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A persistent spatial organization of eddies is identified in the lowest portion of the stably-stratified planetary boundary layer. The analysis uses flow realizations from published large-eddy simulations (Sullivan et al., J Atmos Sci 73(4):1815-1840, 2016) ranging in stability from neutral to nearly z-less stratification. The coherent turbulent structure is well approximated as a series of uniform momentum zones (UMZs) and uniform temperature zones (UTZs) separated by thin layers of intense gradients that are significantly greater than the mean. This pattern yields stairstep-like instantaneous flow profiles whose shape is distinct from the mean profiles that emerge from long-term averaging. However, the scaling of the stairstep organization is closely related to the resulting mean profiles. The differences in velocity and temperature across the thin gradient layers remain proportional to the surface momentum and heat flux conditions regardless of stratification. The vertical thickness of UMZs and UTZs is proportional to height above the surface for neutral and weak stratification, but becomes thinner and less dependent on height as the stability increases. Deviations from the logarithmic mean profiles for velocity and temperature observed under neutral conditions are therefore predominately due to the reduction in zone size with increasing stratification, which is empirically captured by existing Monin-Obukhov similarity relations for momentum and heat. The zone properties are additionally used to explain trends in the turbulent Prandtl number, thus providing a connection between the eddy organization, mean profiles, and turbulent diffusivity in stably stratified conditions., Comment: 35 pages, 12 figures

Measurements collected at two experimental sites, the Amazon Tall Tower Observatory (ATTO) and the tower at the Cuieiras Biological Reserve (ZF2) that was part of the GoAmazon experiment, were considered to study deviations from the law-of-the wall for the roughness sublayer (RSL) above the Amazon Forest. A plethora of physical, chemical, and biological processes are influenced by the flow structure in the RSL. Further, above tall and dense canopies the handshake between the land and the atmosphere in numerical Weather Predictions and Earth Systems Models occurs in the RSL. For the mean velocity, the RSL effects are operationally accommodated using a dimensionless roughness sublayer correction function (φ) to the law-of-the wall. For dense canopies the mixing-layer analogy is assumed, and the correction function φ depends on the vorticity thickness, Ls. φ measures the ratio of the eddy viscosity from attached eddies to a zero-plane displacement (d) and the actual eddy viscosity in the RSL at (z-d), for (z-d)/Ls>>1, φ∼1. However, this formulation remains only plausible for dense forested canopies and its extension to sparse and urban canopies difficult at the least. In the Amazon the experimental determination and modeling of φ may also be challenging for the forest topography that introduce z-dependent mean pressure gradients that then lead to variability in second-order flow velocity statistics with z. In this work an original formulation of φ is proposed based on a scale-wise co-spectral budget model that balances mechanical production to pressure-decorrelation terms in the co-spectral budget. Because the turbulent kinetic energy dissipation rate (ε) is conserved across the vertical velocity spectrum, the co-spectral budget model reveals a novel link between φ and a macro-scale dissipation length, Ld=u*3/ε.The friction velocity, u*, is interpreted from the moving-equilibrium hypothesis to be appropriately defined at the canopy top. The analysis shows that the estimation of φ with the new formulation agrees with independent estimates of φ using measured turbulent momentum flux and mean velocity gradient. Further, Ld emerges as a key length scale in the RSL, being more efficient than the vorticity thickness in the estimation of the peaks of the wind vertical velocity spectra.

Boundary layer turbulence in coastal regions differs from that in deep ocean because of bottom interactions. In this paper, we focus on the merging of surface and bottom boundary layers in a finite-depth coastal ocean by numerically solving the wave-averaged equations using a large eddy simulation method. The ocean fluid is driven by combined effects of wind stress, surface wave, and a steady current in the presence of stable vertical stratification. The resulting flow consists of two overlapping boundary layers, i.e. surface and bottom boundary layers, separated by an interior stratification. The overlapping boundary layers evolve through three phases, i.e. a rapid deepening, an oscillatory equilibrium and a prompt merger, separated by two transitions. Before the merger, internal waves are observed in the stratified layer, and they are excited mainly by Langmuir turbulence in the surface boundary layer. These waves induce a clear modulation on the bottom-generated turbulence, facilitating the interaction between the surface and bottom boundary layers. After the merger, the Langmuir circulations originally confined to the surface layer are found to grow in size and extend down to the sea bottom (even though the surface waves do not feel the bottom), reminiscent of the well-organized Langmuir supercells. These full-depth Langmuir circulations promote the vertical mixing and enhance the bottom shear, leading to a significant enhancement of turbulence levels in the vertical column., 20 pages, 15 figures

Modification to the law of the wall represented by a dimensionless correction function [Formula: see text] is derived using atmospheric turbulence measurements collected at two sites in the Amazon in near-neutral stratification, where z is the distance from the forest floor and h is the mean canopy height. The sites are the Amazon Tall Tower Observatory for [Formula: see text] and the Green Ocean Amazon (GoAmazon) site for [Formula: see text]. A link between the vertical velocity spectrum [Formula: see text] ( k is the longitudinal wavenumber) and [Formula: see text] is then established using a co-spectral budget (CSB) model interpreted by the moving-equilibrium hypothesis. The key finding is that [Formula: see text] is determined by the ratio of two turbulent viscosities and is given as [Formula: see text], where [Formula: see text], [Formula: see text], [Formula: see text] is a scale-dependent decorrelation time scale between velocity components, [Formula: see text] is predicted from the Rotta constant [Formula: see text], and the isotropization of production constant [Formula: see text] given by rapid distortion theory, [Formula: see text] is the von Kármán constant, [Formula: see text] is the friction velocity at the canopy top, and d is the zero-plane displacement. Because the transfer of energy across scales is conserved in [Formula: see text] and is determined by the turbulent kinetic energy dissipation rate ([Formula: see text]), the CSB model also predicts that [Formula: see text] scales with [Formula: see text], where [Formula: see text] is the length scale of attached eddies to [Formula: see text], and [Formula: see text] is a macro-scale dissipation length.

