Showing papers on "Convection published in 1998"

Simulations of Solar Granulation. I. General Properties

Abstract: Numerical simulations provide information on solar convection not available by direct observation. We present results of simulations of near surface solar convection with realistic physics: an equation of state including ionization and three-dimensional, LTE radiative transfer using a four-bin opacity distribution function. Solar convection is driven by radiative cooling in the surface thermal boundary layer, producing the familiar granulation pattern. In the interior of granules, warm plasma ascends with ≈ 10% ionized hydrogen. As it approaches and passes through the optical surface, the plasma cools, recombines, and loses entropy. It then turns over and converges into the dark intergranular lanes and further into the vertices between granulation cells. These vertices feed turbulent downdrafts below the solar surface, which are the sites of buoyancy work that drives the convection. Only a tiny fraction of the fluid ascending at depth reaches the surface to cool, lose entropy, and form the cores of these downdrafts. Granules evolve by pushing out against and being pushed in by their neighboring granules, and by being split by overlying fluid that cools and is pulled down by gravity. Convective energy transport properties that are closely related to integral constraints such as conservation of energy and mass are exceedingly robust. Other properties, which are less tightly constrained and/or involve higher order moments or derivatives, are found to depend more sensitively on the numerical resolution. At the highest numerical resolution, excellent agreement between simulated convection properties and observations is found. In interpreting observations it is crucial to remember that surfaces of constant optical depth are corrugated. The surface of unit optical depth in the continuum is higher above granules and lower in the intergranular lanes, while the surface of optical depth unity in a spectral line is corrugated in ways that are influenced by both thermal and Doppler effects.

Large-scale imaging of high-latitude convection with Super Dual Auroral Radar Network HF radar observations

Abstract: The HF radars of the Super Dual Auroral Radar Network (SuperDARN) provide measurements of the E × B drift of ionospheric plasma over extended regions of the high-latitude ionosphere. With the recent augmentation of the northern hemisphere component to six radars, a sizable fraction of the entire convection zone (approximately one-third) can be imaged nearly instantaneously (∼2 min). To date, the two-dimensional convection velocity has been mapped by combining line-of-sight velocity measurements obtained from pairs of radars within common-volume areas. We describe a new method of deriving large-scale convection maps based on all the available velocity data. The measurements are used to determine a solution for the distribution of electrostatic potential, Φ, expressed as a series expansion in spherical harmonics. The addition of data from a statistical model constrains the solution in regions of no data coverage. For low-order expansions the results provide a gross characterization of the global convection. We discuss the processing of the radar velocity data, the factors that condition the fitting, and the reliability of the results. We present examples of imaging that demonstrate the response of the global convection to variations in the interplanetary magnetic field (IMF). In the case of a sudden polarity change from northward to southward IMF, the convection is seen to reconfigure globally on very short (<6 min) timescales.

Satellite-Based Insights into Precipitation Formation Processes in Continental and Maritime Convective Clouds

Abstract: Multispectral analyses of satellite images are used to calculate the evolution of the effective radius of convective cloud particles with temperature, and to infer from that information about precipitation forming processes in the clouds. Different microphysical processes are identified at different heights. From cloud base to top, the microphysical classification includes zones of diffusional droplet growth, coalescence droplet growth, rainout, mixed-phase precipitation, and glaciation. Not all zones need appear in a given cloud system. Application to maritime clouds shows, from base to top, zones of coalescence, rainout, a shallow mixed-phase region, and glaciation starting at −10°C or even warmer. In contrast, continental clouds have a deep diffusional growth zone above their bases, followed by coalescence and mixed-phase zones, and glaciation at −15° to −20°C. Highly continental clouds have a narrow or no coalescence zone, a deep mixed-phase zone, and glaciation occurring between −20° and −30°C. Limit...

Handbook of Heat Transfer

Abstract: Handbook of Numerical Heat Transfer Free Full Download Links from Multiple Mirrors added by DL4W on 2015-04-10 02:13:35. Handbook of heat transfer / editors, W.M. Rohsenow, J.P. Hartnett. Y.I. Cho. m 3rd ed. p. cm. Includes bibliographical references and index. ISBN 0-07053555-8. Students investigate heat transfer by conduction, convection and radiation. The analogy between heat and mass transfer is covered and applied in the analysis.

