Mean Plume Research Articles

Statistics of Bubble Plumes Generated by Breaking Surface Waves

AbstractWe examine the dependence of the penetration depth and fractional surface area (e.g., whitecap coverage) of bubble plumes generated by breaking surface waves on various wind and wave parameters over a wide range of sea state conditions in the North Pacific Ocean, including storms with sustained winds up to 22 m s−1 and significant wave heights up to 10 m. Our observations include arrays of freely drifting SWIFT buoys together with shipboard systems, which enabled concurrent high‐resolution measurements of wind, waves, bubble plumes, and turbulence. We estimate bubble plume penetration depth from echograms extending to depths of more than 30 m in a surface‐following reference frame collected by downward‐looking echosounders integrated onboard the buoys. Our observations indicate that mean and maximum bubble plume penetration depths exceed 10 and 30 m beneath the surface during high winds, respectively, with plume residence times of many wave periods. They also establish strong correlations between bubble plume depths and wind speeds, spectral wave steepness, and whitecap coverage. Interestingly, we observe a robust linear correlation between plume depths, when scaled by the total significant wave height, and the inverse of wave age. However, scaled plume depths exhibit non‐monotonic variations with increasing wind speeds. Additionally, we explore the dependencies of the combined observations on various non‐dimensional predictors used for whitecap coverage estimation. This study provides the first field evidence of a direct relation between bubble plume penetration depth and whitecap coverage, suggesting that the volume of bubble plumes could be estimated by remote sensing.

Assessment of smoke plume height products derived from multisource satellite observations using lidar-derived height metrics for wildfires in the western US

Abstract. As wildfires intensify and fire seasons lengthen across the western US, the development of models that can predict smoke plume concentrations and track wildfire-induced air pollution exposures has become critical. Wildfire smoke plume height is a key indicator of the vertical placement of plume mass emitted from wildfire-related aerosol sources in climate and air quality models. With advancements in Earth observation (EO) satellites, spaceborne products for aerosol layer height or plume injection height have recently emerged with increased global-scale spatiotemporal resolution. However, to evaluate column radiative effects and refine satellite algorithms, vertical profiles of regionally representative aerosol properties from wildfires need to be measured directly. In this study, we conducted the first comprehensive evaluation of four passive satellite remote-sensing techniques specifically designed for retrieving plume height. We compared these satellite products with the airborne Wyoming Cloud Lidar (WCL) measurements during the 2018 Biomass Burning Flux Measurements of Trace Gases and Aerosols (BB-FLUX) field campaign in the western US. Two definitions, namely, “plume top” and “extinction-weighted mean plume height”, were used to derive the representative heights of wildfire smoke plumes, based on the WCL-derived vertical aerosol extinction coefficient profiles. Using these two definitions, we performed a comparative analysis of multisource satellite-derived plume height products for wildfire smoke. We provide a discussion related to which satellite product is most appropriate for determining plume height characteristics near a fire event or estimating downwind plume rise equivalent height, under multiple aerosol loadings. Our findings highlight the importance of understanding the sensitivity of different passive remote-sensing techniques on space-based wildfire smoke plume height observations, in order to resolve ambiguity surrounding the concept of “effective smoke plume height”. As additional aerosol-observing satellites are planned in the coming years, our results will inform future remote-sensing missions and EO satellite algorithm development. This bridges the gap between satellite observations and plume rise modeling to further investigate the vertical distribution of wildfire smoke aerosols.

Investigating the Spatial and Temporal Limitations for Remote Sensing of Wildfire Smoke Using Satellite and Airborne Imagers During FIREX‐AQ

