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Gravity currents contribute to the transport of heat and mass in atmospheric and aquatic environments. In aquatic systems subject to daily surface cooling, gravity currents propagate through turbulent convective surroundings. Yet, the effects of thermal convection on aquatic gravity currents remain to be quantified. This paper demonstrates how the interaction between penetrative convection and downslope gravity currents impacts the fluid dynamics and transport across littoral aquatic systems. We performed field experiments in a wind‐sheltered lake experiencing differential cooling to resolve the dynamics of thermally driven gravity currents in convective environments. Our in situ observations reveal that convective plumes penetrate gravity currents, generating large vertical fluctuations that foster the erosion of the stratified layer. This enhanced vertical mixing destroys the stratified downslope flow and limits the basin‐scale transport. Our results demonstrate that the interaction between penetrative convection and downslope gravity currents controls the littoral‐pelagic connectivity in aquatic ecosystems. Plain Language Summary: Horizontal differences in fluid density generate flows called gravity currents. These currents transport heat and mass across environmental systems. The dynamics of gravity current and the resulting transport are well understood when the surroundings are quiescent, which is rarely the case in nature. In aquatic systems, turbulent processes such as convection energize the ambient water. Cooling‐driven convection occurs when the surface of aquatic systems loses heat to the atmosphere, which generates sinking thermal plumes. In this paper, we used a lake experiencing differential cooling as an ideal field‐scale laboratory to investigate the effects of convection on the dynamics of gravity currents. Our in situ observations reveal that convective plumes distort the upper interface of gravity currents and limit the flow intensity. These results demonstrate that convection constrains the horizontal transport induced by gravity currents in natural systems. Key Points: Convective plumes penetrate downslope gravity currents and generate vertical interface fluctuationsVertical convective mixing erodes the stratified downslope flow and limits lateral transportThe time of maximal lateral transport is delayed to the relaxation phase once penetrative convection weakens [ABSTRACT FROM AUTHOR]

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Since the operation of Xiaolangdi Reservoir, great social and economic benefits have been brought into play in ensuring the flood control and silt reduction, the safety of living, production, irrigation and ecological water use. Based on the measured data, this paper analyzed the outflow conditions of high water level density flow and the influence factors of sediment discharge of Xiaolangdi Reservoir. The results show that during the high water level operation (above 235 m) of Xiaolangdi Reservoir, two conditions shall be met for the density flow to reach the front of the dam. a) The inflow flood process shall not be less than 3 days and the fine sediment content shall be higher than 40%. b) Inflow discharge Qi shall not be less than 4000 m³/s. And meet the sediment concentration shall not be less than 13.0 kg/m³. Alternatively, if inflow discharge Qi is between 1700 m³/s and 4000 m³/s, and meet the sediment concentration Si shall not be less than 86 minus 0.018Qi. Sediment discharge ratio of reservoir η is inversely proportional to the ratio of the water storage body and the outflow discharge and the ratio of the inflow and outflow discharge. The larger the maximum continuous 3-day inflow sediment transport rate, the stronger the flow power and the better the sediment discharge effect. The research results can provide a certain technical support for the high water level density flow sediment discharge operation of Xiaolangdi Reservoir. [ABSTRACT FROM AUTHOR]

The work presented in this paper is devoted to the study of the phenomena of electron emission that take place at the cathode in an electric arc discharge. This problem remains a subject of careful research conducted by several specialists. Nevertheless, it seems interesting to make a comparison between all the theories currently available. In an electric arc, electrons are emitted from the surface of the cathode. The cathode spot is the place from which the electrons emerge. The current density of the electrons emitted represents 90% of the total current density. The remaining 10% is provided by the positive ions present in front of the cathode. Thus, the determination of the total current density at the cathode in an arc discharge is reduced to the calculation of that of the emitted electrons. The choice of the method for calculating the current density of the electrons emitted at the cathode is crucial for the evaluation of the total current density at the cathode. In our model, the current density of emitted electrons is evaluated using the Murphy-Good formulation which is in the current state, the most general and the most accurate equation. [ABSTRACT FROM AUTHOR]

