Convective Structures Research Articles

Chromospheric plasma ejection above a pore

ABSTRACT We present high spatial resolution observations of short-lived transients, ribbons and jet-like events above a pore in Ca ii H images where fine structure, like umbral dots, light bridges and penumbral microfilaments, is present in the underlying photosphere. We found that current layers are formed at the edges of the convective fine structure, due to the shear between their horizontal field and the ambient vertical field. High vertical electric current density patches are observed in the photosphere around these events, which indicates the formation of a current sheet at the reconnection site. In the framework of past studies, low altitude reconnection could be the mechanism that produces such events. The reconnection is caused by an opposite polarity field produced by the bending of field lines by convective downflows at the edge of pore fine structure.

The Importance of Horizontal Rolls in the Rapid Intensification of Tropical Cyclones

The present laboratory study is focused on the role of convective rolls in the enhancement of the heat flux from the sea and triggering of the process of rapid intensification of tropical cyclones. The appearance of coherent convective structures such as thermals and rolls is registered by different optical techniques and temperature measurements. Two-dimensional velocity fields are used for the investigation of the structure and characteristics of the flow. The heat flux from the heating plate to the fluid is measured directly. Obtained results clearly show that rapid intensification of a laboratory analog of a tropical cyclone is tightly linked with the heat transfer process in the boundary layer. Formation of secondary convective structures strongly increases the heat transfer and intensity of convective circulations. The intensity of radial inflow is a crucial aspect for the intensification of cyclonic vortex, hence rapid variation of the heat transfer is a factor that has a substantial influence on the dynamics of a laboratory vortex. Time series of maximal radial and azimuthal velocities are in a good qualitative agreement with those from a theoretical model. It is shown that the mean square of the velocity depends linearly on the temperature difference between the inflow and the heating surface.

On the Forecast Sensitivity of MCS Cold Pools and Related Features to Horizontal Grid Spacing in Convection-Allowing WRF Simulations

Abstract While the implementation of convection-allowing models has improved the representation of convective features, a consensus is lacking regarding what horizontal grid spacing most appropriately resolves convective structures, is computationally feasible, and provides the most useful output to forecasters. The present study evaluates 14 simulated MCSs with 3-, 1- and 0.333-km horizontal grid spacing in order to understand sensitivity in simulated MCS forward propagation speeds and cold pool behavior with decreased grid spacing. MCS cold pools were found to be significantly larger in runs using finer grid spacing. In addition, a greater similarity in solutions occurred when grid spacing was refined to 1 km and less, with 1- and 0.333-km MCS cold pools more similar in magnitude, depth, length, and areal coverage, than 3-km cold pools. The 1-km simulations demonstrated a small increase in forecast skill for 3-h QPF throughout MCS evolution compared to 3-km runs. The 1-km MCS 9-h precipitation swaths were also better aligned with observations compared to 3-km simulations. When evaluating MCS forward propagation speeds, however, 3-km simulated MCS speeds were more similar to observations compared to 1 km.

Slow diffusive structure in Nikolaevskii turbulence.

Weak turbulence has been investigated in nonlinear-nonequilibrium physics to understand universal characteristics near the transition point of ordered and disordered states. Here the one-dimensional Nikolaevskii turbulence, which is a mathematical model of weak turbulence, is studied theoretically. We calculate the velocity field of the Nikolaevskii turbulence assuming a convective structure and carry out tagged-particle simulations in the flow to clarify the Nikolaevskii turbulence from the Lagrangian description. The tagged particle diffuses in the disturbed flow and the diffusion is superdiffusive in an intermediate timescale between ballistic and normal-diffusive scale. The diffusion of the slow structure is characterized by the power law for the control parameter near the transition point of the Nikolaevskii turbulence, suggesting that the diffusive characteristics of the slow structure remain scale invariant. We propose a simplified model, named two-scale Brownian motion, which reveals a hierarchy in the Nikolaevskii turbulence.

Phase Transition of Ice at High Pressures and Low Temperatures.

