Showing papers on "Convection published in 1994"

Convective boiling and condensation

Abstract: Introduction 1. The basic models 2. Empirical treatments of two-phase flow 3. Introduction to convective boiling 4. Subcooled boiling heat transfer 5. Void fraction and pressure drop in subcooled boiling 6. Saturated boiling heat transfer 7. Critical heat flux in forced convective flow - 1. Vertical uniformly heated tubes 8. Critical heat flux in forced convective flow - 2. More complex situations 9. Condensation 10. Conditions influencing the performance of boiling and condensing systems 11. Multi-component boiling and condensation Appendix Index

Principles of Enhanced Heat Transfer

Abstract: Heat Transfer Fundamentals Performance Evaluation for Single-Phase Flows Performance Evaluation Criteria for Two-Phase Heat Exchangers Plate-and-Fin Extended Surfaces Externally Finned Tubes Insert Devices for Single-Phase Flow Internally Finned Tubes and Annuli Integral Roughness Fouling on Enhanced Surfaces Pool Boiling Vapor Space Condensation Convective Vaporization Convective Condensation Enhancement Using Electric Fields Simultaneous Heat and Mass Transfer Additives for Gases and Liquids Problem Supplement Index.

On large-scale circulations in convecting atmospheres

Abstract: The dominant thinking about the interaction between large-scale atmospheric circulations and moist convection holds that convection acts as a heat source for the large-scale circulations, while the latter supply water vapour to the convection. We show that this idea has led to fundamental misconceptions about this interaction, and offer an alternative paradigm, based on the idea that convection is nearly in statistical equilibrium with its environment. According to the alternative paradigm, the vertical temperature profile itself, rather than the heating, is controlled by the convection, which ties the temperature directly to the subcloud-layer entropy. The understanding of large-scale circulations in convecting atmospheres can, therefore, be regarded as a problem of understanding the distribution in space and time of the subcloud-layer entropy. We show that the subcloud-layer entropy is controlled by the sea surface temperature, the surface wind speed, and the large-scale vertical velocity in the convecting layer, and demonstrate how the recognition of this control leads to a simple, physically consistent view of large-scale flows, ranging from the Hadley and Walker circulations to the 30–50-day oscillation. In particular, we argue that the direct effect of convection on large-scale circulations is to reduce by roughly an order of magnitude the effective static stability felt by such circulations, and to damp all of them.

A coupled ocean‐atmosphere model of relevance to the ITCZ in the eastern Pacific

Abstract: The intertropical convergence zone (ITCZ) stays in the northern hemisphere over the Atlantic and eastern Pacific, even though the annual mean position of the sun is on the equator. To study some processes that contribute to this asymmetry about the equator, we use a two-dimensional model which neglects zonal variations and consists of an ocean model with a mixed layer coupled to a simple atmospheric model. In this coupled model, the atmosphere not only transports momentum into the ocean, but also directly affects sea surface temperature by means of wind stirring and surface latent heat flux. Under equatorially symmetric conditions, the model has, in addition to an equatorially symmetric solution, two asymmetric solutions with a single ITCZ that forms in only one hemisphere. Strong equatorial upwelling is essential for the asymmetry. Local oceanic turbulent processes involving vertical mixing and surface latent heat flux, which are dependent on wind speed, also contribute to the asymmetry. DOI: 10.1034/j.1600-0870.1994.t01-1-00001.x

