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Turbulence closure schemes, besides their intrinsic theoretical importance, represent a fundamental component in the atmospheric numerical models. Among his numerous and diverse scientific contributions, Prof. Sergej S. Zilitinkevich, with his coauthors, elaborated a turbulence closure model for stably-stratified geophysical flows, the Energy and Flux Budget (EFB) model. This closure has been verified and applied on many different experimental datasets and case studies, for steady state and homogeneous conditions. Having available observational datasets for urban and suburban sites in different cities in Italy, we investigate the deviation of the observations of turbulent kinetic energy and momentum flux from the EFB turbulence closure model in heterogeneous conditions. This allows addressing and interpreting the features that induce such deviation between the model and the observations. The EFB model is then revisited including residual terms that can account for the non-stationarity and heterogeneity of the considered cases. The correction with the residual terms leads to improve the agreement between the theoretical formulations and the observed behaviour for the turbulent kinetic energy shares and for the vertical momentum flux. [ABSTRACT FROM AUTHOR]

Modification to the law of the wall represented by a dimensionless correction function ϕ RSL (z / h) 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 z / h ∈ [ 1 , 2.3 ] and the Green Ocean Amazon (GoAmazon) site for z / h ∈ [ 1 , 1.4 ]. A link between the vertical velocity spectrum E w w (k) (k is the longitudinal wavenumber) and ϕ RSL is then established using a co-spectral budget (CSB) model interpreted by the moving-equilibrium hypothesis. The key finding is that ϕ RSL is determined by the ratio of two turbulent viscosities and is given as ν t , B L / ν t , RSL , where ν t , RSL = (1 / A) ∫ 0 ∞ τ (k) E w w (k) d k , ν t , B L = k v (z − d) u * , τ (k) is a scale-dependent decorrelation time scale between velocity components, A = C R / (1 − C I) = 4.5 is predicted from the Rotta constant C R = 1.8 , and the isotropization of production constant C I = 3 / 5 given by rapid distortion theory, k v is the von Kármán constant, u * 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 E w w (k) and is determined by the turbulent kinetic energy dissipation rate (ε), the CSB model also predicts that ϕ RSL scales with L B L / L d , where L B L is the length scale of attached eddies to z = d , and L d = u * 3 / ε is a macro-scale dissipation length. [ABSTRACT FROM AUTHOR]

One of the hallmarks of the stable boundary layer is the switching between turbulent (active) and non-turbulent (passive) states. In very stable conditions, the boundary layer becomes layered with fully-developed turbulence confined to a shallow region near the surface. In the quiescent region above this near-surface layer, the turbulence is weak, intermittent and detached from the ground. These conditions promote the development of a low-level jet that re-energizes the turbulence through an elevated shear layer. The Monin–Obukhov similarity theory fails in the layered stable boundary layer thereby making the quantification of mixing and transport properties challenging for numerical models. In the present study, multi-level time series from a tall (140 m) meteorological tower are analyzed using the telegraphic approximation to investigate analogies with a general class of intermittency models that include self-organized criticality. The analogy between turbulence and self-organized criticality is restricted to clustering properties of sign changes of flow variables for describing switching between turbulent and non-turbulent states. The telegraphic approximation provides a new perspective on clustering and on external and internal intermittency for periods dominated by turbulent motions, a low-level jet and submeso motions. Some of these periods are characterized by the absence of turbulence but occasionally punctuated by bursts of intermittent turbulent events. The switching probability of active–inactive states and the lifetimes of inactive states (related to intermittent turbulent bursts) show evidence of self-organized-criticality like behaviour in terms of scaling laws. The coexistence of self-organized criticality and intermittent turbulence may offer new perspectives on the genesis of scaling laws and similarity arguments, thereby improving the performance of numerical models in the stable boundary layer. [ABSTRACT FROM AUTHOR]

Submeso motions add complexities to the structure of the stable boundary layer. Such motions include horizontal meandering and gravity waves, in particular when the large-scale flow is weak. The coexistence and interaction of such submeso motions is investigated through the analysis of data collected in Antarctica, in persistent conditions of strong atmospheric stratification. Detected horizontal meandering is frequently associated with temperature oscillations characterized by similar time scales (30 min) at all levels (2, 4.5 and 10 m). In contrast, dirty gravity waves superimposed on horizontal meandering are detected only at the highest level, characterized by time scales of a few minutes. The meandering produces an energy peak in the low-frequency spectral range, well fitted by a spectral model previously proposed for low wind speeds. The coexistence of horizontal and vertical oscillations is observed in the presence of large wind-direction shifts superimposed on the gradual flow meandering. Such shifts are often related to the variation of the mean flow dynamics, but also to intermittent events, localized in time, which do not produce a variation in the mean wind direction and that are associated with sharp decreases in wind speed and temperature. The noisy gravity waves coexisting with horizontal meandering persist only for a few cycles and produce bursts of turbulent mixing close to the ground, affecting the exchange processes between the surface and the stable boundary layer. The results confirm the importance of sharp wind-direction changes at low wind speed in the stable atmosphere and suggest a possible correlation between observed gravity waves and dynamical instabilities modulated by horizontal meandering. [ABSTRACT FROM AUTHOR]