Desert dust accounts for most of the atmosphere’s aerosol burden by mass and produces numerous important impacts on the Earth system. However, current global climate models (GCMs) and land surface models (LSMs) struggle to accurately represent key dust emission processes, in part because of inadequate representations of soil particle sizes that affect the dust emission threshold, surface roughness elements that absorb wind momentum, and boundary-layer characteristics that control wind fluctuations. Furthermore, because dust emission is driven by small-scale (~1 km or smaller) processes, simulating the global cycle of desert dust in GCMs with coarse horizontal resolutions (~100 km) presents a fundamental challenge. This representation problem is exacerbated by dust emission fluxes scaling nonlinearly with wind speed above a threshold wind speed that is sensitive to land surface characteristics. Here, we address these fundamental problems underlying the simulation of dust emissions in GCMs and LSMs by developing improved descriptions of (1) the effect of soil texture on the dust emission threshold, (2) the effects of non-erodible roughness elements (both rocks and green vegetation) on the surface wind stress, and (3) the effects of boundary-layer turbulence on driving intermittent dust emissions. We then use the resulting revised dust emission parameterization to simulate global dust emissions in a standalone model forced by reanalysis meteorology and land surface fields. We further propose (4) a simple methodology to scale up high-resolution gridded dust emissions to the coarse resolution of GCMs. The resulting dust emission simulation shows substantially improved agreement against regional dust emissions observationally constrained by inverse modeling. We thus find that our revised dust emission parameterization can substantially improve dust emission simulations in GCMs and LSMs.

Large-eddy simulations of flow over a “horizontally” uniform model forest are used to investigate the effects of gentle topography on the turbulent kinetic energy (TKE) budget within the canopy roughness sublayer. Despite significant differences between simulations using idealized sinusoidal topography and real topography of the Amazon forest, results indicate that the effects of topography are located predominantly in the upper canopy and above, and are mostly caused by mean advection of TKE. The “horizontally” averaged TKE budget from idealized and real gentle topographies are almost identical to that for flat terrain, including a clear inertial layer above the roughness sublayer in which shear production is balanced by local dissipation. At topography crests, where observational towers are usually located, mean vertical advection of TKE can be as important as horizontal advection. We propose the use on an approximate TKE balance equation to estimate mean advection from single tower measurements, and introduce a new advection index that can be used as a proxy to quantify the importance of the topography on the TKE budget.

The K-profile parameterization (KPP) is a common method to model turbulent fluxes in regional and global oceanic models. Many versions of KPP exist in the oceanic sciences community, and one of their main differences is how they take the effects of nonbreaking waves into account. Although there is qualitative consensus that nonbreaking waves enhance vertical mixing due to the ensuing Langmuir circulations, there is no consensus on the quantitative aspects and modeling approach. In this paper we use a recently developed method to estimate both components of KPP (the diffusive term, usually called local, and the nondiffusive component, usually called nonlocal) based on numerically simulated turbulent fluxes without any a priori assumptions about their scaling or their shape. Through this method we show that the cubic shape usually used in KPP is not optimal for wavy situations and propose new ones. Furthermore, we show that the formulation for the nondiffusive fluxes, which currently only depend on the presence of surface buoyancy fluxes, should also take wave effects into account. We also investigate how the application of these changes to KPP improves the representation of turbulent fluxes in a diagnostic approach when compared with previous models.

Eddy diffusivity models are a common method to parameterize turbulent fluxes in the atmospheric sciences community. However, their inability to handle convective boundary layers leads to the addition of a nondiffusive flux component (usually called nonlocal) alongside the original diffusive term (usually called local). Both components are often modeled for convective conditions based on the shape of the eddy diffusivity profile for neutral conditions. This assumption of shape is traditionally employed due to the difficulty of estimating both components based on numerically simulated turbulent fluxes without any a priori assumptions. In this manuscript we propose a novel method to avoid this issue and estimate both components from numerical simulations without having to assume any a priori shape or scaling for either. Our approach is based on optimizing results from a modeling perspective and taking as much advantage as possible from the diffusive term, thus maximizing the eddy diffusivity. We use our method to diagnostically investigate four different large-eddy simulations spanning different stability regimes, which reveal that nondiffusive fluxes are important even when trying to minimize them. Furthermore, the calculated profiles for both diffusive and nondiffusive fluxes suggest that their shapes change with stability, which is an effect that is not included in most models currently in use. Finally, we use our results to discuss modeling approaches and identify opportunities for improving current models.