The behavior of ice under extreme conditions undergoes the change of intermolecular binding patterns and leads to the structural phase transitions, which are needed for modeling the convection and internal structure of the giant planets and moons of the solar system as well as H2O-rich exoplanets. Such extreme conditions limit the structural explorations in laboratory but open a door for the theoretical study. The ice phases IX and XIII are located in the high pressure and low temperature region of the phase diagram. However, to the best of our knowledge, the phase transition boundary between these two phases is still not clear. In this work, based on the second-order Møller–Plesset perturbation (MP2) theory, we theoretically investigate the ice phases IX and XIII and predict their structures, vibrational spectra and Gibbs free energies at various extreme conditions, and for the first time confirm that the phase transition from ice IX to XIII can occur around 0.30 GPa and 154 K. The proposed work, taking into account the many-body electrostatic effect and the dispersion interactions from the first principles, opens up the possibility of completing the ice phase diagram and provides an efficient method to explore new phases of molecular crystals.

A pressure decomposition framework for aeroacoustic analysis of turbulent jets

Aeroacoustic analyses of jet flows have benefited greatly from a decomposition of turbulent pressure fluctuations into hydrodynamic and acoustic components. This is typically accomplished using signal processing techniques based on phase speeds, coherence properties or spectral analyses. We present an approach, building on the Momentum Potential Theory (MPT) approach of (Doak, 1989), to split pressure fluctuations into their hydrodynamic, acoustic and entropic, collectively designated fluid-thermodynamic (FT), components. Key advantages are that the approach is applicable everywhere in the jet i.e, not restricted to the near-acoustic field, and does not need user-defined thresholds. The effectiveness of the technique is demonstrated by analyzing the flowfields of three simulated jets, to encompass moderate-compressible to supersonic conditions. The statistical properties and wavepacket dynamics of each pressure component, and their relationships with the unsplit pressure are elaborated. The acoustic pressure field has the form of a wavepacket that attenuates downstream and whose modal analysis reveals low-rank behavior. At each Mach number examined, the acoustic pressure also identifies the relative prominence of each of three components: i) waves with upstream propagating energy content (negative group velocity), ii) supersonically traveling radiating downstream waves, and iii) subsonically convected evanescent waves, which follow the convection pattern of hydrodynamic eddies in the turbulent region. With increasing Mach number, the radiating and convected bands of energy move closer to each other. The hydrodynamic pressure also displays a wavepacket structure, but its features are different: it displays large-scale subsonically convected structures even past the core collapse region. Thus, in the turbulent region of the jet, the acoustic pressure displays smaller integral time scales of fluctuations than the hydrodynamic component. The acoustic pressure field, which includes a zero-crossing in its radial profiles, displays larger wavelengths than the hydrodynamic pressure field, correlates better with the near-field pressure signal and captures the radiated component of noise, especially at shallow angles. These properties make it a suitable field for informing pressure-based wavepacket models for jet noise.

NUMERICAL INVESTIGATION OF LAMINAR MIXED CONVECTION IN A SQUARE CROSS-SECTIONED CYLINDRICAL ANNULAR ENCLOSURE

Steady-state laminar mixed convection of Newtonian fluids in a square cross-sectioned cylindrical annular enclosure with rotating inner wall and heated top cover has been numerically analysed based on axisymmetric incompressible flow simulations. Richardson number, Reynolds number and r_i/R effects on heat and momentum transport have been investigated for the range of Richardson number 0 ≤ Ri ≤ 1, Reynolds number 500 ≤ Re ≤ 2000 and 0.25≤r_i/R≤8 at a representative value of Prandtl number (i.e. Pr=1.0). A scaling analysis has been also carried out in order to elucidate the possible influences of Reynolds, Richardson and Prandtl numbers and r_i/R on the mean Nusselt number. It has been found that the mean Nusselt number (Nu) ̅ demonstrates a monotonically decreasing trend with increasing Ri whereas (Nu) ̅ increases with increasing r_i/R and Re which is consistent with scaling estimation. It is also observed that the flow pattern in the case of purely forced convection (i.e. Ri = 0) is significantly different from those in mixed convection (i.e. Ri> 0). In the case of Ri = 0 (i.e. purely forced convection), a one-cell flow structure with two small vortexes on the top corners is observed for r_i/R≤1, whereas a second cell appears in the flow field for r_i/R> 1 at Re = 1000. On the other hand, in the case of mixed convection (i.e. Ri> 0), two-cell and four-cell flow structures occur in the flow field depending on Ri and r_i/R for the range of Ri, Re and r_i/R considered here at Pr⁡=1.0. Based these observations, a flow regime diagram has been proposed here for mixed convection (i.e. Ri> 0) for the range of Ri, Re and r_i/R analysed in this study.