An Along‐the‐Channel Model for Proton Exchange Membrane Fuel Cells

Abstract: An along-the-channel model is developed for evaluating the effects of various design and operating parameters on the performance of a proton exchange membrane (PEM) fuel cell. The model, which is based on a previous one, has been extended to include the convective water transport across the membrane by a pressure gradient, temperature distribution in the solid phase along the flow channel, and heat removal by natural convection and coflow and counterflow heat exchangers. Results from the model show that the performance of a PEM fuel cell could be improved by anode humidification and positive differential pressure between the cathode and anode to increase the back transport rate of water across the membrane. Results also show that effective heat removal is necessary for preventing excessive temperature which could lead to local membrane dehydration. For heat removal and distribution, the counterflow heat exchanger is most effective.

Inevitability of a magnetic field in the Sun's radiative interior

Abstract: The gas in the convective outer layers of the Sun rotates faster at the equator than in the polar regions, yet deeper inside (in the radiative zone) the gas rotates almost uniformly1,2,3. There is a thin transition layer between these zones, called the tachocline4. This structure has been measured seismologically1,2,3, but no purely fluid-dynamical mechanism can explain its existence. Here we argue that a self-consistent model requires a large-scale magnetic field in the Sun's interior, as well as consideration of the Coriolis effects in the convection zone and in the tachocline. Turbulent stresses in the convection zone induce (through Coriolis effects) a meridional circulation, causing the gas from the convection zone to burrow downwards, thereby generating the horizontal and vertical shear that characterizes the tachocline. The interior magnetic field stops the burrowing, and confines the shear, as demanded by the observed structure of the tachocline. We outline a dynamical theory of the flow, from which we estimate a field strength of about 10−4 tesla just beneath the tachocline. An important test of this picture, after numerical refinement, will be quantitative consistency between the predicted and observed interior angular velocities.

Frictional Moisture Convergence in a Composite Life Cycle of the Madden–Julian Oscillation

Abstract: A composite life cycle of the Madden–Julian oscillation (MJO) is constructed using an index based on the first two EOFs of the bandpass-filtered (20–80 days) 850-mb zonal wind averaged from 5°N to 5°S every 2.5° around the equator. Precipitation, 1000-mb convergence, 850-mb wind, and 200-mb wind are composited for the period 1979–95. Water vapor integrated from the surface to 300 mb is composited for the period 1988–92. Frictional moisture convergence at the equator is shown to play an important role in the life cycle of the Madden–Julian oscillation (MJO). Regions of boundary layer convergence foster growth of positive water vapor anomalies to the east of convection. This convergence coincides with 850-mb easterly wind anomalies, as is consistent with Kelvin wave dynamics. Drying of the atmosphere occurs rapidly after the passage of convection with the onset of 850-mb westerly perturbations. Possible mechanisms for this drying include boundary layer divergence and subsidence or horizontal advect...

A Molecular Theory for Fast Flows of Entangled Polymers

Abstract: The Doi−Edwards (DE) theory for the rheological properties of entangled polymer melts and solutions successfully predicts the response to large step-shear strains but fails to predict other nonlinear shear properties, such as the steady-state viscosity or the relaxation of stress after cessation of steady shearing. Many of these failures remain even in the extension of the theory by Marrucci and Grizzuti (Gazz. Chim. Ital. 1988, 118, 179)1 to allow deformation-induced “tube stretch”. Here, we find that a much more successful theory can be obtained by also accounting for “convective constraint release”, i.e., the loss of entanglement constraints caused by the retraction of surrounding chains in their tubes. (Marrucci, G. J. Non-Newtonian Fluid Mech. 1996, 62, 279 and Ianniruberto, G.; Marrucci, G. J. Non-Newtonian Fluid Mech. 1996, 65, 241).2,3 In the molecular model developed here, convective constraint release can both shorten the reptation tube and allow reorientation of interior tube segments. The revi...

Gravitational, Symmetric, and Baroclinic Instability of the Ocean Mixed Layer

Abstract: A hierarchy of hydrodynamical instabilities controlling the transfer of buoyancy through the oceanic mixed layer is reviewed. If a resting ocean of horizontally uniform stratification is subject to spatially uniform buoyancy loss at the sea surface, then gravitational instability ensues in which buoyancy is drawn from depth by upright convection. But if spatial inhomogeneities in the ambient stratification or the forcing are present (as always exist in nature), then horizontal density gradients will be induced and, within a rotation period, horizontal currents in thermal-wind balance with those gradients will be set up within the mixed layer. There are two important consequences on the convective process: Upright convection will become modified by the presence of the thermal wind shear; fluid parcels are exchanged not along vertical paths but, rather, along slanting paths in symmetric instability. Theoretical considerations suggest that this slantwise convection sets the potential vorticity of th...