The Life Cycle of the Madden–Julian Oscillation

Abstract: A composite life cycle of the Madden-Julian oscillation (MJO) is constructed from the cross covariance between outgoing longwave radiation (OLR), wind, and temperature. To focus on the role of convection, the composite is based on episodes when a discrete signal in OLR is present. The composite convective anomaly possesses a predominantly zonal wavenumber 2 structure that is confined to the eastern hemisphere. There, it propagates eastward at about 5 m/s and evolves through a systematic cycle of amplification and decay. Unlike the convective anomaly, the circulation anomaly is not confined to the eastern hemisphere. The circulation anomaly displays characteristics of both a forced response, coupled to the convective anomaly as it propagates across the eastern hemisphere, and a radiating response, which propagates away from the convective anomaly into the western hemisphere at about 10 m/s. The forced response appears as a coupled Rossby-Kelvin wave while the radiating response displays predominantly Kelvin wave features. When it is amplifying, the convective anomaly is positively correlated to the temperature perturbation, which implies production of eddy available potential energy (EAPE). A similar correlation between upper-tropospheric divergence and temperature implies conversion of EAPE to eddy kinetic energy during this time. When it is decaying, temperature has shifted nearly into quadrature with convection, so their correlation and production of EAPE are then small. The same correspondence to the amplification and decay of the disturbance is mirrored in the phase relationship between surface convergence and anomalous convection. The correspondence of surface convergence to the amplification and decay of the convective anomaly suggests that frictional wave- Conditional Instability of the Second Kind (CISK) plays a key role in generating the MJO.

A Comparison of Shear- and Buoyancy-Driven Planetary Boundary Layer Flows

Abstract: Planetary boundary layer (PBL) flows are known to exhibit fundamental differences depending on the relative combination of wind shear and buoyancy forces. These differences are not unexpected in that shear instabilities occur locally, while buoyancy force sets up vigorous thermals, which result in nonlocal transport of heat and momentum. At the same time, these two forces can act together to modify the flow field. In this study, four large-eddy simulations (LESs) spanning the shear and buoyancy flow regimes were generated; two correspond to the extreme cases of shear and buoyancy-driven PBLs, while the other two represent intermediate PBLs where both forces are important. The extreme cases are used to highlight and quantify the basic differences between shear and convective PBLs in 1) flow structures, 2) overall statistics, and 3) turbulent kinetic energy (TKE) budget distributions. Results from the two intermediate LES cases are used to develop and verify a velocity scaling and a TKE budget mode...

High Rayleigh Number Convection

Abstract: Turbulent convection exemplifies many of the startling aspects of turbulent flows that have been uncovered in the past two decades, but frequently exhibits a novel twist. Thus, as in the case of free shear flows, convection can organize into large-scale vortical structures, but these then react back in subtle ways on the boundary layers which ultimately sustain them. Thermal plumes are a coherent mode of heat transport, analogous to boundary layer bursts, yet their overall effect can be surprisingly close to the structureless predictions of mixing length theory. Convection cells are closed, which facilitates their experimental control, but fluctuations never exit and there is a dynamically determined bulk forcing. While the single­ pass mode characteristic of wind tunnel experiments seems simpler, the convection cell is, in ways to be discussed, more constrained. This review aims to familiarize the turbulence researcher with con­ vergent lines of investigation in convection and also to remind those working in convection that turbulence is not a new subject. To situate convection within the gamut of other turbulent flows, let us by way of introduction contrast the directions in which convection has developed with research on the turbulent boundary layer. From the onset of convection up to Rayleigh numbers Ra � 1 0 times critical, there is a great wealth of information about flow structures (which can be visualized from above), and their relative stabilities (Busse 198 1 ) . Turbulence, in the sense of many coupled modes, and not just sensitive dependence on initial conditions, can arise for low Ra in large aspect ratio

Advection-Dominated Accretion: Self-Similarity and Bipolar Outflows

Abstract: We consider axisymmetric viscous accretion flows where a fraction f of the viscously dissipated energy is advected with the accreting gas as stored entropy and a fraction 1-f is radiated. When f is small (i.e. very little advection), our solutions resemble standard thin disks in many respects except that they have a hot tenuous corona above. In the opposite {\it advection-dominated} limit ($f\rightarrow1$), the solutions approach nearly spherical accretion. The gas is almost at virial temperature, rotates at much below the Keplerian rate, and the flow is much more akin to Bondi accretion than to disk accretion. We compare our exact self-similar solutions with approximate solutions previously obtained using a height-integrated system of equations. We conclude that the height- integration approximation is excellent for a wide range of conditions. We find that the Bernoulli parameter is positive in all our solutions, especially close to the rotation axis. This effect is produced by viscous transport of energy from small to large radii and from the equator to the poles. In addition, all the solutions are convectively unstable and the convection is especially important near the rotation axis. For both reasons we suggest that a bipolar outflow will develop along the axis of the flows, fed by material from the the surface layers of the equatorial inflow.