AbstractStarting from point sources, wildfire smoke is important in the global aerosol system. The ability to characterize smoke near‐source is key to modeling smoke dispersion and predicting air quality. With hemispheric views and 10‐min refresh, imagers in Geostationary (GEO) orbit have advantages monitoring smoke over once‐per‐day sensors in low‐earth orbit (LEO). However, both can be inadequate in capturing the characteristics of smoke plumes close to their sources due to too‐coarse spatial resolution (both detector and product resolution), too‐sparse temporal resolution (from LEO sensors), and too‐conservative masking. In addition to satellite observations, the Fire Influence on Regional to Global Environments and Air Quality experiment offered sub‐orbital enhanced‐MODIS Airborne Simulator (eMAS) imagery at 50 m pixel resolution—including multiple eMAS flight tracks over individual fires in short time periods. It provided opportunity to explore smoke plume characterization at various spatial and temporal scales and quantify the limitations of space sensors for describing smoke magnitude near source as well as its temporal evolution. Here we applied modified aerosol algorithm to different imagers, relaxing its masking to estimate smoke's aerosol optical depth (AOD) as close as possible to its source. We found that GEO sensors with nominal 1 km spatial resolution can match the much finer resolution eMAS retrieved mean plume AOD, as long as the retrieval spatial resolution is finer than the width of the plumes. However, the plume's maximum AOD may be drastically underestimated by satellite products.

Quantifying particulate matter optical properties and flow rate in industrial stack plumes from the PRISMA hyperspectral imager

Abstract. Industrial activities such as metallurgy, coal and oil combustion, cement production, and petrochemistry release aerosol particles into the atmosphere. We propose analyzing the aerosol composition of plumes emitted by different industrial stacks using PRISMA (PRecursore IperSpettrale della Missione Applicativa) satellite hyperspectral observations. Three industrial sites have been observed: a coal-fired power plant in Matla, South Africa (imaged on 25 September 2021); a steel plant in Wuhan, China (24 March 2021); and gas flaring at an oil extraction site in Hassi Messaoud, Algeria (9 July 2021). Below-plume surface reflectances are constrained using a combination of PRISMA and Sentinel-2/MSI images. Radiative transfer simulations are performed for each scene including the surface, background atmosphere, and plume optical properties. The plume aerosol optical thickness (AOT), particle radius, volume of coarse-mode aerosol, and soot are then retrieved within the plumes following an optimal estimation framework. The mean plume retrieved AOT at 500 nm ranges between 0.27 and 1.27 and the median radius between 0.10 and 0.12 µm. We found a volume fraction of soot of 3.6 % and 10.4 % in the sinter plant and coal-fired plant plumes, respectively. The mass flow rate of particulate matter at a point source estimated by an integrated mass enhancement method varies from 840 ± 155 g s−1 for the flaring emission to 1348 ± 570 g s−1 at the coal-fired plant.

PLUME STRUCTURES IN NATURAL CONVECTION WITH TRANSPIRATION

We present plume structures at high Rayleigh numbers (Ra<sub>H</sub> ~ 10<sup>11</sup>) and Schmidt numbers (Sc ~ 675), in convection driven by density differences across a horizontal membrane, when vertical transpiration velocities in the range of 0.002 cm/s ≤ V<sub>o</sub> ≤ 0.065 cm/s are imposed. The density differences across the membrane are created by having brine with higher concentrations above the membrane and ammonium chloride with lower concentrations below it, with the bottom fluid being forced upward across the membrane by gravitational heads to create transpiration. Planar laser induced fluorescence (PLIF) visualizations in a horizontal plane grazing the top of the membrane, with refractive index matching, brings out the plume structures and their evolution with Ra<sub>H</sub> and V<sub>o</sub>. The plume structures show complex dendritic patterns of line plumes with plume-free patches; the plume-free patches decrease in area and number, with decrease in Ra<sub>H</sub> and V<sub>o</sub>. At low V<sub>o</sub>, these plume-free patches have aligned plumes around them, while at larger V<sub>o</sub>, such aligned plumes are not seen. We expect these patches to be due to the impingement of large-scale flow at low V<sub>o</sub>, while they could be due to the modification of boundary layer instability at large V<sub>o</sub>. These plumes become closer with increase in Ra<sub>H</sub>, while they become more separated with increase in V<sub>o</sub>. We quantify this variation by measuring the total plume length (L<sub>p</sub>) by skeletonizing the plume structures and calculate the mean plume spacing (λ) from L<sub>p</sub>; λ show nontrivial, non-power law dependence on Ra<sub>H</sub> and V<sub>o</sub>, different from those observed for the case of no throughflow and low throughflows.