Modeling Density-Driven Flow in Porous Media

Abstract: 1 Introduction.- 1.1 Density-Driven Flow.- 1.2 Modeling.- 1.3 Modeling Density-driven Flow in Porous Media.- 1.4 FAST-C(2D) Modeling Software.- 2 Density and Other Water Properties.- 2.1 Dependence on Temperature.- 2.1.1 Density.- 2.1.2 Thermal Expansion Coefficient.- 2.1.3 Viscosity.- 2.1.4 Specific Heat Capacity.- 2.1.5 Thermal Conductivity.- 2.1.6 Diffusivity.- 2.2 Dependence on Salinity.- 2.2.1 Density.- 2.2.2 Viscosity.- 2.3 Dependence on Pressure.- 2.3.1 Density.- 2.3.2 Compressibility.- 3 Analytical Description.- 3.1 Basic Principles.- 3.2 Oberbeck-Boussinesq Assumption.- 3.3 Hydraulic Head Formulation.- 3.4 Streamfunction Formulation.- 3.5 Vorticity Equation.- 3.6 Extended Oberbeck-Boussinesq Assumption.- 3.7 Dimensionless Formulation.- 3.8 Boundary Layer Formulation.- 3.9 Heat and Mass Transfer.- 4 Numerical Modeling (Fast-C(2D)).- 4.1 Spatial Discretization.- 4.2 Temporal Discretization.- 4.3 Boundary Conditions.- 4.4 Initial Conditions and RESTART.- 4.5 Solution of the Nonlinear System.- 4.5.1 Newton Method and Variations.- 4.5.2 Picard Iterations.- 4.6 Solution of Linear Systems.- 4.6.1 Conjugate Gradients.- 4.7 Postprocessing.- 5 Steady Convection.- 5.1 Benard Experiments in Porous Medium.- 5.2 Linear Analysis.- 5.2.1 Isotropic Porous Medium.- 5.2.2 Anisotropic Porous Medium.- 5.3 Bifurcation Analysis.- 5.4 Numerical Experiments.- 5.4.1 Isotropic Porous Medium.- 5.4.2 Anisotropic Porous Medium.- 6 Special Topics in Convection.- 6.1 Thermal Convection in Slender Boxes.- 6.1.1 Analytical Studies.- 6.1.2 Numerical Experiments.- 6.2 Variable Viscosity Effects on Convection.- 6.2.1 Introduction.- 6.2.2 Onset of Convection.- 6.2.3 Heat Transfer.- 6.3 Convection in Cold Groundwater.- 6.3.1 Streamfunction Formulation.- 6.3.2 Onset of Convection.- 6.3.3 Flow Patterns.- 6.4 Relevance of Convection in Natural Systems.- 7 Oscillatory Convection.- 7.1 Hopf Bifurcation.- 7.2 Simulation.- 7.3 Influence of Numerical Parameters.- 8 Horizontal Heat and Mass Transfer.- 8.1 Analytical Approximations and Solutions.- 8.1.1 Convection.- 8.1.2 Conduction.- 8.2 Numerical Experiments.- 8.2.1 Conduction.- 8.2.2 Convection.- 9 Elder Experiment.- 9.1 Laboratory Experiment.- 9.2 Numerical Experiments.- 9.2.1 Elder's Model.- 9.2.2 FAST-C(2D) Model.- 9.2.3 Further Models.- 9.3 Related Problems.- 10 Geothermal Flow (Yusa's Example).- 10.1 Hypothetical Situation and Analytical Description.- 10.2 Flow Pattern Characterization.- 10.3 Sensitivity Analysis.- 10.4 Other Geothermal Systems.- 11 Saltwater Intrusion (Henry's Example).- 11.1 Problem Description.- 11.2 Sharp Interface Approach.- 11.3 Henry's Example.- 11.4 Modeling Saltwater Intrusion.- 11.4.1 Henry's Example.- 11.4.2 Parameter Variation.- 11.4.3 Layered Aquifers.- 12 Saltwater Upconing.- 12.1 Problem Description.- 12.2 Modeling Saltwater Upconing.- 12.2.1 Sharp Interface Approach.- 12.2.2 Miscible Displacement.- 12.2.3 Variable Density Effects.- 12.3 Case Study.- 13 Flow Across a Salt-Dome.- 13.1 Salt Formations and Scenarios.- 13.2 HYDROCOIN Test-Case.- 13.3 Modeling the HYDROCOIN Test-Case.- 13.4 FAST-C(2D) Model.- 14 Desert Sedimentary Basins.- 14.1 System Description.- 14.2 Numerical Modeling.- Concluding Remark.- References.- Appendix I: Fast-C(2D) Input- and Output-Files.- Input-File for FAST-C(2D).- Output-Files.

