Showing papers on "Convection published in 1997"
TL;DR: In this paper, the Navier Stokes model on the sphere has been used to model the global circulation of the ocean, from the convective scale to the global scale, and a solution strategy has been proposed to deal with small-scale phenomena which are not in hydrostatic balance.
Abstract: Ocean models based on consistent hydrostatic, quasi-hydrostatic, and nonhydrostatic equation sets are formulated and discussed. The quasi-hydrostatic and nonhydrostatic sets are more accurate than the widely used hydrostatic primitive equations. Quasi-hydrostatic models relax the precise balance between gravity and pressure gradient forces by including in a consistent manner cosine-of-latitude Coriolis terms which are neglected in primitive equation models. Nonhydrostatic models employ the full incompressible Navier Stokes equations; they are required in the study of small-scale phenomena in the ocean which are not in hydrostatic balance. We outline a solution strategy for the Navier Stokes model on the sphere that performs efficiently across the whole range of scales in the ocean, from the convective scale to the global scale, and so leads to a model of great versatility. In the hydrostatic limit the Navier Stokes model involves no more computational effort than those models which assume strict hydrostatic balance on all scales. The strategy is illustrated in simulations of laboratory experiments in rotating convection on scales of a few centimeters, simulations of convective and baroclinic instability of the mixed layer on the 1- to 10-km scale, and simulations of the global circulation of the ocean.
1,302 citations
TL;DR: In this article, a two-component lattice Boltzmann equation (LBE) method was used to simulate Rayleigh-B\'enard convection in two and three dimensions.
Abstract: Rayleigh-B\'enard convection is numerically simulated in two and three dimensions using a recently developed two-component lattice Boltzmann equation (LBE) method. The density field of the second component, which evolves according to the advection-diffusion equation of a passive scalar, is used to simulate the temperature field. A body force proportional to the temperature is applied, and the system satisfies the Boussinesq equation except for a slight compressibility. A no-slip, isothermal boundary condition is imposed in the vertical direction, and periodic boundary conditions are used in horizontal directions. The critical Rayleigh number for the onset of the Rayleigh-B\'enard convection agrees with the theoretical prediction. As the Rayleigh number is increased higher, the steady two-dimensional convection rolls become unstable. The wavy instability and aperiodic motion observed, as well as the Nusselt number as a function of the Rayleigh number, are in good agreement with experimental observations and theoretical predictions. The LBE model is found to be efficient, accurate, and numerically stable for the simulation of fluid flows with heat and mass transfer.
519 citations
TL;DR: In this article, a two-dimensional (zonally or azimuthally averaged) model with parameterized horizontal and vertical fluxes is compared to three-dimensional numerical calculations in which the eddy transfer is resolved.
Abstract: Parametric representations of oceanic geostrophic eddy transfer of heat and salt are studied ranging from horizontal diffusion to the more physically based approaches of Green and Stone (GS) and Gent and McWilliams (GM). The authors argue for a representation that combines the best aspects of GS and GM: transfer coefficients that vary in space and time in a manner that depends on the large-scale density fields (GS) and adoption of a transformed Eulerian mean formalism (GM). Recommendations are based upon a two-dimensional (zonally or azimuthally averaged) model with parameterized horizontal and vertical fluxes that is compared to three-dimensional numerical calculations in which the eddy transfer is resolved. Three different scenarios are considered: 1) a convective ‘‘chimney’’ where the baroclinic zone is created by differential surface cooling; 2) spindown of a frontal zone due to baroclinic eddies; and 3) a wind-driven, baroclinically unstable channel. Guided by baroclinic instability theory and calibrated against eddy-resolving calculations, the authors recommend a form for the horizontal transfer coefficient given by 2 fM 2 2 k 5 a l 5 a l, N ˇRi where Ri 5 f2N2/M4 is the large-scale Richardson number and f is the Coriolis parameter; M2 and N2 are measures of the horizontal and vertical stratification of the large-scale flow, l measures the width of the baroclinic zone, and a is a constant of proportionality. In the very different scenarios studied here the authors find a to be a ‘‘universal’’ constant equal to 0.015, not dissimilar to that found by Green for geostrophic eddies in the atmosphere. The magnitude of the implied k, however, varies from 300 m2 s21 in the chimney to 2000 m2 s21 in the wind-driven channel.
485 citations
TL;DR: The present study used an articulated thermal manikin with 16 body segments to generate radiative heat transfer coefficients as well as natural- and forced-mode convective coefficients to simulate both transient and spatial inhomogeneities in the thermal environment.
