Dense Plume Research Articles

Structure of dense plumes from a finite-height cylinder in laminar crossflow

Structure of dense plumes from a finite-height cylinder in laminar crossflow

Megafire smoke exposure jeopardizes tree carbohydrate reserves and yield.

The global incidence of megafires is on the rise, leading to extensive areas being shrouded in dense smoke for prolonged periods, spanning days or weeks1. Here, by integrating long-term regional observations of non-structural carbohydrate content in trees across California's Central Valley with spatiotemporal satellite data, we present compelling evidence that dense smoke plumes negatively impact carbohydrate stores in three tree species: Prunus dulcis, Pistacia vera and Juglans regia. Our findings show that the presence of smoke causes a significant decrease in total non-structural carbohydrates, with reductions in the accumulation of both soluble sugar and starch reserves. This decline in carbohydrate levels persists through the trees' dormancy period into the next season's bloom, culminating in a reduced yield. Our results highlight a previously unrecognized wildfire threat that could affect plant health and ecosystem stability in both agricultural and natural environments.

Effects of a surface layer cross-flow and slope steepness on the rate of descent of dense water flows along a slope

The pathways and mixing in dense water overflows have been addressed in a range of numerical studies based on the DOME (Dynamics of Overflow Mixing and Entrainment) setup. These studies are motivated by overflows such as the flow of dense and cold water over the ridge between Iceland and Scotland. The main route of this water is through the Faroe-Shetland channel and further through the Faroe Bank Channel. After leaving the Faroe Bank Channel, the dense water continues along the slope along the Iceland-Faroe Ridge. Across this ridge there is also a strong flow of Atlantic water that may affect the pathway and mixing in the dense plume. In the DOME investigations, the slope steepness is assumed to be constant and there is no active background flow in the ambient water masses above the dense plume. With the situation along the Iceland-Faroe Ridge in mind, the setup from the DOME experiment is adjusted to study the effects of i) the slope profile, ii) flows crossing the slope on top of the along-slope flow of dense water, and iii) the topography near the overflow channel of dense water. It is found that the rate of descent is depending on slope steepness, and for a curved slope going from flat on top of a ridge to much steeper away from the ridge, dense water parcels near the ridge top or crest will sink slower than the faster flowing dense water further down along the slope. A cross-flow over the ridge, mimicking the flow of Atlantic water over the Iceland-Faroe Ridge, will force stronger mixing between the dense water and the ambient water above and dense water mixed up into the surface water can be allowed to flow out across the ridge. Furthermore, the dynamical situation along the slope will change and fluid parcels in the deeper part of the plume can experience a faster rate of descent. By removing a barrier in the topography downstream of the overflow channel, dense water can be steered towards the ridge and the starting point of the center of mass of the descending overflow water will be higher up on the slope.

Dense Contaminants Mixing Into the Saltwater Wedge in Coastal Aquifers: Laboratory and Numerical Investigations

AbstractThe saltwater‐freshwater mixing zones in coastal aquifers can host complex physical exchange and biogeochemical transformations. The land‐sourced dense contaminant plumes could be transferred into the mixing zone of the saltwater wedge due to the density effect prior to discharge to the sea. However, the mixing process between dense contaminants and the saltwater wedge has not received much attention, largely due to the lack of physical evidence. This study used laboratory experiments and numerical simulations to investigate the transport and discharge behaviors of variable‐density contaminant plumes in tidally influenced unconfined coastal aquifers. Results demonstrate that the highly dense contaminants mix with the underlying saltwater and finally merge with the saltwater wedge. This process significantly extends the contaminant discharge durations, thereby reducing the peak value of contaminant efflux. The dense contaminants are elongated along the landward margin of the saltwater wedge, leading to a larger spreading area (Ms) than that of constant‐density contaminants. The sensitivity analysis indicates that the high density of contaminants acts as a trigger to induce the mixing of them and wedges. The higher hydraulic conductivity, lower dispersivities and reduced inland freshwater flux significantly increase the residence times (Rt) and discharge duration (Dt) by enhancing the mixing of dense contaminants with seawater. In contrast, both Rt and Dt values are not only non‐monotonic functions of tidal amplitudes but also less sensitive to tidal effects. Compared with the non‐tidal condition, however, the addition of tides significantly increases both Rt and Ms values of dense contaminant plumes. The results presented herein provide valuable insights into the mechanisms of dense contaminants mixing into saltwater wedges, which could guide practitioners in designing effective strategies to protect coastal environments from land‐sourced contaminants.