Convection partition and dynamics in inclined Poiseuille-Rayleigh-Bénard flow

Since Henri Bénard first carried out experiments on convection in the fluid layer heated from below at the beginning of last century, and Lord Rayleigh first analyzed small disturbance theoretically, Rayleigh-Bénard convection has received much attention from many researchers, and has become one of the models to study the spatiotemporal structure, flow stability and dynamic characteristics of convection. The methods of studying the Rayleigh-Bénard convection are divided into experimental research, theoretical analysis and numerical simulation. With the development of computer, the research of numerical simulation has made great progress. Because the Rayleigh-Bénard convection can be accurately described by continuity equation, momentum equation and energy equation of hydrodynamics. Therefore, the numerical simulation based on hydrodynamics equations has aroused a lot of research interest. Based on the classical Rayleigh-Bénard convection, the influence of horizontal flow on the Rayleigh-Bénard convection can be studied by applying horizontal flow to one end of the horizontal cavity. On the other hand, the influence of cavity inclination on Rayleigh-Bénard convection can be studied by considering the variation of inclined angles in the cavity. Some valuable convective properties have been obtained. In order to reveal some new convection structures or phenomena, the effects of cavity inclination and through-flow on Rayleigh-Bénard convection are considered at the same time in this paper.By using the numerical simulation of the basic equations of hydrodynamics, the convection partition and dynamic characteristics of the fluid with Prandtl number <i>Pr</i> = 6.99 in the inclined cavity with through-flows are discussed. The results show that for the reduced Rayleigh number <i>r</i> = 9, the system presents uniform traveling wave convection, non-uniform traveling wave convection and single roll convection pattern at the through-flow Reynolds number <i>Re</i> = 1.5 with the increase of the inclined angle <i>θ</i> in the cavity, that for the through-flow Reynolds number <i>Re</i> = 12.5, the system presents the localized traveling wave convection, parallel flow and localized single roll convection pattern with the increase of the inclined angle <i>θ</i> in the cavity, that furthermore, the numerical simulation of different values of through-flow Reynolds number <i>Re</i> and inclined angle <i>θ</i> in the cavity shows that on the plane composed of through flow Reynolds number <i>Re</i> and inclined angle <i>θ</i> in the cavity, the convection in the inclined cavity with through-flow can be divided into six kinds of pattern regions, namely, uniform traveling wave convection region, non-uniform traveling wave convection region, single roll convection region, localized traveling wave convection region, parallel flow region, and localized single roll convection region. The characteristics of the maximum vertical velocity <i>w</i><sub>max</sub> and Nusselt number <i>Nu</i> of convection varying with time in different convection regions are studied. The dynamic properties of convective amplitude <i>A</i> and Nusselt number <i>Nu</i> in different convective regions varying with inclined angle <i>θ</i> in the cavity are discussed.