The enhancement of warming rates with elevation, so-called elevation-dependent warming (EDW), is one of the regional, still not completely understood, expressions of global warming. Sentinels of climate and environmental changes, mountains have experienced more rapid and intense warming trends in the recent decades, leading to serious impacts on mountain ecosystems and downstream. In this paper we use a state-of-the-art Global Climate Model (EC-Earth) to investigate the impact of model spatial resolution on the representation of this phenomenon and to highlight possible differences in EDW and its causes in different mountain regions of the Northern Hemisphere. To this end we use EC-Earth climate simulations at five different spatial resolutions, from ∼ 125 to ∼ 16 km, to explore the existence and the driving mechanisms of EDW in the Colorado Rocky Mountains, the Greater Alpine Region and the Tibetan Plateau-Himalayas. Our results show that the more frequent EDW drivers in all regions and seasons are the changes in albedo and in downward thermal radiation and this is reflected in both daytime and nighttime warming. In the Tibetan Plateau-Himalayas and in the Greater Alpine Region, an additional driver is the change in specific humidity. We also find that, while generally the model shows no clear resolution dependence in its ability to simulate the existence of EDW in the different regions, specific EDW characteristics such as its intensity and the relative role of different driving mechanisms may be different in simulations performed at different spatial resolutions. Moreover, we find that the role of internal climate variability can be significant in modulating the EDW signal, as suggested by the spread found in the multi-member ensemble of the EC-Earth experiments which we use. [ABSTRACT FROM AUTHOR]

In this paper we analyze the concentration higher-order moments of a dispersing plume in a neutral wind tunnel boundary layer. They are evaluated through a fluctuating plume model in which the plume barycenter is directly calculated from the measured mean concentration field. The reliability of the model is firstly verified by comparing the simulated and measured moments up to the fourth order. Then the relationships between higher and lower-order moments are investigated seeking for a proper scaling of the fourth, fifth and sixth-order normalized moments. We found that a general behavior appears when they are expressed as a function of the third-order moments. In this case each order calculated or measured at different distances collapses on the same curve. [ABSTRACT FROM AUTHOR]

A model for the evaluation of the concentration fluctuation variance is coupled with a one-particle Lagrangian stochastic model and results compared to a wind-tunnel simulation experiment. In this model the concentration variance evolves along the particle trajectories according to the same Langevin equation used for the simulation of the velocity field, and its dissipation is taken into account through a decay term with a finite time scale. Indeed, while the mean concentration is conserved, the concentration variance is not and our model takes into account its dissipation. A simple parametrization for the dissipation time scale is proposed and it is found that it depends linearly on time and on the ratio between the size and the height of the source through a scaling factor of 1 / 3. [ABSTRACT FROM AUTHOR]

Investigation of low-wind cases observed during the Urban Turbulent Project campaign (Torino, Italy) and at the Santa Maria meteorological station (Santa Maria, Brazil) provides insight into the wind-meandering phenomenon, i.e. large, non-turbulent oscillations of horizontal wind speed and temperature. Meandering and non-meandering cases are identified through analysis of the Eulerian autocorrelation functions of the horizontal wind-velocity components and temperature. When all three autocorrelation functions oscillate, meandering is present. As with weak turbulence, meandering shows no dependence on stability but is influenced by presence of buildings and depends on wind speed. We show that, while the standard deviation of the horizontal velocity is always large in low-wind conditions, the standard deviation of the vertical velocity shows very different behaviour in meandering and non-meandering conditions. In particular, the value of the ratio of the standard deviations of the vertical and horizontal velocities typifies the meandering condition. [ABSTRACT FROM AUTHOR]

