Convective Plumes Research Articles

Transport by deep convection in basin-scale geostrophic circulation: turbulence-resolving simulations

Direct numerical simulations are used to investigate the nature of fully resolved small-scale convection and its role in large-scale circulation in a rotating $f$-plane rectangular basin with imposed surface temperature difference. The large-scale circulation has a horizontal geostrophic component and a deep vertical overturning. This paper focuses on convective circulation with no wind stress, and buoyancy forcing sufficiently strong to ensure turbulent convection within the thermal boundary layer (horizontal Rayleigh numbers $Ra\approx 10^{12}{-}10^{13}$). The dynamics are found to depend on the value of a convective Rossby number, $Ro_{\unicode[STIX]{x0394}T}$, which represents the strength of buoyancy forcing relative to Coriolis forces. Vertical convection shifts from a mean endwall plume under weak rotation ($Ro_{\unicode[STIX]{x0394}T}>10^{-1}$) to ‘open ocean’ chimney convection plus mean vertical plumes at the side boundaries under strong rotation ($Ro_{\unicode[STIX]{x0394}T}<10^{-1}$). The overall heat throughput, horizontal gyre transport and zonally integrated overturning transport are then consistent with scaling predictions for flow constrained by thermal wind balance in the thermal boundary layer coupled to vertical advection–diffusion balance in the boundary layer. For small Rossby numbers relevant to circulation in an ocean basin, vertical heat transport from the surface layer into the deep interior occurs mostly in ‘open ocean’ chimney convection while most vertical mass transport is against the side boundaries. Both heat throughput and the mean circulation (in geostrophic gyres, boundary currents and overturning) are reduced by geostrophic constraints.

Kelvin-Helmholtz billows in rising morning inversions

Wavelike motions (similar to Kelvin-Helmholtz billows, KHBs) within the rising inversion layers during the morning transition of the atmospheric boundary layer (ABL) are investigated. The KHBs were depicted in sodar echograms obtained in a rural terrain near Moscow (Russia) in 2011. Power spectra of the intensity of the sodar return signal were calculated from inversion layers and from convective areas underneath. Comparison of the dominant spectral maximums for the inversion and convective parts of the ABL show connection between convective plumes and KHBs.

On the Factors Controlling the Development of Shallow Convection in Eddy-Diffusivity/Mass-Flux Models

Abstract This study addresses key aspects of shallow moist convection, as simulated by a multiplume eddy-diffusivity/mass-flux (EDMF) model. Two factors suggested in the literature to be essential for the development of convective plumes are investigated: surface conditions and lateral entrainment. The model consistently decomposes the subgrid vertical mixing into convective plumes and the nonconvective environment. The modeled convection shows low sensitivity to the surface plume area. The results indicate that plume development in the subcloud layer is controlled by both surface conditions and lateral entrainment. Their impact significantly changes in the cloud layer where the surface conditions are no longer important. The development of shallow convection is dominated by the interactions between the plumes and the large-scale field and is sensitive to the representation of the variability of thermodynamic properties between the plumes. A simple two-layer model of steady-state convection is proposed to help understand the role of these processes in shaping the properties of moist convection.

Experimental and numerical investigation on the convective thermal plume around the head of the standing and lying hu-man body

This paper presents a study for two thermal plumes generated by two humanoid thermal manikins, one standing and one lying down. The research was approached from a numerical and experimental perspective. The numerical model represents an operating room (OR) with two surgeons, a patient and a unidirectional air flow (UAF) diffuser. The experimental study was made in a climatic chamber, having a similar air distribution system, using particle image velocimetry (PIV) and infrared thermography (IR) measurements. The purpose of the study was to characterize the thermal plumes of the two manikins by numerical and experimental studies. The results obtained from these different approaches were compared with each other and with the ones from the literature in order to validate our numerical models.