Abstract: Human thermal physiological and comfort models will soon be able to simulate both transient and spatial inhomogeneities in the thermal environment With this increasing detail comes the need for anatomically specific convective and radiative heat transfer coefficients for the human body The present study used an articulated thermal manikin with 16 body segments (head, chest, back, upper arms, forearms, hands, pelvis, upper legs, lower legs, feet) to generate radiative heat transfer coefficients as well as natural- and forced-mode convective coefficients The tests were conducted across a range of wind speeds from still air to 50 m/s, representing atmospheric conditions typical of both indoors and outdoors Both standing and seated postures were investigated, as were eight different wind azimuth angles The radiative heat transfer coefficient measured for the whole-body was 45 W/m2 per K for both the seated and standing cases, closely matching the generally accepted whole-body value of 47 W/m2 per K Similarly, the whole-body natural convection coefficient for the manikin fell within the mid-range of previously published values at 34 and 33 W/m2 per K when standing and seated respectively In the forced convective regime, heat transfer coefficients were higher for hands, feet and peripheral limbs compared to the central torso region Wind direction had little effect on convective heat transfers from individual body segments A general-purpose forced convection equation suitable for application to both seated and standing postures indoors was hc=103v06 for the whole-body Similar equations were generated for individual body segments in both seated and standing postures
369 citations
TL;DR: A global set of 714 mesoscale convective complexes is compiled and some of the common properties of the convective systems are identified and examined from a global perspective as discussed by the authors.
Abstract: A global set of 714 mesoscale convective complexes is compiled and some of the common properties of the convective systems are identified and examined from a global perspective. the data set includes date of occurrence, time of first storms, initiation, maximum extent, termination, duration, cold-cloud shield areas, and tracks from initiation to termination.
It is found that the typical convective complex is nocturnal, generates a cold-cloud shield area of approximately 350 000 km2, and persists for about 10 h. the largest systems and most persistent systems tend to occur near the summer solstices. For the globe, about 400 systems occur each year, primarily over land areas. Most systems develop in favoured zones, although some activity occurs over every continent (except Antarctica) and all major oceans. the concentration of activity into favoured zones indicates that there must be special dynamic and/or thermodynamic conditions necessary for convection to organize into convective complexes.
Activity is strongly tied to the solar day, and shifts from 35°S in early January to about 50°N during the boreal summer and back to 35°S by December. Within the northern hemisphere there is a pronounced poleward migration as the jet stream shifts northward. Relatively little migration occurs in the ocean-dominated southern hemisphere where the subtropical jet remains quasi-stationary over the convective-complex regions.
The nocturnal life cycles, copious rainfall, large cloud shields, and great frequency of mesoscale convective complexes suggest that they may be significant contributors to the global hydrologic cycle and earth-system energy budget.
339 citations
TL;DR: In this article, the authors compare chemical transport simulations performed in a model using archived meteorological data (an off-line transport model) to those performed in an on-line model in which the data are predicted every time step (an online model).
Abstract: We compare chemical transport simulations performed in a model using archived meteorological data (an off-line transport model) to those performed in a model in which the meteorological data are predicted every time step (an on-line model). We identify the errors associated with using data sampled at timescales much longer than those operating in the atmosphere or in the on-line model, and strategies for ameleorating those errors. The evaluation is performed in the context of a global off-line chemical transport model called the Model of Atmospheric Transport and Chemistry (MATCH) for three test problems: (1) the passive advection of blobs initially concentrated in the lower and upper troposphere; (2) the surface emission of radon and its decay to lead; and (3) the removal of lead from the atmosphere by wet and dry deposition processes. These problems exercise the important processes of transport by resolved scale winds, rapid transport by smaller scale convection processes, and wet removal (which depends on the representation of the hydrologic cycle). We show that the errors in off-line model simulations (compared to the on-line simulations) can be made small when the sampling interval is order 6 hours or less. We also show that one can accurately reproduce the subgrid-scale processes within the off-line model, rather than needing to archive the results of those processes as input to the off-line model. This suggests that for the spatial and temporal scales treated in global models it is possible to treat many problems nearly as accurately in an off-line mode as one can with an on-line treatment.
308 citations
TL;DR: In this article, the basic equation for volume, heat and CO2 flux in a porous medium which is subject to both a temperature field and molecular diffusion have been analyzed with respect to the stability criteria for convectional vertical flow.
308 citations
TL;DR: In this paper, an experimental study of Rayleigh-Benard convection in the strongly turbulent regime is presented, where the authors report results obtained at low Prandtl number (in mercury, Pr = 0.025) and compare them with results at Pr∼1.
Abstract: An experimental study of Rayleigh–Benard convection in the strongly turbulent regime is presented. We report results obtained at low Prandtl number (in mercury, Pr = 0.025), covering a range of Rayleigh numbers 5 × 106 < Ra < 5 × 109, and compare them with results at Pr∼1. The convective chamber consists of a cylindrical cell of aspect ratio 1.Heat flux measurements indicate a regime with Nusselt number increasing as Ra0.26, close to the 2/7 power observed at Pr∼1, but with a smaller prefactor, which contradicts recent theoretical predictions. A transition to a new turbulent regime is suggested for Ra ≃ 2 × 109, with significant increase of the Nusselt number. The formation of a large convective cell in the bulk is revealed by its thermal signature on the bottom and top plates. One frequency of the temperature oscillation is related to the velocity of this convective cell. We then obtain the typical temperature and velocity in the bulk versus the Rayleigh number, and compare them with similar results known for Pr∼1.We review two recent theoretical models, namely the mixing zone model of Castaing et al. (1989), and a model of the turbulent boundary layer by Shraiman & Siggia (1990). We discuss how these models fail at low Prandtl number, and propose modifications for this case. Specific scaling laws for fluids at low Prandtl number are then obtained, providing an interpretation of our experimental results in mercury, as well as extrapolations for other liquid metals.
