Showing papers on "Convection published in 1989"

Numerical Study of Convection Observed during the Winter Monsoon Experiment Using a Mesoscale Two-Dimensional Model

Abstract: A two-dimensional version of the Pennsylvania State University mesoscale model has been applied to Winter Monsoon Experiment data in order to simulate the diurnally occurring convection observed over the South China Sea. The domain includes a representation of part of Borneo as well as the sea so that the model can simulate the initiation of convection. Also included in the model are parameterizations of mesoscale ice phase and moisture processes and longwave and shortwave radiation with a diurnal cycle. This allows use of the model to test the relative importance of various heating mechanisms to the stratiform cloud deck, which typically occupies several hundred kilometers of the domain. Frank and Cohen's cumulus parameterization scheme is employed to represent vital unresolved vertical transports in the convective area. The major conclusions are: Ice phase processes are important in determining the level of maximum large-scale heating and vertical motion because there is a strong anvil componen...

A Comprehensive Mass Flux Scheme for Cumulus Parameterization in Large-Scale Models

Abstract: Observational studies indicate that a mass flux approach may provide a realistic framework for cumulus parameterization in large-scale models, but this approach, through the introduction of a spectral cloud ensemble, leads normally to rather complex schemes. In this paper the question is addressed whether much simpler schemes can already provide realistic values of the thermal forcing by convection under various synoptic conditions. This is done through verifying such a scheme first on data from field experiments for periods of tropical penetrative convection (GATE, Marshall Islands), tradewind cumuli (ATEX, BOMEX) and extratropical organized convection (SESAME-79) and then in a NWP model. The scheme considers a population of clouds where the cloud ensemble is described by a one-dimensional bulk model as earlier applied by Yanai et al. in a diagnostic study of tropical convection. Cumulus scale downdrafts are included. Various types of convection are represented, i.e., penetrative convection in c...

Scaling of hard thermal turbulence in Rayleigh-Bénard convection

Abstract: An experimental study of Rayleigh-Benard convection in helium gas at roughly 5 K is performed in a cell with aspect ratio 1. Data are analysed in a ‘hard turbulence’ region (4 × 107 < Ra < 6 × 1012) in which the Prandtl number remains between 0.65 and 1.5. The main observation is a simple scaling behaviour over this entire range of Ra. However the results are not the same as in previous theories. For example, a classical result gives the dimensionless heat flux, Nu, proportional to . A new scaling theory is described. This new approach suggests scaling indices very close to the observed ones. The new approach is based upon the assumption that the boundary layer remains in existence even though its Rayleigh number is considerably greater than unity and is, in fact, diverging. A stability analysis of the boundary layer is performed which indicates that the boundary layer may be stabilized by the interaction of buoyancy driven effects and a fluctuating wind.

Heat Transfer Measurements From a Surface With Uniform Heat Flux and an Impinging Jet

Abstract: There are numerous studies, mostly experimental, on the characteristics and heat transfer associated with jet impingement on surfaces. These studies have considered both single jets and multiple jets (i.e., arrays) and many different aspects of impinging jets including the effects of crossflow, jet orientation (oblique jets), jet temperature, rotating surfaces, and different surface shapes. The present study is concerned with the case of a single circular turbulent air jet at the ambient air temperature impinging on a flat stationary surface. One of the difficulties in comparing recent numerical work with previous experimental results is the lack of data on the jet characteristics and in some cases the mixed thermal boundary conditions at the surface. The present work provides some new experimental results that attempt to overcome this difficulty by using a fully developed jet and a well-controlled thermal boundary condition (i.e., a uniform heat flux). No other similar measurements were found in the literature.

Large-Eddy Simulation of the Convective Atmospheric Boundary Layer

Abstract: Large-eddy simulations of a free convective atmospheric boundary layer with an overlying capping inversion are considered. Attention is given to the dependence of the results upon the various factors influencing the simulation: the subgrid model, the domain size, and the mesh resolution. By providing artificial constraints upon the convection the results also provide extra insight into the underlying dynamics. The gross features of the boundary layer, such as the overall energy budget, are not sensitive to the details of the simulations but a number of important factors are revealed. It has been found that near the surface the subgrid diffusivity must be larger than is usually supposed, in order for the vertical velocity skewness to have the correct sign. This region of the flow has a significant subgrid-scale heat flux and it seems that the subgrid model requires improvement in such cases. A revised model which under statically unstable conditions allows the mixing-length of the subgrid-scale tu...