Wildfire plume ageing in the Photochemical Large Aerosol Chamber (PHOTO-LAC).

Plumes from wildfires are transported over large distances from remote to populated areas and threaten sensitive ecosystems. Dense wildfire plumes are processed by atmospheric oxidants and complex multiphase chemistry, differing from processes at typical ambient concentrations. For studying dense biomass burning plume chemistry in the laboratory, we establish a Photochemical Large Aerosol Chamber (PHOTO-LAC) being the world's largest aerosol chamber with a volume of 1800 m3 and provide its figures of merit. While the photolysis rate of NO2 (jNO2) is comparable to that of other chambers, the PHOTO-LAC and its associated low surface-to-volume ratio lead to exceptionally low losses of particles to the walls. Photochemical ageing of toluene under high-NOx conditions induces substantial formation of secondary organic aerosols (SOAs) and brown carbon (BrC). Several individual nitrophenolic compounds could be detected by high resolution mass spectrometry, demonstrating similar photochemistry to other environmental chambers. Biomass burning aerosols are generated from pine wood and debris under flaming and smouldering combustion conditions and subsequently aged under photochemical and dark ageing conditions, thus resembling day- and night-time atmospheric chemistry. In the unprecedented long ageing with alternating photochemical and dark ageing conditions, the temporal evolution of particulate matter and its chemical composition is shown by ultra-high resolution mass spectrometry. Due to the spacious cavity, the PHOTO-LAC may be used for applications requiring large amounts of particulate matter, such as comprehensive chemical aerosol characterisation or cell exposures under submersed conditions.

Ozone pollution episodes and PBL height variation in the NYC urban and coastal areas during LISTOS 2019

In summer, frequent ozone (O3) episodes occur in New York City (NYC) and coastal areas along Long Island Sound (LIS) due to complex interplay of urban pollutants transport, land-sea breezes, and chemistry. The Long Island Sound Tropospheric Ozone Study (LISTOS) was carried out to address this issue; here we present an integrated analysis of the diurnal variation of planetary-boundary-layer height (PBLH) and O3 in the NYC urban and coastal area during LISTOS in summer 2019. A persistent O3 episode (July 28–31, 2019) is investigated which shows much higher O3 (90–120 ppb with 5-min average maximum) along the North-shore of Long Island (NLI) than the NYC urban and South-shore of Long Island (SLI). We evaluate the PBLH estimate with a multi-platform observation in the complex coastal environment, and compare the PBLH diurnal variation in the NYC urban to coastal areas. The results indicate lower PBLH in the mid-morning and noon in the NLI than the NYC urban area. With the simultaneously measured O3 and PBLH, the integrated O3 content or total content in the PBL defined as the O3 concentrations multiplied by PBLH show good consistency in the urban and NLI coast at noon on July 28–29, 2019. A dense O3 plume, rich in CO, aerosols, black carbon, HCHO and NOx appeared in the PBL as detected from the UMD Cessna research aircraft, satellite TROPOMI and ozonesonde; their coincidently large values imply the combined effects of urban plumes transport, lower PBLH and land-sea breezes in the NLI area. Finally, we evaluated the forecast of O3 and PBLH from the NOAA NAM-CMAQ (12 km grid) and WRF-Chem (1.3 km grid) models with the observations. Both models show good agreement with the observations at the NYC urban site; but at the NLI coastal site, the NAM-CMAQ model forecast indicates a timing bias for the maximum O3 at noon. These results provide insight into the impacts of the sea breeze and urban heat island on air quality.

Bacterial bioconvection confers context-dependent growth benefits and is robust under varying metabolic and genetic conditions.

