Showing papers on "Convection published in 2007"

Liquid Vapor Phase Change Phenomena : An Introduction to the Thermophysics of Vaporization and Condensation Processes in Heat Transfer Equipment

Abstract: Part 1:Thermodynamic and Mechanical Aspects of Interfacial Phenomena and Phase Transitions 1.Introductory Concepts 2.Interfacial Tension 3.Wetting Phenomena and Contact Angles 4.Transport Effects and Dynamic Behavior of Interfaces 5.Phase Stability Part 2:Boiling and Condensation Near Immersed Bodies 6.Heterogenous Nucleation and Bubble Growth in Liquids 7.Pool Boiling 8.Other Aspects of Boiling and Evaporation in an Extensive Ambient 9.External Condensation Part 3:Internal Flow Convective Boiling and Condensation 10.Introduction to Two-Phase Flow 11.Internal Convective Condensation 12.Cnvective Boiling in Tubes and Channels Part 4:Special Topics 13.Special Topics and Applications Appendix I:Basic Elements of the Kinetic Theory of Gases Appendix II:Saturation Properties of Selected Fluids Index

Alfven Waves in the Solar Corona

Abstract: Alfven waves, transverse incompressible magnetic oscillations, have been proposed as a possible mechanism to heat the Sun's corona to millions of degrees by transporting convective energy from the photosphere into the diffuse corona. We report the detection of Alfven waves in intensity, line-of-sight velocity, and linear polarization images of the solar corona taken using the FeXIII 1074.7-nanometer coronal emission line with the Coronal Multi-Channel Polarimeter (CoMP) instrument at the National Solar Observatory, New Mexico. Ubiquitous upward propagating waves were seen, with phase speeds of 1 to 4 megameters per second and trajectories consistent with the direction of the magnetic field inferred from the linear polarization measurements. An estimate of the energy carried by the waves that we spatially resolved indicates that they are too weak to heat the solar corona; however, unresolved Alfven waves may carry sufficient energy.

Influence of substrate conductivity on circulation reversal in evaporating drops.

Abstract: Nonuniform evaporation from sessile droplets induces radial convection within the drop, which produces the well-known ‘‘coffee-ring’’ effect. The evaporation also induces a gradient in temperature and consequently a gradient in surface tension, generating a Marangoni flow. Here we investigate theoretically and experimentally the thermal Marangoni flow and establish criteria to gauge its influence. An asymptotic analysis indicates that the direction of the flow depends on the relative thermal � ;

Turbulent convection in stellar interiors. I. Hydrodynamic simulation

Abstract: We describe the results of 3D numerical simulations of oxygen shell burning and hydrogen core burning in a 23 M☉ stellar model. A detailed comparison is made to stellar mixing-length theory (MLT) for the shell-burning model. Simulations in 2D are significantly different from 3D, in terms of both flow morphology and velocity amplitude. Convective mixing regions are better predicted using a dynamic boundary condition based on the bulk Richardson number than by purely local, static criteria like Schwarzschild or Ledoux. MLT gives a good description of the velocity scale and temperature gradient for shell convection; however, there are other important effects that it does not capture, mostly related to the dynamical motion of the boundaries between convective and nonconvective regions. There is asymmetry between upflows and downflows, so the net kinetic energy flux is not zero. The motion of convective boundaries is a source of gravity waves; this is a necessary consequence of the deceleration of convective plumes. Convective overshooting is best described as an elastic response by the convective boundary, rather than ballistic penetration of the stable layers by turbulent eddies. The convective boundaries are rife with internal and interfacial wave motions, and a variety of instabilities arise that induce mixing through a process best described as turbulent entrainment. We find that the rate at which material entrainment proceeds at the boundaries is consistent with analogous laboratory experiments and simulation and observation of terrestrial atmospheric mixing. In particular, the normalized entrainment rate E = uE/σH is well described by a power-law dependence on the bulk Richardson number RiB = ΔbL/σ for the conditions studied, 20 RiB 420. We find E = ARi, with best-fit values log A = 0.027 ± 0.38 and n = 1.05 ± 0.21. We discuss the applicability of these results to stellar evolution calculations.