Double diffusion in oceanography

Abstract: The modern study of double-diffusive convection began with Melvin Stern's article on "The Salt Fountain and Thermohaline Convection" in 1960. In that paper, he showed how opposing stratifications of two component species could drive convection if their diffusivities differed. Stommel ct al (1956) had earlier noted that there was significant potential energy available in the decrease of salinity with depth found in much of the tropical and subtropical ocean. While they suggested that a flow (the salt fountain) would be driven in a thermally-conducting pipe, it was Stern who realized that the two orders of magnitude difference in heat and salt diffusivities allowed the ocean to form its own pipes. These later came to be known as "salt fingers." Stern also identified the potential for the oscillatory instability when cold, fresh water overlies warm, salty water in the 1960 paper, though only in a footnote. Turner & Stommel (1964) demonstrated the "diffusive-convection" process a few years later. From these beginnings in oceanography over three decades ago, double diffusion has come to be recognized as an important convection process in a wide variety of fluid media, including magmas, metals, and stellar interiors (Schmitt 1983, Turner 1985). However, it is interesting to note that about one hundred years before Stern's paper, W. S. Jevons (1857) reported on the observation of long, narrow convection cells formed when warm, salty water was introduced over cold, fresh water. He correctly attributed the phenomenon to a difference in the diffusivities for heat and

Basic Heat and Mass Transfer

Abstract: 1. Introduction and Elementary Heat Transfer. 2. Steady One-Dimensional Heat Conduction. 3. Multidimensional and Unsteady Conduction. 4. Convection Fundamentals and Correlations. 5. Convection Analysis. 6. Thermal Radiation. 7. Condensation, Evaporation, and Boiling. 8. Heat Exchangers. 9. Mass Transfer. A. Property Data. B. Unit, Conversion Factors, and Mathematics. C. Charts. Bibliography. Nomenclature. Index.

Intraseasonal behavior of clouds, temperature, and motion in the tropics

Abstract: The spectral character of tropical convection is investigated in an 11-yr record of outgoing longwave radiation from the Advanced Very High Resolution Radiometer (AVHRR) to identify interaction with the tropical circulation. Along the equator in the eastern hemisphere, the space-time spectrum of convection possesses a broad peak at wavenumbers 1-3 and eastward periods of 35-95 days. Significantly broader than the dynamical signal of the Madden-Julian oscillation (MJO), this quasi-discrete convective signal is associatd with a large-scale anomaly that propagates across and modulates time mean or 'climatological convection' over the equatorial Indian Ocean and western Pacific. Outside that region the convective signal is small, even though, under amplified conditions, coherence can be found east of the date line and in the subtropics. Having a zonal scale of approximately wavenumber 2, anomalous convection propagates eastward at some 5 m/s and suppresses as well as reinforces climatological in the eastern hemisphere. The convective signal amplifies to a seasonal maximum near vernal equinox and, to a weaker degree, again near autumnal equinox, when climatological convection and warm sea surface temperature (SST) cross the equator. Contemporaneous records of motion from European Center for medium-Range Weather Forecasts (ECMWF) analyses and tropospheric-mean temperature from Microwave Sounding Unit reveal an anomalous component of the tropical circulation that coexists with the convective signal and embodies many of the established properties of the MJO. In the eastern hemisphere, subtropical Rossby gyres and zonal Kelvin structure along the equator flank the convective anomaly as it tracks eastward, giving the anomalous circulation to form of a 'forced response.' In the western hemisphere, the dynamical signal is composed chiefly of wavenumber-1 Kelvin structure, which as the form of a 'propagating response' that is excited in and radiates away from anomalous convection at some 10 m/s. Kelvin structure comprising the propagating response appears in 850-mb and 200-mb zonal winds even when the convective signal is absent, albeit with much smaller amplitude. In contrast, the signal in 1000-mb convergence appears only when accompanied by anomalous convection, which suggests that convergence in the boundary layer is instrumental in achieving strong interaction with the convective pattern.