• New method based on Eulerian auto-correlation functions to evaluate the coherent structures. • Estimation and parameterization of the impact of stability on the shear length scale. • Parametrization of the separation length scale between coherent vortices at the canopy top. Observations of the vertical structure of the turbulent flow in different stability regimes above and within the Amazon Forest at the Amazon Tall Tower Observatory (ATTO) site have been presented in Part I. Here, the influence of stability on the inception and development of coherent structures is investigated. According to the mixing-layer analogy the coherent vortices that dominate the turbulent flows at the canopy-atmosphere interface are generated by hydrodynamical instabilities triggered by an inflection in the vertical profile of the mean wind speed at or near the canopy top. The coherent motions time, T p e a k and separation length scale, Λ , depend on the shear length scale, L s , close to the canopy top. The present analysis studies the characteristics of L s , T p e a k and Λ in the five stability regimes defined in Part I. The behaviour of L s with stability is evaluated and parameterized. L s increases with decreasing stability, presenting two asymptotes for large unstable and stable stratification and a linear behaviour close to neutral stratification. Coherent structures and their timescale are detected with an original method based on the autocorrelation functions of 5-min subsets of turbulent quantities. The vertical time scale is larger in neutral conditions and decreases for both increasing and decreasing stability. At the canopy top the separation length scale presents a linear dependence on L s , whose slope is maximum in neutral conditions and decreases departing from neutrality. An original parameterization of the dependence of this slope on h / L , where h is the canopy height and L is the Obukhov length, is shown. Combining the parameterizations, the dependence of the separation length scale is finally presented. Λ has a maximum for slightly unstable conditions, linearly decreases towards neutral and weakly stable stratifications and then it tends to zero for very intense unstable and stable stratifications. [ABSTRACT FROM AUTHOR]

The joint concentration probability density function of two reactive chemical species is modelled using a bivariate Gamma distribution coupled with a three-dimensional fluctuating plume model able to simulate the diffusion and mixing of turbulent plumes. A wind-tunnel experiment (Brown and Bilger, J Fluid Mech 312:373-407, ), carried out in homogeneous unbounded turbulence, in which nitrogen oxide is released from a point source in an ozone doped background and the chemical reactions take place in non-equilibrium conditions, is considered as a test case. The model is based on a stochastic Langevin equation reproducing the barycentre position distribution through a proper low-pass filter for the turbulence length scales. While the meandering large-scale motion of the plume is directly simulated, the internal mixing relative to the centroid is reproduced using a bivariate Gamma density function. The effect of turbulence on the chemical reaction (segregation), which in this case has not yet attained equilibrium, is directly evaluated through the covariance of the tracer concentration fields. The computed mean concentrations and the O-NO concentration covariance are also compared with those obtained by the Alessandrini and Ferrero Lagrangian single particle model (Alessandrini and Ferrero, Physica A 388:1375-1387, ) that entails an ad hoc parametrization for the segregation coefficient. [ABSTRACT FROM AUTHOR]

Boundary-layer instabilities are studied by analyzing the results of laboratory simulations of wall turbulence in a shear-driven rotating flow. The experiments were carried out in the Turin University Laboratory rotating water tank, where a circular flow was generated by either increasing (spin-up) or decreasing (spin-down) the rotation speed of the platform. The flow was measured using a Particle Image Velocimetry technique and the developed turbulence analyzed. Two cases were accounted for, in the former the measurements were performed over a smooth surface (bottom of the tank), while in the latter a rough-to-smooth transition was considered. The turbulent boundary layer developed inside the tank is analyzed by means of vertical profiles of mean and turbulent quantities and on the basis of drag coefficients. Then turbulent structures developed in the different cases are shown and discussed in terms of the vorticity fields. Finally, an analysis based on the concept of swirling strength was carried out to select among the vortex extremes those associated with a coherent structure. [ABSTRACT FROM AUTHOR]

High‐frequency measurements obtained at two micrometeorological towers are investigated for a rare northward surging gust front that impacted the Amazon Tall Tower Observatory (ATTO), in central Amazon. The gust front originated from a decaying mesoscale convective system (MCS) during the morning hours of 27 December 2021 near Manaus, Amazonas state, northern Brazil, and surged north‐eastward towards the ATTO site. Large temperature drops and vigorous, persistent winds were observed at the towers which lasted for over 4 h despite the gust front being detached from its parent, decaying MCS. More importantly, the gust front was responsible for drastic increases of CO2 concentrations throughout the tower depths, which suggests that the gust front winds horizontally advected CO2‐rich air from a source upstream from the ATTO site. The CO2‐rich outflow is hypothesized to originate from downward transport and/or biomass burning from forest fires in southeastern Amazon, both ideas that are supported by large increases of aerosol concentrations measured at ATTO following the gust front passage. Our results stress the need for further investigations addressing the role played by mesoscale convective circulations in the redistribution of trace gases and aerosols in the Amazon. [ABSTRACT FROM AUTHOR]