Monsoon convection in the Himalayan region as seen by the TRMM Precipitation Radar

Abstract: Three-dimensional structure of summer monsoon convection in the Himalayan region and its overall variability are examined by analyzing data from the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar over the June–September seasons of 2002 and 2003. Statistics are compiled for both convective and stratiform components of the observed radar echoes. Deep intense convective echoes (40 dBZ echo reaching heights > 10 km) occur primarily just upstream (south) of and over the lower elevations of the Himalayan barrier, especially in the northwestern concave indentation of the barrier. The deep intense convective echoes are vertically erect, consistent with the relatively weak environmental shear. They sometimes extend above 17 km, indicating that exceptionally strong updraughts loft graupel to high altitudes. Occasionally, scattered isolated deep intense convective echoes occur over the Tibetan Plateau. Wide intense convective echoes (40 dBZ echo > 1000 km2 in horizontal dimension) also occur preferentially just upstream of and over the lower elevations of the Himalayas, most frequently in the northwestern indentation of the barrier. The wide intense echoes have an additional tendency to occur along the central portion of the Himalayas, and they seldom if ever occur over the Tibetan Plateau. The wide intense echoes exhibit three mesoscale structures: amorphous areas, lines parallel to the mountain barrier, and arc-shaped squall lines perpendicular to and propagating parallel to the steep Himalayan barrier. The latter are rare, generally weaker than those seen in other parts of the world, and occur when a midlevel jet is aligned with the Himalayan escarpment. Deep and wide intense convective echoes over the northwestern subcontinent tend to occur where the low-level moist layer of monsoon air from the Arabian Sea meets dry downslope flow, in a manner reminiscent of severe convection leeward of the Rocky Mountains in the central USA. As the low-level layer of moist air from the sea moves over the hot arid northwestern subcontinent, it is capped by an elevated layer of dry air advected off the Afghan or Tibetan Plateau. The capped low-level monsoonal airflow accumulates instability via surface heating until this instability is released by orographically induced lifting immediately adjacent to or directly over the foothills of the Himalayas. Broad (>50 000 km2 in area) stratiform echoes occur in the eastern and central portions of the Himalayan region in connection with Bay of Bengal depressions. Their centroids are most frequent just upstream of the Himalayas, in the region of the concave indentation of the barrier at the eastern end of the range. The steep topography apparently enhances the formation and longevity of the broad stratiform echoes. Monsoonal depressions provide a moist maritime environment for the convection, evidently allowing mesoscale systems to develop larger stratiform echoes than in the western Himalayan region. Copyright © 2007 Royal Meteorological Society

Role of Phase Transitions in a Dynamic Mantle

Abstract: Summary The interaction of solid-solid phase transitions with convection in the Earth's mantle involves, for univariant systems: (1) effects of latent heat and advection of ambient temperature on the position of the phase boundary and on its associated body force, and (2) the coupling of latent heat with the ordinary thermal expansivity of the material. For divariant systems, an effective thermal expansion coefficient and a modified adiabatic temperature gradient may be defined for the phase transition zone. Linear stability theory for a fluid layer with a univariant phase change is reviewed and applied to the endothermic spinel-oxide transformation. The theory of the stability of a fluid layer with a divariant phase transformation is developed and critical Rayleigh numbers are given for a model of the olivine-spinel transition. Of special interest is the case where the Earth's temperature gradient exceeds the adiabatic temperature gradient outside the phase transition zone but is smaller than the increased adiabatic temperature gradient in the two-phase olivine-spinel region. The thermal structure of the descending lithosphere is calculated, including the effects of frictional heating on the slip zone and of the olivine-spinel and spinel-oxide transitions; temperature contrasts of 700 °K can exist between the slab and adjacent mantle at 800 km depth. The net body force on the descending slab due to thermal contraction and the major mineralogical phase changes is downward. The olivine-spinel transition may be responsible for the tensional focal mechanisms of intermediate depth earthquakes while the spinel-oxide transformation may be related to the compressional focal mechanisms of deep earthquakes.

A Madden-Julian oscillation event realistically simulated by a global cloud-resolving model.

Abstract: A Madden-Julian Oscillation (MJO) is a massive weather event consisting of deep convection coupled with atmospheric circulation, moving slowly eastward over the Indian and Pacific Oceans. Despite its enormous influence on many weather and climate systems worldwide, it has proven very difficult to simulate an MJO because of assumptions about cumulus clouds in global meteorological models. Using a model that allows direct coupling of the atmospheric circulation and clouds, we successfully simulated the slow eastward migration of an MJO event. Topography, the zonal sea surface temperature gradient, and interplay between eastward- and westward-propagating signals controlled the timing of the eastward transition of the convective center. Our results demonstrate the potential making of month-long MJO predictions when global cloud-resolving models with realistic initial conditions are used.