Mantle convection simulations with rheologies that generate plate-like behaviour

Abstract: A long-standing problem in geodynamics is how to incorporate surface plates in numerical models of mantle convection. Plates have usually been inserted explicitly in convection models as rigidrafts1,2,3, as a separate rheological layer4,5 or as a high-viscosity region within weak zones6,7,8,9,10,11. Plates have also been generated intrinsically through the use of a more complex (non-newtonian) rheology for the entire model12,13 but with a prescribed mantle flow. However, previous attempts to generate plates intrinsically and in a self-consistent manner (without prescribed flow) have not produced surface motions that appear plate-like14,15,16. Here we present a three-dimensional convection model that generates plates in a self-consistent manner through the use of a rheology that is temperature and strain-rate dependent, and which incorporates the concept of a yield stress. This rheology induces a stiff layer on top of a convecting fluid, and we find that this layer breaks at sufficiently high stresses. The model produces a style of convection that contains some of the important features of plate tectonics, such as the subduction of the stiff layer and plate-like motion on the surface of the fluid mantle. However, the model also produces some non-Earth-like features, such as episodic subduction followed by the slow growth of a new stiff layer, which may be more consistent with the style of convection found on Venus.

Thermal Vibrational Convection

Abstract: Basic Equations: Mechanical "Quasi-equilibrium" and Its Stability. Plane-Parallel Flows and Their Stability. Non-linear Problems. Internal Heat Sources. Vibrations of Finite Frequencies. Thermovibrational Convection in the General Case of Arbitrary Vibrations. The Problem of Boundary Conditions. The Second-order Effects. Some Particular Problems. Index.

Radiative–convective equilibrium in a three-dimensional cloud-ensemble model

Abstract: A knowledge of radiative convective interactions is key to an understanding of the tropical climate. In an attempt to address this a cloud-resolving model has been run to a radiative-convective equilibrium state in three dimensions. The model includes a three-phase bulk microphysical scheme and a fully interactive two-stream broadband radiative-transfer scheme for both the infrared and solar radiation. The simulation is performed using a fixed sea surface temperature, and cyclic lateral boundary conditions. No ‘large-scale’ convergence, mean wind shear or background vorticity was imposed. The total integration lasted 70 days, and a statistical equilibrium state was reached at all heights after 30 days of simulation in all model variables. It is seen that some variables, such as vertical mass flux, adjust quickly to their equilibrium values while others, such as column-integrated water amount, domain-mean temperature and convective available potential energy (CAPE) display variation on a longer 30-day time-scale. The equilibrium state had a column-integrated vapour amount of 42.3 kg m−2, a mean temperature of 258.7 K and a pseudo-adiabatic CAPE value of 1900 J kg−1. The equilibrium-state statistics are consistent with tropical observations. The convection does not remain randomly distributed but instead becomes organized, aligning in a band structure associated with high moisture values in the boundary layer. This organization seems to result from interactions between radiation, convection and surface fluxes. The surface-flux feedback is due to higher boundary-layer winds, associated with convection, increasing surface fluxes of moisture locally. Horizontally inhomogeneous radiation can act to make clouds longer lasting and also increase convergence into cloudy region. Replacing the wind-sensitive surface-flux calculation with a linear relaxation to surface values appeared to largely destroy this organization, as did the use of an imposed horizontally uniform radiative-heating rate.

The Role of Environmental Shear and Thermodynamic Conditions in Determining the Structure and Evolution of Mesoscale Convective Systems during TOGA COARE

Abstract: A collection of case studies is used to elucidate the influence of environmental soundings on the structure and evolution of the convection in the mesoscale convective systems sampled by the turboprop aircraft in the Tropical Ocean Global Atmosphere (TOGA) Coupled Ocean–Atmosphere Response Experiment (COARE). The soundings were constructed primarily from aircraft data below 5–6 km and primarily from radiosonde data at higher altitudes. The well-documented role of the vertical shear of the horizontal wind in determining the mesoscale structure of tropical convection is confirmed and extended. As noted by earlier investigators, nearly all convective bands occurring in environments with appreciable shear below a low-level wind maximum are oriented nearly normal to the shear beneath the wind maximum and propagate in the direction of the low-level shear at a speed close to the wind maximum; when there is appreciable shear at middle levels (800–400 mb), convective bands form parallel to the shear. With...

