Showing papers on "Convection published in 1995"

Sensitivity of climate simulations to the parameterization of cumulus convection in the Canadian climate centre general circulation model

Abstract: A simplified cumulus parameterization scheme, suitable for use in GCMs, is presented. This parameterization is based on a plume ensemble concept similar to that originally proposed by Arakawa and Schubert (1974). However, it employs three assumptions which significantly simplify the formulation and implementation of the scheme. It is assumed that an ensemble of convective‐scale updrafts with associated saturated downdrafts may exist when the atmosphere is locally conditionally unstable in the lower troposphere. However, the updraft ensemble is comprised only of those plumes which are sufficiently buoyant to penetrate through this unstable layer. It is assumed that all such plumes have the same upward mass flux at the base of the convective layer. The third assumption is that moist convection, which occurs only when there is convective available potential energy (CAPE) for reversible ascent of an undiluted parcel from the sub‐cloud layer, acts to remove CAPE at an exponential rate with a specified...

DARN/SUPERDARN : A global view of the dynamics of high-latitude convection

Abstract: The Dual Auroral Radar Network (DARN) is a global-scale network of HF and VHF radars capable of sensing backscatter from ionospheric irregularities in the E and F-regions of the high-latitude ionosphere. Currently, the network consists of the STARE VHF radar system in northern Scandinavia, a northern-hemisphere, longitudinal chain of HF radars that is funded to extend from Saskatoon, Canada to central Finland, and a southern-hemisphere chain that is funded to include Halley Station, SANAE and Syowa Station in Antarctica. When all of the HF radars have been completed they will operate in pairs with common viewing areas so that the Doppler information contained in the backscattered signals may be combined to yield maps of high-latitude plasma convection and the convection electric field. In this paper, the evolution of DARN and particularly the development of its SuperDARN HF radar element is discussed. The DARN/SupperDARN network is particularly suited to studies of large-scale dynamical processes in the magnetosphere-ionosphere system, such as the evolution of the global configuration of the convection electric field under changing IMF conditions and the development and global extent of large-scale MHD waves in the magnetosphere-ionosphere cavity. A description of the HF radars within SuperDARN is given along with an overview of their existing and intended locations, intended start of operations, Principal Investigators, and sponsoring agencies. Finally, the operation of the DARN experiment within ISTP/GGS, the availability of data, and the form and availability of the Key Parameter files is discussed.

Advection-dominated accretion: Self-similarity and bipolar outflows

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

Scaling of temperature‐ and stress‐dependent viscosity convection

Abstract: Simple scaling analysis of temperature‐ and stress‐dependent viscosity convection with free‐slip boundaries suggests three convective regimes: the small viscosity contrast regime which is similar to convection in a fluid whose viscosity does not depend on temperature, the transitional regime characterized by self‐controlled dynamics of the cold boundary layer and the asymptotic regime in which the cold boundary becomes stagnant and convection involves only the hottest part of the lid determined by a rheological temperature scale. The first two regimes are usually observed in numerical experiments. The last regime is similar to strongly temperature‐dependent viscosity convection with rigid boundaries studied in laboratory experiments.

Equations governing convection in earth's core and the geodynamo

Abstract: Convection in Earth's fluid core is regarded as a small deviation from a well-mixed adiabatic state of uniform chemical composition. The core is modeled as a binary alloy of iron and some lighter constituent, whose precise chemical composition is unknown but which is here assumed to be FeAd, where Ad = Si, O or S. The turbulent transport of heat and light constituent is considered, and a simple ansatz is proposed in which this is modeled by anisotropic diffusion. On this basis, a closed system of equations and boundary conditions is derived that governs core convection and the geodynamo. The dual (thermal + compositional) nature of core convection is reconsidered. It is concluded that compositional convection may not dominate thermal convection, as had previously been argued by Braginsky (Soviet Phys. Dokl., v. 149, p. 8, 1963; Geomag, and Aeron., v. 4, p. 698, 1964), but that the two mechanisms are most probably comparable in importance. The key parameters leading to this conclusion are isolated...