Experimentation, modelling, and simulation of hydrochory in an Amazonian river

Abstract Seed transport by hydrochory is a key mechanism of long‐distance dispersal constrained by attributes of the seed and hydrodynamics of the river, influenced by seasonal precipitation and hydrological pulses. However, the extent to which a hydrodynamic model can predict seed dispersal influenced by a tributary is unknown. The study was conducted along a 10‐km stretch of the Falsino River in Amapá, Brazil. Hydrodynamic parameters from the 2021 rainy season were used to calibrate a three‐dimensional numerical model (SisBaHia) and simulate hydrochory of Macrolobium bifolium, a widely distributed species in the Amazon floodplains. This model was coupled with a Lagrangian dispersal model to estimate the average transport distance of the fruit plume. The simulated results were compared statistically with those of dispersal quantified in the field. The field experiment coincided with the maximum hydrological pulse, providing with a maximum potential distance of longitudinal dispersal fruit of c. 10 km in 2 hr. The orders of magnitude of the mean plume transport (observed and numerically simulated centre of mass) were compatible with each other over six longitudinal tracking sections (4.0% ≤ estimated × observed error ≤ 16.5%). Different channel stretches had distinct hydraulic characteristics that influenced spatial dispersal dynamics and are likely to be factors influencing the distribution of M. bifolium in these environments. The present research is a contribution to understanding fluvial hydrodynamics and hydrochory by M. bifolium, whose seed dispersal syndrome is an adaptive characteristic that might explain its abundance and richness in these Amazonian riparian zones. We used M. bifolium as a model species to understand the role of seasonal flood pulse and fluvial hydrodynamics related to hydrochory favouring.

Study of turbulent dispersion of a concentration plume emitting from a line source over a rib‐roughened surface

Turbulent dispersion of a concentration plume emitting from a horizontal line source over a rib-roughened surface has been studied using direct numerical simulations (DNS). Four test cases have been compared to investigate the effects of the source elevation and location on the plume dispersion in a turbulent boundary layer. The transport process of the concentration is investigated in both physical and spectral spaces which includes analyses of statistical moments of the concentration field, decay rate, plume width, pre-multiplied spectra of the velocity and concentration fields, and the probability density function (PDF) of concentration fluctuations. It is observed that as the mean plume development enters the long-range dispersion stage, the decay rate of the mean concentration field begins to feature a constant slope of −3/2, while the vertical spread of the mean plume exhibits a constant slope of 1/3 in all four line source release cases in a ribbed wall flow. It is also observed that the profiles of both the mean concentration field and the RMS of concentration fluctuations exhibit a self-similarity pattern downstream of the line source in all four test cases. By comparing the characteristic length scales of the spanwise velocity and scalar fields, two distinct stages of the instantaneous plume development are observed, dominated by the turbulent convective and diffusive mechanisms. It is discovered that the transition from the turbulent convective stage to the turbulent diffusive stage occurs more rapidly for line sources placed near the wall and inside the cavities.

A Novel Approach to Estimating Time-Averaged Volcanic SO2 Fluxes from Infrared Satellite Measurements

Long-term continuous time series of SO2 emissions are considered critical elements of both volcano monitoring and basic research into processes within magmatic systems. One highly successful framework for computing these fluxes involves reconstructing a representative time-averaged SO2 plume from which to estimate the SO2 source flux. Previous methods within this framework have used ancillary wind datasets from reanalysis or numerical weather prediction (NWP) to construct the mean plume and then again as a constrained parameter in the fitting. Additionally, traditional SO2 datasets from ultraviolet (UV) sensors lack altitude information, which must be assumed, to correctly calibrate the SO2 data and to capture the appropriate NWP wind level which can be a significant source of error. We have made novel modifications to this framework which do not rely on prior knowledge of the winds and therefore do not inherit errors associated with NWP winds. To perform the plume rotation, we modify a rudimentary computer vision algorithm designed for object detection in medical imaging to detect plume-like objects in gridded SO2 data. We then fit a solution to the general time-averaged dispersion of SO2 from a point source. We demonstrate these techniques using SO2 data generated by a newly developed probabilistic layer height and column loading algorithm designed for the Cross-track Infrared Sounder (CrIS), a hyperspectral infrared sensor aboard the Joint Polar Satellite System’s Suomi-NPP and NOAA-20 satellites. This SO2 data source is best suited to flux estimates at high-latitude volcanoes and at low-latitude, but high-altitude volcanoes. Of particular importance, IR SO2 data can fill an important data gap in the UV-based record: estimating SO2 emissions from high-latitude volcanoes through the polar winters when there is insufficient solar backscatter for UV sensors to be used.