Experimental Analysis of Chaotic Transport and Lagrangian Coherent Structures in Rayleigh-Benard Convection

Experimental Analysis of Chaotic Transport and Lagrangian Coherent Structures in Rayleigh-Benard Convection

Experimental Study of the Convective Motions by the PIV Technique within an Evaporating Liquid Layer into the Gas Flow

We present the experimental study of convection in a horizontal liquid layer (ethanol, 3-mm deep), evaporating from a localized surface (10 × 10 mm2) into the gas flow (air). Visualization and measurements of the two-component velocity field in the liquid layer has been carried out with the Particle Image Velocimetry (PIV) technique. In our experiments we consider a novel configuration in which the gas-liquid interface is maintained in the flat position in the confined square area and the volatile liquid evaporates from the planar surface into the gas flowing along the surface. We also consider the effect of the gas velocity (0.0138–0.138 m/s) and the gas and the liquid temperature (20 °C - 50 °C) on the convective flow structure within the liquid layer. It is shown that the gas velocity and both, the gas and the liquid temperatures induce significant changes in the convective flow structure. We give the first experimental proof of the phenomenon that the motion of the gas-liquid interface goes along the counter-current direction to the gas flow as theoretically predicted. The analysis of the experimental data shows that the influence of the gas flow velocity on the Marangoni convection at the maximum temperature (50 °C) is significantly reduced owing to the growth of the diffusion resistance for the gas flow under a strong evaporation from the interface. As a result, it leads to the low surface temperature gradient, which decreases thermocapillary stresses and the circulation velocity of the first (thermocapillary) vortex within the fluid layer. Further, we observe that the disappearance of the second convective vortex circulating in the same direction with the gas flow. The governing factor, determining the structure of convective flows within the liquid is the thermocapillary effect due to the intensive evaporation provided that the cooling and the temperature distribution are uniform on the gas-liquid interface.

Long-term Behaviors of a Single Particle Forming a Coherent Structure in Thermocapillary-driven Convection in Half-zone Liquid Bridge of High Prandtl-number Fluid

Long-term Behaviors of a Single Particle Forming a Coherent Structure in Thermocapillary-driven Convection in Half-zone Liquid Bridge of High Prandtl-number Fluid

Study of convective structures and phase transition induced in a water layer by non-stationary boundary conditions

The problem of reconstructing interference and Hilbert structures from a numerical model of the evolution of the thermal field of convective flows in a water layer bounded by flat heat-exchanging surfaces under unsteady boundary conditions in the monotonic cooling mode and taking into account density inversion at a temperature of +4 °C was solved. The simulation of the thermal fields of convective flows in the form of the dynamic structure of isotherms was carried out taking into account the nonlinear dependence of thermal conductivity and water density on temperature. The field of the phase function, its Hilbert image and the interference field, which are compared with the results of the interference and Hilbert visualization of the fields of phase optical density obtained in the experiment, were reconstructed from the isotherms field supplemented by calculating of the velocity field and of the temperature gradients field. The presented films illustrate the qualitative adequacy of the coevolution of numerical models and real processes.

Heat transfer and structure of a turbulized flow in a converging channel

The paper deals with experimental studies of convective heat transfer and kinematic flow structure in a plane converging channel. Different turbulence intensities were generated by a variety of arrangements at the channel inlet. Distributions of heat transfer coefficient over the wall of the converging channel were obtained in different regimes of the air flow. Optical measurements yielded the profiles of hydrodynamic parameters at characteristics locations.

Area-Averaged Surface Moisture Flux over Fragmented Sea Ice: Floe Size Distribution Effects and the Associated Convection Structure within the Atmospheric Boundary Layer

Sea ice fragmentation results in the transformation of the surface from relatively homogeneous to highly heterogeneous. Atmospheric boundary layer (ABL) rapidly responds to those changes through a range of processes which are poorly understood and not parametrized in numerical weather prediction (NWP) models. The aim of this work is to increase our understanding and develop parametrization of the ABL response to different floe size distributions (FSD). The analysis is based on the results of simulations with the Weather Research and Forecasting model. Results show that FSD determines the distribution and intensity of convection within the ABL through its influence on the atmospheric circulation. Substantial differences between various FSDs are found in the analysis of spatial arrangement and strength of ABL convection. To incorporate those sub-grid effects in the NWP models, a correction factor for the calculation of surface moisture heat flux is developed. It is expressed as a function of floe size, sea ice concentration and wind speed, and enables a correction of the flux computed from area-averaged quantities, as is typically done in NWP models. In general, the presented study sheds some more light on the sea ice–atmosphere interactions and provides the first attempt to parametrize the influence of FSD on the ABL.