Ориентационные эффекты конвекции в полости между частично нагретыми цилиндрами различной формы

This paper reports the results of numerical simulation of a steady-state convective flow of incompressible fluid in the gap between two coaxial cylinders. We assume that only one side of the inner triangular cylinder is heated, and the external circular cylinder is kept at constant temperature. It has been established that the rotation of the heater through a certain angle causes the convective plume axis to deviate. This strongly affects the flow pattern and the related heat transfer process. The maximum integral heat flux is observed when the heater is in its side position, and the minimum heat flux is observed when the heater is in the bottom position. Approximating the results, we obtain the dependence of the Nusselt number on the Rayleigh parameter and the rotation angle.

The Carr Fire Vortex: A Case of Pyrotornadogenesis?

AbstractRadar and satellite observations document the evolution of a destructive fire‐generated vortex during the Carr fire on 26 July 2018 near Redding, California. The National Weather Service estimated that surface wind speeds in the vortex were in excess of 64 m/s, equivalent to an EF‐3 tornado. Radar data show that the vortex formed within an antecedent region of cyclonic wind shear along the fire perimeter and immediately following rapid vertical development of the convective plume, which grew from 6 to 12 km aloft in just 15 min. The rapid plume development was linked to the release of moist instability in a pyrocumulonimbus (pyroCb). As the cloud grew, the vortex intensified and ascended, eventually reaching an altitude of 5,200 m. The role of the pyroCb in concentrating near‐surface vorticity distinguishes this event from other fire‐generated vortices and suggests dynamical similarities to nonmesocyclonic tornadoes.

Inverse modeling of the natural state of geothermal reservoirs using adjoint and direct methods

The application of the adjoint and direct methods to inverse modeling of the natural state of a convective geothermal system is discussed. The methods have been applied to other subsurface modeling problems, but they have seldom been used in the geothermal context and not at all for high-enthalpy, two-phase systems.There are two important features of the natural state problem that make it an interesting challenge for inverse modeling. The first is that good downhole temperature measurements are typically available from 1 to 2 km deep wells, scattered over a large area of the geothermal system. Secondly, the geological structure of the system controls how the convective plume develops over geological time and thus downhole temperatures can be used to infer the large-scale permeability distribution. This interaction of large-scale, heat induced convection and geological structure determines the size and shape of hot geothermal systems and makes inverse modeling of their natural state particularly useful, no matter what method is used.Another important issue with natural state geothermal modeling is the challenge of the computational task involved. Most previous inverse modeling studies of the natural state of high-enthalpy geothermal reservoirs have used the derivative-based Levenberg-Marquardt method with derivatives of model outputs evaluated by finite differences. This is a very time-consuming process since many lengthy forward simulations of a transient approach to the steady natural state are required. In the present study adjoint and direct methods are used for the efficient evaluation of derivatives and prove to be very effective.A synthetic two-dimensional vertical slice model is used to test the applicability of the adjoint and direct methods. The results show that the methods offer much faster inversions compared to those based on finite differencing.

Nonlinear Gravitational Waves and Atmospheric Instability

This is an outline of the main results of the study of dust devils (a type of atmospheric vortices) with a focus on the mechanism of vortex generation in an unstable stratified atmosphere. In the approximation of ideal hydrodynamics, a new nonlinear model of the generation of convective motions and dust devils in an unstable stratified atmosphere has been developed. Using nonlinear equations for internal gravity waves, the model of generation of convective cell plumes has been investigated in the axially symmetric approximation. It has been shown that, in a convectively unstable atmosphere with large-scale seed vorticity, the plumes extremely rapidly generate small-scale intense vertical vortices. The structure of radial, vertical, and toroidal velocity components in these vortices has been investigated. The structure of vertical vorticity and toroidal velocity in vortex areas that are limited by radius has been examined.