Instability of thermocapillary–buoyancy convection in shallow layers. Part 1. Characterization of steady and oscillatory instabilities

Abstract: Combined thermocapillary–buoyancy convection in a thin rectangular geometry is investigated experimentally, with an emphasis on the generation of hydrothermal-wave instabilities. For sufficiently thin layers, pure hydrothermal waves are observed, and are found to be oblique as predicted by a previous linear-stability analysis (Smith & Davis 1983). For thicker layers, both a steady multicell state and an oscillatory state are found to exist, but the latter is not in the form of a pure hydrothermal wave.

Dynamical Interaction of Solar Magnetic Elements and Granular Convection: Results of a Numerical Simulation

Abstract: Nonstationary convection in the solar photosphere and its interaction with photospheric magnetic structures (flux sheets in intergranular lanes) have been simulated using a numerical code for two-dimensional MHD with radiative energy transfer. Dynamical phenomena are identified in the simulations, which may contribute to chromospheric and coronal heating. Among these are the bending and horizontal displacement of a flux sheet by convective flows and the excitation and propagation of shock waves both within and outside the magnetic structure. Observational signatures of these phenomena are derived from calculated Stokes profiles of Zeeman-sensitive spectral lines. We suggest that the extended red wings of the observed Stokes V profiles are due to downward coacceleration of magnetized material in a turbulent boundary layer between the flux sheet and the strong external downflow. Upward-propagating shocks in magnetic structures should be detectable if a time resolution of about 10 s is achieved, together with a spatial resolution that allows one to isolate individual magnetic structures. Determination of the complicated internal dynamics of magnetic elements requires observations with a spatial resolution better than 100 km in the solar photosphere.

Convective Trigger Function for a Mass-Flux Cumulus Parameterization Scheme

Abstract: A precipitation physics package for the National Centers for Environmental Prediction Regional Spectral Model designed to improve the skill of precipitation forecasts is proposed. The package incorporates a prognostic grid-resolvable precipitation scheme and a parameterized convection scheme with a convective trigger function that explicitly couples boundary layer and convective precipitation processes. Comprehensive sensitivity experiments were conducted with a grid spacing of approximately 25 km for a heavy rain case over the United States during 15–17 May 1995. In this paper, the trigger function setup in the convective parameterization scheme and its impact on the predicted precipitation are discussed. Special attention is given to the interaction of cloud properties in the parameterized convection with the evolution of grid-resolvable precipitation physics. The impact of convective forcing due to different convective triggers on the large-scale pattern downstream is also discussed. The imple...

Coriolis effect on gravity-driven convection in a rotating porous layer heated from below

Abstract: Linear stability and weak nonlinear theories are used to investigate analytically the Coriolis effect on three-dimensional gravity-driven convection in a rotating porous layer heated from below. Major differences as well as similarities with the corresponding problem in pure fluids (non-porous domains) are particularly highlighted. As such, it is found that, in contrast to the problem in pure fluids, overstable convection in porous media is not limited to a particular domain of Prandtl number values (in pure fluids the necessary condition is Pr<1). Moreover, it is also established that in the porous-media problem the critical wavenumber in the plane containing the streamlines for stationary convection is not identical to the critical wavenumber associated with convection without rotation, and is therefore not independent of rotation, a result which is quite distinct from the corresponding pure-fluids problem. Nevertheless it is evident that in porous media, just as in the case of pure fluids subject to rotation and heated from below, the viscosity at high rotation rates has a destabilizing effect on the onset of stationary convection, i.e. the higher the viscosity the less stable the fluid. Finite-amplitude results obtained by using a weak nonlinear analysis provide differential equations for the amplitude, corresponding to both stationary and overstable convection. These amplitude equations permit one to identify from the post-transient conditions that the fluid is subject to a pitchfork bifurcation in the stationary convection case and to a Hopf bifurcation associated with the overstable convection. Heat transfer results were evaluated from the amplitude solution and are presented in terms of Nusselt number for both stationary and overstable convection. They show that rotation has in general a retarding effect on convective heat transfer, except for a narrow region of small values of the parameter containing the Prandtl number where rotation enhances the heat transfer associated with overstable convection.