Observational data from two field campaigns in the Amazon forest were used to study the vertical structure of turbulence above the forest. The analysis was performed using the reduced turbulent kinetic energy (TKE) budget and its associated two-dimensional phase space. Results revealed the existence of two regions within the roughness sublayer in which the TKE budget cannot be explained by the canonical flat-terrain TKE budgets in the canopy roughness sublayer or in the lower portion of the convective ABL. Data analysis also suggested that deviations from horizontal homogeneity have a large contribution to the TKE budget. Results from LES of a model canopy over idealized topography presented similar features, leading to the conclusion that flow distortions caused by topography are responsible for the observed features in the TKE budget. These results support the conclusion that the boundary layer above the Amazon forest is strongly impacted by the gentle topography underneath.

The partitioning of ecosystem evapotranspiration and carbon dioxide fluxes into their plant and ground components is a critical research priority to better understand the water cycle and ecosystem function. Despite advances in different measurement techniques and partitioning models in the last decades, much is still unknown regarding the importance of different components of H2O and CO2 fluxes in ecosystems. In this work, we compare three partitioning methods that are based on analysis of conventional high frequency eddy-covariance (EC) data: the flux variance similarity method, the modified relaxed eddy accumulation methods, and the conditional eddy covariance method. First, we test these methods using fields experimental data, comparing them to other reference measurements for the components fluxes (gas chambers and leaf levels measurements). Subsequently, we develop a novel approach for simulating these fluxes in large eddy simulations and apply it to further probe the performance, assumptions, and relative skill of the three methods. The findings allow us to recommend partitioning best practices for their implementation, and to develop methods for the joint analyses of the various approaches.

The inertial subrange of turbulent scales is commonly reflected by a power law signature in ensemble statistics such as the energy spectrum and structure functions - both in theory and from observations. Despite promising findings on the topic of fractal geometries in turbulence, there is no accepted image for the physical flow features corresponding to this statistical signature in the inertial subrange. The present study uses boundary layer turbulence measurements to evaluate the self-similar geometric properties of velocity isosurfaces and investigate their influence on statistics for the velocity signal. The fractal dimension of streamwise velocity isosurfaces, indicating statistical self-similarity in the size of "wrinkles" along each isosurface, is shown to be constant only within the inertial subrange of scales. For the transition between the inertial subrange and production range, it is inferred that the largest wrinkles become increasingly confined by the overall size of large-scale coherent velocity regions such as uniform momentum zones. The self-similarity of isosurfaces yields power law trends in subsequent one-dimensional statistics. For instance, the theoretical 2/3 power law exponent for the structure function can be recovered by considering the collective behavior of numerous isosurface level sets. The results suggest that the physical presence of inertial subrange eddies is manifested in the self-similar wrinkles of isosurfaces., 30 pages, 11 figures

The presence of coarse mineral dust in the atmosphere has been substantiated in several recent measurement campaigns, which include observations of particles up to and above 100μm in diameter. Yet, atmospheric dust models either do not include particles larger than 20μm or severely underestimate their concentrations. One possibility for the underestimated concentrations is that models do not represent enhancements of particle transport due to subgrid-scale topography. Here, large-eddy simulations are used in combination with Lagrangian particle tracking to assess the impact of gentle two-dimensional topography with 50 and 100m elevation on the vertical transport of coarse dust in neutrally stratified conditions. The presence of topography significantly increases the likelihood that 5 and 20μm particles reach several hundred meters in altitude. Further, topography increases this likelihood by orders of magnitude for larger 60μm particles. Three mechanisms are observed to contribute to the increased vertical transport: a strong upward mean flow region on the uphill side of the topography, ejection of particles downwind of the topography crest, and enhanced vertical dispersion in the wake of the crest. The compounding effects of these mechanisms provide a pathway for coarse dust emitted from the surface to reach elevations where they can be further transported into the free atmosphere by large-scale motions such as convective plumes. While these findings are motivated by mineral dust observations, they are generally applicable to other heavy aerosols such as pollen.

The turbulence-kinetic-energy dissipation rate is a fundamental property in turbulent flows, but its direct measurement in the atmospheric surface layer is still a challenge. Indirect estimates are often obtained from inertial-subrange laws using turbulence statistics of the longitudinal velocity component. In this study, synthetic turbulence data are used to investigate the impact of path-averaging effects present in sonic anemometer data on the inertial subrange of the second-order structure function. Path averaging reduces the energy levels in the second-order structure function, creating a negative bias in the estimates of the dissipation rate. The effect is dependent on the path-averaging transfer function, mean wind speed and path length. A simple correction for the bias on the basis of existing transfer functions is applied and tested with data obtained from two separate sonic anemometers. Compared to the spectrum, the second-order structure function after the correction becomes the optimum statistical measure for indirect estimation of the dissipation rate, due to its lower random error and insensitivity to aliasing effects.