Wildfire smoke-plume rise: a simple energy balance parameterization

Abstract. The buoyant rise and the resultant vertical distribution of wildfire smoke in the atmosphere have a strong influence on downwind pollutant concentrations at the surface. The amount of smoke injected vs. height is a key input into chemical transport models and smoke modelling frameworks. Due to scarcity of model evaluation data as well as the inherent complexity of wildfire plume dynamics, smoke injection height predictions have large uncertainties. In this work we use the coupled fire–atmosphere model WRF-SFIRE configured in large-eddy simulation (LES) mode to develop a synthetic plume dataset. Using this numerical data, we demonstrate that crosswind integrated smoke injection height for a fire of arbitrary shape and intensity can be modelled with a simple energy balance. We introduce two forms of updraft velocity scales that exhibit a linear dimensionless relationship with the plume vertical penetration distance through daytime convective boundary layers. Lastly, we use LES and prescribed burn data to constrain and evaluate the model. Our results suggest that the proposed simple parameterization of mean plume rise as a function of vertical velocity scale offers reasonable accuracy (30 m errors) at little computational cost.

Characterisation of African biomass burning plumes and impacts on the atmospheric composition over the south-west Indian Ocean

Abstract. We present an investigation of biomass burning (BB) plumes originating from Africa and Madagascar based on measurements of a suite of volatile organic compounds (VOCs), carbon monoxide (CO), ozone (O3) and nitrogen dioxide (NO2) obtained during the dry season of 2018 and 2019 at the high-altitude Maïdo observatory (21.1∘ S, 55.4∘ E, 2160 ma.s.l.), located on the remote island of La Réunion in the south-west Indian Ocean (SWIO). Biomass burning plume episodes were identified from increased acetonitrile (CH3CN) mixing ratios. Enhancement ratios (EnRs) – relative to CO – were calculated from in situ measurements for CH3CN, acetone (CH3COCH3), formic acid (HCOOH), acetic acid (CH3COOH), benzene (C6H6), methanol (CH3OH) and O3. We compared the EnRs to emission ratios (ERs) – relative to CO – reported in the literature in order to estimate loss or production of these compounds during transport. For CH3CN and CH3COOH, the calculated EnRs are similar to the ERs. For C6H6 and CH3OH, the EnR is lower than the ER, indicating a net sink of these compounds which was found to be in line with the expected atmospheric lifetime. For CH3COCH3 and HCOOH, the calculated EnRs are larger than the ERs. The discrepancy reaches an order of magnitude for HCOOH (18–34 pptv ppbv−1 compared to 1.8–4.5 pptv ppbv−1). This points to significant secondary production of HCOOH during transport. The Copernicus Atmospheric Monitoring Service (CAMS) global model simulations reproduce the temporal variation of CO mixing ratios well at the observatory but underestimate O3 and NO2 mixing ratios in the plumes by on average 16 ppbv and 60 pptv respectively. This discrepancy between modelled and measured O3 mixing ratios was attributed to (i) large uncertainties in VOC and NOx (NO+NO2) emissions due to BB in CAMS and (ii) misrepresentation of NOx recycling in the model during transport. Finally, transport of pyrogenically emitted CO is calculated with FLEXPART in order to (i) determine the mean plume age during the intrusions at the observatory and (ii) estimate the impact of BB on the pristine marine boundary layer (MBL). By multiplying the excess CO in the MBL with inferred EnRs at the observatory, we calculated the expected impact of BB on CH3CN, CH3COCH3, CH3OH and C6H6 concentrations in the MBL. These excesses constitute increases of ∼20 %–150 % compared to background measurements in the SWIO MBL reported in the literature.