Microbes often respond to environmental cues by adopting collective behaviors-like biofilms or swarming-that benefit the population. During "bioconvection," microbes gather in dense groups and plume downward through fluid environments, driving flow and mixing on the scale of millions of cells. Though bioconvection was observed a century ago, the effects of differing physical and chemical inputs and its potential selective advantages for different species of microbes remain largely unexplored. In Bacillus subtilis, vertical oxygen gradients that originate from air-liquid interfaces create cell-density inversions that drive bioconvection. Here, we develop Escherichia coli as a complementary model for the study of bioconvection. In the context of a still fluid, we found that motile and chemotactic genotypes of both E. coli and B. subtilis bioconvect and show increased growth compared to immotile or non-chemotactic genotypes, whereas in a well-mixed fluid, there is no growth advantage to motility or chemotaxis. We found that fluid depth, cell concentration, and carbon availability affect the emergence and timing of bioconvective patterns. Also, whereas B. subtilis requires oxygen gradients to bioconvect, E. coli deficient in aerotaxis (Δaer) or energy-taxis (Δtsr) still bioconvects, as do cultures that lack an air-liquid interface. Thus, in two distantly related microbes, bioconvection may confer context-dependent growth benefits, and E. coli bioconvection is robustly elicited by multiple types of chemotaxis. These results greatly expand the set of physical and metabolic conditions in which this striking collective behavior can be expected and demonstrate its potential to be a generic force for behavioral selection across ecological contexts. IMPORTANCE Individual microorganisms frequently move in response to gradients in their fluid environment, with corresponding metabolic benefits. At a population level, such movements can create density variations in a fluid that couple to gravity and drive large-scale convection and mixing called bioconvection. In this work, we provide evidence that this collective behavior confers a selective benefit on two distantly related species of bacteria. We develop new methods for quantifying this behavior and show that bioconvection in Escherichia coli is surprisingly robust to changes in cell concentration, fluid depth, interface conditions, metabolic sensing, and carbon availability. These results greatly expand the set of conditions known to elicit this collective behavior and indicate its potential to be a selective pressure across ecological contexts.

A benchmark for variably saturated variable-density variable-viscosity flow and solute transport in porous media

In natural environments, fluid density and viscosity can be affected by spatial and temporal variations of solute concentration, for example, due to saltwater intrusion in coastal aquifers, leachate infiltration from waste disposal sites, and upconing of saline water from deep aquifers. Potentially unstable situations may arise in which a dense fluid overlies a less dense fluid. This situation can produce instabilities manifested by dense plume fingers moving vertically downwards counterbalanced by vertical upward flow of the less dense fluid. The resulting free convection increases solute transport rates over large distances and times relative to constant-density flow. Unstable brine flow is further complicated if the porous medium is variably saturated. The results from a laboratory experiment of variably saturated variable-density flow and solute transport from Simmons et al. (2002) are used as the physical basis to define a new mathematical benchmark. This benchmark aims at realistically reproducing the experimental fingering patterns. Random hydraulic conductivity fields were used in the simulations as a numerical perturbation method to realistically mimic the observed dense plume fingering. The HydroGeoSphere code coupled with PEST are used to calibrate the parameter set that defines the benchmark. A grid convergence analysis is performed to obtain the adequate spatial and temporal discretizations. The new mathematical benchmark is useful for model comparison and testing of variably saturated variable-density flow in porous media. Simmons CT, Pierini ML, Hutson JL (2002) Laboratory investigation of variable-density flow and solute transport in unsaturated–saturated porous media. Transp Porous Media. 47(2): 215–244, 10.1023/A:1015568724369.

Ionization detection of neutral 2,5-dihydroxy benzoic acid molecules in the matrix-assisted laser desorption/ionization plume by ultraviolet laser post-ionization: Correlation between internal energy distribution and thermal decomposition rate

We probed neutral 2,5-dihydroxy benzoic acid (DHB) crystals in the matrix-assisted laser desorption/ionization (MALDI) plume using ultraviolet (UV) laser post-ionization at 266 nm to investigate the initial MALDI process involving fast phase transitions. Mainly three ion signals, DHB+, [DHB–H2O]+ produced by the dissociative ionization of neutral DHB, and [DHB–CO2]+ formed by pyrolysis in the dense plume, were detected using time-of-flight mass spectrometry. The [DHB–H2O]+/DHB+ ratio was utilized as the fraction of highly excited neutral DHB molecules produced by photoexcitation and survived after phase explosion, and the [DHB–CO2]+/DHB+ ratio indicated the extent of pyrolysis of neutral DHB molecules in the thermally heated condensed phase before phase explosion. The two indexes showed an inverse relationship with respect to the laser delay corresponding to the position in the plume, i.e., the [DHB–H2O]+/DHB+ ratio revealed high values at the tip of the plume, whereas the [DHB–CO2]+/DHB+ ratio increased in the rear part of the plume, reflecting the molecular dynamics of solid-to-gas phase transition involving the production of most MALDI analyte ions.