Liquid flow in heterogeneous biofilms

Abstract: Liquid flow was studied in aerobic biofilms, consisting of microbial cell clusters (discrete aggregates of densely packed cells) and interstitial voids. Fluorescein microinjection was used as a qualitative technique to determine the presence of flow in cell clusters and voids. Flow velocity profiles were determined by tracking fluorescent latex spheres using confocal microscopy. Liquid was flowing through the voids and was stagnant in the cell clusters. Consequently, in voids both diffusion and convection may contribute to mass transfer, whereas in cell clusters diffusion is the dominant factor. The flow velocity in the biofilm depended on the average flow velocity of the bulk liquid. The velocity profiles in biofilms were linear and the velocity was zero at the substratum surface. The velocity gradients within biofilms were 50% of that near walls without biofilm coverage. The influence of the biofilm roughness on the flow velocity profiles was similar to that caused by rigid roughness elements.

Impact of Land-Surface Moisture Variability on Local Shallow Convective Cumulus and Precipitation in Large-Scale Models

Abstract: Numerical experiments using state-of-the-art high-resolution mesoscale cloud model showed that land-surface moisture significantly affects the timing of onset of clouds and the intensity and distribution of precipitation. In general, landscape discontinuity enhances shallow convective precipitation. Two mechanisms that are strongly modulated by land-surface moisture-namely, random turbulent thermal cells and organized sea-breeze-like mesoscale circulations-also determine the horizontal distribution of maximum precipitation. However, interactions between shallow cumulus and land-surface moisture are highly nonlinear and complicated by different factors, such as atmospheric thermodynamic structure and large-scale background wind. This analysis also showed that land-surface moisture discontinuities seem to play a more important role in a relatively dry atmsophere, and that the strongest precipitation is produced by a wavelength of land-surface forcing equivalent to the local Rossby radius of deformation. A general trend between the maximum precipitation and the normalized maximum latent heat flux was identified. In general, large values of mesoscale latent heat flux imply strongly developed mesoscale circulations and intense cloud activity, accompanied by large surface latent heat fluxes that transport more water vapor into the atmosphere.

Aspects of Wellbore Heat Transfer During Two-Phase Flow (includes associated papers 30226 and 30970 )

Abstract: Wellbore fluid temperature is governed by the rate of heat loss from the wellbore to the surrounding formation, which in turn is a function of depth and production/injection time The authors present an approach to estimate wellbore fluid temperature during steady-state two-phase flow The method incorporates a new solution of the thermal diffusivity equation and the effect of both conductive and convective heat transport for the wellbore/formation system For the multiphase flow in the wellbore, the Hasan-Kabir model has been adapted, although other mechanistic models may be used A field example is used to illustrate the fluid temperature calculation procedure and shows the importance of accounting for convection in the tubing/casing annulus A sensitivity study shows that significant differences exist between the predicted wellhead temperature and the formation surface temperature and that the fluid temperature gradient is nonlinear This study further shows that increased free gas lowers the wellhead temperature as a result of the Joule-Thompson effect In such cases, the expression for fluid temperature developed earlier for single-phase flow should not be applied when multiphase flow is encountered An appropriate expression is presented in this work for wellbores producing multiphase fluids

Convection enhanced diffusion for periodic flow

Abstract: This paper studies the influence of convection by periodic or cellular flows on the effective diffusivity of a passive scalar transported by the fluid when the molecular diffusivity is small. The flows are generated by two-dimensional, steady, divergence-free, periodic velocity fields.

Onset of thermal convection in fluids with temperature‐dependent viscosity: Application to the oceanic mantle