Development of limited-view tomography for measurement of Spray G plume direction and liquid volume fraction

The method for direct injection of fuel in the cylinder of an IC engines is important to high-efficiency and low-emission performance. Optical spray diagnostics plays an important role in understanding plume movement and interaction for multi-hole injectors, and providing baseline understanding used for computational optimization of fuel delivery. Traditional planar or line-of-sight diagnostics fail to capture the liquid distribution because of optical thickness concerns. This work proposes a high-speed (67 kHz) extinction imaging technique at various injector rotations coupled to computed tomography (CT) for time-resolved reconstruction of liquid volume fraction in three dimensions. The number of views selected and processing were based on synthetic (modeled) liquid volume fraction data where extinction and CT adequately reconstructed each plume. The exercise showed that for an 8-hole, symmetric-design injector (ECN Spray G), only three different views are enough to reproduce the direction of each plume, and particularly the mean plume direction. Therefore, the number of views was minimized for experiments to save expense. Measurements applying this limited-view technique confirm plume–plume variations also detected with mechanical patternation, while providing better spatial and temporal resolution than achieved previously. Uncertainties due to the limited view within pressurized spray chambers, the droplet size, and optically thick regions are discussed.Graphic abstract

MAIAC Thermal Technique for Smoke Injection Height From MODIS

We present a new algorithm to derive smoke plume height ( $\text{H}^{\mathrm {a}}$ ) using the thermal contrast from the rising mixture of aerosol and emitted gases in the Moderate Resolution Imaging Spectroradiometer (MODIS) 11- $\mu \text{m}$ channel. Validation shows good agreement with the wind-corrected Multi-angle Imaging SpectroRadiometer (MISR)-MISR Interactive Explorer (MINX) values, with about 60% of the MODIS Terra thermal retrievals within 500 m of MISR $\text{H}^{\mathrm {a}}$ and 450 m lower on average. The bias is expected because the thermal technique represents an effective rather than a top plume height from MISR MINX. The comparison of MODIS Aqua retrievals with Cloud-Aerosol LiDAR with Orthogonal Polarization (CALIOP) Cloud-Aerosol LiDAR and Infrared Pathfinder Satellite Observations (CALIPSO) shows similar statistics, with a standard deviation of 458 m for the mean plume height and 216 m lower on average. $\text{H}^{\mathrm {a}}$ is part of the Multi-Angle Implementation of Atmospheric Correction (MAIAC) MODIS Collection 6 suite of products (MCD19), accessible via the Land Product Distributed Active Archive Center (LP DAAC). Aerosol injection height is reported in the daily MAIAC atmospheric product MCD19A2 along with the cloud mask (CM), column water vapor, aerosol optical depth (AOD), AOD uncertainty, and aerosol type, at 1-km resolution on a global sinusoidal grid. Despite some limitations, the vastly increased coverage from MODIS observations makes it a valuable data set complementing the established MISR and CALIOP products.

Numerical study of plume emission from an elevated line source in a wall-bounded flow

Purpose This paper aims to study turbulent dispersion of a passive plume emitting from a single elevated line source of different elevations in a plane channel flow by using direct numerical simulation (DNS). Design/methodology/approach The investigation was conducted in both physical and spectral spaces, which includes an analysis of statistical moments and pre-multiplied spectra of the velocity and concentration fields. The pre-multiplied power spectra of the velocity and concentration fields are compared to identify the transition of the plume development from the turbulent convective stage to the turbulent diffusive stage. Findings It is observed that due to the presence of wall shear, the mean plume drifts toward the wall for the near-wall source release case. It is also observed that streamwise development of the plume is sensitive to both the source elevation and the downstream distance from the source. For the line source placed near the center of the channel, the plume development is dominated by the bulk meandering effects. However, for the plume emitting from the near-wall line source, it hits the ground soon after its release and becomes dominated by the wall shear. As the downstream distance from the line source increases, the streamwise development of the plume released from the near-wall line source transitions from a turbulent convective stage to a turbulent diffusive stage. Originality/value This paper represents an original DNS study of turbulent mixing and dispersion of a passive plume emitting from a line source of different elevations in a wall-bounded flow. This paper proposes a practical method to identify the transition of the plume development from the turbulent convective to the turbulent diffusive stages.