306 citations
TL;DR: In this article, an objective scheme is developed to process atmospheric soundings of winds, temperature, and water vapor mixing ratio over a network of a small number of stations, given the inevitable uncertainties in the original data, state variables of the atmosphere are adjusted by the smallest possible amount in this scheme to conserve columnintegrated mass, moisture, static energy, and momentum.
Abstract: For the purpose of deriving grid-scale vertical velocity and advective tendencies from sounding measurements, an objective scheme is developed to process atmospheric soundings of winds, temperature, and water vapor mixing ratio over a network of a small number of stations. Given the inevitable uncertainties in the original data, state variables of the atmosphere are adjusted by the smallest possible amount in this scheme to conserve column-integrated mass, moisture, static energy, and momentum. The scheme has the capability of incorporating a variety of supplemental measurements to constrain large-scale vertical velocity and advective tendencies derived from state variables. The method has been implemented to process the Atmospheric Radiation Measurement Program’s (ARM) soundings of winds, temperature, and water vapor mixing ratio at the boundary facilities around the Cloud and Radiation Testbed site in northern Oklahoma in April 1994. It is found that state variables are adjusted by an amount comparable to their measurement uncertainties to satisfy the conservation requirements of mass, water vapor, heat, and momentum. Without these adjustments, spurious residual sources and sinks in the column budget of each quantity have the same magnitudes as other leading components. Sensitivities of the diagnosed vertical velocity and apparent heat, moisture, and momentum sources to the number of conservation constraints are presented. It is shown that constraints of column budget of moisture and dry static energy can make large differences to these diagnostics, especially when some original sounding data are missing and have to be interpolated. Analysis of the moisture budget shows that large-scale convergence often corresponds to precipitation, but there are occasions when precipitation corresponds to a large reduction of column precipitable water and columnmoisture divergence. Analysis of momentum budget shows large magnitudes of subgrid-scale momentum sources and sinks (about 4 m s21 h21) in the convective events.
289 citations
TL;DR: In this article, the authors present a systematic study of three potentially important effects: depth-dependent viscosity, an endothermic phase change, and bottom versus internal heating.
Abstract: Mantle convection is influenced simultaneously by a number of physical effects: brittle failure in the surface plates, strongly variable viscosity, mineral phase changes, and both internal heating (radioactivity) and bottom heating from the core. Here we present a systematic study of three potentially important effects: depth-dependent viscosity, an endothermic phase change, and bottom versus internal heating. We model three-dimensional spherical convection at Rayleigh Ra=108 thus approaching the dynamical regime of the mantle. An isoviscous, internally heated reference model displays point-like downwellings from the cold upper boundary layer, a blue spectrum of thermal heterogeneity, and small but rapid time variations in flow diagnostics. A modest factor 30 increase in lower mantle viscosity results in a planform dominated by long, linear downwellings, a red spectrum, and great temporal stability. Bottom heating has the predictable effect of adding a thermal boundary layer at the base of the mantle. We use a Clapeyron slope of γ=−4 MPa °K−1 for the 670 km phase transition, resulting in a phase buoyancy parameter of P=−0.112. This phase change causes upwellings and downwellings to pause in the transition zone but has little influence on the inherent time dependence of flow and only a modest reddening effect on the heterogeneity spectrum. Larger values of P result in stronger effects, but our choice of P is likely already too large to be representative of the mantle transition zone. Combinations of all three effects are remarkably predictable in terms of the single-effect models, and the effect of depth-dependent viscosity is found to be dominant.
271 citations
TL;DR: In this paper, a parametrization of convective momentum-transports for deep convection based upon the mass-flux convection-scheme discussed by Gregory and Rowntree is presented.
Abstract: Diagnostics derived from cloud-resolving-model simulations in part I of this study, relating to the vertical transport of horizontal momentum by convection, are used to develop a parametrization of convective momentum-transports for deep convection based upon the mass-flux convection-scheme discussed by Gregory and Rowntree. the importance of cloud pressure-gradients to the flow within the cloud is emphasised, and a simple method of representing their effect is suggested. the scheme is able to reproduce the fluxes derived from the cloud-resolving model studies where cloud organization by the flow is unimportant. Inclusion of the parametrization in a version of the Meteorological Office Unified Model demonstrates that convective momentum-transports play a large role in the momentum balance of the atmosphere. Generally, simulation of the mean atmospheric circulation by the Unified Model is improved by the inclusion of such transports.