As human encroachment increases in the Amazon rain forest, it is important to determine how anthropogenic emissions of reactive gases affect regional atmospheric chemistry. In the present study, we investigate the extent to which urban air plumes modify the levels of ozone (O3) and nitrogen oxides (NOX) in the downwind rain forest. The median mixing ratios of the background O3, NOX, and NOX oxidation products (NOZ) were 20 (15 parts per billion on a volume basis, ppbv), 0.6 ppbv (0.6 ppbv), and 1.0 ppbv (0.5 ppbv) during the dry (wet) season at the study site. Compared to the background environment, air plumes from the city of Manaus had enhanced median mixing ratios for O3 and NOZ by 30–50% and 40–90%, respectively. However, the enhancements of NOX in the air plumes were less than 20%, indicating that the majority of NOX was chemically converted to O3 and NOZ during transport. Results from a photochemical model showed that an injection of 8 ppbv of NOX into the rain forest can cause up to 260% and 150% increases in O3 and hydroxyl radical (OH) levels compared to the background conditions, indicating the likely extent that NOX can modify the air quality and oxidative capacity in the Amazon rain forest. Slopes of the O3-temperature linear relationships increased with NOX levels from 3.7 to 6.5 ppbv per degree Kelvin during the dry season and 1.7–5.5 ppbv per degree Kelvin during the wet season. Average rates of change of the slope with respect to NOZ were approximately 1.8 and 2.3 times higher than those with respect to NOX for the dry and wet seasons. One key conclusion of this study is that NOZ substantially contributed to the O3 formation in response to temperature under enhanced NOX conditions in the forested environment.

A series of GOMRI-sponsored experimental and computational studies have discovered, elucidated and quantified the impact of small-scale processes on the dispersion, transport and weathering of crude oil slicks and subsurface plumes. Physical interfacial phenomena occurring at micron-scales include the formation of particle-stabilized emulsions, penetration of particles into oil droplets, formation of compound water-containing oil droplets during plume breakup, and the mechanisms affecting the breakup of oil into micro-droplet by tip streaming resulting from the drastic reduction in interfacial tension upon introduction of dispersant. Efforts aimed at development targeted delivery of surfactants have introduced solvent-free halloysite nanotubes that can be filled with surfactants, and preferentially released at oil-water interface. Buoyant surfactant-based gels, which enhance their encounter rates with oil slicks and adhere to weathered oil have also been developed. Studies of oil-bacteria interactions during early phases of biodegradation and shown how the bacteria, some highly active, attach to the oil-water interfaces and form complex films. Clay-decorated droplets sequester these bacteria and promote the propagation of these biofilm. Long extracellular polymeric substance (EPS) streamers generated by these biofilms form connected networks involving multiple droplets and debris, as well as increase the drag on the oil droplets. At 0.01–10 m scales, the generation of subsurface and airborne crude oil droplets by breaking waves, subsurface plumes and raindrop impact have been quantified. For waves, premixing the oil with dispersant reduces the droplets sizes to micron- and submicron-scales, and changes the slope of their size distribution. Without dispersant, the droplet diameters can be predicted based on the turbulence scales. With dispersant, the droplets are much smaller than the turbulence scales owing to the abovementioned tip-streaming. Aerosolization of oil is caused both by the initial splash and by subsequent bubble bursting, as entrained bubbles rise to the surface. Introduction of dispersant increases the airborne nano-droplet concentration by orders of magnitude, raising health questions. Dispersant injection also reduces the size of droplets in subsurface plumes, affecting the subsequent dispersion of these plume by currents and turbulence. Advancements have also been made in modeling of dissolution of oil in plumes, as well as in applications of Large Eddy Simulations (LES) to model plumes containing oil droplets and gas bubbles. The new multiscale framework, which accounts for the droplet size distribution and mass diffusion, can simulate the near- and far-fields of plumes, and predict the effect of vertical mixing promoted by turbulence on the transport of dispersed oil.

Computer simulations are advancing our understanding of how natural and anthropogenic materials are transported around the upper layer of the ocean.

A framework to describe transport and deposition of ultrafine particles (UFP) within forests using large eddy simulation (LES) is presented. Comparison with measurements collected within and above a Scots pine stand in Southern Finland are used to explore the plausibility of the simulations. The numerical model is then employed to quantify the effects of canopy morphology (leaf area index and leaf area density), turbulence intensity, and particle size on the partitioning between upper canopy and subcanopy deposition and the overall deposition velocities. Results show a complex interplay between canopy morphology and turbulence, which is reflected on the particle flux profiles within the canopy. However, mean particle concentration profiles, total deposition, and deposition velocities at the canopy top are insensitive to the leaf area density profile but show dependence mostly on leaf area index, turbulence levels, and particle size. Finally, with the goal of understanding the sensitivity of the deposition velocity to all these parameters, an analytical model is developed that shows good agreement with LES results (within ± 20 % ) for all conditions simulated here.

Large-eddy simulations of an observed single-layer Arctic mixed-phase cloud are analyzed to study the value of forward modeling of profiling millimeter-wave cloud radar Doppler spectral width for model evaluation. Individual broadening terms and their uncertainties are quantified for the observed spectral width and compared to modeled broadening terms. Modeled turbulent broadening is narrower than the observed values when the turbulent kinetic energy dissipation rate from the subgrid-scale model is used in the forward model. The total dissipation rates, estimated with the subgrid-scale dissipation rates and the numerical dissipation rates, agree much better with both the retrieved dissipation rates and those inferred from the power spectra of the simulated vertical air velocity. The comparison of the microphysical broadening provides another evaluative measure of the ice properties in the simulation. To accurately retrieve dissipation rates as well as each broadening term from the observations, we suggest a few modifications to previously presented techniques. First, we show that the inertial subrange spectra filtered with the radar sampling volume is a better underlying model than the unfiltered −5/3 law for the retrieval of the dissipation rate from the power spectra of the mean Doppler velocity. Second, we demonstrate that it is important to filter out turbulence and remove the layer-mean reflectivity-weighted mean fall speed from the observed mean Doppler velocity to avoid overestimation of shear broadening. Finally, we provide a method to quantify the uncertainty in the retrieved dissipation rates, which eventually propagates to the uncertainty in the microphysical broadening.

