The mean flow is directed eastwards across the narrow channel, parallel to the ice front and at right angles to the major axes of the tidal ellipses. We speculate on reasons for this unusual behaviour.

Diurnal Variation in the Troposphere's Energy Balance

The tropospheric energy balance is examined in an effort to ascertain the diurnal variation of its components. The mean local diurnal temperature change in the 850–300 mb layer was observationally determined utilizing upper air data collected at 142 Northern Hemisphere radiosonde stations. Temperature data from the GATE, ATEX, BOMEX and LIE experiments, and Operations Redwing and Hardtack in the tropical west Pacific were also analyzed. Diurnal radiative cooling values are computed for specific geographical regions, and the magnitude of the required diurnal warming for each region is determined. Required warming is composed of a variety of atmospheric processes such as condensation energy release, surface to air sensible heat transfer, horizontal advection and vertical motion. A large morning maximum and late afternoon-early evening minimum of tropospheric required warming is found for all data sets, at all latitudes, in all seasons, and for both oceanic and land areas. These morning versus late afternoon required warming differences are of magnitudes of 2–3 to 1. It is hypothesized that the subsidence warming contribution to the diurnal variation of required warming dominates the other components. Increased morning subsidence and weaker afternoon–early evening subsidence is apparently a generally operating phenomenon in the troposphere.

A Comparison Of Numerical Methods For Computing The Smooth Solution Of A System With Multiple Time Scales

Many physical systems allow motions with different time scales, e.g. advective, inertial-gravity and sound waves in the atmosphere. In most of these cases, the majority of the energy is in the first class of motions (defined as smooth solutions in the bounded derivative theory) and the interactions between the various classes of motion are very weak. If advective motions were the only ones present, then an explicit second order method with a time step satisfying the CFL criterion based on the maximum speed of the winds would be sufficiently accurate and efficient for most simulations. However, when multiple waves are present the time step must satisfy the CFL criterion based on the fastest speed in the system. This results in a less efficient computation of the smooth solution and produces an imbalance between the spatial and temporal truncation errors for that solution. A number of continuum and numerical methods to overcome these problems are discussed and compared.

THE MECHANICS OF INSTABILITY-LINE FORMATION

Abstract It is proposed that under certain conditions of wind flow that temperature changes due to horizontal advection and vertical motion produce a trend towards the formation within a few hours of a line or narrow band of vertical thermal instability associated with a marked increase in low-level convergence along the same line or narrow band. An assumed distribution of winds, chosen to represent typical conditions observed preceding by a few hours the formation of an instability line, is studied and a model presented. The model agrees well with synoptic observation and studies. The atmospheric mechanism derived is of the mesoscale and is directly applicable to instability line formation in the “High Plains” and most pre-cold frontal instability lines in other areas can be explained by it.

THE ROLE OF VERTICAL MOTIONS IN OZONE-WEATHER RELATIONSHIPS

Abstract The relative importance of horizontal advection and vertical motion in producing the day-to-day changes in total ozone amount is calculated, and the manner in which these two factors combine to produce the well-known ozone-weather relationships is explained. The calculations show that at most one third of the range of daily values is attributable to vertical motions, the remainder presumably being the result of horizontal advection.