TL;DR: In this paper, the authors show that the Northern Hemisphere summer monsoon displays statistically significant climatological intraseasonal oscillations (CISOs), which characterize monsoon singularities.
Abstract: Using climatological pentad mean outgoing longwave radiation (OLR) and European Centre for Medium-Range Weather Forecasts analysis winds, the authors show that the Northern Hemisphere summer monsoon displays statistically significant climatological intraseasonal oscillations (CISOs). The extreme phases of CISO characterize monsoon singularities—monsoon events that occur on a fixed pentad with usual regularity, whereas the transitional phases of CISO represent the largest year-to-year monsoon variations. The CISO results from a phase-locking of transient intraseasonal oscillation to annual cycle. It exhibits a dynamically coherent structure between enhanced convection and low-level convergent (upper-level divergent) cyclonic (anticyclonic) circulation. Its phase propagates primarily northward from the equator to the northern Philippines during early summer (May–July), and westward along 15°N from 170°E to the Bay of Bengal during August and September. The propagation of CISO links monsoon singular...
TL;DR: In this article, the authors show that the convection generated SST gradient plays an important role in cluster propagation and development, and the relative importance of radiative and evaporative fluxes in SST regulation is also discussed.
Abstract: Existing theories of the Madden–Julian oscillation neglect the feedback between the modification of sea surface temperature by the convection and development of a convective cluster itself. The authors show that the convection-generated SST gradient plays an important role in cluster propagation and development. The relative importance of radiative and evaporative fluxes in SST regulation is also discussed. Various Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment and Central Equatorial Pacific Experiment observation platforms are used to estimate the effects of equatorial convection on SST changes during March 1993. The data include drifting buoys and TAO-buoy array measurements, combined with the Navy Operational Global Atmospheric Prediction System analyzed surface wind fields and Geostationary Meteorological Satellite cloud-top temperatures. It is shown that during the equatorial convection episode SST is decreasing under and to the west of the convective heat sour...
TL;DR: In this article, a two-layer nonlinear theory is developed to account properly for the effect of deformation on the interface temperature profile, which is shown to be significant in microgravity and for thin liquid layers.
Abstract: Surface-tension-driven Benard (Marangoni) convection in liquid layers heated from below can exhibit a long-wavelength primary instability that differs from the more familiar hexagonal instability associated with Benard. This long-wavelength instability is predicted to be significant in microgravity and for thin liquid layers. The instability is studied experimentally in terrestrial gravity for silicone oil layers 0.007 to 0.027 cm thick on a conducting plate. For shallow liquid depths ( 0.024 cm), the system forms only the hexagonal convection cells. A two-layer nonlinear theory is developed to account properly for the effect of deformation on the interface temperature profile. Experimental results for the long-wavelength instability are compared to our two-layer theory and to a one-layer theory that accounts for the upper gas layer solely with a heat transfer coefficient. The two-layer model better describes the onset of instability and also predicts the formation of localized elevations, which the one-layer model does not predict. A weakly nonlinear analysis shows that the bifurcation is subcritical. Solving for steady states of the system shows that the subcritical pitchfork bifurcation curve never turns over to a stable branch. Numerical simulations also predict a subcritical instability and yield long-wavelength states that qualitatively agree with the experiments. The observations agree with the onset prediction of the two-layer model, except for very thin liquid layers; this deviation from theory may arise from small non-uniformities in the experiment. Theoretical analysis shows that a small non-uniformity in heating produces a large steady-state deformation (seen in the experiment) that becomes more pronounced with increasing temperature difference across the liquid. This steady-state deformation becomes unstable to the long-wavelength instability at a smaller temperature difference than that at which the undeformed state becomes unstable in the absence of non-uniformity.
TL;DR: In this article, a review of the many techniques that have been developed to enhance convective heat transfer is presented, including compound techniques, pool boiling, convective boiling/evaporation, vapor-space condensation, and convective condensation.
Abstract: This review considers the many techniques that have been developed to enhance convective heat transfer. After introducing the techniques, the applications to most of the modes of heat transfer (single-phase forced convection, including compound techniques, pool boiling, convective boiling/evaporation, vapor-space condensation, and convective condensation) are described. Comments are offered regarding commercial introduction of this technology and the generations of heat transfer technology : advanced enhancement represents third-generation heat transfer tehnology.
TL;DR: In this article, the authors examined the relationship between intraseasonal (periods <100 days) variations of convection, sea surface temperature (SST), surface wind stress, and surface fluxes of latent heat and radiation in the warm pool of the equatorial Indian and western Pacific Oceans.