Self-similar spectra of point-source scalar plumes in a turbulent boundary layer

Measurements of concentration fluctuations in a passive scalar plume released within a turbulent boundary layer are utilised to ascertain the scaling of concentration spectra. It is observed that the concentration spectra in a narrow meandering plume has a self-similar behaviour in both transverse ($y$) and vertical ($z$, i.e. wall-normal) directions. Experimental data reveal self-similarity when the magnitude of concentration spectra is scaled by the local concentration variance whereas frequency is suitably scaled utilising the integral length scale of the streamwise velocity or the boundary layer thickness and the source velocity as length and velocity scales, respectively. Furthermore, our data show that at each frequency, the concentration energy is distributed across the$y$and$z$directions that is proportional to concentration variance at that location. These results are consistent with our non-dimensional analysis. Based on these observations, if the mean plume statistics are known, a model is proposed with which concentration spectrum at any position within the plume can be calculated using the spectrum at any another location as the input. The model is tested extensively for point-source plumes released at various heights and streamwise distances in a turbulent boundary layer, and is found to predict spectra at different$y$and$z$locations in close agreement with measurements.

Parameterisation study of chemically reactive pollutant dispersion over idealised urban areas based on the Gaussian plume model

Dispersion of pollutants emitted from vehicles over urban areas largely affects pedestrian-level air quality. In this study, turbulent dispersion of reactive pollutants in the ABL over hypothetical urban areas in the form of idealised street canyons is investigated using large-eddy simulation (LES). Nitric oxide (NO) is emitted from the first street canyon into the urban ABL doped with ozone (O3). First of all, we make use of the advection-diffusion equation with chemical kinetics to derive the theoretical relation between the dispersion coefficients of tracer and reactive pollutants. Next, the source depletion analogy is used to determine the plume shape instead of the conventional Gaussian plume model. Finally, regression to the LES output unveils that the vertical dimensionless NO concentration profiles exhibit self-similarity for a range of background O3 concentrations. A new parameterisation of reactive plume dispersion over urban areas, whose performance is remarkable over the mean plume rise, is thus suggested.

Parameterisation study of chemically reactive pollutant dispersion over idealised urban areas based on the Gaussian plume model

Dispersion of pollutants emitted from vehicles over urban areas largely affects pedestrian-level air quality. In this study, turbulent dispersion of reactive pollutants in the ABL over hypothetical urban areas in the form of idealised street canyons is investigated using large-eddy simulation (LES). Nitric oxide (NO) is emitted from the first street canyon into the urban ABL doped with ozone (O3). First of all, we make use of the advection-diffusion equation with chemical kinetics to derive the theoretical relation between the dispersion coefficients of tracer and reactive pollutants. Next, the source depletion analogy is used to determine the plume shape instead of the conventional Gaussian plume model. Finally, regression to the LES output unveils that the vertical dimensionless NO concentration profiles exhibit self-similarity for a range of background O3 concentrations. A new parameterisation of reactive plume dispersion over urban areas, whose performance is remarkable over the mean plume rise, is thus suggested.

Time-dependent variations in vertical fluxes of hydrothermal plumes at mid-ocean ridges

We investigated the short-period variations in vertical fluxes of hydrothermal plumes at mid-ocean ridges through quantitative analysis of digital video images of plumes using the particle image velocimetry method. The analyzed digital video images of hydrothermal plumes were selected from the V Vent and Biotransect Vent at the fast-spreading East Pacific Rise, the Grotto Mound of the Endeavour Segment at the intermediate fast-spreading Juan de Fuca Ridge, and the TAG vent at the slow-spreading Northern Mid-Atlantic Ridge. The PIV was able to track the motion of turbulent parcels instead of individual particles within a hydrothermal plume. The mean plume vertical flux was shown to increase with increasing height above the vent orifice, revealing significant turbulent disperse of the rising hydrothermal plumes. The dominant periods of oscillation in the mean vertical flux were observed to be within a relatively narrow band width of 0.5–5 s despite diverse geological settings. Such pervasive short-period variations in plume flux are hypothesized to be caused by the pressure-drop oscillation within a network of compressible two-phase fluid within a sub-seafloor hydrothermal source zone. It is further argued that both the period and amplitude of plume flux oscillation might increase with decreasing plume vertical flux. The relatively narrow band width of the observed oscillation periods might reflect common characteristics in the plume dynamics despite diverse geological settings.

