Formation Of Convection Cells Research Articles

Exploring the use of 3D radar measurements in predicting the evolution of single-core convective cells

Object-based radar rainfall nowcasting is a widely used technique for convective storm prediction. Currently, most existing object-based nowcasting methods primarily focus on predicting cell movements, neglecting the temporal evolution of cell properties such as size, shape, and intensity. Incorporating this evolution is critical for improving predictability in convective storms. While previous studies have used three-dimensional (3D) radar observations to capture vertical changes during convective cell formation, these efforts often analyse or reconstruct specific convective events. Integrating 3D radar information into operational object-based radar rainfall nowcasting remains an open challenge. This research addresses this challenge using deep learning (DL) techniques. More specifically, a DL-based prediction model is developed, which uses 2D and 3D cells' properties retrieved from 3D radar reflectivity data at the current time and across the past 15 min to predict the evolution of these properties over the next 15 min. This model could eventually be integrated into existing object-based nowcasting models. A total of 4708 cell lifecycles, extracted from high-resolution (5-min, 1-km, 24 levels at 0.5 km intervals) 3D radar data across the UK, are used to train the model, and a total of 1177 lifecycles are used for testing. The proposed model is shown to predict the evolution of single-core convective cells effectively, including changes in 2D projected geometry and mean 2D and 3D reflectivity. In particular, by incorporating information on the vertical evolution of convective cores, the prediction errors of mean reflectivity (in both 2D and 3D) can be reduced by approximately 50% at 15-min forecast lead time, as compared to a persistence forecast. Keywords: radar, tracking, convective cell, nowcasting, 3D, deep learning, lstm.

Boundary-bound reactions: Pattern formation with and without hydrodynamics.

We study chemical pattern formation in a fluid between two flat plates and the effect of such patterns on the formation of convective cells. This patterning is made possible by assuming the plates are chemically reactive or release reagents into the fluid, both of which we model as chemical fluxes. We consider this as a specific example of boundary-bound reactions. In the absence of coupling with fluid flow, we show that the two-reagent system with nonlinear reactions admits chemical instabilities equivalent to diffusion-driven Turing instabilities. In the other extreme, when chemical fluxes at the two bounding plates are constant, diffusion-driven instabilities do not occur but hydrodynamic phenomena analogous to Rayleigh-Bénard convection are possible. Assuming we can influence the chemical fluxes along the domain and select suitable reaction systems, this presents a mechanism for the control of chemical and hydrodynamic instabilities and pattern formation. We study a generic class of models and find necessary conditions for a bifurcation to pattern formation. Afterwards, we present two examples derived from the Schnakenberg-Selkov reaction. Unlike the classical Rayleigh-Bénard instability, which requires a sufficiently large unstable density gradient, a chemohydrodynamic instability based on Turing-style pattern formation can emerge from a state that is uniform in density. We also find parameter combinations that result in the formation of convective cells whether gravity acts upwards or downwards relative to the reactive plate. The wave number of the cells and the direction of the flow at regions of high/low concentration depend on the orientation, hence, different patterns can be elicited by simply inverting the device. More generally, our results suggest methods for controlling pattern formation and convection by tuning reaction parameters. As a consequence, we can drive and alter fluid flow in a chamber without mechanical pumps by influencing the chemical instabilities.

Effects of Convective Mergers on the Evolution of Microphysical and Electrical Activity in a Severe Squall Line Simulated by WRF Coupled With Explicit Electrification Scheme

AbstractTo investigate the process of convective merger (CM) and its effect on thunderstorms evolution and corresponding electrical activity, a severe squall line that took place on 27 July 2015 over the Beijing Metropolitan Region (BMR) is simulated and studied using the Weather Research and Forecasting (WRF) model coupled with an explicit electrification lightning scheme (E‐WRF). The model‐simulated radar reflectivity reasonably captured the whole squall line evolution, including the merging of individual cells and storms. The variation of normalized flash frequency simulated by E‐WRF is highly consistent with the occurrence of a prominent flash rate surge during the CM both in the observation and the simulation. Cloud bridge is one of the regions where lightning events increase most. The upstream anvil and the sinking outflow led to the rapid connection and formation of new convective cells between the two older main storms. Subsequently, the mass of graupel and snow increased substantially at the middle level, and the mass of upper‐level ice was horizontally advected from upstream. During the merger, the in‐cloud charge distribution evolved from a staggered charge pocket structure into a vertically stratified five‐layer structure. This study supports the hypothesized role of CM in enhancing lightning activity, and further expands our understanding of CM effects on microphysics and charge‐redistribution.