The enhancement of warming rates with elevation, a phenomenon which is referred to as Elevation-Dependent Warming (EDW), is one of the regional, still not completely understood, manifestations of global warming. Sentinels of climate and environmental changes, mountain regions have overall experienced more rapid and intense warming trends compared to the globally-averaged temperature increase in the recent decades, leading to serious impacts both on high-altitude mountain ecosystems and downstream. In this work we analyse an ensemble of simulations from one state-of-the-art Global Climate Model (EC-Earth) run at five different spatial resolutions, from ∼125 to ∼16 km. The aim of the study is to investigate the impact of the model resolution on the representation of EDW, and to highlight possible differences in EDW characteristics and causes in three mountain regions of the Northern Hemisphere mid-latitudes - the Colorado Rocky Mountains, the Greater Alpine Region and the Tibetan Plateau–Himalayas.Our results show that the changes in albedo and in downward longwave radiation are the more frequent EDW drivers in all regions and seasons and that this is reflected in both daytime and nighttime warming. In the Tibetan Plateau-Himalayas and in the Greater Alpine Region, an additional driver is the change in specific humidity. We also find that, while generally the model ability in simulating the existence of EDW in the different regions does not (clearly) depend on the spatial resolution which is employed, specific EDW characteristics such as its intensity and the relative role of different driving mechanisms may be different in simulations performed at different spatial resolutions. Overall, our results seem to indicate that the model resolution plays a crucial role in small areas such as the Alps, where a too coarse resolution would lead to an underrepresentation of the highest altitudes. In fact, elevational dependence of warming, as well as of other mechanisms or variables, could not be easily identified if the range of altitudes is too limited.We also find that the role of internal climate variability in modulating the EDW signal can be significant, as suggested by the spread found in the multi-member ensemble of the EC-Earth experiments used. [ABSTRACT FROM AUTHOR]

This work investigates the characteristics of submeso motions and their influence on the dynamics of the atmospheric boundary layer in different stability conditions. Submeso motions generally refer to a complex mix of non-stationary stochastic processes of different nature on scales that separate the main turbulent eddies (few meters in stable conditions) and the smallest mesoscale motions (few kilometres). Submeso motions can take a variety of forms including gravity waves, density currents or drainage flows. During low-speed conditions submeso activities manifest themselves as horizontal meandering of the wind velocity vector. Meandering generally refers to large non-turbulent oscillations of the horizontal wind velocity components, associated to the oscillation of the wind direction and temperature. Although horizontal meandering is typically associated to stable stratifications of the boundary layer, wavelike oscillations associated to low turbulent activity have also been observed in diurnal conditions. Some insight in the understanding of the meandering dynamic has been achieved, but the causes of the meandering phenomenon and its complex interactions with turbulence are still poorly understood. While these modes may have a strong impact on the horizontal transport of pollutants in the presence of weak and intermittent turbulent mixing, their accurate parameterisation within weather and climate models remains an open question. In this work we study the role of radiative forcing on the generation of horizontal meandering in different stability conditions. To this purpose, measurements of wind velocity components, temperature, radiation, CO2 and water vapour concentrations carried out at two distinct sites (Santa Maria, Brazil; Ny-Ålesund, Svalbard, Arctic) were analysed. To detect meandering occurrences and their characteristic time‐scales Eulerian autocorrelation functions and spectral analysis of the wind components and temperature are employed. Horizontal meandering is identified by a clear negative lobe in the Eulerian autocorrelation functions corresponding to a peak located at low-frequency in the spectra of wind components and temperature. The performed analysis confirmed that horizontal modes represents a distinctive feature of the very stable boundary layer, characterised by a strong radiative cooling, low wind conditions and a weak and intermittent mechanical turbulent production. Horizontal meandering has been observed also during daytime in presence of low positive radiative forcing, often related to cloudy conditions and weak large scale flows. In all atmospheric conditions meandering appears strongly correlated to scalar oscillations, confirming its important role in the horizontal transport of pollutant in low-wind conditions. [ABSTRACT FROM AUTHOR]

One year of continuous measurements on 10 levels carried out at a coastal site in Espirito Santo State, south-eastern Brazil, and the FLOSSII dataset were analysed to characterize the wind meandering phenomenon in the Stable Boundary Layer. During low-wind episodes the oscillations of the horizontal components of the wind velocity associated to the oscillation of the wind temperature are easily recognizable by a clear negative lobe in their autocorrelation functions. We used this distinctive feature to identify meandering and non-meandering case. We show that the ratio of the variance of the wind velocity vertical component over the variance of the composite of the wind velocity horizontal components splits the frequency distribution of the turbulence statistics and divides the nocturnal boundary layer in two different regimes characterized by different turbulent properties. In particular, we showed how during meandering conditions the turbulent heat flux is negligible and the Boundary Layer decouples thereby enabling the onset of wavelike phenomena. [ABSTRACT FROM AUTHOR]