An ocean large‐eddy simulation of Langmuir circulations and convection in the surface mixed layer

Abstract: Numerical experiments were performed using a three-dimensional large-eddy simulation model of the ocean surface mixed layer that includes the Craik-Leibovich vortex force to parameterize the interaction of surface waves with mean currents. Results from the experiments show that the vortex force generates Langmuir circulations that can dominate vertical mixing. The simulated vertical velocity fields show linear, small-scale, coherent structures near the surface that extend downwind across the model domain. In the interior of the mixed layer, scales of motion increase to eddy sizes that are roughly equivalent to the mixed-layer depth. Cases with the vortex force have stronger circulations near the surface in contrast to cases with only heat flux and wind stress, particularly when the heat flux is positive. Calculations of the velocity variance and turbulence dissipation rates for cases with and without the vortex force, surface cooling, and wind stress indicate that wave-current interactions are a dominant mixing process in the upper mixed layer. Heat flux calculations show that the entrainment rate at the mixed-layer base can be up to two times greater when the vortex force is included. In a case with reduced wind stress, turbulence dissipation rates remained high near the surface because of the vortexmore » force interaction with preexisting inertial currents. In deep mixed layers ({approximately}250 m) the simulations show that Langmuir circulations can vertically transport water 145 m during conditions of surface heating. Observations of turbulence dissipation rates and the vertical temperature structure support the model results. 42 refs., 20 figs., 21 tabs.« less

The Gravity Wave Response above Deep Convection in a Squall Line Simulation.

Abstract: High-frequency gravity waves generated by convective storms likely play an important role in the general circulation of the middle atmosphere. Yet little is known about waves from this source. This work utilizes a fully compressible, nonlinear, numerical, two-dimensional simulation of a midlatitude squall line to study vertically propagating waves generated by deep convection. The model includes a deep stratosphere layer with high enough resolution to characterize the wave motions at these altitudes. A spectral analysis of the stratospheric waves provides an understanding of the necessary characteristics of the spectrum for future studies of their effects on the middle atmosphere in realistic mean wind scenarios. The wave spectrum also displays specific characteristics that point to the physical mechanisms within the storm responsible for their forcing. Understanding these forcing mechanisms and the properties of the storm and atmosphere that control them are crucial first steps toward developing a parameterization of waves from this source. The simulation also provides a description of some observable signatures of convectively generated waves, which may promote observational verification of these results and help tie any such observations to their convective source.

Optimum design and selection of heat sinks

Abstract: An analytical simulation model has been developed for predicting and optimizing the thermal performance of bidirectional fin heat sinks in a partially confined configuration. Sample calculations are carried out, and parametric plots are provided, illustrating the effect of various design parameters on the performance of a heat sink. It is observed that the actual convection flow velocity through fins is usually unknown to designers, yet, is one of the parameters that greatly affect the overall thermal performance of a heat sink. In this paper, a simple method of determining the fin flow velocity is presented, and the development of the overall thermal model is described. An overview of different types of heat sinks and associated design parameters is provided. Optimization of heat-sink designs and typical parametric behaviors are discussed based on the sample simulation results.

The Behavior of a Simple Hurricane Model Using a Convective Scheme Based on Subcloud-Layer Entropy Equilibrium

Abstract: Recent work on the interaction of convection with large-scale flows suggests that a closure based on a presumed equilibrium surface enthalpy fluxes and input of low-entropy air into the subcloud layer by convective downdrafts works well in models of the tropical atmosphere. Such a convective representation is here used in a simple numerical tropical cyclone model. This further simplifies the model, while in many respects improving its performance.

Regulation of moist convection over the West Pacific warm pool

Abstract: The mechanisms that regulate moist convection over the warm tropical oceans are not well understood. One school of thought holds that convection is caused by the convergence of moisture, which in turn is produced by an independent dynamical mechanism. Another school maintains that convection occurs as needed to just balance the production of convective instability and that the timescales to establish this balance is much less than the timescales of tropical disturbances. This is called the quasiequilibrium hypothesis. This paper explores how convection is actually governed over the west Pacific warm pool. Convection appears to be initiated there when the boundary-layer equivalent potential temperature exceeds a threshold value that is determined by conditions just above cloud base. Given known surface flux values and the propensity for convection to inject low equivalent potential temperature air into the boundary layer, it is shown that under most circumstances convection is regulated by a balan...