Reduction in RF sheath rectification with insulating antenna enclosure walls

Radiofrequency (RF) sheath rectification is one of the most prominent deleterious effects associated with ion cyclotron range of frequencies (ICRF) heating in fusion plasmas. RF sheaths and associated effects, such as impurity generation and convective cell generation, need to be mitigated to ensure that ICRF is a viable option for heating in future fusion devices. Experiments were performed on the Large Plasma Device (LAPD) at UCLA to explore the effects of using electrically-insulating antenna enclosures on RF rectified sheaths. Three different enclosure side-wall materials were used, including copper, MACOR (electrically insulating), and MACOR over copper. In the case of the MACOR–copper side walls, the non-conductive MACOR material was exposed to the bulk plasma but a layer of copper was added below to allow for image currents to flow. All three of the experiments had similar plasma density, temperature, and background magnetic field. In the case of the copper enclosure, RF rectified potentials, many times the local electron temperature, and associated formation of convective cells were observed and reported Martin M. et al (2017 Phys. Rev. Lett. 119 205002). In the experiments with MACOR and MACOR–copper enclosures, RF rectification was significantly reduced. Additionally, these latter two experiments showed no evidence of convective cell formation. Although the results from the MACOR experiment are reminiscent of the results obtained in ASDEX-U with a three-strap antenna optimized to reduce image currents on the antenna limiters Bobkov V. et al (2016 Nucl. Fusion 56 084001), the MACOR–copper experiment seems to suggest that insulating plasma facing materials have at least an equally strong impact on reducing potential rectification.

Compressibility effects on the linear-stability of centrifugal buoyancy-induced flow

The focus of this study is to understand the evolution of instability in centrifugal buoyancy-induced flow in a rotating system. The problem is of interest in atmospheric flows as well as in engineering applications. In this study, we perform direct numerical simulations (DNS) by solving the compressible Navier–Stokes equations and multi-dimensional stability analyses by using a forced DNS approach. We systematically and independently vary the Rayleigh and Mach numbers. The heat transfer by thermal conduction is used as a base state and maintained as a reference, upon which the growth of small perturbations is investigated. It is found that the critical wavenumber obtained from the linear stability analysis at the onset of convection has a much shorter wavelength than the one that eventually appears in the non-linear regime. Further, the investigations show that compressibility effects lead to a reduction of the growth rate of the dominant mode, and it modifies the overall formation of convection cells in the flow.

Transition from Dendritic to Cell-like Crystalline Structures in Drying Droplets of Fetal Bovine Serum under the Influence of Temperature.

The desiccation of biofluid droplets leads to the formation of complex deposits which are morphologically affected by the environmental conditions, such as temperature. In this work, we examine the effect of substrate temperatures between 20 and 40 °C on the desiccation deposits of fetal bovine serum (FBS) droplets. The final dried deposits consist of different zones: a peripheral protein ring, a zone of protein structures, a protein gel, and a central crystalline zone. We focus on the crystalline zone showing that its morphological and topographical characteristics vary with substrate temperature. The area of the crystalline zone is found to shrink with increasing substrate temperature. Additionally, the morphology of the crystalline structures changes from dendritic at 20 °C to cell-like for substrate temperatures between 25 and 40 °C. Calculation of the thermal and solutal Bénard–Marangoni numbers shows that while thermal effects are negligible when drying takes place at 20 °C, for higher substrate temperatures (25–40 °C), both thermal and solutal convective effects manifest within the drying drops. Thermal effects dominate earlier in the evaporation process leading, we believe, to the development of instabilities and, in turn, to the formation of convective cells in the drying drops. Solutal effects, on the other hand, are dominant toward the end of drying, maintaining circulation within the cells and leading to crystallization of salts in the formed cells. The cell-like structures are considered to form because of the interplay between thermal and solutal convection during drying. Dendritic growth is associated with a thicker fluid layer in the crystalline zone compared to cell-like growth with thinner layers. For cell-like structures, we show that the number of cells increases and the area occupied by each cell decreases with temperature. The average distance between cells decreases linearly with substrate temperature.