Thermal convection in a volumetrically heated, infinite Prandtl number fluid with strongly temperature‐dependent viscosity: Implications for planetary thermal evolution

Abstract: Parameterized models of the thermal evolution of planets are usually based on the assumption that the lithosphere-convecting mantle boundary can be defined by an isotherm at a temperature below which viscosity is infinite on geologic timescales. Recent experimental results argue against this assumption. We have investigated both the definition of the lithosphere-convecting mantle boundary and the power law relation describing convecting heat transfer, based on numerical experiments of thermal convection in a volumetrically heated fluid with temperature-dependent viscosity. Other recent studies have treated only the heating from below, but volumetric heating is likely to be the dominant mode of heating in planetary mantles, either as a consequence of radioactive heating or as a proxy for secular cooling. Convection can occur either in the whole box or be located under a stagnant lid. In the lid regime, convection is driven by a temperature contrast depending on the rheology of the fluid and the interior temperature. This result, in agreement with experimental studies, indicates that boundary between the stagnant lid and the convecting layer (similar to the lithosphere-convecting mantle boundary) cannot be defined as a fixed isotherm. During thermal evolution of planets, the viscosity contrast in the convecting mantle remains constant, not the temperature at the bottom of the lithosphere. We present an example showing that the evolution of planets is strongly dependent on the criterion chosen to define the lithosphere-convecting mantle boundary. For reasonable values of the activation energy for thermally activated creep, the temperature defining the lithosphere-convecting mantle boundary, the mantle temperature, and the thickness of the lithosphere could be larger than expected from previous models which treat the base of the lithosphere as a fixed isotherm.

An Investigation of Neutrino-driven Convection and the Core Collapse Supernova Mechanism Using Multigroup Neutrino Transport

Abstract: We investigate neutrino-driven convection in core collapse supernovae and its ramifications for the explosion mechanism. We begin with a postbounce model that is optimistic in two important respects: (1) we begin with a 15 M☉ precollapse model, which is representative of the class of stars with compact iron cores; (2) we implement Newtonian gravity. Our precollapse model is evolved through core collapse and bounce in one dimension using multigroup (neutrino energy-dependent) flux-limited diffusion (MGFLD) neutrino transport and Newtonian Lagrangian hydrodynamics, providing realistic initial conditions for the postbounce convection and evolution. Our two-dimensional simulation begins at 12 ms after bounce and proceeds for 500 ms. We couple two-dimensional piecewise parabolic method (PPM) hydrodynamics to precalculated one-dimensional MGFLD neutrino transport. (The neutrino distributions used for matter heating and deleptonization in our two-dimensional run are obtained from an accompanying one-dimensional simulation. The accuracy of this approximation is assessed.) For the moment, we sacrifice dimensionality for realism in other aspects of our neutrino transport. MGFLD is an implementation of neutrino transport that simultaneously (1) is multigroup and (2) simulates with sufficient realism the transport of neutrinos in opaque, semitransparent, and transparent regions. Both are crucial to the accurate determination of postshock neutrino heating, which sensitively depends on the luminosities, spectra, and flux factors of the electron neutrinos and antineutrinos emerging from their respective neutrinospheres. By 137 ms after bounce, we see neutrino-driven convection rapidly developing beneath the shock. By 212 ms after bounce, this convection becomes large scale, characterized by higher entropy, expanding upflows and lower entropy, denser, finger-like downflows. The upflows reach the shock and distort it from sphericity. The radial convection velocities at this time become supersonic just below the shock, reaching magnitudes in excess of 109 cm s-1. Eventually, however, the shock recedes to smaller radii, and at ~500 ms after bounce there is no evidence in our simulation of an explosion or of a developing explosion. Our angle-averaged density, entropy, electron fraction, and radial velocity profiles in our two-dimensional model agree well with their counterparts in our accompanying one-dimensional MGFLD run above and below the neutrino-driven convection region. In the convection region, the one-dimensional and angle-averaged profiles differ somewhat because (1) convection tends to flatten the density, entropy, and electron fraction profiles, and (2) the shock radius is boosted somewhat by convection. However, the differences are not significant, indicating that, while vigorous, neutrino-driven convection in our model does not have a significant impact on the overall shock dynamics. The differences between our results and those of other groups are considered. These most likely result from differences in (1) numerical hydrodynamics methods; (2) initial postbounce models, and, most important; (3) neutrino transport approximations. We have compared our neutrino luminosities, rms energies, and inverse flux factors with those from the exploding models of other groups. Above all, we find that the neutrino rms energies computed by our multigroup (MGFLD) transport are significantly lower than the values obtained by Burrows and coworkers, who specified their neutrino spectra by tying the neutrino temperature to the matter temperature at the neutrinosphere and by choosing the neutrino degeneracy parameter arbitrarily, and by Herant and coworkers in their transport scheme, which (1) is gray and (2) patches together optically thick and thin regions. The most dramatic difference between our results and those of Janka and Muller is exhibited by the difference in the net cooling rate below the gain radii: Our rate is 2-3 times greater during the critical 50-100 ms after bounce. We have computed the mass and internal energy in the gain region as a function of time. Up to ~150 ms after bounce, we find that both increase as a result of the increasing gain region volume, as the gain and shock radii diverge. However, at all subsequent times, we find that the mass and internal energy in the gain region decrease with time in accordance with the density falloff in the preshock region and with the flow of matter into the gain region at the shock and out of the gain region at the gain radius. Therefore, we see no evidence in the simulations presented here that neutrino-driven convection leads to mass and energy accumulation in the gain region. We have compared our one- and two-dimensional densities, temperatures, and electron fractions in the region below the electron neutrino and antineutrino gain radii, above which the neutrino luminosities are essentially constant (i.e., the neutrino sources are entirely enclosed), in an effort to assess how spherically symmetric our neutrino sources remain during our two-dimensional evolution, and therefore, in an effort to assess our use of precalculated one-dimensional MGFLD neutrino distributions in calculating the matter heating and deleptonization. We find no difference below the neutrinosphere radii. Between the neutrinosphere and gain radii we find no differences with obvious ramifications for the supernova outcome. We note that the interplay between neutrino transport and convection below the neutrinospheres is a delicate matter and is discussed at greater length in another paper (Mezzacappa and coworkers). However, the results presented therein do support our use of precalculated one-dimensional MGFLD in the present context. Failure in our "optimistic" 15 M☉ Newtonian model leads us to conclude that it is unlikely, at least in our approximation, that neutrino-driven convection will lead to explosions for more massive stars with fatter iron cores or in cases in which general relativity is included.