Granular Convection Observed by Magnetic Resonance Imaging

Abstract: Vibrations in a granular material can spontaneously produce convection rolls reminiscent of those seen in fluids. Magnetic resonance imaging provides a sensitive and noninvasive probe for the detection of these convection currents, which have otherwise been difficult to observe. A magnetic resonance imaging study of convection in a column of poppy seeds yielded data about the detailed shape of the convection rolls and the depth dependence of the convection velocity. The velocity was found to decrease exponentially with depth; a simple model for this behavior is presented here.

On a Babcock-Leighton dynamo model with a deep-seated generating layer for the toroidal magnetic field

Abstract: A dynamo model of the Babcock-Leighton type having the following features is studied. The toroidal fieldB φ is generated in a thin layer (the GL), located at the lower solar convection zone, by a shear in the angular velocity acting on the poloidal fieldB p (= ∇ × [0, 0,A φ ].) If, in this layer, and for a certain value of the polar angle,θ, |B O | exceeds a critical field,B cr , then the eruption of a flux tube occurs. This flux tube, which is assumed to rise radially, generates, when reaching the surface, a bipolar magnetic region (BMR) with fluxes Φ p and Φ f for the preceding and following spot respectively. For the purpose of the numerical calculations this BMR is replaced by its equivalent axisymmetrical magnetic ring doublet. The ensemble of these eruptions acts as the source term for the poloidal field. This field, generated in the surface layers, reaches the lower solar convection by transport due to meridional motions and by diffusion. The meridional motions are the superpositions of a one-cell velocity field that rises at the equator and sinks at the poles and of a two-cell circulation that rises at the equator and poles and sinks at mid latitudes. The toroidal field andA O were expanded in Legendre polynomials, and the coupled partial differential equations (int andr; time and radial coordinate) satisfied by the coefficients in these expansions were solved by a finite difference method. In the expansions, Legendre polynomials up to order thirty were included. In spite of an exhaustive search no solutions were found with Φ p = −Φ f . The solutions presented in this paper were obtained with Φp = −0.5 Φ f . In this case, the northern and southern hemisphere are not entirely decoupled since lines of force join both hemispheres. Most of the solutions found were periodic. For the one-cell meridional flow described above and for a purely radial shear in the GL (the angular velocity increasing inwards) the dynamo wave propagates from the pole towards the equator. The new cycle starts at the poles while the old cycle is still present in the equatorial regions.

Effect of adiabatic deceleration on the focused transport of solar cosmic rays

Abstract: In the framework of focused transport theory, adiabatic deceleration arises from adiabatic focusing in the solar wind frame and from differential solar wind convection. An explicit formula is given for the deceleration of individual particles as a function of the pitch angle. Deceleration and other first-order effects of the solar wind, including convection, are incorporated into a numerical code for simulating the transport of energetic particles along the interplanetary magnetic field. We use this code to model the transport of solar flare protons. We find that including deceleration can increase the decay rate of the near-Earth intensity by 75\% more than would be expected based on advection from higher momenta, due to an interplay with diffusive processes. Improved response functions are derived for the impulsive injection of particles near the Sun, and it is found that neglecting deceleration leads to incorrect estimates of the scattering mean free path based on the intensity decay alone, especially for lower-energy particles.

Effects of boundary conditions on non-Darcian heat transfer through porous media and experimental comparisons

Abstract: The present work centers around the numerical simulation of forced convective incompressible flow through porous beds. Inertial as well as viscous effects are considered in the momentum equation. The mathematical model for energy transport was based on the two-phase equation model, which does not employ local thermal equilibrium assumption between the fluid and the solid phases. The transport processes for two different types of boundary conditions are studied. The analysis was performed in terms of nondimensional parameters that successfully cast together all the pertinent influencing effects. Comparisons were made between our numerical findings and experimental results. Overall, the comparisons that were made for the constant wall heat flux boundary condition display good agreement.