Abstract: The relationships between intraseasonal (periods <100 days) variations of convection, sea surface temperature (SST), surface wind stress, and surface fluxes of latent heat and radiation in the warm pool of the equatorial Indian and western Pacific Oceans are examined using 7 yr of gridded outgoing longwave radiation (OLR), SST, and surface stress and latent heat flux based on European Centre for Medium-Range Weather Forecasts analyses. In the warm pool region enhanced evaporation, which results from enhanced surface westerlies, lags enhanced convection by ∼1 week. Intraseasonal SST fluctuations lag decreased evaporation by ∼1 week and decreased convection (which implies increased insolation) by ∼2 weeks, suggesting that anomalous latent heat flux and surface insolation drive SST changes on intraseasonal timescale. The relationship between anomalous SST, surface wind stress and surface fluxes of latent heat and shortwave radiation for the Madden–Julian oscillation (MJO), which dominates the intras...
TL;DR: In this paper, the authors examined the environmental conditions necessary for sustaining rolls and for influencing the aspect ratio, ratio of roll wavelength to convective boundary layer (CBL) depth, and orientation.
Abstract: Data from the Convection and Precipitation/Electrification (CaPE) project, as well as results from numerical simulations, are used to study horizontal convective rolls. The environmental conditions necessary for sustaining rolls and for influencing the aspect ratio, ratio of roll wavelength to convective boundary layer (CBL) depth, and orientation are examined. Observations and numerical model simulations both suggest that a moderate surface sensible heat flux and some vertical wind shear are necessary for roll existence. Unlike some previous studies, however, it is shown that rolls occurred within very low CBL shear conditions (∼2 × 10−3 s−1). In addition, the low-level (i.e., ∼200 m) shear seems to be more important than the shear through the depth of the CBL in roll sustenance. The aspect ratio is shown to be proportional to the CBL instability, measured in terms of the Monin–Obukhov length. The roll orientation is similar to the wind direction at 10 m AGL, the CBL wind direction, the inversio...
TL;DR: In this paper, the authors investigate the nature and onset of convection for a range of boundary conditions typically found in saline lakes and provide insight into the predictive behavior of groundwater dynamics and solute distributions in many natural systems.
Abstract: Convective groundwater motion can be generated in aquifers beneath and adjacent to saline lakes. Such motions strongly control the subsurface distribution of salts, the formation of saline minerals, and the distribution of contaminants that may be disposed of in the saline lake. In particular, schemes to use saline lakes as evaporation basins for irrigation waste waters must consider the groundwater dynamics associated with convection. The convection is driven by the evaporative concentration of salts at the land surface, leading to an unstable distribution of density. However, the evaporative groundwater discharge can dynamically stabilize this saline boundary layer and inhibit convection. In this work we investigate the nature and onset of convection for a range of boundary conditions typically found in saline lakes. Processes involved in the accumulation of salt at the surface of a groundwater discharge zone and in the gravitational instability of the near-surface groundwater are considered. Results of theoretical stability analysis are applied through laboratory and numerical experimentation to the more complex geometries typically found in the field. These comparisons indicate that for large saline lakes, the stability of the saline boundary layer can be parameterized by traditional Rayleigh criteria, in which the aquifer permeability and the evaporation rate from the lake bed are principal controlling factors. For typical saline lake environments, convection will dominate in sediments whose permeability exceeds approximately 10−14 m2. Below this threshold permeability, the boundary layer should be stabilized by the evaporative flux, resulting in the accumulation of salts and evaporites at the land surface. These results provide insight into the predictive behavior of groundwater dynamics and solute distributions in many natural systems.
TL;DR: In this paper, a nonlinear shallow water model on the sphere is used to simulate barotropic aspects of ITCZ breakdown, where a prescribed zonally elongated mass sink near the equator produces a cyclonic potential vorticity (PV) anomaly that has a reversed meridional PV gradient on its poleward side.
Abstract: In satellite images the ITCZ (intertropical convergence zone) is sometimes observed to undulate and break down into a series of tropical disturbances. Tropical cyclones may later develop within these disturbances and move into higher latitudes allowing the ITCZ to reform. It has been proposed that ITCZ breakdown results from a convectively modified form of combined barotropic and baroclinic instability of the mean flow. An unstable mean flow can be produced by ITCZ convection in just a couple of days. In this sense, the ITCZ produces favorable conditions for its own instability and breakdown. In this study, a nonlinear shallow water model on the sphere is used to simulate barotropic aspects of ITCZ breakdown. In the shallow-water model, the ITCZ is simulated by a prescribed zonally elongated mass sink near the equator. The mass sink produces a cyclonic potential vorticity (PV) anomaly that has a reversed meridional PV gradient on its poleward side. According to the Ripa theorem, a flow that has a...
TL;DR: In this article, a complete model is used for the prediction of the daily and annual variation of ground surface temperature, based on the transient heat conduction differential equation using as boundary condition the energy balance equation at the ground surface.
TL;DR: In this article, a self-consistent model for cooling of the Earth's core is presented, in which the thermal history of the core is computed as a function of the time evolution of the heat flux delivered to the mantle across the core-mantle boundary.