Copyright of Larhyss Journal is the property of Biskra University, Research Laboratory in Subterranean & Surface Hydraulics and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

We prove an upper bound for the volume of maximal analytic sets on which the generic Lelong number of a closed positive current is positive. As a particular case, we give a uniform upper bound on the volume of the singular locus of an analytic set in terms of its volume on a compact Kähler manifold. [ABSTRACT FROM AUTHOR]

On 30 June 2013, 19 Granite Mountain Hotshots firefighters were killed fighting a wildfire near Yarnell in the mountains of Central Arizona. They succumbed when the wildfire, driven by erratic winds, blocked their escape route and overran their location. A previous study is extended to simulate and analyze the downscale organization of convective circulations that redirected the wildfire, which started from the scale of the Rossby Wave Breaking over North America to a convective gust front that redirected the wildfire, trapping the firefighters. Five stages are found: Stage I, the initial deep prolonged gust front; Stage II, a front-to-rear jet and its ascending motions that organized high-based convection; Stage III, high-based dry microburst-induced downdrafts organized initially by ascending flow in Stage II that transported mass and entropy to the surface; Stage IV; multiple meso-γ-scale high centers and confluence zones formed that encompassed the firefighters' location, which established a favorable environment leading to Stage V, canyon-scale circulations formed surrounding the fire. The atmosphere thus transitioned from supporting a deep and long-lived convective density current to elevated dry microbursts with mass and wind outflow into a canyon, redirecting the ongoing wildfire. [ABSTRACT FROM AUTHOR]

Abstract Gravity currents contribute to the transport of heat and mass in atmospheric and aquatic environments. In aquatic systems subject to daily surface cooling, gravity currents propagate through turbulent convective surroundings. Yet, the effects of thermal convection on aquatic gravity currents remain to be quantified. This paper demonstrates how the interaction between penetrative convection and downslope gravity currents impacts the fluid dynamics and transport across littoral aquatic systems. We performed field experiments in a wind‐sheltered lake experiencing differential cooling to resolve the dynamics of thermally driven gravity currents in convective environments. Our in situ observations reveal that convective plumes penetrate gravity currents, generating large vertical fluctuations that foster the erosion of the stratified layer. This enhanced vertical mixing destroys the stratified downslope flow and limits the basin‐scale transport. Our results demonstrate that the interaction between penetrative convection and downslope gravity currents controls the littoral‐pelagic connectivity in aquatic ecosystems.

The paper presents the results of a long-term research program on the improvement of existing shallow circular secondary settling tanks (SSTs) based on the computational fluid dynamics (CFD) investigation of their inner hydrodynamic processes. The results provide insight into the flow and concentration pattern within such SSTs and highlight some important details that largely determine their performance. As a novel detail of the research of SSTs, a direct dependence was found between the inlet height and the length of the radial density jet induced by the entering water–sludge suspension. A hydrodynamically optimized (low positioned) inlet facility decreases the kinetic energy and the length of the density jet, as a result of which it vanishes before hitting outer boundary of the SST. This results in an enhanced performance indicators even of such shallow SSTs, under both dry weather and peak load conditions as well. [ABSTRACT FROM AUTHOR]

We introduce the notion of exotic periodic points of a meromorphic self-map. We then establish the expected asymptotic for the number of isolated or exotic periodic points for holomorphic self-maps with a simple action on the cohomology groups on a compact Kähler manifold. [ABSTRACT FROM AUTHOR]