An Observational Overview of Dusty Deep Convection in Martian Dust Storms.

Deep convection, as used in meteorology, refers to the rapid ascent of air parcels in the Earth's troposphere driven by the buoyancy generated by phase change in water. Deep convection undergirds some of the Earth's most important and violent weather phenomena and is responsible for many aspects of the observed distribution of energy, momentum, and constituents (particularly water) in the Earth's atmosphere. Deep convection driven by buoyancy generated by the radiative heating of atmospheric dust may be similarly important in the atmosphere of Mars but lacks a systematic description. Here we propose a comprehensive framework for this phenomenon of dusty deep convection (DDC) that is supported by energetic calculations and observations of the vertical dust distribution and exemplary dusty deep convective structures within local, regional, and global dust storm activity. In this framework, DDC is distinct from a spectrum of weaker dusty convective activity because DDC originates from pre-existing or concurrently forming mesoscale circulations that generate high surface dust fluxes, oppose large-scale horizontal advective-diffusive processes, and are thus able to maintain higher dust concentrations than typically simulated. DDC takes two distinctive forms. Mesoscale circulations that form near Mars's highest volcanoes in dust storms of all scales can transport dust to the base of the upper atmosphere in as little as two hours. In the second distinctive form, mesoscale circulations at low elevations within regional and global dust storm activity generate freely convecting streamers of dust that are sheared into the middle atmosphere over the diurnal cycle.

A Climatological Analysis of Tropical Cyclone Rapid Intensification in Environments of Upper-Tropospheric Troughs

AbstractTropical cyclone (TC)–trough interactions are a common occurrence in the North Atlantic basin and lead to a variety of TC intensity changes, from rapid intensification (RI) to rapid weakening. To test whether certain TC–trough configurations are more favorable for RI than others, the upper-tropospheric troughs involved in such interactions were objectively classified into one of three clusters through the implementation of a machine-learning, dimensionality-reduction technique in conjunction with a k-means clustering algorithm. Through composite analyses, the upper-tropospheric potential vorticity structure, the TC convective structure, and the TC environment were examined for both rapidly intensifying TCs and nonrapidly intensifying (non-RI) TCs. As a whole, RI episodes were associated with upper-tropospheric troughs of shorter zonal wavelengths and greater upstream TC–trough displacements than non-RI episodes. RI was found to occur most frequently when an upper-tropospheric cutoff low was located approximately 500–1000 km southwest of the TC location. RI occurred preferentially in environments associated with less ventilation of the TC warm core with low-entropy environmental air. An examination of potential trough-induced forcing for convection revealed little relationship between RI and eddy flux convergence of angular momentum. Nonetheless, RI episodes were associated with anomalously vigorous convective activity within the TC inner core, as diagnosed by infrared and passive microwave satellite imagery.

Validating Time–Distance Helioseismic Inversions for Nonseparable Subsurface Profiles of an Average Supergranule

Abstract Supergranules are divergent 30 Mm-sized cellular flows observed everywhere at the solar photosphere. Their place in the hierarchy of convective structures and their origin remain poorly understood. Estimating supergranular depth is of particular interest because this may help point to the underlying physics. However, their subsurface velocity profiles have proven difficult to ascertain. Birch et al. suggested that helioseismic inferences would benefit from an ensemble average over multiple realizations of supergranules due to the reduction in realization noise. Bhattacharya et al. used synthetic forward-modeled seismic wave travel times and demonstrated the potential of helioseismic inversions to recover the flow profile of an average supergranule that is separable in the horizontal and vertical directions, although the premise of this calculation has since been challenged by Ferret. In this work we avoid this assumption and carry out a validation test of helioseismic travel-time inversions starting from plausible synthetic nonseparable profiles of an average supergranule. We compute seismic wave travel times and sensitivity kernels by simulating wave propagation through this background. We find that, while the ability to recover the exact profile degrades based on the number of parameters involved, we are nevertheless able to recover the peak depth of our models in a few iterations where the measurements are presumably above the noise cutoff. This represents an important step toward unraveling the physics behind supergranules, as we start appreciating the parameters that we may reliably infer from a time–distance helioseismic inversion.