TL;DR: In this article, boundary mixing is implemented in an ocean general circulation model such that the vertical mixing coefficient ky is nonzero only near side boundaries and in convection regions, and the model is used in a highly idealized configuration with no wind forcing and very nearly fixed surface density to investigate the three-dimensional dynamics of the thermohaline circulation.
Abstract: Boundary mixing is implemented in an ocean general circulation model such that the vertical mixing coefficient ky is nonzero only near side boundaries and in convection regions. The model is used in a highly idealized configuration with no wind forcing and very nearly fixed surface density to investigate the three-dimensional dynamics of the thermohaline circulation. For ky 5 20 3 1024 m2 s21 and lower, the meridional overturning strength to great accuracy is proportional to ; meridional heat transport is proportional to . The circulation 2/3 1/2 kk y y patterns resemble those from runs with uniform vertical mixing, but vertical motion is entirely confined to the boundary regions. Near the western boundary, there is upwelling everywhere. Near the eastern boundary, there is a consistent pattern of downwelling above upwelling, with downwelling reaching deeper at high latitudes; this pattern is explained by convection and vertical advective‐diffusive balance underneath. For ky 5 30 3 1024 m2 s21 and higher, no steady solutions have been found; the meridional overturning oscillates on a timescale of about 25 years. A time-averaged thermally direct overturning cell is not supported dynamically because convection extends longitudinally across the entire basin, and upwelling near the western boundary does not lead to densities higher than at the eastern boundary. Assuming uniform upwelling in the west, level isopycnals near the equator, and level isopycnals along the eastern boundary south of the outcropping latitude permits the analytic determination of convection depth at the eastern wall and hence the density difference between the eastern and western walls. This difference is at most one-quarter the surface density difference between high and low latitudes, and agrees in magnitude and latitudinal dependence with the numerical experiments. Scaling arguments estimate overturning strength as of the order of 10 3 106 m3 s21 and confirm the 2/3 power dependence onky. The derivation also gives a dependence of overturning strength with latitude that agrees qualitatively with the numerical results. The scaling for the dependence of meridional heat transport on latitude agrees well with the model results; scaling for heat transport amplitude agrees less well but correctly predicts a weaker dependence on ky than maximum overturning.
TL;DR: In this article, it is argued that the convection results from the low-level divergence-convergence doublet centred about the η = 0 contour which is the mitigating response to the inertial instability.
Abstract: There are two major organized cloud configurations in the vicinity of the equator. Where there is a small cross-equatorial surface pressure gradient, convection is close to the equator and is generally tied to the location of the lowest sea-level pressure (SLP) and warmest sea-surface temperature (SST), in agreement with arguments based upon simple thermodynamical considerations. However, when there is a substantial cross-equatorial pressure gradient, such as occurs in the monsoon regions, organized convection appears off the equator in the summer hemisphere, equatorward of the SLP minimum and not necessarily collocated with the warmest SSTs. Thus, in this instance, simple thermodynamical considerations alone cannot explain 2he location of the convection. In this situation, the zero absolute vorticity contour (η = 0) also lies in the summer hemisphere. Therefore, between the equator and the η = 0 contour is a region of locally-anticyclonic absolute vorticity and an inertially unstable regime. It is argued that the convection results from the low-level divergence-convergence doublet centred about the η = 0 contour which is the mitigating response to the inertial instability. The associated latitude-height secondary circulation should provide subsidence (suppressed convection) over the equator and rising motion (enhanced convection) to the north of the zero absolute vorticity contour.
Signatures of the inertial instability predicted by theory are found in observations supporting the hypothesis. Wherever a strong cross-equatorial pressure gradient exists, the η = 0 contour bisects a maximum in the divergent wind field. Divergence is found equatorward of the zero contour and convergence on the poleward side. Latitude-height cross sections show strong local meridional circulations with maximum rising motion on the poleward side of η = 0. As the regions where the rising motions occur are conditionally unstable, there is deep convection and the vertical circulations extend throughout the troposphere. It is noted that the intensity of the off-equator convection is deeper (and probably stronger) than convection located at the equator. This is probably because the convection associated with the inertial instability is more efficient. Necessary conditions for the location of near-equatorial convection are listed.
Arguments are presented whereby inertial instability is established as the cause, rather than an effect, of off-equatorial convection. These include an outline of the sequence of processes leading up to the convection. The factors that limit the encroachment of the η = 0 contour into the summer hemisphere are discussed and an explanation for the existence of the low-level westerly monsoon wind maximum is suggested. The possible role played by the instability mechanism (or the lack of it) in coupled model simulations that produce seasonally migrating and/or double ITCZs in the eastern Pacific Ocean is discussed. Finally, it is proposed that the instability mechanism is important in the initiation of westward-moving disturbances found in the eastern Pacific and in determining active and break periods in the summer Indian monsoon.