Shallow-to-Deep Transition of Continental Moist Convection: Cold Pools, Surface Fluxes, and Mesoscale Organization

Abstract Large-eddy simulation is used to investigate the effects of cold pools driven by rain evaporation on the shallow-to-deep convection transition over land. The physically consistent methodologies are developed to obtain a time-dependent reference ensemble without cold pools and to apply interactive surface heat fluxes without modeling of surface energy and water budgets. Three different simulation ensembles are contrasted. The reference ensemble, in the spirit of one-dimensional single-column models, eliminates cold pools by horizontally homogenizing negative buoyancy production due to rain evaporation. The additional ensembles complement the reference cold-pool-free ensemble by including cold pools and by applying either interactive or prescribed surface fluxes. Contrasting these ensembles suggests possible improvements of convection parameterization in large-scale models of weather and climate. Without cold pools, the reference ensemble preserves key features of buoyancy-driven cellular convection associated with a field of convective plumes, as assumed in a typical convection parameterization. With cold pools, a significant enhancement of surface heat and moisture fluxes and about an hour delay of their daily maximum is simulated. Cold pools enhance near-surface temperature and moisture standard deviations as well as maxima of the near-surface updraft velocity. They also lead to the reduction of cloud lateral entrainment, deeper vertical development of the cloud layer, and a few-times-larger accumulated surface precipitation. Interactive surface fluxes provide a damping mechanism that noticeably suppresses all these effects. Perhaps surprisingly, cold pools do not significantly change the cloud-base convective mass flux that approximately follows the evolution of surface heat fluxes.

Experimental study of rotating convection in the presence of bi-directional thermal gradients with localized heating

We conduct experiments on the convective dynamics of a rotating fluid in a novel configuration, which comprises of a cylindrical annulus with peripheral spot heating at the bottom on the outer edge and uniform cooling on the inner edge. This system naturally provides an additional vertical gradient on the outer edge of the annulus, along with a radial gradient, thereby mimicking the thermal gradient patterns encountered in a real atmosphere. Localized heating is carried out at the bottom using a thin annular metal strip of thickness, h=5 mm. Water is used as the working fluid. Once the system reaches a statistically steady state, localized temperature measurements are carried out at various radial locations along the vertical fluid height. We measure time series of the temperature at several locations and visualize the flow structures over a range of Taylor number Ta (spanning 6.5 × 108- 2.7 × 109) and Rayleigh number Ra (spanning 2.2 × 108- 6.2 × 108). Temperature time series data at different heights, z, near the outer edge, along with two point correlations of temperature data indicate that, in the presence of rotation, columnar convective plumes (CCP) exist above the heating zone that aid in the vertical transport of heat. Qualitative flow field and temperature measurements in the bulk fluid also indicate the existence of baroclinic waves, which aid in the horizontal transport of cold and hot fluids between the annuli. The two point temperature correlations show that these waves break into eddies as the value of Ta increases. Overall, it is speculated that heat transport in this new configuration is governed by the co-existence and interplay of convective plumes and baroclinic waves/eddies, which closely simulates the dynamics of geophysical motions. It is found that the heat transport near the outer edge and in the fluid bulk is a strong function of Ra and Ta. For the highest value of Ra=6.2 × 108, the radial heat transport was most effective at Ta=0. However, at lower values of Ra, the radial heat transport in the outer and bulk regions was highest at Ta=1.5 × 109. These results indicate that an optimum value of Ra and Ta exists at which the convection driven radial heat transport is most energetic through the interaction of CCP and baroclinic waves/eddies.

Three‐Dimensional Turbulence‐Resolving Modeling of the Venusian Cloud Layer and Induced Gravity Waves: Inclusion of Complete Radiative Transfer and Wind Shear

AbstractVenus' convective cloud layers and associated gravity waves strongly impact the local and global budget of heat, momentum, and chemical species. Here we use for the first time three‐dimensional turbulence‐resolving dynamical integrations of Venus' atmosphere from the surface to 100‐km altitude, coupled with fully interactive radiative transfer computations. We show that this enables to correctly reproduce the vertical position (46‐ to 55‐km altitude) and thickness (9 km) of the main convective cloud layer measured by Venus Express and Akatsuki radio occultations, as well as the intensity of convective plumes (3 m/s) measured by VEGA balloons. Both the radiative forcing in the visible and the large‐scale dynamical impact play a role in the variability of the cloud convective activity with local time and latitude. Our model reproduces the diurnal cycle in cloud convection observed by Akatsuki at the low latitudes and the lack thereof observed by Venus Express at the equator. The observed enhancement of cloud convection at high latitudes is simulated by our model, although underestimated compared to observations. We show that the influence of the vertical shear of horizontal superrotating winds must be accounted for in our model to allow for gravity waves of the observed intensity (>1 K) and horizontal wavelength (up to 20 km) to be generated through the obstacle effect mechanism. The vertical extent of our model also allows us to predict for the first time a 7‐km‐thick convective layer at the cloud top (70‐km altitude) caused by the solar absorption of the unknown ultraviolet absorber.