Speculations on the Thermal and Tectonic History of the Earth

Abstract: Summary The connection between the Earth’s thermal history and convection in the mantle is exploited to elucidate the early evolution of the Earth. It appears probable that convection extending over almost all of the mantle has dominated vertical heat transport throughout the whole of the Earth’s history. Only in boundary layers at the surface and at a depth of 650-700 km is conduction likely to be important. The resulting evolution appears to be consistent with geological observations on early Precambrian rocks. Various arguments are put forward in favour of two horizontal scales of convective flow in the mantle at depths less than 650 km. The large scale flow is related to the motion of major plates, and must be ordered over distances of more than 5000 km. Its evolution and energetics are discussed and there are no obvious problems in maintaining the proposed convective motions. Small scale flow with an extent of the order of 500 km appears necessary both to explain the heat flow through older parts of the Earth‘s surface and to reconcile the geophysical observations with the results of numerical experiments. Though the existence of the small scale flow is at present speculative, various tests of its presence are proposed.

A solar surface dynamo

Abstract: Context: Observations indicate that the `quiet' solar photosphere outside active regions contains considerable amounts of magnetic energy and magnetic flux, with mixed polarity on small scales. The origin of this flux is unclear. Aims: We test whether local dynamo action of the near-surface convection (granulation) can generate a significant contribution to the observed magnetic flux. Methods: We have carried out MHD simulations of solar surface convection, including the effects of strong stratification, compressibility, partial ionization, radiative transfer, as well as an open lower boundary. Results: Exponential growth of a weak magnetic seed field (with vanishing net flux through the computational box) is found in a simulation run with a magnetic Reynolds number of about 2600. The magnetic energy approaches saturation at a level of a few percent of the total kinetic energy of the convective motions. Near the visible solar surface, the (unsigned) magnetic flux density reaches at least a value of about 25 G. Conclusions: A realistic flow topology of stratified, compressible, non-helical surface convection without enforced recirculation is capable of turbulent local dynamo action near the solar surface.

Transport above the Asian summer monsoon anticyclone inferred from Aura Microwave Limb Sounder tracers

Abstract: [1] Tracer variability above the Asian summer monsoon anticyclone is investigated using Aura Microwave Limb Sounder (MLS) measurements of carbon monoxide, ozone, water vapor, and temperature during Northern Hemisphere summer (June to August) of 2005. Observations show persistent maxima in carbon monoxide and minima in ozone within the anticyclone in the upper troposphere–lower stratosphere (UTLS) throughout summer, and variations in these tracers are closely related to the intensity of underlying deep convection. Temperatures in the UTLS are also closely coupled to deep convection (cold anomalies are linked with enhanced convection), and the three-dimensional temperature patterns are consistent with a dynamical response to near- equatorial convection. Upper tropospheric water vapor in the monsoon region is strongly coherent with deep convection, both spatially and temporally. However, at the altitude of the tropopause, maximum water vapor is centered within the anticyclone, distant from the deepest convection, and is also less temporally correlated with convective intensity. Because the main outflow of deep convection occurs near 12 km, well below the tropopause level (16 km), we investigate the large-scale vertical transport within the anticyclone. The mean vertical circulation obtained from the ERA40 reanalysis data set and a free-running general circulation model is upward across the tropopause on the eastern end of the anticyclone, as part of the balanced threedimensional monsoon circulation. In addition to deep transport from the most intense convection, this large-scale circulation may help explain the transport of constituents to tropopause level.

Tropical Cyclone Intensification from Asymmetric Convection: Energetics and Efficiency

Abstract: Prior studies of the linear response to asymmetric heating of a balanced vortex showed that the resulting intensity change could be very closely approximated by computing the purely symmetric response to the azimuthally averaged heating. The symmetric response to the purely asymmetric part of the heating was found to have a very small and most often negative impact on the intensity of the vortex. This result stands in contrast to many previous studies that used asymmetric vorticity perturbations, which suggested that purely asymmetric forcing could lead to vortex intensification. The issue is revisited with an improved model and some new methods of analysis. The model equations have been changed to be more consistent with the anelastic approximation, but valid for a radially varying reference state. Expressions for kinetic and available potential energies are presented for both asymmetric and symmetric motions, and these are used to quantify the flow of energy from localized, asymmetric heat sour...