Broadband attenuation-based measurements of bubble size distributions

Oceanic bubbles generated by breaking waves are a key mechanism for air-sea gas exchange. Dense bubble plumes can also interfere with acoustic propagation in shallow water due to their high scattering and attenuation. Most studies of oceanic bubbles have used narrow-band acoustical and optical techniques to quantify bubble size distributions under breaking waves in open water. To study bubble distributions at estuarine tidal fronts, where localized downwelling and advection can generate large bubbles plumes, we developed a measurement system for estimating bubble size distributions via broadband excess attenuation. The system consists of two transducers (3–30 kHz and 30–110 kHz) and two hydrophones mounted on a towable frame. Testing was performed in a laboratory wave tank to measure bubble distributions below a field-scale (1 + meter) breaking wave. This presentation will discuss the bubble size distributions observed at different locations along the breaking wave crest in the direction of propagation and their associated time scales and sound speed anomalies. We also compare these distributions with those previously observed in open water and will present preliminary results from a field deployment of the system in the Connecticut River estuary.

A Numerical Investigation into the Spread Characteristics of a Human Virus-Carrying Droplet in a Classroom Environment.

In public health, the transmission characteristics and laws of highly infectious virus-carrying particles in the air environment have become a hot topic. The study on the spread characteristics of human virus-carrying droplets in a typical densely populated space is necessary. As such, a classroom space lattice Boltzmann method (LBM) model with a dense population is established to simulate and analyze the spreading and diffusing behavior of pathogenic droplets. The results show that the dispersion density is mainly affected by the mainstream wind direction in the area of concern, and particle aggregation is more likely to form in the area close to the wind disturbance. Due to the dense thermal plumes, the droplet movement is a clear convergence towards the upper space of the classroom. This could explain the fact that people living above confirmed cases are now more likely to be infected.

Enabling direct kinetic simulation of dense plasma plume expansion for laser ablation plasma thrusters

Laser ablation plasma thrusters are an emerging space propulsion concept that provides promise for lightweight payload delivery. Predicting the lifetime and performance of these thrusters hinges on a comprehensive characterization of the expansion dynamics of the ablated plasma plume. While state-of-the-art techniques for simulating plasmas are often particle-based, a grid-based direct kinetic solver confers advantages in such a transient and inhomogeneous problem by eliminating statistical noise. A direct kinetic solver including interparticle collisions is employed on a plume expansion model problem spanning one dimension each in configuration and velocity space. The high degree of thermodynamic nonequilibrium inherent in plume expansion is characterized, justifying the need for a kinetic rather than a hybrid or fluid solver. Thruster-relevant metrics such as the momentum flux are also computed. The plume dynamics are observed to be highly inhomogeneous in space with insufficient time for thermalization in the region preceding the expansion front, and the theoretical possibility of reducing the local grid resolution by up to two orders of magnitude at the far end of the domain is established. These grid-point requirements are verified via the employment of nonuniform grids of various expansion ratios, several of which also employ coarsening in velocity space. Longer domain lengths are explored to characterize thruster-scale phenomena and larger ambient pressures are simulated as a testbed to probe facility effects due to collisions with background particles.