Influence of a Salt Plume Parameterization in a Coupled Climate Model

AbstractSea ice formation is accompanied by the rejection of salt which in nature tends to be mixed vertically by the formation of convective plumes. Here we analyze the influence of a salt plume parameterization in an atmosphere‐sea ice‐ocean model. Two 330‐yearlong simulations have been conducted with the AWI Climate Model. In the reference simulation, the rejected salt in the Arctic Ocean is added to the uppermost ocean layer. This approach is commonly used in climate modeling. In another experiment, employing salt plume parameterization, the rejected salt is vertically redistributed within the mixed layer based on a power law profile that mimics the penetration of salt plumes. We discuss the effects of this redistribution on the simulated mean state and on atmosphere–ocean linkages associated with the intensity of deepwater formation. We find that the salt plume parameterization leads to simultaneous increase of sea ice (volume and concentration) and decrease of sea surface salinity in the Arctic. The salt plume parameterization considerably alters the interplay between the atmosphere and the ocean in the Nordic Seas. The parameterization modifies the ocean ventilation; however, resulting changes in temperature and salinity largely compensate each other in terms of density so that the overturning circulation is not significantly affected.

Application of Geoid Anomalies to the Tectonic Research in the East Asian Continental Margin

In this paper, we calculated multi-scale residual geoid anomalies with the method of geoid separation processing, according to EGM2008 ultra-high order gravity field model, remove-restore technique and Stokes integral. The East Asian continental margin was selected as the study area. The residual geoid anomalies have been calculated by programming. On the basis of residual geoid anomalies at various orders, the interlayer geoid anomalies at different depths were calculated to depict the spatial distribution characteristics of the residual geoid. Finally, we conducted a detailed geophysical interpretation for the study area according to the geoid anomalies in combination with other geophysical datasets. Four conclusions can be outlined as follows: 1) it is impracticable that geoid anomalies are used in the interpretation of the shallow objects due to the influence of the terrain; 2) the anomalies of residual geoid can reflect the intensity of small-scale mantle convection in the asthenosphere; 3) the interlayer geoid anomalies can reflect the magmatic activities associated with the mantle convection and mantle plume in different scales; 4) the study of the geoid may provide an approach for the research of the subduction zone, mantle convection and mantle plume.

Convection: a neglected pathway for downward transfer of wind energy in the oceanic mixed layer

Upper-ocean turbulent mixing plays a vital role in mediating air-sea fluxes and determining mixed-layer properties, but its energy source, especially that near the base of the mixed layer, remains unclear. Here we report a potentially significant yet rarely discussed pathway to turbulent mixing in the convective mixed layer. During convection, as surface fluid drops rapidly in the form of convective plumes, intense turbulence kinetic energy (TKE) generated via surface processes such as wave breaking is advected downward, enhancing TKE and mixing through the layer. The related power, when integrated over the global ocean except near the surface where the direct effect of breaking waves dominates, is estimated at O (1)TW, comparable to that required by maintaining the Meridional Overturning Circulation (MOC). The mechanism in question therefore deserves greater research attention, especially in view of the potential significance of its proper representation in climate models.

Unsteady processes in a natural convective plume

The paper presents the results of an experimental and numerical study of the structure of a plume formed over a heated disk under conditions of conjugate heat transfer in a range of small and average Grashof numbers. Numerical simulation was performed using the Ansys Fluent code, where the problem of the flow of a compressible gas in conditions of conjugate heat transfer in an unsteady formulation was solved under the assumption of laminar flow regime. One of the main goal of this paper is to compare results of modelling of the plume in assumption of axisymmetric and three-dimensional flow regime. As a result of a comparison of the results of physical and numerical experiments, there is a conclusion about a good qualitative concurrence of the flow structure in a range of a small Grashof numbers.