Isolating Large‐Scale Smoke Impacts on Cloud and Precipitation Processes Over the Amazon With Convection Permitting Resolution

AbstractAbsorbing aerosol from biomass burning impacts the hydrological cycle and radiation fluxes both directly and indirectly via modifications to convective processes and cloud development. Using the ICOsahedral Non‐hydrostatic modelling framework in a regional configuration with 1,500 m convection‐permitting resolution, we isolate the response of the Amazonian atmosphere to biomass burning smoke via enhanced cloud droplet number concentrations Nd (aerosol‐cloud interactions; ACI) and changes to radiative fluxes (aerosol‐radiation interactions; ARI) over a period of 8 days. We decompose ARI into contributions from reduced shortwave radiation and localized heating of the smoke. We show ARI influences the formation and development of convective cells: surface cooling below the smoke drives suppression of convection that increases with smoke optical depth, while the elevated heating promotes initial suppression and subsequent intensification of convection overnight; a corresponding diurnal response (repeating temporal response day‐after‐day) from high precipitation rates is shown. Enhanced Nd (ACI) perturbs the bulk cloud properties and suppresses low‐to‐moderate precipitation rates. Both ACI and ARI result in enhanced high‐altitude ice clouds that have a strong positive longwave radiative effect. Changes to low‐cloud coverage (ARI) and albedo (ACI) drive an overall negative shortwave radiative effect, that slowly increases in magnitude due to a moistening of the boundary layer. The overall net radiative effect is dominated by the enhanced high‐altitude clouds, and is sensitive to the plume longevity. The considerable diurnal responses that we simulate cannot be observed by polar orbiting satellites widely used in previous work, highlighting the potential of geostationary satellites to observe large‐scale impacts of aerosols on clouds.

Hydrogeological research in the Tuyajto Lake at the Flamingo National Reserve (Atacama, Chile)

The Tuyajto Lake is a saline lake located in the Andean Altiplano of northern Chile, at the foot of the volcano of the same name. It is fed by springs located on its eastern and northern boundaries. These springs discharge groundwater from a volcanic aquifer. Arid conditions dominate in the area, with an average precipitation of less than 200 mm/year. The tritium content in some groundwater samples shows the contribution of modern recharge to the total groundwater flow. Recharge occurs by infiltration of snowmelt in the austral winter months and to a lesser extent by short but intense precipitation events during the summer. According to the vertical gradients of precipitation and its rainfall isotopic content (δ18O, δ2H), the recharge zone of the springs is located at the northern area of the lake, above 4,900 m a.s.l., along the slopes of the Tuyajto volcano, whereas the recharge to the springs discharging on the eastern area of the lake originates in the adjacent basins of Pampa Colorada and Pampa Las Tecas, at altitudes of between 4,400 and 4,700 m a.s.l. The water of these springs may contain measurable tritium. The chemical composition of groundwater is the result of meteoric water evaporation processes, high temperature water-rock interaction and the dissolution of buried old salt flat deposits. The groundwater flow is shallow, due to the presence of a regional low permeability ignimbrite formation, which precludes the formation of deep convective groundwater flow cells due to the high density. At the local scale, the Laguna Tuyajto behaves as a flow-through system compared to the regional groundwater flow. The persistence of the springs is essential for the existence of the brine sheet and maintaining the ecological conditions for the waterfowl.