Here we present observations of a dust storm that occurred on 22 February 2020 in the northwestern Sonoran Desert. In‐situ and remotely sensed measurements and output from numerical simulations suggest that evaporative cooling from cold frontal orographic precipitation spilling over an upwind mountain range generated a density current, with dust uplift occurring as the density current traveled over the emissive desert surface. Because the density current was laden with dust, time series of vertical profiles of aerosol backscatter and extinction from a ceilometer located 25 km downwind of the initial dust emission event show a well‐developed density current structure, including an overturning frontal head with a vertical extent of 1.2 km. Ceilometer measurements and soundings suggest a density current body depth of 400–500 m exhibiting a two‐layer structure that consisted of a positively sheared and dusty lower‐level, and a negatively sheared and pristine upper level. Kelvin‐Helmholtz instability at the top of the density current cold pool generated quasi‐regular oscillations in the height of the dust and pristine‐sky interfacial layer. Ridges and troughs in the height of this interfacial layer were coupled to maxima and minima in surface wind speed and near surface dust concentrations, respectively, with peak dust concentrations located directly under the interfacial layer ridges. These results corroborate several findings from model studies of dust emission and transport by density currents, and suggest that the internal circulation of a density current modifies the timing of dust emission and the patterns of dust concentration within the current body. Plain Language Summary: Here we discuss the case of a unique dust storm that occurred in the western end of the Sonoran Desert. Remote sensing and in‐situ measurements are used to demonstrate that the dust storm was generated by a density current. Lidar backscatter measurements during passage of this density current over a research site in the region are used to trace the path of dust laden air through the density current head and body, and reveal the internal circulation of this feature. These observational results corroborate findings from modeling studies of dust emission and transport by density currents, but also suggest a complex distribution of dust within the density current body. Key Points: Measurements of a dust storm in the western Sonoran Desert that was generated by a density current are presentedThe density current exhibits a 2‐layer internal circulation with a lower dusty layer and an upper pristine layerThese observations corroborate several findings from numerical simulations of dust emission and transport by density currents [ABSTRACT FROM AUTHOR]

Cold pool outflows (CPOs) are thought to be the most significant meteorological mechanism of mineral dust emission from the world's largest source in the central and western Sahara in boreal summer. An absence of CPOs from numerical models and reanalyses used to simulate Saharan dust emission leads to considerable error in modeling of dust fluxes from the Sahara. As such, the role of CPOs in the observed variability of dust through the monsoon season remains unclear. To remedy these issues, an improved observational benchmark is needed. In this research, an automated approach to identify and track CPOs in dust imagery from the Spinning Enhanced Visible and Infrared Imager (SEVIRI) is derived. The approach is found to flag 74.2% of events identified manually (26/35). 1,559 events are tracked for June, July and August of 2004–2017. CPOs follow a clear diurnal cycle, peaking at 1700–1900 Universal Time Coordinated. Propagation speeds decay exponentially through their lifetime, but on average speeds are 1.5 ms−1 higher at night. About 22.5% of the observed events exceed a total traveled distance of 300 km, with an overwhelming preference for northwestwards propagation. Common across the southern central and western Sahara, CPO activity shifts north through summer in line with observed dust emission. The exception to this is the development of an intense hotspot of CPO activity in southern Algeria in August, which does not parallel any known late season outbreaks of dust. The results underline the importance of the southernmost Saharan dust sources, activated by frequent CPO occurrence in early summer. Key Points: An objective satellite detection and tracking scheme is developed and used to characterize Saharan cold pool outflows across 14 summersCold pool outflows are widespread across the central and western Sahara, peaking in August close to the Mali‐Niger‐Algeria triple pointPatterns of cold pool outflow activity can help explain an observed decline in late summer dust emission [ABSTRACT FROM AUTHOR]

Copyright of Larhyss Journal is the property of Biskra University, Research Laboratory in Subterranean & Surface Hydraulics and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Deep-Sea Tailings Placement (DSTP) systems have been used at several mines to discharge diluted mine tailings at great depths. DSTP often provides considerable environmental and engineering benefits compared to traditional on-land tailings storage. The key element in the viability of a DSTP system is the ability to discharge mine tailings at sufficient depths and at such a location that a density current is formed that carries the tailings along the seabed to the abyssal ocean, and such that adequate dilution can be achieved and environmental impacts minimized. Depending on the composition of the slurry, sub-sea plumes might also form from the density current at different levels of neutral buoyancy, further diluting the effluent. The engineering, environmental and construction aspects of the deep-sea outfall system are illustrated through a case study of the DSTP outfall system on Woodlark Island in Papua New Guinea, located in the tropical south Pacific. [ABSTRACT FROM AUTHOR]