Abstract: Heat flow measurements through old seafloor demonstrate that the oceanic lithosphere is heated from below away from hot spot tracks. We reevaluate the hypothesis of small-scale convection beneath the lithosphere with laboratory experiments in fluids whose viscosity depends strongly on temperature. Rayleigh numbers were between 106 and 108 and viscosity contrasts were up to 106. A layer of fluid was impulsively cooled from above, and a cold boundary layer grew at the top of the fluid layer. After a finite time, convective instabilities developed in the lowermost part of the boundary layer, while the upper part remained stagnant. The variation of surface heat flow as a function of time reflects the three-dimensional nature of the flow and the presence of a thick lid. At viscosity contrasts greater than 103, this variation is very similar to what is observed on the oceanic lithosphere. For small times, heat flow follows the behavior of a half-space cooled from above by conduction. Some time after the onset of convection, it deviates from the conductive evolution and settles to a value which seems almost constant over a length of time equal to a few multiples of the onset time. The occurrence of small-scale convection is difficult to detect in global data sets of seafloor depths. The onset of convection is marked by a small “trough” in the local subsidence curve but does not occur at the same time everywhere because of the probabilistic nature of the instability process. Later instabilities occur independently of each other and, at any given age, involve a region of small horizontal extent below a thick lid. The characteristics of the instability depend on the function describing the variation of viscosity with temperature. Scaling laws are derived for the onset time and for the surface heat flow. The requirement that small-scale convection supplies 45 mW m−2 to the oceanic lithosphere provides a relationship between the activation enthalpy for creep and the asthenosphere viscosity. For a range of activation enthalpy of 250 to 600 kJ mol−1, the asthenosphere viscosity must be between 3×1018 and 4×1017 Pa s. The thickness of the stagnant lid and the temperature difference driving small-scale convection are predicted to be about 80 km and 200°C, respectively.

Convection driven zonal flows and vortices in the major planets

Abstract: The dynamical properties of convection in rotating cylindrical annuli and spherical shells are reviewed. Simple theoretical models and experimental simulations of planetary convection through the use of the centrifugal force in the laboratory are emphasized. The model of columnar convection in a cylindrical annulus not only serves as a guide to the dynamical properties of convection in rotating sphere; it also is of interest as a basic physical system that exhibits several dynamical properties in their most simple form. The generation of zonal mean flows is discussed in some detail and examples of recent numerical computations are presented. The exploration of the parameter space for the annulus model is not yet complete and the theoretical exploration of convection in rotating spheres is still in the beginning phase. Quantitative comparisons with the observations of the dynamics of planetary atmospheres will have to await the consideration in the models of the effects of magnetic fields and the deviations from the Boussinesq approximation.

Dynamics of emerging active region flux loops

Abstract: The buoyant rise of a magnetic flux loop arising from a single perturbed segment of a toroidal flux ring lying slightly beneath the base of the convection zone is studied by way of numerical simulations. We have considered flux loop evolution assuming both solid-body rotation, and differential rotation consistent with recent results from helioseismology. Our major results are presented, and we offer some speculations on the decay of active regions, based on the results of our studies. We speculate that as plasma in the tube attempts to establish hydrostatic equilibrium along the field lines after the flux emergence has taken place, the tube field strength at some intermediate depths below the surface becomes sufficiently small at the surface portions of the tube (which have cooled and undergone convective collapse) become dynamically disconnected from those portions near the base of the convection zone. The surface proportions of the emerged flux tubes are then transported by motions near the photosphere, such as supergranular convection and meridional flow.

Warm Core Vortex Amplification over Land

Abstract: A convectively generated mesoscale vortex that was instrumental in initiating and organizing five successive mesoscale convective systems over a period of three days is documented. Two of these convective systems were especially intense and resulted in widespread heavy rain with localized flooding. Based upon radar and satellite data, the detectable size of the vortex became much larger following the strong convective developments, nearly tripling its initial diameter over its three-day life cycle. During nighttime, when convection typically intensified within the vortex, movement of the system tended to slow. Following dissipation of the convection in the morning, the daytime movement accelerated. Cross sections of potential vorticity taken through the vortex center clearly show a maximum at midlevels and a well-defined minimum directly above. The vortex and the potential vorticity maximum were essentially colocated and the system was nearly axisymmetric in the vertical. Over the three-day life ...