Assessing the Grell-Freitas Convection Parameterization in the NASA GEOS Modeling System.

We implemented and began to evaluate an alternative convection parameterization for the NASA Goddard Earth Observing System (GEOS) general circulation model (GCM). The proposed parameterization follows the mass flux approach with several closures, for equilibrium and nonequilibrium convection, and includes scale and aerosol aware functionalities. Recently, we extended the scheme to a trimodal spectral size distribution of allowed convective plumes to simulate the transition among shallow, congestus, and deep convection regimes. In addition, the inclusion of a new closure for nonequilibrium convection resulted in a substantial gain of realism in the model representation of the diurnal cycle of convection over the land. We demonstrated the scale‐dependence functionality with a cascade of global‐scale simulations from a nominal horizontal resolution of 50 km down to 6 km. The ability to realistically simulate the diurnal cycle of precipitation over various regions of the earth was verified against several remote sensing‐derived intradiurnal precipitation estimates. We extended the model performance evaluation for weather‐scale applications by bringing together some available operational short‐range weather forecast models and global atmospheric reanalyses. Our results demonstrate that the GEOS GCM with the alternative convective parameterization has good properties and competitive skill in comparison with state‐of‐the‐art observations and numerical simulations.

Driving Solar Giant Cells through the Self-organization of Near-surface Plumes

Abstract Global 3D simulations of solar giant-cell convection have provided significant insight into the processes which yield the Sun’s observed differential rotation and cyclic dynamo action. However, as we move to higher-resolution simulations a variety of codes have encountered what has been termed the convection conundrum. As these simulations increase in resolution and hence the level of turbulence achieved, they tend to produce weak or even anti-solar differential rotation patterns associated with a weak rotational influence (high Rossby number) due to large convective velocities. One potential culprit for this convection conundrum is the upper boundary condition applied in most simulations, which is generally impenetrable. Here we present an alternative stochastic plume boundary condition which imposes small-scale convective plumes designed to mimic near-surface convective downflows, thus allowing convection to carry the majority of the outward solar energy flux up to and through our simulated upper boundary. The use of a plume boundary condition leads to significant changes in the convective driving realized in the simulated domain and thus to the convective energy transport, the dominant scale of the convective enthalpy flux, and the relative strength of the strongest downflows, the downflow network, and the convective upflows. These changes are present even far from the upper boundary layer. Additionally, we demonstrate that, in spite of significant changes, giant cell morphology in the convective patterns is still achieved with self-organization of the imposed boundary plumes into downflow lanes, cellular patterns, and even rotationally aligned banana cells in equatorial regions. This plume boundary presents an alternative pathway for 3D global convection simulations where driving is non-local and may provide a new approach toward addressing the convection conundrum.

Dynamics of thinning and destruction of the continental cratonic lithosphere: Numerical modeling