Self-organization of TiO2 microparticles on the surface of a thin liquid layer due to local heating and the formation of convective cells

The aggregation of TiO2 microparticles followed by self-organization of a cluster of agglomerates at convective cells formation in a thin layer of water is studied experimentally. Complex behavior of the cluster is considered at a strong system deviation from equilibrium as a result of local heating. A periodic character of the behavior of a cluster of particles has been detected. Particle sizes and the number of cells change cyclically over time. Hexagonal convective cells have a very small size of about 50–100 μm (5–10 times less than the layer height). Data on the cluster occurrence and dissociation over time is provided. It is shown that when the particle diameter and the diameter of coagulated particles reach the maximum (the maximum diameter of the cluster corresponds to 100–130 μm, and the maximum diameter of the aggregate is 20–30 μm), the convective cells lose stability and the cluster partially disintegrates. Self-organization control using local heating can be effectively applied to create new materials, as well as in biology and medicine.

Simulations of compressibility effects in centrifugal buoyancy-induced flow in a closed rotating cavity

In this study, we investigate centrifugal buoyancy-induced flow by conducting numerical experiments for Rayleigh numbers in the range of 105 to 108. The primary focus of the study is to understand the fluid flow and heat transfer induced by centrifugal buoyancy in a rapidly rotating system with direct relevance to the internal air system of a gas-turbine engine. We solve the compressible Navier–Stokes equations in an inertial frame of reference. Flow visualization shows the formation of counter-rotating convection cells in the domain analogous to Rayleigh–Bénard convection. The prediction of wall heat transfer shows good agreement with experimental data for low Rayleigh number but deviates for the high Rayleigh number cases. Further, the effect of compressibility or Mach number is studied by running additional simulations with varying flow Mach numbers at fixed Rayleigh and Rossby numbers. A strong dependence of flow structure and induced wall heat transfer rate on the flow Mach number is observed. The increase in flow Mach number causes density stratification, which suppresses the formation of convection rollers and reduces the wall heat transfer rate. Furthermore, a guideline is provided on how to reduce the computational cost for the high Rayleigh number cases.

First predictive simulations for deuterium shattered pellet injection in ASDEX Upgrade

First simulations of deuterium shattered pellet injection into an ASDEX Upgrade H-Mode plasma with the JOREK MHD code are presented. Resistivity is increased by one order of magnitude in most simulations to reduce computational costs and allow for extensive parameter scans. The effect of various physical parameters on MHD activity and thermal quench (TQ) dynamics is studied and MHD influence on ablation is shown. TQs are obtained quickly after injection in most simulations with a typical duration of 100 microseconds, which slows down at lower resistivity. Although the n = 1 magnetic perturbation dominates in the simulations, toroidal harmonics up to n = 10 contribute to stochastization and stochastic transport in the plasma core. The post-TQ density profile remains hollow for a few hundred microseconds. However, when flux surfaces re-form around the magnetic axis, the density becomes monotonic, again, suggesting beneficial behavior for runaway electron avoidance/mitigation. With 1021 atoms injected, TQ is typically incomplete and triggered when the shards reach the q = 2 rational surface. At a larger number of injected atoms, TQ can set in even before the shards reach this surface. For low field side injection considered here, repeated formation of outward convection cells is observed in the ablation region reducing material assimilation. This is due to sudden rise of pressure in the high density cloud when the stochastic region expands further releasing heat from the hot core. After TQ, strong sheared poloidal rotation is created by Maxwell stress, which contributes to re-formation of flux surfaces.

Hydrogeological research in the Tuyajto Lake at the Flamingo National Reserve (Atacama, Chile)

The Tuyajto Lake is a saline lake located in the Andean Altiplano of northern Chile, at the foot of the volcano of the same name. It is fed by springs located on its eastern and northern boundaries. These springs discharge groundwater from a volcanic aquifer. Arid conditions dominate in the area, with average precipitation less than 200 mm/year. The tritium content in some groundwater samples shows the contribution of modern recharge to the total groundwater flow. Recharge occurs by infiltration of snowmelt in the austral winter months and to a lesser extent by short but intense precipitation events during the summer. The vertical gradients of rainfall isotopic content (δ18O, δ2H) and precipitation points to the recharge zone of the springs being located at the northern area of the lake, above 4,900 m a.s.l. along the slopes of the Tuyajto volcano, whereas recharge to the springs discharging on the eastern area of the lake originates in the adjacent basins of Pampa Colorada and Pampa Las Tecas at altitudes between 4,400 and 4,700 m a.s.l. The water of these springs may contain measurable tritium. The chemical composition of groundwater is the result of meteoric water evaporation processes, high temperature water-rock interaction and dissolution of buried old salt flat deposits. The groundwater flow is shallow, due to the presence of a regional low permeability ignimbrite formation that precludes the formation of deep convective groundwater flow cells due to the high density of brines. At a local scale, the Tuyajto lake behaves as a flow-through system with respect to the regional groundwater flow. The persistence of the springs is essential for the existence of the brine sheet and maintaining the ecological conditions for the waterfowl.