The Finite-Amplitude Nature of Tropical Cyclogenesis

Abstract: We have constructed a simple, balanced, axisymmetric model as a means of understanding the existence of the threshold amplitude for tropical cyclogenesis discovered by Rotunno and Emanuel. The model is similar to Ooyama's but is phrased in Schubert and Hack's potential radius coordinates. The essential difference between this and other balanced models lies in the representation of convective clouds. In the present model the cumulus updraft mass flux depends simply and directly on the buoyancy (on angular momentum surfaces) of lifted subcloud-layer air and is not explicitly constrained by moisture convergence. The downdraft mass flux is equal to the updraft flux multiplied by (1−ϵ), where ϵ is the precipitation efficiency. The complete spectrum of convective clouds in nature is here represented by two extremes: deep clouds with a precipitation efficiency of one, and shallow, nonprecipitating clouds. The former stabilize the atmosphere both by heating the free atmosphere and drying out the subcloud...

Evaluation of turbulent transport and dissipation closures in second-order modeling

Abstract: We show that the turbulence statistics from our (96)3 large-eddy-simulation (LES) studies of a convective boundary layer are in excellent agreement with those from the Deardorff–Willis laboratory convection tank. Using these LES data, we evaluate contemporary parameterizations for turbulent transport and dissipation in second-order closure models of the convective boundary layer. The gradient-diffusion parameterization for turbulent transport fares poorly, due in large part to the direct influence of buoyancy. This leads to poor predictions of the vertical profiles of some turbulence statistics. We also find that the characteristic length scales for the mechanical and thermal dissipation rates typically used in second-order closure models are a factor of 2–3 too small; this leads to underpredictions of turbulence kinetic energy levels. Finally, we find that the flux and variance budgets for conservative scalars are substantially different in top-down and bottom-up diffusion. In order to reproduce...

Constraints on the Structure of Mantle Convection Using Seismic Observations, Flow Models, and the Geoid

Abstract: The establishment of the theory of plate tectonics in the late 1960s has left little doubt that the mantle is convecting. The plates themselves form the cold upper thermal boundary layer of the mantle convection system; the cooling of oceanic plates as they move away from midoceanic ridges provides the mechanism by which the Earth loses most of its heat (e.g., Sclater et al., 1980; O'Connell and Hager, 1980). The mantle is in turn cooled by the cold slabs that plunge into Earth's interior at subduction zones. Although plate tectonics implies that convective motions in the mantle are the dominant mechanism for heat transport, and we can measure the surface motions associated with it, we are remarkably ignorant of even the gross features of the interior flow field associated with this mantle circulation. Only at subduction zones, where seismicity presumably marks the particle trajectories of the cold descending boundary layer, do we have direct evidence for the interior flow pattern and state of stress. Most of what is understood, or thought to be understood, about convection in the Earth's interior is based on comparison of simplified models to observations taken at the surface. Examples of these models of mantle convection are given in the other chapters of this book, as well as in the general geophysical literature. These include studies of convection in media with uniform rheology (Busse, this volume; Jarvis and Peltier, this volume), interpretation of travel time anomalies from deep earthquakes in terms of simple thermal models of subducted slabs (Jordan eta!., this volume), interpretation of geochemical anomalies in terms of models of the distribution of mantle heterogeneities (Hart and Zindler, this volume), and interpretation of changes in the Earth's shape and rotational parameters in terms of models of mantle rheology (Peltier, this volume). In order to be useful, models must be simple enough to understand, and yet contain enough of the essential physics to be applicable. The line between oversimplification and overwhelming complexity is a fine one, and its positioning is a matter of subjective judgement, particularly when some observations have a fairly small signal to noise ratio. The ultimate test of a particular model is whether it can satisfy, within their uncertainties, the observations. If it cannot it must be rejected, although unfortunately, the converse is not true. The more types of observations a model can satisfy, however, the more likely it is to be correct.

Magnetic Flux Transport on the Sun

Abstract: Although most of the magnetic flux observed on the sun originates in the low-latitude sunspot belts, this flux is gradually dispersed over a much wider range of latitudes by supergranular convective motions and meridional circulation. Numerical simulations show how these transport processes interact over the 11-year sunspot cycle to produce a strong "topknot" polar field, whose existence near sunspot minimum is suggested by the observed strength of the interplanetary magnetic field and by the observed areal extent of polar coronal holes. The required rates of diffusion and flow are consistent with the decay rates of active regions and with the rotational properties of the large-scale solar magnetic field.