Thinning of the cratonic lithosphere is common in nature, but its destruction is not. In either case, the mechanisms for both thinning and destruction are still widely under debate. In this study, we have made a review on the processes and mechanisms of thinning and destruction of cratonic lithosphere according to previous studies of geological/geophysical observations and numerical simulations, with specific application to the North China Craton (NCC). Two main models are suggested for the thinning and destruction of the NCC, both of which are related to subduction of the oceanic lithosphere. One is the “bottom-up” model, in which the deeply subducting slab perturbs and induces upwelling from the hydrous mantle transition zone (MTZ). The upwelling produces mantle convection and erodes the bottom of the overriding lithosphere by the fluid-melt-peridotite reaction. Mineral compositions and rheological properties of the overriding lithospheric mantle are changed, allowing downward dripping of lithospheric components into the asthenosphere. Consequently, lithospheric thinning or even destruction occurs. The other is the “top-down” model, characterized by the flat subduction of oceanic slab beneath the overriding cratonic lithosphere. Dehydration reactions from the subducting slab would significantly hydrate the lithospheric mantle and decrease its rheological strength. Then the subduction angle may be changed from shallow to steep, inducing lateral upwelling of the asthenosphere. This upwelling would heat and weaken the overriding lithospheric mantle, which led to the weakened lithospheric mantle dripping into the asthenosphere. These two models have some similarities, in that both take the subducting oceanic slab and relevant fluid migration as the major driving mechanism for thinning or destruction of the overriding cratonic lithosphere. The key difference between the two models is the effective depth of the subducting oceanic slab. One is stagnation and flattening in the MTZ, whereas the other is flat subduction at the bottom of the cratonic lithosphere. In the NCC, the eastern lithosphere was likely affected by subduction of the Izanagi slab during the Mesozoic, which would have perturbed the asthenosphere and the MTZ, and induced fluid migration beneath the NCC lithosphere. The upwelling fluid may largely have controlled the reworking of the NCC lithosphere. In order to discuss and analyze these two models further, it is crucial to understand the role of fluids in the subduction zone and the MTZ. Here, we systematically discuss phase transformations of hydrous minerals and the transport processes of water in the subduction system. Furthermore, we analyze possible modes of fluid activity and the problems to explore the applied feasibility of each model. In order to achieve a comprehensive understanding of the mechanisms for thinning and destruction of cratonic lithosphere, we also consider four additional possible dynamic models: extension-induced lithospheric thinning, compression-induced lithospheric thickening and delamination, large-scale mantle convection and thermal erosion, and mantle plume erosion. Compared to the subduction-related models presented here, these four models are primarily controlled by the relatively simple and single process and mechanism (extension, compression, convection, and mantle plume, respectively), which could be the secondary driving mechanisms for the thinning and destruction of lithosphere.

Geomagnetic acceleration and rapid hydromagnetic wave dynamics in advanced numerical simulations of the geodynamo

Geomagnetic secular acceleration is a unique window on the dynamics taking place in Earth's core. In this study, the behaviours of the secular acceleration and underlying core dynamics are examined in new numerical simulations of the geodynamo that reside on a theoretical path in parameter space connecting the region where most classical models are found to the natural conditions. The typical time scale for geomagnetic acceleration is found to be invariant along this path, at a value close to 10 years that matches Earth's core estimates. Despite this invariance, the spatio-temporal properties of secular acceleration show significant variability along the path, with an asymptotic regime of rapid rotation reached after 30% of this path (corresponding to a model Ekman number $E=3~10^{-7}$). In this regime, the energy of secular acceleration is entirely found at periods longer than that of planetary rotation, and the underlying flow acceleration patterns acquire a two-dimensional columnar structure representative of the rapid rotation limit. The spatial pattern of the secular acceleration at the core-mantle boundary shows significant localisation of energy within an equatorial belt. Rapid hydromagnetic wave dynamics is absent at the start of the path but can be clearly exhibited in the asymptotic regime. This study reports on ubiquitous axisymmetric geostrophic torsional waves of weak amplitude relatively to convective transport, and also stronger, laterally limited, quasi-geostrophic Alfv\'en waves propagating in the cylindrical radial direction from the tip of convective plumes towards the core-mantle boundary. Quasi-geostrophic Alfv\'en waves are shown to be an important carrier of flow acceleration to the core surface that links with the generation of strong, short-lived and intermittent equatorial pulses in the secular acceleration energy.

Synthetic schlieren—application to the visualization and characterization of air convection

Synthetic schlieren is a digital image processing optical method relying on the variation of optical index to visualize the flow of a transparent fluid. In this article, we present a step-by-step, easy-to-implement and affordable experimental realization of this technique. The method is applied to air convection caused by a warm surface. We show that the velocity of rising convection plumes can be linked to the temperature of the warm surface and propose a simple physical argument to explain this dependence. Moreover, using this method, one can reveal the tenuous convection plumes rising from one’s hand, a phenomenon invisible to the naked eye. This spectacular result may help students to realize the power of careful data acquisition combined with astute image processing techniques. This spectacular result may help students to realize the power of careful data acquisition combined with astute image processing techniques (refer to the video abstract).