Source and boundary condition effects on unconfined and confined vertically distributed turbulent plumes

Plumes generated by vertically distributed sources of buoyancy have been observed to have substantially lower entrainment coefficients than their equivalent-geometry constant buoyancy flux plumes. Two differences between distributed and localized sources of buoyancy are the presence of a wall shear stress at the source and that non-ideal source conditions are distributed over the whole height of the enclosure for a vertically distributed source. Herein the impact of non-ideal source and boundary conditions on vertically distributed plumes is analysed. It is shown that, at small heights, the plume volume flow rate is significantly influenced by the wall-source volume flux. At larger heights the wall-source buoyancy is greater than the mean plume buoyancy, creating a non-self-similar horizontal buoyancy distribution within the plume. Recent experiments into the behaviour of a vertically distributed source of buoyancy in a confined region have also shown that the plume partially detrains in the stratified region of the enclosure. This detrainment has not been observed for constant buoyancy flux plumes in a confined region. Although models have been proposed to quantify the detrainment process, it is still unclear why vertically distributed buoyancy sources detrain while constant buoyancy flux plumes do not in the same physical geometry. The impact of source and boundary effects on previously published experiments on vertically distributed plumes are reviewed and the possible implications for plume entrainment and detrainment are discussed.

Evaluation of fast atmospheric dispersion models in a regular street network

The need to balance computational speed and simulation accuracy is a key challenge in designing atmospheric dispersion models that can be used in scenarios where near real-time hazard predictions are needed. This challenge is aggravated in cities, where models need to have some degree of building-awareness, alongside the ability to capture effects of dominant urban flow processes. We use a combination of high-resolution large-eddy simulation (LES) and wind-tunnel data of flow and dispersion in an idealised, equal-height urban canopy to highlight important dispersion processes and evaluate how these are reproduced by representatives of the most prevalent modelling approaches: (i) a Gaussian plume model, (ii) a Lagrangian stochastic model and (iii) street-network dispersion models. Concentration data from the LES, validated against the wind-tunnel data, were averaged over the volumes of streets in order to provide a high-fidelity reference suitable for evaluating the different models on the same footing. For the particular combination of forcing wind direction and source location studied here, the strongest deviations from the LES reference were associated with mean over-predictions of concentrations by approximately a factor of 2 and with a relative scatter larger than a factor of 4 of the mean, corresponding to cases where the mean plume centreline also deviated significantly from the LES. This was linked to low accuracy of the underlying flow models/parameters that resulted in a misrepresentation of pollutant channelling along streets and of the uneven plume branching observed in intersections. The agreement of model predictions with the LES (which explicitly resolves the turbulent flow and dispersion processes) greatly improved by increasing the accuracy of building-induced modifications of the driving flow field. When provided with a limited set of representative velocity parameters, the comparatively simple street-network models performed equally well or better compared to the Lagrangian model run on full 3D wind fields. The study showed that street-network models capture the dominant building-induced dispersion processes in the canopy layer through parametrisations of horizontal advection and vertical exchange processes at scales of practical interest. At the same time, computational costs and computing times associated with the network approach are ideally suited for emergency-response applications.

Short-term passive tracer plume dispersion in convective boundary layer using a high-resolution WRF-ARW model

High-resolution Advanced Research Weather Research and Forecast model is used to understand the role of atmospheric stability on the short-term transport of a continuous release passive scalar plume in three different convective boundary layer regimes: highly convective, combined shear- and buoyancy and shear dominated. Friction velocity to convective velocity ratio and atmospheric stability parameter are used to classify the boundary layer regimes. The effect of release height on the plume transport is addressed by releasing the plumes at surface, near-surface and elevated heights. Total 144 simulations are performed by releasing the plume in the morning and afternoon times of January and August months and at three release heights. Results show that horizontal transport of the plume scales with the initial wind conditions for surface and near-surface releases, and the vertical transport scales with atmospheric stability parameter. Mean plume height and vertical dispersion parameter obtained by convective scaling laws reached their asymptotic values after getting well-mixed in the boundary layer. The dimensionless downwind distance for the mean plume height to reach its asymptote is found to follow a power-law with respect to the atmospheric stability paramter. The coefficient and exponent of the power-law observed are found to be functions of the plume release height normalized by the boundary layer depth.