Magnetosphere‐ionosphere‐thermosphere coupling: Effect of neutral winds on energy transfer and field‐aligned current

Abstract: The assimilative mapping of ionospheric electrodynamics (AMIE) algorithm has been applied to derive the realistic time-dependent large-scale global distributions of the ionospheric convection and particle precipitation during a recent Geospace Environment Modeling (GEM) campaign period: March 28-29, 1992. The AMIE outputs are then used as the inputs of the National Center for Atmospheric Research thermosphere-ionosphere general circulation model to estimate the electrodynamic quantities in the ionosphere and thermosphere. It is found that the magnetospheric electromagnetic energy dissipated in the high-latitude ionosphere is mainly converted into Joule heating, with only a small fraction (6%) going to acceleration of thermospheric neutral winds. Our study also reveals that the thermospheric winds can have significant influence on the ionospheric electrodynamics. On the average for these 2 days, the neutral winds have approximately a 28% negative effect on Joule heating and approximately a 27% negative effect on field-aligned currents. The field-aligned currents driven by the neutral wind flow in the opposite direction to those driven by the plasma convection. On the average, the global electromagnetic energy input is about 4 times larger than the particle energy input.

Fluid Flow in Fault Zones: Analysis of the Interplay of Convective Circulation and Topographically Driven Groundwater Flow

Abstract: High-permeability faults, acting as preferential pathways for fluid migration, are important geological structures for fluid, energy, and solute transport. This paper examines the interaction of thermally driven convective circulation in a steeply dipping fault zone and groundwater flow through the surrounding country rock that is driven by a regional topographic gradient. We consider a geometry where a fault zone with a homogeneous, isotropic permeability is located beneath a narrow valley in a region with substantial topographic relief. System behavior is best characterized in terms of the large-scale permeabilities of the country rock and the fault zone. Using three-dimensional numerical simulations, we map in permeability space four fluid flow and heat transfer regimes within a fault zone: conductive, advective, steady convective, and unsteady convective. The patterns of fluid flow and/or heat transfer are substantially different in each of these regimes. Maximum discharge temperatures can also be plotted in permeability space; the maximum discharge temperature in the advective regime is in general lower than that in the steady convective regime. A higher basal heat flux expands the convective regime in permeability space, as does a greater fault depth. Higher topographic relief on the regional water table compresses the convective regime, with the advective regime suppressing convective circulation at lower country rock permeabilities. If convective cells with aspect ratios close to 1 cannot form, the steady convective regime is smaller in permeability space, and the boundary between steady and unsteady convection occurs at lower values of fault zone permeability. At low country rock permeabilities a water table gradient along the surface trace of the fault of approximately 0.3% suppresses convective cells; at higher country rock permeabilities, convection can be suppressed by smaller gradients on the water table.

Integral Effects of Deep Convection

Abstract: The large-scale, integral effect of convective elements (plumes) constituting an open-ocean chimney is investigated both theoretically and with a plume-resolving numerical model. The authors consider an initially homogeneous “patch” of ocean of depth H, with Coriolis parameter f, in which buoyancy is lost from the surface at a rate B. Both vorticity constraints on the convection patch and model analyses imply that, irrespective of the details of the plumes themselves, the mean vertical transport resulting from their action must be vanishingly small. Plumes are best thought of as mixing agents, which efficiently homogenize properties of the chimney. Scaling laws are derived from dynamical arguments and tested against the model. Using an expression for the vertical mixing timescale, they relate the chimney properties, the strength of the geostrophic rim-current setup around it, and its breakup timescale by baroclinic instability to the external parameters B, f, and H. After breakup, the instability...

The Relationship between Sea Surface Temperature and Latent Heat Flux in the Equatorial Pacific

Abstract: Moored buoy data from the equatorial Pacific are analyzed to investigate the relationship between sea surface temperature (SST) and latent heat flux from the ocean. It is found that at low SST the latent heat flux increases with SST; at high SST the latent heat flux decreases with increasing SST, a relationship that cannot be explained by thermodynamic considerations alone. Analysis of the wind speeds and humidity differences between the surface air and the saturation humidity at the sea surface temperature indicates that while at low SST the humidity difference primarily determines the latent heat flux, and at high SST a sharp decrease in wind speed is mostly responsible for the low latent heat flux. A mechanism that leads to low latent heat flux at high SST is suggested; it involves the interaction between convection and the large-scale circulation. The longitudinal distribution of SST, wind speed, humidity difference, and latent heat flux is found to be similar to that in previous studies. In ...