Mesoscale characteristics of a rare severe hailstorm event

This study analyzed the environmental field, physical quantify fields, convection parameters and radar echo characteristics of a severe hailstorm that occurred on June 18, 2013, due to a mesoscale convective system. The analysis was performed using ground meteorological data, sounding data and GFS-ANL (gfs 0.5° × 0.5°) reanalysis data in combination with satellite cloud imagery and Doppler radar observations. The results showed that this rare hailstorm event was generated under the influence of a weakening and eastwardly moving low vortex over Lake Balkhash; and a distinct shear line in the middle and lower troposphere over the western South Xinjiang. Strong vertical wind shears, deep conditionally unstable stratification, ample water vapor in the lower troposphere and suitable wet-bulb zero heights are important contributors to the formation of severe hailstorms. An increase in convective available potential energy (CAPE) and a decrease in the lifting index are strong indicators of a severe hailstorm. For the studied hailstorm event, the time corresponding to the maximum value of the normalized CAPE was close to the timing of the actual hailstorm. The severe hailstorm, consisting of multiple supercells, exhibited notable characteristics of hail clouds and supercells. The formation, disappearance and replacement of strong convection cells occurred while the supercells were sustained. The tracking and forecasting results obtained using the Thunderstorm Identification, Tracking, Analysis and Nowcasting system are found to be, to a certain extent, able to indicate of the direction of the supercell storms during this severe hailstorm event.

Behavior of confined granular beds under cyclic thermal loading

We investigate the mechanical behavior of a confined granular packing of irregular polyhedral particles under repeated heating and cooling cycles by means of numerical simulations with the non-smooth contact dynamics method. Assuming a homogeneous temperature distribution as well as constant temperature rate, we study the effect of the container shape, and coefficients of thermal expansions on the pressure buildup at the confining walls and the density evolution. We observe that small changes in the opening angle of the confinement can lead to a drastic peak pressure reduction. Furthermore, the displacement fields over several thermal cycles are obtained and we discover the formation of toroidal convection cells inside the granular material. The root mean square of the vorticity is then calculated from the displacement fields and a quadratic dependency on the ratio of thermal expansion coefficients is established.

Combined effects of inclination angle and fin number on thermal performance of a PCM-based heat sink

Combined effects of fin number and inclination angle on the thermal performance of a PCM-based heat sink with longitudinal plate fins are investigated experimentally. Five different heat sink geometries with one, two, three, four and five fins are tested under different inclination angles varied from 0° to 90° at a constant thermal load of 16 W for the cases of with and without PCM inclusion. N-eicosane is used as the phase change material. Simultaneous temperature measurements and solid-liquid interface imaging are made to evaluate cooling performance of each heat sink configuration. The results reveal that the inclination angle and fin number plays a critical role on the formation of convective cells in the liquid PCM domain and consequently on the heat transfer and operating time. It is disclosed that the heat sink configuration with an inclination angle of 60° and three fins performs the optimal result in terms of operating time.

Effect of the Eyewall Cold Pool on the Inner Rainband of a Tropical Cyclone

AbstractThe low‐level cold pool just under a tropical cyclone eyewall and its effect on the formation of inner rainbands are studied, through idealized convection‐permitting simulations. Analysis of the potential temperature budget indicates that the formation of the eyewall cold pool (ECP) results from the evaporation of eyewall rainfall. The ECP is a barrier to boundary layer inflow, which results in clear convergence and vertical updraft at its outer edge. A budget analysis of the vertical motion tendency also indicates that the sum of the buoyancy and vertical pressure gradient terms makes a large contribution to the vertical updraft at the outer edge of the ECP. This then promotes the formation of convection cells and an inner rainband. Two sensitivity experiments further verify that the intensity of the ECP‐triggered inner rainbands is positively correlated with the magnitude of the radial gradient of potential temperature at the outer edge of the ECP.