The Amazon rain forest is a major global isoprene source, but little is known about its seasonal ambient concentration patterns. To investigate the environmental and phenological controls over isoprene seasonality, we measured isoprene mixing ratios, concurrent meteorological data, and leaf area indices from April 2014 to January 2015 above a rain forest in the central Amazon, Brazil. Daytime median isoprene mixing ratios varied throughout the year by a factor of two. The isoprene seasonal pattern was not solely driven by sunlight and temperature. Leaf age and quantity also contributed to the seasonal variations of isoprene concentrations, suggesting leaf phenology was a crucial variable needed to correctly estimate isoprene emissions. A zero-dimensional model incorporating the estimated emissions, atmospheric boundary layer dynamics, and air chemistry was used to assess the contributions of each process on the variability of isoprene. Surface deposition was an important sink mechanism and accounted for 78% of the nighttime loss of isoprene. Also, chemical reactions destroyed isoprene and during 6:00 to 18:00 h local time 56, 77, 69, and 69% of the emitted isoprene was chemically consumed in June, September, December, and January, respectively. Entrainment fluxes from the residual layer contributed 34% to the early-morning above-canopy isoprene mixing ratios. Sensitivity analysis showed that hydroxyl radical (HO) recycling and segregation of isoprene–HO played relatively lesser roles (up to 16%) in regulating ambient isoprene levels. Nitric oxide (NO) levels dominated isoprene chemical reaction pathways associated with consumption and production of HO under low-NO and high volatile organic compound (VOC) conditions. While surface deposition and oxidative processes altered isoprene levels, the relative importance of these factors varied seasonally with leaf phenology playing a more important role.

The Hurst phenomenon is a well-known feature of long-range persistence first observed in hydrological and geophysical time series by E. Hurst in the 1950s. It has also been found in several cases in turbulence time series measured in the wind tunnel, the atmosphere, and in rivers. Here, we conduct a systematic investigation of the value of the Hurst coefficient H in atmospheric surface-layer data, and its impact on the estimation of random errors. We show that usually $$H > 0.5$$ , which implies the non-existence (in the statistical sense) of the integral time scale. Since the integral time scale is present in the Lumley–Panofsky equation for the estimation of random errors, this has important practical consequences. We estimated H in two principal ways: (1) with an extension of the recently proposed filtering method to estimate the random error ( $$H_p$$ ), and (2) with the classical rescaled range introduced by Hurst ( $$H_R$$ ). Other estimators were tried but were found less able to capture the statistical behaviour of the large scales of turbulence. Using data from three micrometeorological campaigns we found that both first- and second-order turbulence statistics display the Hurst phenomenon. Usually, $$H_R$$ is larger than $$H_p$$ for the same dataset, raising the question that one, or even both, of these estimators, may be biased. For the relative error, we found that the errors estimated with the approach adopted by us, that we call the relaxed filtering method, and that takes into account the occurrence of the Hurst phenomenon, are larger than both the filtering method and the classical Lumley–Panofsky estimates. Finally, we found that there is no apparent relationship between H and the Obukhov stability parameter. The relative errors, however, do show stability dependence, particularly in the case of the error of the kinematic momentum flux in unstable conditions, and that of the kinematic sensible heat flux in stable conditions.

The low-wavenumber regime of the spectrum of turbulence commensurate with Townsend’s “attached” eddies is investigated here for the near-neutral atmospheric surface layer (ASL) and the roughness sublayer (RSL) above vegetation canopies. The central thesis corroborates the significance of the imbalance between local production and dissipation of turbulence kinetic energy (TKE) and canopy shear in challenging the classical distance-from-the-wall scaling of canonical turbulent boundary layers. Using five experimental datasets (two vegetation canopy RSL flows, two ASL flows, and one open-channel experiment), this paper explores (i) the existence of a low-wavenumber k−1 scaling law in the (wind) velocity spectra or, equivalently, a logarithmic scaling ln(r) in the velocity structure functions; (ii) phenomenological aspects of these anisotropic scales as a departure from homogeneous and isotropic scales; and (iii) the collapse of experimental data when plotted with different similarity coordinates. The results show that the extent of the k−1 and/or ln(r) scaling for the longitudinal velocity is shorter in the RSL above canopies than in the ASL because of smaller scale separation in the former. Conversely, these scaling laws are absent in the vertical velocity spectra except at large distances from the wall. The analysis reveals that the statistics of the velocity differences Δu and Δw approach a Gaussian-like behavior at large scales and that these eddies are responsible for momentum/energy production corroborated by large positive (negative) excursions in Δu accompanied by negative (positive) ones in Δw. A length scale based on TKE dissipation collapses the velocity structure functions at different heights better than the inertial length scale.