Large-eddy simulation and experimental study of heat transfer, nitric oxide emissions and combustion instability in a swirled turbulent high-pressure burner

Abstract: Nitric oxide formation in gas turbine combustion depends on four key factors: flame stabilization, heat transfer, fuel–air mixing and combustion instability. The design of modern gas turbine burners requires delicate compromises between fuel efficiency, emissions of oxides of nitrogen (NOx) and combustion stability. Burner designs allowing substantial NOx reduction are often prone to combustion oscillations. These oscillations also change the NOx fields. Being able to predict not only the main species field in a burner but also the pollutant and the oscillation levels is now a major challenge for combustion modelling. This must include a realistic treatment of unsteady acoustic phenomena (which create instabilities) and also of heat transfer mechanisms (convection and radiation) which control NOx generation.In this work, large-eddy simulation (LES) is applied to a realistic gas turbine combustion chamber configuration where pure methane is injected through multiple holes in a cone-shaped burner. In addition to a non-reactive simulation, this article presents three reactive simulations and compares them to experimental results. The first reactive simulation neglects effects of cooling air on flame stabilization and heat losses by radiation and convection. The second reactive simulation shows how cooling air and heat transfer affect nitric oxide emissions. Finally, the third reactive simulation shows the effects of combustion instability on nitric oxide emissions. Additionally, the combustion instability is analysed in detail, including the evaluation of the terms in the acoustic energy equation and the identification of the mechanism driving the oscillation.Results confirm that LES of gas turbine combustion requires not only an accurate chemical scheme and realistic heat transfer models but also a proper description of the acoustics in order to predict nitric oxide emissions and pressure oscillation levels simultaneously.

Transport Processes and Separation Process Principles (Includes Unit Operations)