Dynamics of sediment-laden plumes in the ocean

We present a series of experiments to illustrate the dynamics of positively or negatively buoyant particle-laden plumes in a cross-flow, with relevance for the discharge of sediment into the ocean during deep-sea mining operations. In an unstratified ambient fluid, our experiments identify three different regimes, corresponding to (i) a dense particle-laden plume, host to relatively dense saline fluid, in which the particles separate from the descending plume as the flow speed falls below the particle settling speed; (ii) a dense particle-laden plume, host to buoyant fluid, in which the fluid gradually rises from the sinking plume of particles, to form a second rising plume of source fluid; and (iii) a buoyant particle-laden plume, host to buoyant fluid, which rises from the discharge pipe, and from which particles gradually sediment. Classical models of single-phase plumes describe the initial motion of the plumes in cases (i) and (iii), but as the flow speed falls below the particle fall speed, sedimentation leads to a change in the averaged buoyancy, and, hence, the plume speed. Our data also suggest that the sedimentation leads to a reduction in the rate of entrainment of ambient fluid, compared with the classical single-phase plumes. We also show that with a density stratified ambient fluid, the stratification may arrest the plume prior to significant particle sedimentation, and in this case, the plume tends to spread downstream at the level of neutral buoyancy where particle sedimentation proceeds. The bulk density of the residual plume fluid may then remain intermediate between the density of the upper and lower layer fluid, or may become less dense than the upper layer fluid, in which case, following sedimentation, the plume fluid rises through the upper layer. While the dynamics of deep-sea mining plumes in the ocean are more complex, for example, owing to background turbulence and mixing, the results of our new laboratory experiments highlight the range of flow processes which may influence the initial dispersion and sedimentation of particles in such plumes following release into the water, depending on the initial conditions, the ambient density and the particle fall speed. We also discuss the relevance of our work in the context of ash dispersal by volcanic plumes.

Characterization of aerosol particles during a high pollution episode over Mexico City

More than 7 thousand wildfires were recorded over Mexico in 2019, affecting almost 640 thousand hectares. Most of these fires occurred during the spring season generating dense smoke plumes, impacting urban areas in the central part of the Mexican plateau. From May 10 to 17, 2019, biomass burning (BB) plumes affected Mexico City (MC) and diffused across the basin, producing PM2.5 levels ~ 2 times higher than the nation's air quality standards. Average PM2.5 concentrations increased sharply from 29.4 ± 7.2 µg m−3 to 65.1 ± 13.6 µg m−3 when the dense smoke plumes were detected. The higher particle concentration impacted the aerosol optical depth (AOD) as values ~ 3 times greater than the annual mean (0.32 ± 0.12) were measured, which resulted in a 17% loss of global horizontal irradiation (GHI). Under these severe pollution conditions, the visibility (Va) was reduced by ~ 80%. The high incidence of strong absorbent particles, such as soot and tarballs was revealed through electron microscopy and X-ray fluorescence (XRF) analysis. These techniques show chemical similarities between MC aerosols and those from the high-altitude (~ 4010 m. a. g. l.) Altzomoni Atmospheric Observatory, evidencing a strong influence of the BB emissions, suggesting a regional transport of these pollutants.

Nighttime and daytime dark oxidation chemistry in wildfire plumes: an observation and model analysis of FIREX-AQ aircraft data

Abstract. Wildfires are increasing in size across the western US, leading to increases in human smoke exposure and associated negative health impacts. The impact of biomass burning (BB) smoke, including wildfires, on regional air quality depends on emissions, transport, and chemistry, including oxidation of emitted BB volatile organic compounds (BBVOCs) by the hydroxyl radical (OH), nitrate radical (NO3), and ozone (O3). During the daytime, when light penetrates the plumes, BBVOCs are oxidized mainly by O3 and OH. In contrast, at night or in optically dense plumes, BBVOCs are oxidized mainly by O3 and NO3. This work focuses on the transition between daytime and nighttime oxidation, which has significant implications for the formation of secondary pollutants and loss of nitrogen oxides (NOx=NO+NO2) and has been understudied. We present wildfire plume observations made during FIREX-AQ (Fire Influence on Regional to Global Environments and Air Quality), a field campaign involving multiple aircraft, ground, satellite, and mobile platforms that took place in the United States in the summer of 2019 to study both wildfire and agricultural burning emissions and atmospheric chemistry. We use observations from two research aircraft, the NASA DC-8 and the NOAA Twin Otter, with a detailed chemical box model, including updated phenolic mechanisms, to analyze smoke sampled during midday, sunset, and nighttime. Aircraft observations suggest a range of NO3 production rates (0.1–1.5 ppbv h−1) in plumes transported during both midday and after dark. Modeled initial instantaneous reactivity toward BBVOCs for NO3, OH, and O3 is 80.1 %, 87.7 %, and 99.6 %, respectively. Initial NO3 reactivity is 10–104 times greater than typical values in forested or urban environments, and reactions with BBVOCs account for >97 % of NO3 loss in sunlit plumes (jNO2 up to 4×10-3s-1), while conventional photochemical NO3 loss through reaction with NO and photolysis are minor pathways. Alkenes and furans are mostly oxidized by OH and O3 (11 %–43 %, 54 %–88 % for alkenes; 18 %–55 %, 39 %–76 %, for furans, respectively), but phenolic oxidation is split between NO3, O3, and OH (26 %–52 %, 22 %–43 %, 16 %–33 %, respectively). Nitrate radical oxidation accounts for 26 %–52 % of phenolic chemical loss in sunset plumes and in an optically thick plume. Nitrocatechol yields varied between 33 % and 45 %, and NO3 chemistry in BB plumes emitted late in the day is responsible for 72 %–92 % (84 % in an optically thick midday plume) of nitrocatechol formation and controls nitrophenolic formation overall. As a result, overnight nitrophenolic formation pathways account for 56 %±2 % of NOx loss by sunrise the following day. In all but one overnight plume we modeled, there was remaining NOx (13 %–57 %) and BBVOCs (8 %–72 %) at sunrise.