Planetary-Scale Circulations in the Presence of Climatological and Wave-Induced Heating

Abstract: Interaction between the large-scale circulation and the convective pattern is investigated in a coupled system governed by the linearized primitive equations. Convection is represented in terms of two components of heating: A 'climatological component' is prescribed stochastically to represent convection that is maintained by fixed distributions of land and sea and sea surface temperature (SST). An 'induced component' is defined in terms of the column-integrated moisture flux convergence to represent convection that is produced through feedback with the circulation. Each component describes the envelope organizing mesoscale convective activity. As SST on the equator is increased, induced heating amplifies in the gravest zonal wavenumbers at eastward frequencies, where positive feedback offsets dissipation. Under barotropic stratification, a critical SST of 29.5 C results in positive feedback exactly cancelling dissipation in wavenumber 1 for an eastward phase speed of 6 m/s. Sympathetic interaction between the circulation and the induced heating is the basis for 'frictional wave-Conditional Instability of the Second Kind (CISK)', which is distinguished from classical wave-CISK by rendering the gravest zonal dimensions most unstable. Under baroclinic stratification, the coupled system exhibits similar behavior. The critical SST is only 26.5 C for conditions representative of equinox, but in excess of 30 C for conditions representative of solstice. Having the form of an unsteady Walker circulation, the disturbance produced by frictional wave-CISK compares favorably with the observed life cycle of the Madden-Julian oscillation (MJO). SST above the critical value produces an amplifying disturbance in which enhanced convection coincides with upper-tropospheric westerlies and is positively correlated with temperature and surface convergence. Conversely, SST below the critical value produces a decaying disturbance in which enhanced convection coincides with upper-tropospheric easterlies and is nearly in quadrature with temperature and surface convergence. While sharing essential features with the MJO in the Eastern Hemisphere, frictional wave-CISK does not explain observed behavior in the Western Hemisphere, where the convective signal is largely absent. Comprised of Kelvin structure with the same frequency, observed behavior in the Western Hemisphere can be understood as a propagating response that is excited in and radiates away from the fluctuation of convection in the Eastern Hemisphere.

Coupling of microprocesses and macroprocesses due to velocity shear: An application to the low-altitude ionosphere

Abstract: Recent observations indicate that low-altitude (below 1500 km) ion energization and thermal ion upwelling are colocated in the convective flow reversal region. In this region the convective velocity V(sub perpendicular) is generally small but spatial gradients in V(sub perpendicular) can be large. As a result, Joule heating is small. The observed high level of ion heating (few electron volts or more) cannot be explained by classical Joule heating alone but requires additional heating sources such as plasma waves. At these lower altitudes, sources of free energy are not obvious and hence the nature of ion energization remains ill understood. The high degree of correlation of ion heating with shear in the convective velocity (Tsunoda et al., 1989) is suggestive of an important role of velocity shear in this phenomenon. We provide more recent evidence for this correlation and show that even a small amount of velocity shear in the transverse flow is sufficient to excite a large-scale Kelvin-Helmholtz mode, which can nonlinearly steepen and give rise to highly stressed regions of strongly sheared flows. Futhermore, these stressed regions of strongly sheared flows may seed plasma waves in the range of ion cyclotron to lower hybrid frequencies, which are potential sources for ion heating. This novle two-step mechanism for ion energization is applied to typical observations of low-altitude thermal ion upwelling events.

Quasi-two-dimensional dynamics of plasmas and fluids.

Abstract: In the lowest order of approximation quasi‐two‐dimensional dynamics of planetary atmospheres and of plasmas in a magnetic field can be described by a common convective vortex equation, the Charney and Hasegawa–Mima (CHM) equation. In contrast to the two‐dimensional Navier–Stokes equation, the CHM equation admits ‘‘shielded vortex solutions’’ in a homogeneous limit and linear waves (‘‘Rossby waves’’ in the planetary atmosphere and ‘‘drift waves’’ in plasmas) in the presence of inhomogeneity. Because of these properties, the nonlinear dynamics described by the CHM equation provide rich solutions which involve turbulent, coherent and wave behaviors. Bringing in nonideal effects such as resistivity makes the plasma equation significantly different from the atmospheric equation with such new effects as instability of the drift wave driven by the resistivity and density gradient. The model equation deviates from the CHM equation and becomes coupled with Maxwell equations. This article reviews the linear and non...