Abstract: Preface. I. TRANSPORT PROCESSES: MOMENTUM, HEAT, AND MASS. 1. Introduction to Engineering Principles and Units. Classification of Transport Processes and Separation Processes (Unit Operations). SI System of Basic Units Used in This Text and Other Systems. Methods of Expressing Temperatures and Compositions. Gas Laws and Vapor Pressure. Conservation of Mass and Material Balances. Energy and Heat Units. Conservation of Energy and Heat Balances. Numerical Methods for Integration. 2. Principles of Momentum Transfer and Overall Balances. Introduction. Fluid Statics. General Molecular Transport Equation for Momentum, Heat, and Mass Transfer. Viscosity of Fluids. Types of Fluid Flow and Reynolds Number. Overall Mass Balance and Continuity Equation. Overall Energy Balance. Overall Momentum Balance. Shell Momentum Balance and Velocity Profile in Laminar Flow. Design Equations for Laminar and Turbulent Flow in Pipes. Compressible Flow of Gases. 3. Principles of Momentum Transfer and Applications. Flow Past Immersed Objects and Packed and Fluidized Beds. Measurement of Flow of Fluids. Pumps and Gas-Moving Equipment. Agitation and Mixing of Fluids and Power Requirements. Non-Newtonian Fluids. Differential Equations of Continuity. Differential Equations of Momentum Transfer or Motion. Use of Differential Equations of Continuity and Motion. Other Methods for Solution of Differential Equations of Motion. Boundary-Layer Flow and Turbulence. Dimensional Analysis in Momentum Transfer. 4. Principles of Steady-State Heat Transfer. Introduction and Mechanisms of Heat Transfer. Conduction Heat Transfer. Conduction Through Solids in Series. Steady-State Conduction and Shape Factors. Forced Convection Heat Transfer Inside Pipes. Heat Transfer Outside Various Geometries in Forced Convection. Natural Convection Heat Transfer. Boiling and Condensation. Heat Exchangers. Introduction to Radiation Heat Transfer. Advanced Radiation Heat-Transfer Principles. Heat Transfer of Non-Newtonian Fluids. Special Heat-Transfer Coefficients. Dimensional Analysis in Heat Transfer. Numerical Methods for Steady-State Conduction in Two Dimensions. 5. Principles of Unsteady-State Heat Transfer. Derivation of Basic Equation. Simplified Case for Systems with Negligible Internal Resistance. Unsteady-State Heat Conduction in Various Geometries. Numerical Finite-Difference Methods for Unsteady-State Conduction. Chilling and Freezing of Food and Biological Materials. Differential Equation of Energy Change. Boundary-Layer Flow and Turbulence in Heat Transfer. 6. Principles of Mass Transfer. Introduction to Mass Transfer and Diffusion. Molecular Diffusion in Gases. Molecular Diffusion in Liquids Molecular Diffusion in Biological Solutions and Gels. Molecular Diffusion in Solids. Numerical Methods for Steady-State Molecular Diffusion in Two Dimensions. 7. Principles of Unsteady-State and Convective Mass Transfer. Unsteady-State Diffusion. Convective Mass-Transfer Coefficients. Mass-Transfer Coefficients for Various Geometries. Mass Transfer to Suspensions of Small Particles. Molecular Diffusion Plus Convection and Chemical Reaction. Diffusion of Gases in Porous Solids and Capillaries. Numerical Methods for Unsteady-State Molecular Diffusion. Dimensional Analysis in Mass Transfer. Boundary-Layer Flow and Turbulence in Mass Transfer. II. SEPARATION PROCESS PRINCIPLES (INCLUDES UNIT OPERATIONS). 8. Evaporation. Introduction. Types of Evaporation Equipment and Operation Methods. Overall Heat-Transfer Coefficients in Evaporators. Calculation Methods for Single-Effect Evaporators. Calculation Methods for Multiple-Effect Evaporators. Condensers for Evaporators. Evaporation of Biological Materials. Evaporation Using Vapor Recompression. 9. Drying of Process Materials. Introduction and Methods of Drying. Equipment for Drying. Vapor Pressure of Water and Humidity. Equilibrium Moisture Content of Materials. Rate-of-Drying Curves. Calculation Methods for Constant-Rate Drying Period. Calculation Methods for Falling-Rate Drying Period. Combined Convection, Radiation, and Conduction Heat Transfer in Constant-Rate Period. Drying in Falling-Rate Period by Diffusion and Capillary Flow. Equations for Various Types of Dryers. Freeze-Drying of Biological Materials. Unsteady-State Thermal Processing and Sterilization of Biological Materials. 10. Stage and Continuous Gas-Liquid Separation Processes. Types of Separation Processes and Methods. Equilibrium Relations Between Phases. Single and Multiple Equilibrium Contact Stages. Mass Transfer Between Phases. Continuous Humidification Processes. Absorption in Plate and Packed Towers. Absorption of Concentrated Mixtures in Packed Towers. Estimation of Mass-Transfer Coefficients for Packed Towers. Heat Effects and Temperature Variations in Absorption. 11. Vapor-Liquid Separation Processes. Vapor-Liquid Equilibrium Relations. Single-Stage Equilibrium Contact for Vapor-Liquid System. Simple Distillation Methods. Distillation with Reflux and McCabe-Thiele Method. Distillation and Absorption Efficiencies for Tray and Packed Towers. Fractional Distillation Using Enthalpy-Concentration Method. Distillation of Multicomponent Mixtures. 12. Liquid-Liquid and Fluid-Solid Separation Processes. Introduction to Adsorption Processes. Batch Adsorption. Design of Fixed-Bed Adsorption Columns. Ion-Exchange Processes. Single-Stage Liquid-Liquid Extraction Processes. Types of Equipment and Design for Liquid-Liquid Extraction. Continuous Multistage Countercurrent Extraction. Introduction and Equipment for Liquid-Solid Leaching. Equilibrium Relations and Single-Stage Leaching. Countercurrent Multistage Leaching. Introduction and Equipment for Crystallization. Crystallization Theory. 13. Membrane Separation Processes. Introduction and Types of Membrane Separation Processes. Liquid Permeation Membrane Processes or Dialysis. Gas Permeation Membrane Processes. Complete-Mixing Model for Gas Separation by Membranes. Complete-Mixing Model for Multicomponent Mixtures. Cross-Flow Model for Gas Separation by Membranes. Derivation of Equations for Countercurrent and Cocurrent Flow for Gas Separation for Membranes. Derivation of Finite-Difference Numerical Method for Asymmetric Membranes. Reverse-Osmosis Membrane Processes. Applications, Equipment, and Models for Reverse Osmosis. Ultrafiltration Membrane Processes. Microfiltration Membrane Processes. 14. Mechanical-Physical Separation Processes. Introduction and Classification of Mechanical-Physical Separation Processes. Filtration in Solid-Liquid Separation. Settling and Sedimentation in Particle-Fluid Separation. Centrifugal Separation Processes. Mechanical Size Reduction. Appendices. Appendix A.1. Fundamental Constants and Conversion Factors. Appendix A.2. Physical Properties of Water. Appendix A.3. Physical Properties of Inorganic and Organic Compounds. Appendix A.4. Physical Properties of Foods and Biological Materials. Appendix A.5. Properties of Pipes, Tubes, and Screens. Notation. Index.