Specificfeatures of evolution of dense atomized superheated water plumes and peculiarities of its diagnostics

Experimental study of evolution of plumes of atomized metastable superheated water during its discharge through convergent-divergent nozzles is conducted. Dispersion characteristics of dense plumes in micron and submicron droplet diameter ranges are obtained. Theoretical and prediction analyses of different coagulation mechanisms in the considered two-phase flow are performed. The negligible effect of Brown-type coagulation is shown. It is also demonstrated that turbulent coagulation can change the fraction of micron-diameter droplets within 9%. In addition, for the first time, an “inertial” mechanism of coagulation is considered for the studied plumes under the conditions of plume baking in a cocurrent flow or in the ambient air. It can lead to a considerable decrease in the submicron-droplet mass fraction, which is observed in experiments even at a small distance from the nozzle cut. The predicted data are compared with experimental ones obtained at theexperimental setup.

Analysis of air pollution in the atmosphere due to firecrackers in the Diwali period over an urban Indian region

Short-term investigations of atmospheric pollutants (PM10, PM2.5, SO2, NO2, O3, and CO) were performed during the Diwali festival over Varanasi for a period of six years from 2011 to 2016. Aerosol Optical Depth (AOD) observed for the corresponding days of Diwali was found to be considerably much higher and even its value reached 2.0 for some Diwali years, which is basically almost 3-folds than the control days. The total scattering aerosol optical thickness as well as aerosol extinction co-efficient at 550 nm crossed the value of 1.0 in almost all the Diwali day cases. The associated meteorological conditions (low wind speed, declining temperature, lowered night-time boundary layer height, etc.) during the Diwali period leads to the detrimental accumulation of atmospheric pollutants near to the surface layer in Varanasi region. Moreover, PM10 and PM2.5 concentrations were recorded much higher than the safer limits set by NAAQS for 24-hour mean values throughout the period of study. The concentrations of PM10 and PM2.5 crossed beyond the safer limits and crossed 500 µg/m3 (in 2015) and 450 µg/m3 (in 2016) respectively, which is basically 5–6 times higher than the standard NAAQS limit. In comparison with the trace gases concentrations (e.g. SO2, NO2, O3, and CO) on control day, it was observed higher on the respective Diwali day. Satellite data derived from MODIS (Aqua and Terra) have also been taken into account to observe and verify the unpropitious effects of fireworks for the chosen case. MODIS true-color images show dense smoke plumes and haze over the entire Indo-Gangetic Plain (IGP) on Diwali days of 2011–2016 with its continuation in the following days. Proper assessment and regular monitoring is needed in order to mitigate the localized air pollution due to this kind of festival by the local scale authority to the top-level environmentalists.