TL;DR: In this article, numerical simulations of convection with strongly temperature-dependent viscosity suggest that non-Newtonian visco-ity convection (dislocation creep) passes through three convective regimes similar to those observed for Newtonian conveco-flow (diffusion creep): the small viscoity contrast regime, the transitional regime and the stagnant lid regime.
Abstract: Numerical simulations of convection with strongly temperature-dependent viscosity suggest that non-Newtonian viscosity convection (dislocation creep) passes through three convective regimes similar to those observed for Newtonian viscosity convection (diffusion creep): the small viscosity contrast regime, the transitional regime and the stagnant lid regime. For realistic viscosity contrasts, mantle convection is in the stagnant lid regime characterized by formation of a very viscous, slowly creeping lid on top of an actively convecting mantle. This explains the tectonic style observed on the terrestrial planets and the Moon. On the other hand, this eliminates the possibility that the plates on Earth could be mobile due to non-Newtonian viscosity. The nature of the mobility of lithospheric plates on Earth has yet to be explained.
TL;DR: In this article, a quasi-one-dimensional electrohydrodynamically driven capillary jets are analyzed in the parametrical limit of negligible charge relaxation effects, i.e. when the electric relaxation time of the liquid is small compared to the hydrodynamic times.
Abstract: Electrohydrodynamically (EHD) driven capillary jets are analysed in this work in the parametrical limit of negligible charge relaxation effects, i.e. when the electric relaxation time of the liquid is small compared to the hydrodynamic times. This regime can be found in the electrospraying of liquids when Taylor's charged capillary jets are formed in a steady regime. A quasi-one-dimensional EHD model comprising temporal balance equations of mass, momentum, charge, the capillary balance across the surface, and the inner and outer electric fields equations is presented. The steady forms of the temporal equations take into account surface charge convection as well as Ohmic bulk conduction, inner and outer electric field equations, momentum and pressure balances. Other existing models are also compared. The propagation speed of surface disturbances is obtained using classical techniques. It is shown here that, in contrast with previous models, surface charge convection provokes a difference between the upstream and the downstream wave speed values, the upstream wave speed, to some extent, being delayed. Subcritical, supercritical and convectively unstable regions are then identified. The supercritical nature of the microjets emitted from Taylor's cones is highlighted, and the point where the jet switches from a stable to a convectively unstable regime (i.e. where the propagation speed of perturbations become zero) is identified. The electric current carried by those jets is an eigenvalue of the problem, almost independent of the boundary conditions downstream, in an analogous way to the gas flow in convergent–divergent nozzles exiting into very low pressure. The EHD model is applied to an experiment and the relevant physical quantities of the phenomenon are obtained. The EHD hypotheses of the model are then checked and confirmed within the limits of the one-dimensional assumptions.
TL;DR: In this article, the heat transfer conditions that apply at the boundary between a porous medium and a homogeneous fluid are developed as flux jump conditions based on the non-local form of the volume averaged thermal energy equations for both the fluid and the solid.
TL;DR: The upper ocean heat budget in the western equatorial Pacific warm pool is analyzed for a 3-month period from mid-September through mid-December 1992 using data from the Tropical Atmosphere Ocean (TAO) array enhanced for the Coupled Ocean Atmosphere Response Experiment as mentioned in this paper.
Abstract: The upper ocean heat budget in the western equatorial Pacific warm pool is analyzed for a 3-month period from mid-September through mid-December 1992 using data from the Tropical Atmosphere Ocean (TAO) array enhanced for the Coupled Ocean Atmosphere Response Experiment. Surface heat and moisture fluxes were measured from a centrally located TAO current meter mooring at 0°, 156°E. Lateral heat advection was estimated using temperature data from moorings within 150-250 km of 0°, 156°E. Mixing was estimated as the residual of the heat balance and compared to estimates of mixing based on the Niiler-Kraus parameterization of entrainment mixing. The analysis shows that for the diurnal cycle and for daily to weekly timescale variations like those associated with westerly wind bursts, the sea surface temperature (SST) variability is to a large extent controlled by shortwave radiation and latent heat flux. However, three-dimensional processes can also be important. For example, in early October 1992, the SST at 0°, 156°E increased by nearly 1°C in 7 days due predominately to westward heat advection. Also, the dynamical response to a moderately strong wind burst in late October 1992 included a deepening of the pycnocline, which affected the rate of entrainment cooling, and a reversal of the surface current, which affected the zonal heat advection. The importance of three-dimensional processes (particularly heat advection) in the warm-pool heat balance during this 3-month study period is confirmed by comparing the observed temperature variability with that simulated by a one-dimensional mixed layer model.
01 Dec 1997
TL;DR: In this article, the authors present a model of the solar convection zone in evolutionary and seismic models, and show that the model can be used to model the global structure of the Sun.