Non-local turbulent transport: pollution dispersion aspects of coherent structure of connective flows

Abstract: During the last forty years vertical exchange in the atmospheric surface layer has been parameterized with the aid of the Monin-Obukhov similarity theory. Currently it is understood that the concept of local flux-gradient correspondence underlying that theory and most traditional turbulence closures breaks down in convective conditions. Physical essence of the problem is as follows. In strong convection large-scale semi-organised coherent structures embrace the entire convective boundary layer (~1 km in height). They generate pronounced largescale (2-3 km in the horizontal) flow patterns close to the surface, which play an important role in the horizontal dispersion of atmospheric pollutants. The largescale structures also yield local velocity shears and consequently the sheargenerated turbulence, which crucially affects heat/mass transfer. On the contrary, local shears, oriented in the opposite directions, only slightly affect the mean momentum transfer (hence the term "inactive turbulence"). Only first steps have been made in analysing these effects theoretically. In the present paper a review of the problem is given, and a new theoretical model is proposed capable of reproducing the above essential features of convection. The model is verified using data from recent large-eddy simulation of convective boundary layers together with the reference atmospheric data. Recommendations are given as to how to proceed in practical parameterization of the surface fluxes and horizontal velocity variances in pollution dispersion models.

Convection Initiation Associated with a Sea-Breeze Front, a Gust Front, and Their Collision

Abstract: The initiation of convection associated with a sea-breeze front, a gust front, and their collision is analyzed using data collected in east-central Florida during the Convection and Precipitation/Electrification project. In conjunction with satellite, surface, and rawinsonde information, dual-Doppler radar-derived winds are used to determine the three-dimensional kinematic factors critical to storm development. The gust front, which emanated from storms on the western half of the peninsula, propagated more rapidly and was deeper than the sea-breeze front, which originated from the east coast and was characterized by a distinctly scalloped appearance. Convection associated with the sea-breeze front appeared to develop preferentially at the vertices of this scalloped pattern where there were enhanced regions of convergence and upward motion. On the gust front, a Helmholtz shearing instability produced an organized configuration of convergence and updraft maxima along its length. However, these were...

Coupled heat, water and gas flow in deformable porous media

Abstract: A fully coupled numerical model to simulate the slow transient phenomena involving heat and mass transfer in deforming porous media is developed. It makes use of the modified effective stress concept together with the capillary pressure relationship. The heat transfer through conduction and convection as well as the latent heat transfer (evaporation and/or condensation) is taken into account. The governing equations in terms of displacements, temperature, capillary pressure and gas pressure are coupled non-linear differential equatiosn and are solved by the finite element method. The model is validated with respect to a documented experiment on semisaturated soil behaviour. Two other examples involving subsidence due to pumping from a phreatic aquifer and thermoelastic consolidation of saturated and semisaturated media are also presented.

An Advective Atmospheric Mixed Layer Model for Ocean Modeling Purposes: Global Simulation of Surface Heat Fluxes

Abstract: A simple model of the lowest layer of the atmosphere is developed for coupling to ocean models used to simulate sea surface temperature (SST). The model calculates the turbulent fluxes of sensible and latent heat in terms of variables that an ocean model either calculates (SST) or is forced by (winds). It is designed to avoid the need to specify observed atmospheric data (other than surface winds), or the SST, in the surface flux calculations of ocean models and, hence, to allow a realistic representation of the feedbacks between SST and the fluxes. The modeled layer is considered to be either a dry convective layer or the subcloud layer that underlies marine clouds. The turbulent fluxes are determined through a balance of horizontal advection and diffusion, the surface flux and the flux at the mixed layer top, and, for temperature, radiative cooling. Reasonable simulations of the global distribution of latent and sensible heat flux are obtained. This includes the large fluxes that occur east of ...