The suitability of Kerstein's One-Dimensional Turbulence (ODT) model in the representation of atmospheric boundary layer (ABL) flows within and above plant canopies is investigated. The ODT model was adapted to represent a filtered version of flow and scalar fields, equipped with a Smagorinsky-like sub-grid scale model, a wall model, and a parameterization of the canopy effects on the flow. In the filtered ODT, the entire vertical extension of the ABL is modeled and the “resolved” vertical turbulent transport is represented by stochastic eddies that effectively mix fluid parcels across a path length, representing non-local turbulent fluxes that are a critical feature of the canopy roughness sublayer. Simulations for different canopies and stability conditions are performed and vertical profiles of turbulence and scalar statistics are compared with observational and large-eddy simulation data. This new filtered version of the ODT model yields grid-independent results. Model performance for plant canopies is consistent with previous results from ODT for all cases tested without any case-specific parameter adjustment, generating reasonable agreement in profiles of mean velocity, temperature and water vapor mixing ratio, as well as vertical fluxes of momentum and sensible and latent heat, despite the underestimation of all velocity variances. Non-local transport is intrinsic to the formulation of ODT, which represents a significant advantage compared to other reduced-model approaches employed for canopy flows.

In this study a stochastic wall model based on the “large-eddy” version of the One-Dimensional Turbulence (ODT) model was developed for Large-Eddy Simulation (LES) of smooth and rough channel flows, with the primary goal of providing a refined turbulent flow field near the wall. This LES-ODT coupling was tested with the dynamic Smagorinsky and the scale-dependent Lagrangian dynamic subgrid-scale models. When compared to the same LES with a wall model based on a local law-of-the-wall, LES-ODT improved the one-dimensional energy spectra for all three velocity components close to the wall for both subgrid-scale models tested. More importantly, improving the LES wall model had a more positive effect in the near-wall spectra than improving the subgrid-scale model from the traditional dynamic to the scale-dependent Lagrangian dynamic model. For smooth channels, LES-ODT results compared well with DNS of R e λ = 590 and 5200; however, the variance modeled by the ODT presents discrepancies for all three velocity components, an issue inherent to ODT. Finally, the simulation of a channel flow with additional roughness modeled by a drag force was compared to data of atmospheric flow through a maize field, providing evidence of the potential for this approach to directly simulate complex near-wall phenomena. Given its high computational cost, the main use of the LES-ODT coupling is in studies that require a refinement of the near-wall region without the need to refine the entire LES domain.

In many atmospheric flows, a dispersed phase is actively suspended by turbulence, whose competition with gravitational settling ultimately dictates its vertical distribution. Examples of dispersed phases include snow, sea-spray droplets, dust, or sand, where individual elements of much larger density than the surrounding air are carried by turbulent motions after emission from the surface. In cases where the particle is assumed to deviate from local fluid motions only by its gravitational settling (i.e., they are inertialess), traditional flux balances predict a power-law dependence of particle concentration with height. It is unclear, however, how particle inertia influences this relationship, and this question is the focus of this work. Direct numerical simulations are conducted of turbulent open-channel flow, laden with Lagrangian particles of specified inertia; in this way the study focuses on the turbulent transport which occurs in the lowest few meters of the planetary boundary layer, in regions critical for connecting emission fluxes to the fluxes felt by the full-scale boundary layer. Simulations over a wide range of particle Stokes number, while holding the dimensionless settling velocity constant, are performed to understand the role of particle inertia on vertical dispersion. It is found that particles deviate from their inertialess behaviour in ways that are not easily captured by traditional theory; concentrations are reduced with increasing Stokes number. Furthermore, a similarity-based eddy diffusivity for particle concentration fails as particles experience inertial acceleration, precluding a closed-form solution for particle concentration as in the case of inertialess particles. The primary consequence of this result is that typical flux parametrizations connecting surface emission models (e.g., saltation models or sea-spray generation functions) to elevated boundary conditions may overestimate particle concentrations due to the reduced vertical transport caused by inertia in between; likewise particle emission may be underestimated if inferred from concentration measurements aloft.

We present a theoretical model, based on a simple model of turbulent diffusion and first-order chemical kinetics, to determine air-parcel residence times and the out-of-canopy export of reactive gases emitted within forest canopies under neutral conditions. Theoretical predictions of the air-parcel residence time are compared to values derived from large-eddy simulation for a range of canopy architectures and turbulence levels under neutral stratification. Median air-parcel residence times range from a few sec in the upper canopy to approximately 30 min near the ground and the distribution of residence times is skewed towards longer times in the lower canopy. While the predicted probability density functions from the theoretical model and large-eddy simulation are in good agreement with each other, the theoretical model requires only information on canopy height and eddy diffusivities inside the canopy. The eddy-diffusivity model developed additionally requires the friction velocity at canopy top and a parametrized profile of the standard deviation of vertical velocity. The theoretical model of air-parcel residence times is extended to include first-order chemical reactions over a range of of Damkohler numbers (Da) characteristic of plant-emitted hydrocarbons. The resulting out-of-canopy export fractions range from near 1 for $$Da =10^{-3}$$ to less than 0.3 at $$Da = 10$$ . These results highlight the necessity for dense and tall forests to include the impacts of air-parcel residence times when calculating the out-of-canopy export fraction for reactive trace gases.