A preliminary study of turbulent coherent structures and ozone air quality in Seoul using the WRF-CMAQ model at a 50 m grid spacing

The effects of turbulent coherent structures on daytime ozone air quality in Seoul, South Korea are investigated through a case study using the Weather Research and Forecasting-Community Multiscale Air Quality (WRF-CMAQ) model at a horizontal grid spacing of 50 m. In Seoul, sea breeze and convective structures develop in the daytime. Due to the different wind directions above and below the planetary boundary layer (PBL) top, eddies are formed at the PBL top when updrafts related to convective structures reach above the PBL top. Air at lower level, which has higher concentrations of O3 precursors and lower O3 concentration than air at upper level, is transported upward by convective structures and eddies at the PBL top. Some of the transported air reaches above the PBL top, resulting in the vigorous chemical production of O3 above the PBL top in the afternoon. An integrated process rate analysis is performed to examine the impacts of turbulent coherent structures on O3 concentration in detail. The chemical production of O3 generally appears in updraft areas except near the surface because O3 precursors at lower level are transported by updrafts. Below the height of ~1 km, the horizontal advection of air from other areas to updraft areas increases O3 concentration in updraft areas because O3 concentration in updraft areas is generally lower than that in other areas. Slightly above the PBL top, air with low O3 concentration diverges from updraft areas and air with high O3 concentration at upper level is transported downward by downdrafts around updraft areas.

Avalanches triggered by Kelvin-Helmholtz instability in a cylindrical plasma device.

A profile-evolving simulation of the Controlled Shear Decorrelation Experiment (CSDX) linear device is performed with our newly developed code. The simulation result shows an excellent agreement with the experimental observations of profiles and fluctuations of plasma density and electric potential in the B=1000 G standard discharges, suggesting the mechanism of their evolutions. According to our simulation, an avalanche of plasma density, featuring a rapid destruction of particle profile, is triggered every time the dominant instability transits from near adiabatic collisional drift wave to non-adiabatic Kelvin-Helmholtz instability. The avalanches always start at the point where the local vorticity is the maximum among the whole device. A critical vorticity is found for any avalanche to happen. The avalanches always lead to intermittent particle and heat convective structures outside the main plasma column, and these structures are ejected out as avaloids when zonal flow intensity at birth time is weak.

Pair dispersion in inhomogeneous turbulent thermal convection

Due to large scale flow inhomogeneities and the effects of temperature, turbulence small-scale structure in thermal convection is still an active field of investigation, especially considering sophis- ticated Lagrangian statistics. Here we experimentally study Lagrangian pair dispersion (one of the canonical problems of Lagrangian turbulence) in a Rayleigh-B enard convection cell. A sufficiently high temperature difference is imposed on a horizontal layer of fluid to observe a turbulent flow. We perform Lagrangian tracking of sub-millimetric particles on a large measurement volume in- cluding part of the Large Scale Circulation (LSC) revealing some large inhomogeneities. Our study brings to light several new insights regarding our understanding of turbulent thermal convection: (i) by decomposing particle Lagrangian dynamics into the LSC contribution and the turbulent fluc- tuations, we highlight the relative impact of both contributions on pair dispersion; (ii) using the same decomposition, we estimate the Eulerian second-order velocity structure functions from pair statistics and show that after removing the LSC contribution, the remaining statistics recover usual homogeneous and isotropic behaviours which are governed by a local energy dissipation rate to be distinguished from the global dissipation rate classically used to characterise turbulence in thermal convection; and (iii) we revisit the super-diffusive Richardson-Obukhov regime of particle dispersion and propose a refined estimate of the Richardson constant.