Mesoscale Convective Systems over Southeastern South America and Their Relationship with the South American Low-Level Jet

Abstract: Prior studies have shown that the low-level jet is a recurrent characteristic of the environment during the initiation and mature stages of mesoscale convective systems (MCSs) over the Great Plains of the United States. The South American low-level jet (SALLJ) over southeastern South America (SESA) has an analogous role, advecting heat and moisture from the Amazon basin southward into the central plains of southeastern South America, generating ideal environmental conditions for convection initiation and growth into MCSs. This research has two purposes. One is to describe the characteristics of a 3-yr MCS sample in South America, south of the equator, and its related geographical distribution of convection frequency. The other is to advance the knowledge of the evolution of favorable environmental conditions for the development of large MCSs, specifically those that mature under SALLJ conditions. High horizontal and temporal resolution satellite images are used to detect MCSs in the area for the ...

Marangoni flow in an evaporating water droplet

Abstract: Marangoni effect has been observed in many liquids, but its existence in pure water is still a debated problem. In the present work, the Marangoni flow in evaporating water droplets has been observed by using fluorescent nanoparticles. Flow patterns indicate that a stagnation point where the surface flow, the surface tension gradient, and the surface temperature gradient change their directions exists at the droplet surface. The deduced nonmonotonic variation of the droplet surface temperature, which is different from that in some previous works, is explained by a heat transfer model considering the adsorbed thin film of the evaporating liquid droplet.

The Dynamics of Idealized Convection Schemes and Their Effect on the Zonally Averaged Tropical Circulation

Abstract: In this paper, the effect of a simple convection scheme on the zonally averaged tropical general circulation is examined within an idealized moist GCM to obtain broad classifications of the influence of convection on the Tropics. This is accomplished with a simplified convection scheme in the style of Betts and Miller. The scheme is utilized in a moist GCM with simplified physical parameterizations (gray radiation, with zonally symmetric, slab mixed layer ocean boundary conditions). Comparisons are made with simulations without a convection scheme [i.e., with large-scale condensation (LSC) only], with the moist convective adjustment (MCA) parameterization, and with various formulations and parameter sets with a simplified Betts–Miller (SBM) scheme. With the control run using the SBM scheme, the Tropics become quieter and less dependent on horizontal resolution as compared with the LSC or MCA simulations. The Hadley circulation mass transport is significantly reduced with the SBM scheme, as is the ITCZ precipitation. An important factor determining this behavior is the parameterization of shallow convection: without shallow convection, the convection scheme is largely ineffective at preventing convection from occurring at the grid scale. The sensitivities to convection scheme parameters are also examined. The simulations are remarkably insensitive to the convective relaxation time, and only mildly sensitive to the relative humidity of the reference profile, provided significant large-scale condensation is not allowed to occur. The changes in the zonally averaged tropical circulation that occur in all the simulations are understood based on the convective criteria of the schemes and the gross moist stability of the atmosphere.

Mesoscale Predictability of Moist Baroclinic Waves: Convection-Permitting Experiments and Multistage Error Growth Dynamics

Abstract: A recent study examined the predictability of an idealized baroclinic wave amplifying in a conditionally unstable atmosphere through numerical simulations with parameterized moist convection. It was demonstrated that with the effect of moisture included, the error starting from small random noise is characterized by upscale growth in the short-term (0–36 h) forecast of a growing synoptic-scale disturbance. The current study seeks to explore further the mesoscale error-growth dynamics in idealized moist baroclinic waves through convection-permitting experiments with model grid increments down to 3.3 km. These experiments suggest the following three-stage error-growth model: in the initial stage, the errors grow from small-scale convective instability and then quickly [O(1 h)] saturate at the convective scales. In the second stage, the character of the errors changes from that of convective-scale unbalanced motions to one more closely related to large-scale balanced motions. That is, some of the error from convective scales is retained in the balanced motions, while the rest is radiated away in the form of gravity waves. In the final stage, the large-scale (balanced) components of the errors grow with the background baroclinic instability. Through examination of the error-energy budget, it is found that buoyancy production due mostly to moist convection is comparable to shear production (nonlinear velocity advection). It is found that turning off latent heating not only dramatically decreases buoyancy production, but also reduces shear production to less than 20% of its original amplitude.