The transport of plasma sheet material from the distant tail to geosynchronous orbit

Abstract: Several aspects of mass transport in the Earth's plasma sheet are examined The evolution of plasma sheet material as it moves earthward is examined by statistically comparing plasma sheet properties at three different downtail distances: near-Earth plasma sheet properties obtained from measurements by 1989-046 near the geomagnetic equator near midnight at 66 RE, midtail plasma sheet properties obtained from ISEE 2 measurements during 333 encounters with the neutral sheet, and distant-plasma sheet properties obtained from ISEE 2 measurements during 53 encounters with the interface between the plasma sheet and the plasma sheet boundary layer Examination of the evolution of the plasma sheet through pressure-density space shows that the transport is nearly adiabatic (γ = 152), with a loss of entropy observed in the near-Earth region The estimated pressure loss from the plasma-sheet associated with the aurora is able to account for the observed decrease in entropy The near-Earth plasma sheet plasma is also found to be compressed much less than would be expected from magnetic field models Examination of the evolution of the plasma sheet through density-flux tube-volume space (with the aid of the T89c magnetic field model) indicates that there is a substantial loss of mass from plasma sheet flux tubes Global magnetic reconnection during substorms and patchy reconnection at other times is invoked to account (1) for the required mass loss, (2) for the related lack of compression, and (3) for an observed disconnection between ionospheric convection and plasma sheet convection This reconnection must occur closer than 20 RE downtail Selective transport is examined by statistically analyzing the ISEE 2 neutral sheet crossing data set: strong transport is found to be associated with low densities, with weak Bz, and with large flux tube volume A correlation between the direction of the flow in the plasma sheet and the solar wind velocity indicates that earth-ward transport is stronger when the solar wind velocity is lower An examination of near-Earth and of midtail plasma sheet densities, temperatures, and entropies shows that the plasma sheet is usually spatially homogeneous, contrary to a “bubbles and blobs” picture of transport Several new points of view about plasma sheet transport are discussed, including the dominant role of near-Earth reconnection, the importance of auroral zone pressure loss, the control of the plasma sheet properties by the density and speed of the solar wind, and the disconnection of the ionospheric and plasma sheet flow patterns

Evidence for control of Atlantic subtropical humidity by large scale advection

Abstract: The interplay between large scale dynamics and tropospheric moisture is investigated. A simple conceptual model of the sources and sinks of humidity is used to reconstruct, using a backward Lagrangian trajectory technique, the water vapor distribution in the tropical and subtropical free troposphere. Satellite data in the water vapor channel from both Meteosat-3 and Meteosat-4 satellites are then used to validate the model following a model-to-satellite approach over the whole Atlantic ocean. There is excellent agreement between simulations and observations in the drier regions, but the simulated brightness temperature exhibits a warm bias within and near moist, convective regions. This bias is most probably due to the neglect of cloud effects in reconstructing the simulated brightness temperature, rather than to a dry bias in the simulation. A second advective simulation, performed with monthly mean rather than full transient winds, led to a substantially drier subtropics. This calculation demonstrates the importance of synoptic scale transient eddies in determining the humidity of the subtropical dry zones. It is speculated on this basis that discontinuous changes in synoptic eddy activity could provide a mechanism for rapid global climate changes.