Topology of Convection beneath the Solar Surface

Abstract: It is shown that the topology of convection beneath the solar surface is dominated by effects of stratification. Convection in a strongly stratified medium has: (1) gentle expanding structureless warm upflows and (2) strong converging filamentary cool downdrafts. The horizontal flow topology is cellular, with a hierarchy of cell sizes. The small density scale height in the surface layers forces the formation of the solar granulation, which is a shallow surface phenomenon. Deeper layers support successively larger cells. The downflows of small cells close to the surface merge into filamentary downdrafts of larger cells at greater depths, and this process is likely to continue through most of the convection zone. Radiative cooling at the surface provides the entropy-deficient material which drives the circulation. 13 refs.

A benchmark comparison for mantle convection codes

Abstract: Summary We have carried out a comparison study for a set of benchmark problems which are relevant for convection in the Earth's mantle The cases comprise steady isoviscous convection, variable viscosity convection and time-dependent convection with internal heating We compare Nusselt numbers, velocity, temperature, heat-flow, topography and geoid data Among the applied codes are finite-difference, finite-element and spectral methods In a synthesis we give best estimates of the ‘true’ solutions and ranges of uncertainty We recommend these data for the validation of convection codes in the future

Interactions among Radiation, Convection, and Large-Scale Dynamics in a General Circulation Model

Abstract: We have analyzed the effects of radiatively active clouds on the climate simulated by the UCLA/GLA GCM, with particular attention to the effects of the upper tropospheric stratiform clouds associated with deep cumulus convection, and the interactions of these clouds with convection and the large-scale circulation. Several numerical experiments have been performed to investigate the mechanisms through which the clouds influence the large-scale circulation. In the “NODETLQ” experiment, no liquid water or ice was detrained from cumulus clouds into the environment; all of the condensate was rained out. Upper level supersaturation cloudiness was drastically reduced, the atmosphere dried, and tropical outgoing longwave radiation increased. In the “NOANVIL” experiment, the radiative effects of the optically thich upper-level cloud sheets associated with deep cumulus convection were neglected. The land surface received more solar radiation in regions of convection, leading to enhanced surface fluxes and ...

Climatic Equilibrium of the Atmospheric Convective Boundary Layer over a Tropical Ocean

Abstract: A radiative-convective boundary layer model was developed by coupling a thermodynamic model of a partially mixed convective boundary layer (CBL) with a radiation model, and energy balance constraints were used to study coupled boundary layer (CBL) equilibrium over three timescales (about 1 day, about 10 days, and more than 100 days). It is shown that the variation in cloud top decreases with greater coupling to the atmosphere and the ocean. The slope of the latent heat flux with increasing SST decreases with more tropospheric coupling, and reverses sign with a coupled ocean.

Three-Dimensional Spherical Models of Convection in the Earth's Mantle.

Abstract: Three-dimensional, spherical models of mantle convection in the earth reveal that upwelling cylindrical plumes and downwelling planar sheets are the primary features of mantle circulation. Thus, subduction zones and descending sheetlike slabs in the mantle are fundamental characteristics of thermal convection in a spherical shell and are not merely the consequences of the rigidity of the slabs, which are cooler than the surrounding mantle. Cylindrical mantle plumes that cause hotspots such as Hawaii are probably the only form of active upwelling and are therefore not just secondary convective currents separate from the large-scale mantle circulation. Active sheetlike upwellings that could be associated with mid-ocean ridges did not develop in the model simulations, a result that is in agreement with evidence suggesting that ridges are passive phenomena resulting from the tearing of surface plates by the pull of descending slabs.

Similarity scaling of viscous and thermal dissipation in a convecting surface boundary layer

Abstract: By continuously deploying a turbulence profiler in the upper ocean, we observed the rate of viscous dissipation of turbulent kinetic energy, e, and the rate of diffusive smoothing of turbulent temperature fluctuations, χ, during eleven diurnal cycles of the surface boundary layer (SBL). Even though we restricted our analysis to times when the ocean lost buoyancy at a nearly constant rate, we observed a wide range of conditions, including dominance of the turbulent production by the wind stress. Throughout, e was normalized very well by the sum of the similarity scalings for turbulence produced by wind stress and by convection. Scaling of χ was less successful and applied only when turbulent production was dominated either by wind stress or by convection, and then only within part of the SBL.