A strong mesoscale convective system affected northeastern Italy on August 8, 2008. Notable damage and two casualties resulted, mainly due to the strong wind gusts. The event is analysed using observations, including surface data from a meso‐network of meteorological stations, radar reflectivity and velocity data from a C‐band Doppler radar, polar satellite images, lightning measurements from a lightning detection network and the ambient thermodynamic conditions derived from local radiosoundings. The role of the cold front is investigated; in particular, the associated strong wind descending from the Alps, which interacted with the preexisting convection in the plain, is analysed to understand the way it affected the storm development. A simple density current model is applied to describe the flow characteristics and to identify the mechanisms that could support the development of such a high wind speed. Lastly, observed wind speeds are compared with the theoretical estimates from the proposed model. The results show that different factors contribute to the very strong wind gusts registered by surface stations. The main contributor to the windstorm is a density current (a) driven by the cold front, (b) maintained by the interaction between the cold air impinging on the Friuli Venezia Giulia plain from the north and the strong regional density gradient and (c) eventually enhanced by the storm cold pool. This study highlights the complex evolution of severe storms in a region on the lee side of the Alpine chain, and emphasizes the role of the orography in the enhancement of storm. [ABSTRACT FROM AUTHOR]

A density current is a (two-phase) flow of a fluid of density which is caused essentially by the influence of a density difference on the gravity. The previous experimental studies have been focused on investigation into properties of gravity current in straight channels. This paper presents a series of experiments in which saline gravity currents flowed through in physical models laboratory of Shahid Chamran University of Ahvaz. Experiments were carried out in 8.5 m length, 20cm width and 70cm height of flume and three radiuses of curvature (40, 80,120 cm). In this study the effect of three radiuses of curvature of channel bends on density flow behavior were studied. Experiments performed by four discharges (0.5,0.7,0.9 and 1.1 L/s) and four concentrations (8, 12, 16, 20 and 40 g/L). An Acoustic Doppler velocimeter (ADV-Vectrino+) was used to record the instantaneous downstream velocity. The Vectrino Velocimeter measures water speed using the Doppler effect. The results show that the elevation of the interface at the outer bank is in every case greater than the one at the inner bank. The results show that due to increasing the relative radius of channel bend decreases elevation of the interface and transverse interfacial slopes. The maximum and minimum superelevation occur in r/b =2 and r/b=6, respectively.

Density currents occur when fluid of onedensity propagates into a fluid of different density. These currents are the main caude forsediment transport in dam reservoirs and estuaries. Thenegative effects of density currents can be minimized using obstacles. Theobstacles can totally or partially block density currents and thussedimentation will occur far from the dams.In this paper, the effect of cylindrical obstacles and their arrangement ondensity current body is experimentally studied. Obstacles with constantdiameter and height were glued on the bed in staggered form. The staggered formwere constant during all the experiments, but transverse longitudinal distancesbetween obstacles were variable. It caused two beds with different arrangementof obstacles. The results indicated that rate of mixing in bed with obstaclesis greater than bed without obstacles. So velocity and height of headin bed with obstacles decrease about 40 present

Abstract: The gravity current dynamics in the presence of regular surface waves is investigated experimentally. The aim is to determine the influence of the wave‐induced velocity field on gravity current kinematic parameters and on the concentration distribution. In the presence of surface waves, the front shows a pulsating behavior while periodic oscillations are observed at the interface between lighter and denser fluids. The dynamics of such processes is related to the wave parameters. Present results confirm that the lock‐exchange generated front velocity is strongly influenced by the oscillatory motion and it may be increased or decreased depending on the Lagrangian mass transport induced by the surface waves. Compared to the propagation in the absence of waves, the dynamics of the concentration field is governed by an enhancement of mixing process due to the orbital motion. [ABSTRACT FROM AUTHOR]