Effects of Cloud Vertical Structure on Atmospheric Circulation in the GISS GCM

Abstract: Thirteen experiments have been performed using the Goddard Institute for Space Studies General Circulation Model (GISS GCM) to investigate the response of the large-scale circulation to different macroscale cloud vertical structures (CVS). The overall effect of clouds, the role of their geographic variations, and difference between the transient and equilibrium responses of the atmospheric circulation are also studied. Clouds act to suppress the Hadley circulation in the transient response, but intensify it in the equilibrium state. Changing CVS affects the atmospheric circulation directly by modifying the radiative cooling profile and atmospheric static stability, but the effect is opposed, on average, by an indirect effect on the latent heating profile produced by deep (moist) convection. Different interactions of radiation and convection with land and ocean surfaces mean that this cancellation of CVS effects on radiative and latent heating is not the same at all locations. All three parameters...

Mechanisms for the Generation of Mesoscale Vortices within Quasi-Linear Convective Systems

Abstract: Previous idealized simulations of convective systems have demonstrated that the development of mesoscale vortices within quasi-linear convective systems may be a natural consequence of the finite extent of the convective line, as horizontal vorticity is tilted into the vertical at the line ends. However, the source of this horizontal vorticity has not yet been clearly established, either being associated with the ambient shear or else generated within the system. In this paper, results are presented from a series of idealized simulations that demonstrate that the source, strength, and scale of these vortices depends on the strength of the ambient vertical wind shear, the strength of the system-generated cold pool, the scale of the convective line segments, as well as the phase within the life cycle of the convective system. In particular, for systems that develop in an environment with weak-to-moderate shear, a line-end vortex pair is generated primarily via the tilting of horizontal vorticity ge...

Interannual Variability of Deep-Water Formation in the Northwestern Mediterranean

Abstract: In the Gulf of Lions, observations of deep convection have been sporadically carried out over the past three decades, showing significant interannual variability of convection activity. As long time series of meteorological observations of the region are available from coastal stations, heat flux time series for the Gulf of Lions for the individual winters from 1969 to 1994 are derived by calibrating these observations against direct measurements obtained over the convection site. These heat fluxes are also compared against heat fluxes obtained by the French PERIDOT weather model for the winter of 1991/92. A Kraus–Turner one-dimensional mixed layer model is initialized by climatological mean temperature and salinity profiles and then driven by the heat flux time series of the individual years. Resulting convection depths are in satisfactory agreement with existing observational evidence, showing the dominance of interannual variability of local forcing on convection variability. The interannual variability of convection depth causes interannual variations in deep-water properties, and these are also compared with the hydrographic database.

Heat transport efficiency for stagnant lid convection with dislocation viscosity: Application to Mars and Venus

Abstract: Mantle convection on Mars and Venus is likely to occur in the regime known as stagnant lid convection. We perform thermal boundary layer analyses as well as finite element simulations of stagnant lid convection with non-Newtonian viscosity (which is believed to be more appropriate for the lithosphere and upper mantle) and discuss one particular application of the results, the efficiency of heat transport on the terrestrial planets. As in the case of Newtonian viscosity, the efficiency of heat transfer in the stagnant lid regime is extremely low compared to plate tectonics: For example, in the absence of plate tectonics, the mantle temperature on Earth, which is already close to the solidus, would be about 700–1500 K higher for the present-day value of the surface heat flux. For Venus, the critical heat flux which can be removed without widespread melting is only 10–20 m W/m2. For Mars, it is 15–30 m W/m2. Therefore, there are no doubts that in the absence of mobile plates, the mantle temperature would significantly exceed solidus during planetary evolution. It is hypothesized that this could cause one, or a combination, of two possible processes: (1) differentiation of radiogenic isotopes into the crust during early planetary magmatism and (2) initiation of some kind of plate tectonics as a result of plate weakening due to melting.