Heat transport in giant (exo)planets: a new perspective

Abstract: We explore the possibility that large-scale convection is inhibited over some regions of giant planet interiors, as a consequence of a gradient of composition inherited from either their formation history or particular events like giant impacts or core erosion during their evolution. Under appropriate circumstances, the redistribution of the gradient of molecular weight can lead to double diffusive layered or overstable convection. This leads to much less efficient heat transport and compositional mixing than large-scale adiabatic convection. We show that this process can explain the abnormally large radius of the transit planet HD 209458b and similar objects and may be at play in some giant planets, with short-period planets offering the most favorable conditions. Observational signatures of this transport mechanism are a large radius and a reduced heat flux output compared with uniformly mixed objects. If our suggestion is correct, it bears major consequences on our understanding of giant planet formation, structure and evolution, including possibly our own Jovian planets.

A microscale three-dimensional urban energy balance model for studying surface temperatures

Abstract: A microscale three-dimensional (3-D) urban energy balance model, Temperatures of Urban Facets in 3-D (TUF-3D), is developed to predict urban surface temperatures for a variety of surface geometries and properties, weather conditions, and solar angles. The surface is composed of plane-parallel facets: roofs, walls, and streets, which are further sub-divided into identical square patches, resulting in a 3-D raster-type model geometry. The model code is structured into radiation, conduction and convection sub-models. The radiation sub-model uses the radiosity approach and accounts for multiple reflections and shading of direct solar radiation. Conduction is solved by finite differencing of the heat conduction equation, and convection is modelled by empirically relating patch heat transfer coefficients to the momentum forcing and the building morphology. The radiation and conduction sub-models are tested individually against measurements, and the complete model is tested against full-scale urban surface temperature and energy balance observations. Modelled surface temperatures perform well at both the facet-average and the sub-facet scales given the precision of the observations and the uncertainties in the model inputs. The model has several potential applications, such as the calculation of radiative loads, and the investigation of effective thermal anisotropy (when combined with a sensor-view model).

Improving Simulations of Convective Systems from TRMM LBA: Easterly and Westerly Regimes

Abstract: The 3D Goddard Cumulus Ensemble model is used to simulate two convective events observed during the Tropical Rainfall Measuring Mission Large-Scale Biosphere–Atmosphere (TRMM LBA) experiment in Brazil. These two events epitomized the type of convective systems that formed in two distinctly different environments observed during TRMM LBA. The 26 January 1999 squall line formed within a sheared low-level easterly wind flow. On 23 February 1999, convection developed in weak low-level westerly flow, resulting in weakly organized, less intense convection. Initial simulations captured the basic organization and intensity of each event. However, improvements to the model resolution and microphysics produced better simulations as compared to observations. More realistic diurnal convective growth was achieved by lowering the horizontal grid spacing from 1000 to 250 m. This produced a gradual transition from shallow to deep convection that occurred over a span of hours as opposed to an abrupt appearance of...

Parameterization of Convective Transport in a Lagrangian Particle Dispersion Model and Its Evaluation

Abstract: This paper presents the revision and evaluation of the interface between the convective parameterization by Emanuel and Živkovic ´-Rothman and the Lagrangian particle dispersion model “FLEXPART” based on meteorological data from the European Centre for Medium-Range Weather Forecasts (ECMWF). The convection scheme relies on the ECMWF grid-scale temperature and humidity and provides a matrix necessary for the vertical convective particle displacement. The benefits of the revised interface relative to its previous version are presented. It is shown that, apart from minor fluctuations caused by the stochastic convective redistribution of the particles, the well-mixed criterion is fulfilled in simulations that include convection. Although for technical reasons the calculation of the displacement matrix differs somewhat between the forward and the backward simulations in time, the mean relative difference between the convective mass fluxes in forward and backward simulations is below 3% and can therefore be tolerated. A comparison of the convective mass fluxes and precipitation rates with those archived in the 40-yr ECMWF Reanalysis (ERA-40) data reveals that the convection scheme in FLEXPART produces upward mass fluxes and precipitation rates that are generally smaller by about 25% than those from ERA-40. This result is interpreted as positive, because precipitation is known to be overestimated by the ECMWF model. Tracer transport simulations with and without convection are compared with surface and aircraft measurements from two tracer experiments and to 222 Rn measurements from two aircraft campaigns. At the surface no substantial differences between the model runs with and without convection are found, but at higher altitudes the model runs with convection produced better agreement with the measurements in most of the cases and indifferent results in the others. However, for the tracer experiments only few measurements at higher altitudes are available, and for the aircraft campaigns the 222 Rn emissions are highly uncertain. Other datasets better suitable for the validation of convective transport in models are not available. Thus, there is a clear need for reliable datasets suitable to validate vertical transport in models.