A numerical approach to model non‐isothermal viscous flow through fibrous media with free surfaces

Abstract: A numerical simulation is presented to predict the free surface and its interactions with heat transfer and cure for flow of a shear-thinning resin through the fibre preform the flow part of the simulation is based on the finite element/control volume method. Since the traditional control volume approach produces an error associated with a mass balance inconsistency, a new method which overcomes this issue is proposed, the element control volume method. The heat transfer and cure analysis in the simulation are based on the finite difference/control volume method. Since heat conduction is dominant in the through-thickness direction and most of the heat convection is in-plane, heat transfer and cure are solved in fully three-dimensional form. A simple concept of the boundary condition constant is introduced which models a realistic mould configuration with a heating element located at a distance behind the mould wall. The varying viscosity throughout the mould associated with the strain rate, temperature and degree of cure distribution may be accounted for in calculating the mould-filling pattern. This introduces a two-way coupling between momentum and energy transport in fibrous media during mould filling.

A new mechanism for polar patch formation

Abstract: Polar patches are regions within the polar cap where the F-region electron concentration and airglow emission at 630 nm are enhanced above a background level. Previous observations have demonstrated that polar patches can be readily identified in Polar Anglo-American Conjugate Experiment (PACE) data. Here PACE data and those from complementary instruments are used to show that some polar patches form in the dayside cusp within a few minutes of the simultaneous occurrence of a flow channel event (short-lived plasma jets ∼2 km s−1) and azimuthal flow changes in the ionospheric convection pattern. The latter are caused by variations of the y-component of the interplanetary magnetic field. The physical processes by which these phenomena cause plasma enhancements and depletions in the vicinity of the dayside cusp and cleft are discussed. Subsequently, these features are transported into the polar cap where they continue to evolve. The spatial scale of patches when formed is usually 200-1000 km in longitude and 2°-3° wide in latitude. Their motion after formation and the velocity of the plasma within the patches are the same, indicating that they are drifting under the action of an electric field. Occasionally, patches are observed to occur simultaneously in geomagnetic conjugate regions. Since some of these observations are incompatible with the presently-accepted model for patch formation involving the expansion of the high latitude convection pattern entraining solar-produced plasma, further modeling of the effects of energetic particle precipitation in the cusp, the consequences of flow channel events on the plasma concentrations, and the time dependence of plasma convection as a result of interplanetary magnetic field By changes is strongly recommended. Such studies could be used to determine the relative importance of this new mechanism compared with the existing theory for patch formation as a function of universal time and season.

Organization of convection within the Madden-Julian oscillation

Abstract: Convection associated with the Madden-Julian oscillation (MJO) is organized over a broad range of time and space scales within a large-scale envelope of activity The large-scale envelope of convection is characterized as an eastward propagating disturbance, predominantly of zonal wavenumber 2, which is confined over the warmest waters of the Indian and western Pacific oceans Large-scale convection is shifted toward the disturbance's temperature anomaly, which appears to result from frictional effects in the boundary layer Synoptic and mesoscale convective activity is enhanced within the wet phase of the large-scale envelope The increase at synoptic scales occurs uniformly over all scales, but little difference in the character of the variability is observed between dry and wet phases The enhanced mesoscale activity within the wet phases exhibits a marked tendency to be organized into westward propagating clusters with periods near 2 days and half wavelengths of about 1000 km The role of these modes for the evolution of the MJO is discussed

Avalanche effects in phase transition modulated thermal convection: A model of Earth's mantle

Abstract: We describe the development and application of an anelastic, multiphase model of the mantle convection process in axisymmetric spherical shell geometry. The radial structure of the anelastic reference state has been determined on the basis of elastic wave propagation data, primarily those used to construct the preliminary reference Earth model (PREM). The multiphase model is employed to examine the extent to which the pressure-induced phase transitions in the planetary mantle may conspire to cause the flow to become radially layered. We find that the endothermic phase transition at 670 km depth profoundly influences the radial mixing process in the high Rayleigh number regime. In the Earth-like region of parameter space the flow exhibits a low-frequency quasi-periodicity characterized by rather long periods of relative quiescence in which the circulation is predominantly layered followed by short periods of intense radial mixing across the endothermic horizon. These "avalanche" events are controlled by the periodic instability of the internal thermal boundary layer that develops on the endothermic horizon when the flow is layered. This hydrodynamic process appears to have important implications for the understanding of a number of characteristics of planetary evolution, especially thermal and chemical history.