Dynamics of Eddies Generated by Sea Ice Leads

AbstractInteraction between the atmosphere and ocean in sea ice–covered regions is largely concentrated in leads, which are long, narrow openings between sea ice floes. Refreezing and brine rejection in these leads inject salt that plays a key role in maintaining the polar halocline. The injected salt forms dense plumes that subsequently become baroclinically unstable, producing submesoscale eddies that facilitate horizontal spreading of the salt anomalies. However, it remains unclear which properties of the stratification and leads most strongly influence the vertical and horizontal spreading of lead-input salt anomalies. In this study, the spread of lead-injected buoyancy anomalies by mixed layer and eddy processes are investigated using a suite of idealized numerical simulations. The simulations are complemented by dynamical theories that predict the plume convection depth, horizontal eddy transfer coefficient, and eddy kinetic energy as functions of the ambient stratification and lead properties. It is shown that vertical penetration of buoyancy anomalies is accurately predicted by a mixed layer temperature and salinity budget until the onset of baroclinic instability (~3 days). Subsequently, these buoyancy anomalies are spread horizontally by eddies. The horizontal eddy diffusivity is accurately predicted by a mixing-length scaling, with a velocity scale set by the potential energy released by the sinking salt plume and a length scale set by the deformation radius of the ambient stratification. These findings indicate that the intermittent opening of leads can efficiently populate the polar halocline with submesoscale coherent vortices with diameters of ~10 km, and they provide a step toward parameterizing their effect on the horizontal redistribution of salinity anomalies.

Dense Water Formation on the Icelandic Shelf and Its Contribution to the North Icelandic Jet

AbstractThe North Icelandic Jet (NIJ) is the densest component of the Denmark Strait Overflow Water, feeding the abyssal limb of the Atlantic Meridional Overturning Circulation. Here, by using observational and numerical model data, we explore the formation of overflow water on the Icelandic shelf, the mechanisms involved, and its potential contribution to the NIJ. The sparse observational data on the western Icelandic shelf for the month of February shows top‐to‐bottom mixing on the shelf, distinct and well separated from the dense water offshore, with densities larger than 27.8 kg/m3 in some years. Using a 1‐D mixing model and winter heat flux reanalysis, we suggest that waters with densities exceeding 27.8 kg/m3 are likely to be formed on the shelf in most years by the end of winter. High‐resolution numerical model data shows that the transformation of the Atlantic inflow along the northwest Icelandic shelf generates a dense plume whose waters feed into the NIJ. The bulk of the plume cascades down‐slope north of Iceland, funneled through deep cross‐shelf troughs, with some cascading occurring west of Iceland as well. During years of strong cascading events (2008, 2013, and 2016), the modeled dense plume potentially feeds up to 21% of the NIJ transport at the Siglunes and Kögur sections. Back‐tracked Lagrangian particle trajectories confirm that the western Icelandic shelf is a source of the NIJ. The dense plume transport and variability are found to be dependent on the total oceanic heat loss west of Iceland and along Denmark Strait.

Dense gas plume rise and touchdown for Jack Rabbit II trial 8 chlorine field experiment

The series of nine Jack Rabbit II (JR II) chlorine release field experiments in 2015 and 2016 involved releases of pressurized liquefied chlorine from tanks containing 5–20 tons of liquid. This paper concerns JR II Trial 8, where the release hole (15.2 cm diameter) was oriented upwards at the top of the tank. To our knowledge, this is the only full-scale vertically-directed release of pressurized liquefied chlorine that is available for detailed analysis. Our aim is to improve understanding of the characteristics of the dense jet trajectory (e.g., maximum plume rise, downwind distance where the dense plume first touches the ground, and maximum concentration at touchdown). The chlorine emission rate was measured by instruments in the tank, and the plume rise geometry was estimated from photographs, videos, and lidar scans taken from several angles. During the first 10 s of the release, the videos show that the dense jet rises up to about 40 m, then sinks towards the ground, touching down at a distance of about 60 m. The portion of the plume that sinks to the ground then spreads out about 30–40 m in all directions due to gravity slumping. Due to the decrease with time of the momentum jet mass release rate, the jet gradually becomes less dense and less powerful and the plume remains aloft after about 30–60 s. However, the dense cloud that touched down in the early phases moves downwind as a typical dense cloud. The observed dense jet plume rise and touchdown distance are shown to approximately agree within about a factor of two with integral model predictions by 1) the analytical dense gas plume formulas suggested for vertical jets in 1973 by Hoot, Meroney, and Peterka (HMP), 2) the analytical formula for buoyant (positive or negative) plume trajectory suggested by Briggs in 1969, and 3) the DRIFT dispersion software, which is an integral model. Also, HMP and DRIFT model predictions of concentration at touchdown agree with Trial 8 observations within a factor of about two. It is concluded that the dense jet plume models are able to simulate the Trial 8 plume trajectory and ground level maximum concentration within acceptable ranges (i.e., most of time, within a factor of two).