Scaling laws for the longitudinal structure function in the atmospheric surface layer (ASL) are studied using dimensional analysis and matched asymptotics. Theoretical predictions show that the logarithmic scaling for the scales larger than those of the inertial subrange recently proposed for neutral wall-bounded flows also holds for the shear-dominated ASL composed of weakly unstable, neutral, and all stable conditions (as long as continuous turbulence exists). A 2/3 power law is obtained for buoyancy-dominated ASLs. Data from the Advection Horizontal Array Turbulence Study (AHATS) field experiment confirm these scalings, and they also show that the length scale formed by the friction velocity and the turbulent kinetic energy dissipation rate consistently outperforms the distance from the ground z as the relevant scale in all cases regardless of stability. With this new length scale, the production range of the longitudinal structure function collapses for all measurement heights and stability conditions. A new variable to characterize atmospheric stability emerges from the theory: namely, the ratio between the buoyancy flux and the TKE dissipation rate.

In this work, we investigate the ozone dynamics during the occurrence of both downdrafts associated with mesoscale convective storms and non-classical low-level jets. Extensive data sets, comprised of air chemistry and meteorological observations made in the Amazon region of Brazil over the course of 2014–15, are analyzed to address several questions. A first objective is to investigate the atmospheric thermodynamic and dynamic conditions associated with storm-generated ozone enhancements in the Amazon region. A second objective is to determine the magnitude and the frequency of ground-level ozone enhancements related to low-level jets. Ozone enhancements are analyzed as a function of wind shear, low-level jet maximum wind speed, and altitude of jet core. Strong and sudden increases in ozone levels are associated with simultaneous changes in variables such as horizontal wind speed, convective available potential energy, turbulence intensity and vertical velocity skewness. Rapid increases in vertical velocity skewness give support to the hypothesis that the ozone enhancements are directly related to downdrafts. Low-level jets associated with advancing density currents are often present during and after storm downdrafts that transport ozone-enriched air from aloft to the surface.

Author(s): Chamecki, M; Chor, T; Yang, D; Meneveau, C | Abstract: Material transport in the ocean mixed layer (OML) is an important component of natural processes such as gas and nutrient exchanges. It is also important in the context of pollution (oil droplets, microplastics, etc.). Observational studies of small-scale three-dimensional turbulence in the OML are difficult, especially if one aims at a systematic coverage of relevant parameters and their effects, under controlled conditions. Numerical studies are also challenging due to the large-scale separation between the physical processes dominating transport in the horizontal and vertical directions. Despite this difficulty, the application of large eddy simulation (LES) to study OML turbulence and, more specifically, its effects on material transport has resulted in major advances in the field in recent years. In this paper we review the use of LES to study material transport within the OML and then summarize and synthesize the advances it has enabled in the past decade or so. In the first part we describe the LES technique and the most common approaches when applying it in OML material transport investigations. In the second part we review recent results on material transport obtained using LES and comment on implications.

In the context of many applications of turbulent multi-phase flows, knowledge of the dispersed phase size distribution and its evolution is critical to predicting important macroscopic features. We develop a large eddy simulation (LES) model that can predict the turbulent transport and evolution of size distributions, for a specific subset of applications in which the dispersed phase can be assumed to consist of spherical droplets, and occurring at low volume fraction. We use a population dynamics model for polydisperse droplet distributions specifically adapted to a LES framework including a model for droplet breakup due to turbulence, neglecting coalescence consistent with the assumed small dispersed phase volume fractions. We model the number density fields using an Eulerian approach for each bin of the discretized droplet size distribution. Following earlier methods used in the Reynolds-averaged Navier–Stokes framework, the droplet breakup due to turbulent fluctuations is modelled by treating droplet–eddy collisions as in kinetic theory of gases. Existing models assume the scale of droplet–eddy collision to be in the inertial range of turbulence. In order to also model smaller droplets comparable to or smaller than the Kolmogorov scale we extend the breakup kernels using a structure function model that smoothly transitions from the inertial to the viscous range. The model includes a dimensionless coefficient that is fitted by comparing predictions in a one-dimensional version of the model with a laboratory experiment of oil droplet breakup below breaking waves. After initial comparisons of the one-dimensional model to measurements of oil droplets in an axisymmetric jet, it is then applied in a three-dimensional LES of a jet in cross-flow with large oil droplets of a single size being released at the source of the jet. We model the concentration fields using $N_{d}=15$ bins of discrete droplet sizes and solve scalar transport equations for each bin. The resulting droplet size distributions are compared with published experimental data, and good agreement for the relative size distribution is obtained. The LES results also enable us to quantify size distribution variability. We find that the probability distribution functions of key quantities such as the total surface area and the Sauter mean diameter of oil droplets are highly variable, some displaying strong non-Gaussian intermittent behaviour.

This study considered a new approach for representing flexible canopies within large-eddy simulation that captures the impacts of reconfiguration on both the canopy posture and the canopy drag. The unsteady change in plant posture in response to the passage of turbulence structures (monami) was assessed using established steady-reconfiguration models responding to the unsteady velocity at the top of the canopy. The new drag and plant posture models improved the modelling of highly flexible canopies by more accurately capturing the observed vertical distribution of peak Reynolds stress. When compared to models that do not consider reconfiguration, or that represent it only through a velocity-dependent drag coefficient, the addition of a velocity-dependent canopy posture (unsteady reconfiguration) achieved up to a 56% reduction of the root mean square error for mean horizontal flow velocity and Reynolds stress profiles over the canopy. The RMSE for turbulence intensities and skewness were reduced up to 48% and 56%, respectively.