Abstract: Preface. Part I: Global Structure. Effects of Convection on the Mean Solar Structure J. Christensen-Dalsgaard. Convective Overshooting and Mixing I.W. Roxburgh. Effect of Turbulent Pressure on Solar Oscillation Frequencies H.M. Antia, S. Basu. Parameters of the Solar Convection Zone in Evolutionary and Seismic Models V. Baturin, S.V. Ayukov. A Calibration of Mixing Length Theory Based on RHD Simulations of Solar-Type Convection H.-G. Ludwig, et al. A Precision-Controlled Solar Model with Realistic Subatmospheric Stratification H. Schlattl, et al. Solar Models with Convective Overshoot M. Stix, M. Kiefer. Near-Surface Constraints on the Structure of Stellar Convection Zones R. Trampedach, et al. Part II: Basic Properties of Convection. Stellar Convection General Properties A. Nordlund, B. Stein. Compressible Turbulence V.M. Canuto. Sound Speed Variations Near the Photosphere Due to Entropy Perturbations in 3D Numerical Experiments D. Georgobiani, et al. A Hydrodynamic Simulation of the Radiation-Convection Transition Region Y.-C. Kim, K.L. Chan. Photospheric Downflows: How Deep, How Coherent, How Important? M.P. Rast. 'Mesogranulation' - A Convective or an Oscillatory Phenomenon? Th. Straus. Part III: Convective Effects on Oscillations. Convective Effects on Mode Frequencies C.S. Rosenthal. Excitation and Damping of Solar Acoustic Oscillations P. Kumar. On Nonlinear Solar Oscillations H.P. Singh, et al. Effects of Convection on Solar p Modes M. Swisdak, E. Zweibel. The Theory of Anisotropic P-Mode Propagation in the Solar Convection Zone Yu.D. Zhugzhda. Part IV: Rotation and Magnetic Fields. Rotation and Angular Momentum Transport J.-P. Zahn. The Solar Dynamo Problem F. Cattaneo. Differential Rotation in Turbulent Compressible Convection N.H. Brummell, et al. The Effects of Rotation on the Global Dynamics of Turbulent Convection K. Julien, et al. The Effects of Rotation on Convective Overshoot K. Julien, et al. Two-Dimensional Measurements of Sunspot Oscillations J. Staude, T. Horn. Part V: Local Properties of Convection. Acoustic Tomography of Solar Convective Flows and Structures A.G. Kosovichev, T.L. Duvall Jr. Dynamic Behavior of the Solar Atmosphere R.F. Stein, M. Carlsson. The Influence of Radiative Damping on the Modes of a Magnetized Isothermal Atmosphere D. Banerjee, et al. Dynamical Relations Between Photosphere and Chromosphere N.M. Hoekzema. Photospheric Flows as Measured by SOI/MDI N. Hurlburt, et al. The Dutch Open Telescope R.J. Rutten, et al. The Magneto-Optical Filter in Napoli: Perspectives and Test Observations P.F. Moretti, et al. Part VI: Synthesis. Synthesis, Convection, Structure and Evolution E. Schatzman. Index of Authors. Index of Keywords.
TL;DR: In this article, the authors conducted a series of statistical analyses of gridded outgoing longwave radiation and winds to quantify the magnitudes of propagating and standing components of convection and their coherence with the propagating component of the circulation.
Abstract: Two questions related to the intraseasonal variability of tropical convection and circulation remain controversial. 1) To what degree is the convective component of the Madden–Julian oscillation (MJO) a standing oscillation? 2) Is the eastward propagating circulation anomaly of the MJO coherent with a standing oscillation in convection? In an attempt to settle these issues, the authors undertake a series of statistical analyses of gridded outgoing longwave radiation and winds to quantify the magnitudes of the propagating and standing components of convection and their coherence with the propagating component of the circulation. They demonstrate that no dominant standing oscillation in convection can be identified. Instead, intraseasonal variability of convection is dominated by an eastward propagating mode, which the authors interpret as the convective signal of the MJO. This propagating component accounts for almost all of the convective variance that is coherent with the eastward propagating di...
TL;DR: In this article, three-dimensional numerical simulations of convection and magnetic field generation in the Earth's core now span several hundred thousand years; the magnetic field created during most of this time has an intensity, structure and time dependence similar to the present geomagnetic field.
Abstract: Three-dimensional numerical simulations of convection and magnetic field generation in the Earth's core now span several hundred thousand years; the magnetic field created during most of this time has an intensity, structure and time dependence similar to the present geomagnetic field. Five models are described here. The first is a homogeneous Boussinesq model, driven steadily by heat sources on the inner core boundary. At about 36 000 years into the simulation, a reversal of the dipole moment occurs that resembles those seen in the paleomagnetic reversal record. The four subsequent models are inhomogeneous, that is they allow for the varying properties of the Earth with depth. They are also evolutionary, in that they are powered by the secular cooling of the Earth over geological time. This cooling causes the inner core to grow through freezing, with the concomitant release at the inner core boundary of not only latent heat of crystallization but also light constituents of core fluid that provide respect...