The dynamics of density current over a bottom covered by macro-roughness elements were investigated by laboratory experiments and a computational model using large eddy simulations. The macro-roughness considered had significant size in comparison with the scale of density current. Five different roughness conditions were considered, namely flat bottom (for reference), half spheres, fine gravels, medium gravels, and large gravels. These bottom conditions had variations in roughness element size, shape, angularity, and spatial configuration. The density current was a lock-exchange type with a density difference of 1% between the two fluids initially separated by a gate in the middle. In the computational model, the roughness was captured using two different methods depending on the size of the roughness elements. For the large roughness elements, i.e., the half spheres and the medium and large gravels, an immersed boundary method was used to resolve the surface of each gravel, which was obtained through 3D laser scanning. The realistic and physically correct placement of these scanned objects in the simulation domain was achieved using a computer tool which can detect the collision of rigid bodies and simulate their dynamics. For the fine gravels, a rough wall function was used. The computational model was validated with the data measured in the experiments, including front position and velocity, and point velocity measurement within the current. The results show that density currents over macro-roughness have distinct behavior from those over a smooth boundary. The characteristics (size, angularity, and pavement pattern) of the macro-roughness play a key role in the current development. Macro-roughness significantly retards the front propagation and enhances entrainment. [ABSTRACT FROM AUTHOR]

Since the first impoundment of Three Gorges Dam in 2003, algal blooms occur frequently in the near-dam tributaries. It is widely recognized that the impoundment-induced change in hydrodynamic condition with the lower current velocity will make the eutrophication problem even more severe when an excessive amount of nutrients is already loaded into a reservoir and/or its tributaries. Operation tests carried out by Three Gorges Corporation in 2010 point to some feasible reservoir operation schemes that may have positive impacts on reducing the algal bloom level. In our study, an attempt is made to obtain, through a numerical hydrodynamic and water quality modeling and analysis, the reservoir operation rules that would reduce the level of algal blooms in the Xiangxi River (XXR), a near-dam tributary. Water movements and algal blooms in XXR are simulated and analyzed under different scenarios of one-day water discharge fluctuation or two-week water level variation. The model results demonstrate that the reservoir operations can further increase the water exchange between the mainstream of the Three Gorges Reservoir (TGR) and the XXR tributary and thus move a larger amount of algae into the deep water where it will die. Analysis of the model results indicate that the water discharge fluctuation constituted of a lower valley-load flow and a larger flow difference for the short-term operation (within a day), the rise in water level for the medium-term operation (e.g., over weeks), and the combination of the above two for the long-term operation (e.g., over months) can be the feasible reservoir operation rules in the non-flood season for TGR. [ABSTRACT FROM AUTHOR]

We present the results of laboratory experiments on the behaviour of sustained, dense granular flows in a horizontal flume, in which high-gas pore pressure was maintained throughout the flow duration by continuous injection of gas through the flume base. The flows were fed by a sustained (0.5-30 s) supply of fine (75 ± 15 μm) particles from a hopper; the falling particles impacted an impingement surface at concentrations of ~3 to 45 %, where they densified rapidly to generate horizontally moving, dense granular flows. When the gas supplied through the flume base was below the minimum fluidization velocity of the particles (i.e. aerated flow conditions), three flow phases were identified: (i) an initial dilute spray of particles travelling at 1-2 m s−1, followed by (ii) a dense granular flow travelling at 0.5-1 m s−1, then by (iii) sustained aggradation of the deposit by a prolonged succession of thin flow pulses. The maximum runout of the phase 2 flow was linearly dependent on the initial mass flux, and the frontal velocity had a square-root dependence on mass flux. The frontal propagation speed during phase 3 had a linear relationship with mass flux. The total mass of particles released had no significant control on either flow velocity or runout in any of the phases. High-frequency flow unsteadiness during phase 3 generated deposit architectures with progradational and retrogradational packages and multiple internal erosive contacts. When the gas supplied through the flume base was equal to the minimum fluidization velocity of the particles (i.e. fluidized flow conditions), the flows remained within phase 2 for their entire runout, no deposit formed and the particles ran off the end of the flume. Sustained granular flows differ significantly from instantaneous flows generated by lock-exchange mechanisms, in that the sustained flows generate (by prolonged progressive aggradation) deposits that are much thicker than the flowing layer of particles at any given moment. The experiments offer a first attempt to investigate the physics of the sustained pyroclastic flows that generate thick, voluminous ignimbrites. [ABSTRACT FROM AUTHOR]