Traffic-related pollutant concentrations are typically much higher in near-roadway microenvironments, and pedestrian and resident exposures to air pollutants can be substantially increased by the short periods of time spent on and near roadways. The design of the built environment plays a critical role in the dispersion of pollutants at street level; after normalizing for traffic, differences of a factor of ~5 have been observed between urban neighborhoods with different built environment characteristics. We examined the effects of different built environment designs on the concentrations of street-level ultrafine particles (UFP) at the scale of several blocks using the Quick Urban and Industrial Complex (QUIC) numerical modeling system. The model was capable of reasonably reproducing the complex ensemble mean 3D air flow patterns and pollutant concentrations in urban areas at fine spatial scale. We evaluated the effects of several built environment designs, changing building heights and spacing while holding total built environment volumes constant. We found that ground-level open space reduces street-level pollutant concentrations. Holding volume/surface area constant, tall buildings clustered together with larger open spaces between buildings resulted in substantially lower pollutant concentrations than buildings in rows. Buildings arranged on a ‘checkerboard’ grid with smaller contiguous open spaces, a configuration with some open space on one of the sides of the roadway at all locations, resulted in the lowest average concentrations for almost all wind directions. Rows usually prohibit mixing for perpendicular and oblique wind directions, even when there are large spaces between them, and clustered buildings have some areas where buildings border both sides of the roadways, inhibiting mixing. The model results suggest that pollutant concentrations drop off rapidly with height in the first 10 m or so above the roadways. In addition, the simulated vertical concentration profiles show a moderate elevated peak at the roof levels of the shorter buildings within the area. Model limitations and suggestions both for urban design are both discussed.

Squall lines (SLs) are convective systems that cause heavy precipitation and consequently modify the atmospheric thermodynamic structure near the surface. SLs generated along the northern coast of Brazil and their effect upon atmospheric structure during their westward displacement into the Amazon are studied. Satellite imagery was employed to identify an SL above two experimental sites in the central Amazon and to characterize differences in the near-surface turbulent and ozone exchange during the passage of the SLs. The two sites, which are separated by about 100 km, feature contrasting vegetation. One site is tall canopy rainforest and the other is deforested. From our case study, it is noted that: equivalent potential temperature significantly drops, principally in the forested region, the average near-surface wind speed increases 5 fold, the skewness of vertical wind velocity becomes considerably negative, significant increases in turbulence intensity are observed. These changes suggest the presence of strong downdrafts generated by the SL. Shear production and dissipation rate of turbulent kinetic energy are considerably larger during the SL when compared to periods with absence of SL. In this study, we show that SLs are capable of modifying the vertical organization of the turbulence over forested and deforested areas, leading to changes in certain chemical processes that occur near the surface. To the best of our knowledge, this study represents a first in demonstrating that near-surface turbulent flow in the Amazon region is modified by the presence of SLs.

We describe the salient features of a field study whose goals are to quantify the vertical distribution of plant-emitted hydrocarbons and their contribution to aerosol and cloud condensation nuclei production above a central Amazonian rain forest. Using observing systems deployed on a 50-m meteorological tower, complemented with tethered balloon deployments, the vertical distribution of hydrocarbons and aerosols was determined under different boundary layer thermodynamic states. The rain forest emits sufficient reactive hydrocarbons, such as isoprene and monoterpenes, to provide precursors of secondary organic aerosols and cloud condensation nuclei. Mesoscale convective systems transport ozone from the middle troposphere, enriching the atmospheric boundary layer as well as the forest canopy and surface layer. Through multiple chemical transformations, the ozone-enriched atmospheric surface layer can oxidize rain forest–emitted hydrocarbons. One conclusion derived from the field studies is that the rain forest produces the necessary chemical species and in sufficient amounts to undergo oxidation and generate aerosols that subsequently activate into cloud condensation nuclei.

The inadequacy of conventional gradient diffusion in closure modelling of turbulent heat fluxes within the convective atmospheric boundary layer is often alleviated by accounting for non-local transport effects, such as Deardorff's counter-gradient models, Wyngaard's transport asymmetry closures or mass-flux parametrization. This concept of large-eddy flux transport is examined here with the principal aim of unifying these seemingly different models. Using large-eddy simulation (LES) runs for the atmospheric boundary layer, spanning weakly to strongly convective conditions, a generic diagnostic framework that encodes the role of third-order moments in non-local transport is developed and tested. The premise is that these non-local effects are responsible for the inherent asymmetry in vertical transport and hence the necessary non-Gaussian nature of the joint probability density function (JPDF) of vertical velocity and potential temperature must account for these effects. Conditional sampling (quadrant analysis) of this JPDF and the imbalance between the flow mechanisms of ejections and sweeps are used to characterize this asymmetry, which is then linked to the third-order moments using a cumulant-discard method for the Gram–Charlier expansion of the JPDF. While the concept of ejection-sweep events used here is not a simple extension of that commonly used in the surface layer, their connection to third-order moments shows that the concepts of bottom-up/top-down diffusion or updraught/downdraught models are accounted for by various quadrants of the JPDF. An analogy between mass-flux models and the relaxed eddy accumulation method reveals that there is a seemingly implicit assumption of a Gaussian JPDF in the former.