Cyclic ablation of high-emissivity Sm-doped ZrB2/SiC coatings on alumina substrates

Samarium-doped ZrB2/SiC (ZBS) coatings possess properties of high emissivity and excellent ablation performance suitable for hypersonic applications. Of interest in the current study is how cyclic ablation affects the scale development on alumina substrates. ZBS coatings with 3, 5 and 8mol% of samarium (Sm) dopant were prepared via shrouded plasma spray onto alumina substrates and subjected to two 60-s ablation cycles with temperatures reaching up to 1700°C. Blisters were observed on the Sm-doped coatings after the 1st cycle as a result of a local eutectic reaction between the ablation products and alumina substrate. A Sm-stabilized t-ZrO2 phase was identified through X-ray diffraction after the ablation of the Sm-doped coatings. The ZBS with 5mol% of Sm dopant produced a flower-like microstructure after the 2nd cycle due to the formation of convection cells.

Experimental Observation of Convective Cell Formation due to a Fast Wave Antenna in the Large Plasma Device.

An experiment in a linear device, the Large Plasma Device, is used to study sheaths caused by an actively powered radio frequency (rf) antenna. The rf antenna used in the experiment consists of a single current strap recessed inside a copper box enclosure without a Faraday screen. A large increase in the plasma potential was observed along magnetic field lines that connect to the antenna limiter. The electric field from the spatial variation of the rectified plasma potential generated E[over →]×B[over →]_{0} flows, often referred to as convective cells. The presence of the flows generated by these potentials is confirmed by Mach probes. The observed convective cell flows are seen to cause the plasma in front of the antenna to flow away and cause a density modification near the antenna edge. These can cause hot spots and damage to the antenna and can result in a decrease in the ion cyclotron range of frequencies antenna coupling.

Analysis of three echo-trainings of a rainstorm in the South China warm region

A rainstorm which occurred between May 22 and 23, 2014 in Guangdong Province of the South China warm region was simulated by using the ARW-WRF model. Three “echo-trainings” over the rainstorm center were analyzed and the results of both the simulation and observational analysis showed that this rainstorm process was composed of three stages. In the first stage, gravity waves triggered the simultaneous but relatively independent formation of linear convection and convective cells, which moved toward the northeast through the rain center, thus creating the echo-training. In the second stage, with the formation of cold outflow, new convective cells were continuously created in the southwest and northwest of the rain area and then gradually moved to merge into the northeast rain area, thus forming a new echo-training. In the third stage, multiple rain bands above the rain area moved southeastward and passed through the strongest precipitation center, thus creating the third echo-training. The model simulation showed that a substantial warming appeared at 900 hPa before the convective initiation, leading to the formation of a stable layer below 900 hPa, which was the primary cause for the gravity waves that triggered the multiple convective cells. The multiple convective cells formed the convective line, following which new convection was formed from the cold outflow in its southwest and northwest directions. The new convection in the southwest maintained the rain band; however, the new convection in the northwest, combined with the rain band of the north, formed a large radar reflectivity area and consequently, a larger MCS.

Outgassing and eruption of basaltic magmas: The effect of conduit geometry

The flow dynamics of magmas is controlled by several parameters, including magma rheology, density, surface tension, gas and liquid flow rate, and the geometry of the flow field, which is mainly regulated by the conduit shape. For this reason, magmatic flows along fissures and dikes are significantly different from those along axisymmetric conduits. Basaltic eruptions are typically fed by fractures or dike systems that reach the surface, giving rise to fissure eruptions. Scaled experiments show that in these low-viscosity systems, gas transport and segregation (i.e., the outgassing dynamics) are deeply controlled by the fracture geometry. Rapid bubble clustering and formation of bubble plumes determine the formation of lateral magma convection cells involving up to 70 vol% of the melt. In analogy with outgassing in cylindrical conduits, the average vesicularity and size of bubbles increase with increasing gas flow rate and melt viscosity and density, which also control the lateral extent of the bubble plume and convection cells.