Mixing in the Equatorial Surface Layer and Thermocline

Abstract: surprisingly strong effect of both the daily cycle of solar heating and wind on mixing in the upper ocean. Because of limited variations in atmospheric forcing and currents during the experiment, processes in the daily mixing cycle were similar from day to day. Only the intensity of mixing varied. The lower boundary of the diurnal surface layer separated two distinct mixing regimes, the diurnal surface layer and the thermocline. Within the diurnal surface layer (which extended to 10- to 35-m depth), turbulent kinetic energy dissipation rates : varied relatively little. Although variations in surface layer depth coincided with the daily change in direction of air-sea surface buoyancy production of turbulent kinetic energy (or simply, the surface buoyancy flux), e was significantly greater relative to the buoyancy flux than was expected for a simple convective layer. In the thermocline below the diurnal surface layer, e was highly intermittent; the day-night cycle was stronger, and variability was enhanced by turbulent "bursts" of 2-3 hours duration, which may be related to internal wave breaking events. The turbulent heat flux crossing 20-m depth was almost equal to the surface heat flux less the irradiance penetrating below 20 m. Seventy percent of the surface heat flux was transported vertically to the water below 30 m by turbulent mixing. Only a negligible amount penetrated to the core of the Equatorial Undercurrent. The gradient Richardson number Ri distinguishes between statistically different mixing environments. However,  cannot be predicted from the value of Ri, since the intensity of mixing depends on the intensity of forcing in a way not specified by the value of Ri alone.

Dynamic mixing in magma bodies - Theory, simulations, and implications

Abstract: The magma-mixing process is different from the mantle mixing process in that the mixing components of magma are dynamically active, with the melt density depending strongly on composition. This paper describes simulations of time-dependent variable-viscosity double-diffusive convection which were carried out to investigate quantitatively the mixing dynamics of magma in melt-dominated magma bodies. Results show that the dynamics of double-diffusive convection can impart complex patterns of composition, through time and space. The mixing time depends nonlinearly on many factors, including heat flux driving convection, the rate of diffusion of chemical species, the relative importance of thermal and chemical buoyancy, the viscosities of the mixing components, and the shape of the magma body.

Modelling liquid-solid phase changes with melt convection

Abstract: SUMMARY Methods are described for modelling of phase change processes using the finite element method to simulate freezing and melting including convection in the melt. Evaluation of several enthalpy/specific heat methods and time marching schemes is also included. Suppression of velocities in the solid region is described, and example problems are given. Comparison is made to simulations performed by other researchers using finite difference methods. Substantially different results were found for one of these problems, and this result is shown to be caused by numerical problems in the earlier work. strong effect on the resulting microstructure. A number of researchers have shown reorientation of columnar grains,' alteration of the size and location of equiaxed zones' and macro~egregation,~. all due to melt convection. Mathematical models have been used in attempts to better understand the processes and thus control them. Although the most convenient mathematical models would use analytical solutions to the coupled heat and momentum transport equations, very few such solutions exist for these problems, and none would extend to the realistic problems where complicated geometries and temperature dependent material properties are included. For this reason, nearly all of the efforts in this area have been numerical. There are different types of numerical methods which are appropriate to phase change problems, depending on the kind of material involved. In pure materials, eutectics or congruent melting phases, the liquid-solid interface is sharp and corresponds to an isotherm. For these kinds of problems it may be appropriate to have part of the mesh coincide with the solidification front at all times, and distort the mesh in both phases as the boundary moves. A number of these front- tracking methods have been de~eloped,~, but none exists for three-dimensional problems. For alloys which freeze over a range of temperatures, front-tracking methods are no longer applicable. Instead, what is normally done is to specify the evolution of latent heat over a freezing range as part of the material properties. The phase boundaries are then recovered from the solution as the isotherms corresponding to the beginning and end of the transformation. The two

Convection driven magnetohydrodynamic dynamos in rotating spherical shells

Abstract: Finite amplitude solutions for magnetohydrodynamic dynamos driven by convection in rotating spherical fluid shells with a radius ratio of ηequals; 0.4 are obtained numerically by the Galerkin method. Solutions which are twice periodic in the azimuth (case m equals; 2) are emphasized, but a few cases with higher azimuthal wavenumber have also been considered. An electrically insulating space outside the fluid shell has been assumed. A comparison of the dynamo solutions of both, dipolar and quadrupolar, symmetries with the corresponding non-magnetic solutions shows a strong increase of the amplitude of convection owing to the release of the rotational constraint by the Lorentz force. In some cases at low Taylor number the amplitude of convection is decreased, however, owing to the competition of the magnetic degree of freedom for the same energy source. The strength of differential rotation is usually reduced by the Lorentz force, especially in the case of quadrupolar dynamos which differ in this r...