Impact of Labrador Sea Convection on the North Atlantic Meridional Overturning Circulation

Abstract: The overturning and horizontal circulations of the Labrador Sea are deduced from a composite vertical section across the basin. The data come from the late-spring/early-summer occupations of the World Ocean Circulation Experiment (WOCE) AR7W line, during the years 1990–97. This time period was chosen because it corresponded to intense wintertime convection—the deepest and densest in the historical record—suggesting that the North Atlantic meridional overturning circulation (MOC) would be maximally impacted. The composite geostrophic velocity section was referenced using a mean lateral velocity profile from float data and then subsequently adjusted to balance mass. The analysis was done in depth space to determine the net sinking that results from convection and in density space to determine the diapycnal mass flux (i.e., the transformation of light water to Labrador Sea Water). The mean overturning cell is calculated to be 1S v (1 Sv 10 6 m 3 s 1 ), as compared with a horizontal gyre of 18 Sv. The total water mass transformation is 2 Sv. These values are consistent with recent modeling results. The diagnosed heat flux of 37.6 TW is found to result predominantly from the horizontal circulation, both in depth space and density space. These results suggest that the North Atlantic MOC is not largely impacted by deep convection in the Labrador Sea.

Thermal cracking and the deep hydration of oceanic lithosphere: A key to the generation of plate tectonics?

Abstract: [1] The development of transient thermal stress in suboceanic mantle is investigated on the basis of two-dimensional thermoviscoelastic models incorporating composite rheology appropriate for dry oceanic lithosphere. Thermal stress is shown to be sufficiently high to deeply fracture the coldest part of lithosphere, e.g., to the depth of at least ∼30 km (and possibly down to ∼50 km) in 100-Ma-old lithosphere. The release of thermal stress by tension cracking is limited to the vicinity of cracks, and the cascade crack system is suggested to be required given the finite fracture strength of mantle materials. Possible physical and chemical consequences of deep thermal cracking are also discussed. The rheological evolution of oceanic lithosphere is likely to be affected by thermal cracking and subsequent serpentinization, which introduces the localized zones of weakness in the otherwise stiffest part of lithosphere. This localized weakening may help to explain why plate tectonic convection, not stagnant lid convection, operates in Earth's mantle.

Deep Convective System Evolution over Africa and the Tropical Atlantic

Abstract: In the tropical African and neighboring Atlantic region there is a strong contrast in the properties of deep convection between land and ocean. Here, satellite radar observations are used to produce a composite picture of the life cycle of convection in these two regions. Estimates of the broadband thermal flux from the geostationary Meteosat-8 satellite are used to identify and track organized convective systems over their life cycle. The evolution of the system size and vertical extent are used to define five life cycle stages (warm and cold developing, mature, cold and warm dissipating), providing the basis for the composite analysis of the system evolution. The tracked systems are matched to overpasses of the Tropical Rainfall Measuring Mission satellite, and a composite picture of the evolution of various radar and lightning characteristics is built up. The results suggest a fundamental difference in the convective life cycle between land and ocean. African storms evolve from convectively active systems with frequent lightning in their developing stages to more stratiform conditions as they dissipate. Over the Atlantic, the convective fraction remains essentially constant into the dissipating stages, and lightning occurrence peaks late in the life cycle. This behavior is consistent with differences in convective sustainability in land and ocean regions as proposed in previous studies. The area expansion rate during the developing stages of convection is used to provide an estimate of the intensity of convection. Reasonable correlations are found between this index and the convective system lifetime, size, and depth.