The Yarnell Hill Fire, triggered by dry lightning on 28 June 2013, was initiated by hot and dry westerly winds, which rapidly shifted to north-northeast by convective-induced outflows. This sudden wind shift led to the demise of 19 firefighters. This study focuses on the environment and its predictive potential in terms of erratically changing the fire spread. Three numerical sensitivity tests are performed investigating the evolving synoptic-meso-β scale environmental wind flow: (1) deactivating the evaporative cooling, (2) deactivating surface-driven diurnal heating/cooling, and (3) removing the mountain. Results show the strong north-northeasterly wind induced by the density current(s) and the diurnal surface sensible heating played the most significant roles in enhancing the mesoscale environment conducive to the rapid change in the fire spread direction. While the mountain played a less significant role in weakening the magnitude of the airflow affecting the fire, it still had an impact. Additionally, the Hot-Dry-Windy (HDW) index is calculated to determine its predictor role with respect to the atmosphere affecting the fire. The focus is not on feedback from explicit fire heating on the larger environment but rather the role of the environmental physical processes in causing the convectively induced rapid wind shifts. [ABSTRACT FROM AUTHOR]

The first 10 days of September 2015 were marked by intense dust activity over the Middle East and the Arabian Peninsula. This study examines the atmospheric conditions at the origin of the large dust storms during this period. We particularly investigate the atmospheric dynamics leading to the development of a large dry cyclone over Iraq on 31 August 2015 which in turn generated an intense dust storm that affected most of the countries around the Arabian Gulf and lasted for 5 days. We found that the cyclone developed over Northwest Iraq as a transfer to low levels of a cut-off low which had formed two days earlier at upper levels over Turkey. Large dust loads exceeding 250 tons were emitted and moved southeast in a cyclonic shape toward the Arabian sea. The second large dust storm on 6-8 September 2015 occurred over Syria and affected all the coastal countries on the eastern side of the Mediterranean Sea. It was associated with the occurrence of a series of density currents over northeast Syria emanating from deep convection over the mountainous border between Syria and Turkey. The unusual development of deep convection over this area was associated with a blocking high and interaction with orography. Both the cut-off high and the cut-off low occurred during a period characterized by a meandering polar jet and an enhanced subtropical jet causing unstable weather over mid-latitudes which in turn led to highly polluted atmosphere by natural dust in the affected countries. [ABSTRACT FROM AUTHOR]

Tributary bays of the Three Gorges Reservoir (TGR) are suffering from environmental problems, e.g., eutrophication and algae bloom, which could be related to the limited water exchange capacity of the tributary bays. To understand and quantify the water exchange capacity of a tributary bay, this study investigated the water residence time (RT) in a typical tributary bay of TGR, i.e., the Zhuyi Bay (ZB), using numerical simulation and the adjoint method to obtain the RT. The results show that RT of ZB with an annual mean of 16.7 days increases from the bay mouth to the bay top where the maximum can reach 50 days. There is a significant seasonal variation in RT, with higher RT (average 20 days) in spring and autumn and lower RT (average < 5 days) in the summer. The sensitivity experiments show that the TGR water level regulation has a strong influence on RT. The increase in the water level could increase RT of ZB to some extent. Density currents induced by the temperature difference between the mainstream and tributaries play an important role in the water exchange of ZB, while the impacts of the river discharges and winds on RT are insignificant. [ABSTRACT FROM AUTHOR]