Double-diffusive convection in a porous medium

Abstract: An experimental study has been carried out to examine double-diffusive convection in a porous medium. The experiments were performed in a horizontal layer of porous medium consisting of 3 mm diameter glass beads contained in a box 24 cm × 12 cm × 4 cm high. The top and bottom walls were made of brass and were kept at different constant temperatures by separate baths, with the bottom temperature higher than that of the top. The onset of convection was detected by a heat flux sensor and by the temperature distribution in the porous medium. When the porous medium was saturated with distilled water, the onset of convection was marked by a change in slope of the heat flux curve. The temperature distribution in the longitudinal direction in the middle of the layer indicated a convection pattern consisting of two-dimensional rolls with axes parallel to the short side. This pattern was confirmed by flow visualization. When the porous medium was saturated with salinity gradients of 0.15% cm−1 and 0.225% cm−1, the onset of convection was marked by a dramatic increase in heat flux at the critical ΔT, and the convection pattern was three-dimensional. When the temperature difference was reduced from supercritical to subcritical values, the heat flux curve established a hysteresis loop. Results from linear stability theory, taking into account effects of temperature-dependent viscosity, volumetric expansion coefficients, and a nonlinear basic state salinity profile, are discussed.

Anomalous diffusion of tracer in convection rolls

Abstract: The dispersion of a passive tracer in a two‐dimensional, spatially periodic stationary flow, such as convection rolls, is studied in the large Peclet number limit. In the case where injection, at time t=0, is localized in one roll, two regimes exist. First, there is an anomalous diffusion regime in which the number of invaded rolls grows like t1/3. This regime is due to the presence of separatrices between rolls that induce trapping of tracer within each roll. At a later time, when t≫Td (the diffusion time within a roll), the usual diffusion regime is recovered, yet with an effective diffusive coefficient κeff that is greater than the molecular diffusivity κ by a factor proportional to the square root of the Peclet number.

Convective and Radiative Heat Transfer in Porous Media

Abstract: Publisher Summary This chapter describes the convective and radiative heat transfer in porous media. Convective and radiative heat transfer and multiphase transport processes in porous media, both with or without phase change, have gained extensive attention. Several applications related to porous media require a detailed analysis of convective heat transfer in different geometrical shapes, orientations, and configurations. Based on the specific application, the flow in the porous medium may be internal or external. Forced convection over external boundaries in the presence of a porous medium constitutes a very important subject area. Analysis of convective heat transfer from this type of external boundary embedded in a porous medium has many important applications. It is shown that for the flow field the boundary effect is confined within a thin momentum boundary layer, which often plays an insignificant role in the overall flow consideration. The models developed for simultaneous heat and mass transfer processes in multiphase porous systems may have differences due to the various assumptions made in developing each model. The thermal radiation characteristics of porous beds are also elaborated.

Large-Eddy Simulation of Turbulent Sheared Convection

Abstract: A series of large-eddy simulators of free and sheared convective flow between moving flat plates is presented. Results for free convection are compared with laboratory data. The ratio of friction velocity to the convective velocity sale,u*/w*,is identified as an important parameter in sheared convective flow determining the formation of longitudinal rolls. Rolls are found for u*/w* ≥ 0.35, with aspect ratio decreasing as this parameter increases. It is shown that, in this regime, two-dimensional simulations with a proper choice of roll orientation and turbulence length-scale can produce correct velocity variances and roll aspect ratio.

Natural Convection Along a Vertical Wavy Surface With Uniform Heat Flux

Abstract: Laminar free convection along a semi-infinite vertical wavy surface has been studied by Yao (1983) for the case of uniform surface temperature. This is a model problem for the investigation of heat transfer from roughened surfaces in order to understand heat transfer enhancement. In many applications of practical importance, however, the surface temperature is nonuniform. In this note, the case of uniform surface heat flux rate, which is often approximated in practical applications and is easier to measure in a laboratory, has been investigated. Numerical results have been obtained for a sinusoidal wavy surface. The results show that the Nusselt number varies periodically along the wavy surface. The wavelength of the Nusselt number variation is half of that of the wavy surface, while the amplitude gradually decreases downstream where the boundary layer grows thick. It is hoped that experimental results will become available in the near future to verify the results of this investigation.