This study assesses the performance of historical simulations from CMIP6 models in representing precipitation patterns in southern Brazil between 1982 and 2014, comparing these simulations with ERA5 reanalysis data. The results demonstrate a generally good agreement in the amplitudes of seasonal precipitation between CMIP6 and ERA5, except for Winter, where climate models underestimated rainfall percentiles. During the Summer, the ACCESS-CM2 and BCC-ESM1 models exhibited the strongest correlations. In the Winter and Autumn seasons, the NorESM2-MM model demonstrated the best performance in terms of correlation and standard deviation. However, for the Spring season, despite some models exhibiting strong correlation, there were disparities in the standard deviation. [ABSTRACT FROM AUTHOR]

The ability of the relaxed eddy accumulation (REA) method to estimate the kinematic fluxes of temperature, water vapor and carbon dioxide was assessed for the dry season (3 months) at the ATTO (Amazon Tall Tower Observatory) site from turbulence measurements. The measurements were performed at 50 m above ground within the roughness sublayer. Non-conformity with inertial sublayer conditions was confirmed one more time by analyzing dimensionless scalar standard deviations. Over the scale of the whole dry season, REA and EC (eddy covariance) estimates are essentially equal. Recently found results that the REA method outperforms Monin–Obukhov-based approaches are confirmed. However, we also verify that such results fail to reveal significant variability and scatter of the REA estimates when the fluxes are of small magnitude. On the basis of previous studies, we conjecture that this is caused by a likely imbalance between scalar gradient production and molecular dissipation. Confirmation of our results to trace gases, therefore, requires further study. [ABSTRACT FROM AUTHOR]

The present study introduces a new boundary layer parameterization for weather and forecasting models. It is implemented here as a boundary layer module in Weather Research and Forecasting (WRF) model. The main novelty in the new scheme is that it includes prognostic equations for the heat flux and temperature variance, being the first WRF boundary layer scheme with that feature. This is specially aimed at improving the representation of nocturnal stable boundary layer and of its turbulence regimes, weakly and very stable. The effort is supported by previous studies that found that the two regimes and the transitions between them are better represented by simplified numerical schemes that represent the interactions between the surface and the air adjacent to it when the heat flux and temperature variance are solved prognostically. The results show that the two regimes are adequately simulated by the new scheme. Such an evaluation is presented in terms of the relationship between the turbulence velocity scale and mean wind speed, of the dependence of the potential temperature gradient near the surface and the mean wind speed, and by the relationship between flux and gradient Richardson numbers. In the new scheme, the relationship between thermal structure and the mean and turbulent flows arises naturally from the heat flux prognostic equation, not being arbitrarily imposed by an empirical stability function. [ABSTRACT FROM AUTHOR]

We identify two nocturnal turbulence regimes at different heights above the Amazon Forest and their variation in relation to the proximity of the forest canopy. We analyzed fast response data measured at the Uatumã Sustainable Development Reserve in central Amazonia, Brazil, during September 2021 to January 2022. Our results show that the threshold wind speed that separate the weak turbulence regime from strong turbulence regime increased as a function of the distance above the canopy (43 m, 50 m, 75 m, 100 m, 127 m, 151 m e 172 m). However, at 196 m, 223 m, 247 m, 274 m e 298 m height the change of the turbulence regime occurred at the same threshold wind speed. In contrast, at 316 m we evidenced only situations where the turbulence was weak. This pattern indicated that the structure and roughness of dense forest affect the turbulence structure. [ABSTRACT FROM AUTHOR]

The aim of this work is the evaluation of the influence of the rotation on the turbulence and on the entrainment within a buoyant plume. A series of laboratory experiments are performed in the rotating water tank of the Turlab in Torino (Italy) producing negatively buoyant plumes emitted from a finite circular source, with varying plume density and background rotations. Thanks to the particle image velocimetry technique, we obtain a detailed description of the velocity field within the plume. We present the results in term of mean velocity, turbulence and vorticity fields. The swirling strength analysis is also performed to identify the coherent structures. Finally, the velocity spectra are examined. We find that the rotation affects the dynamics of the plume increasing the turbulence and, in particular, the number of structures developed along the plume edges, which feeds the entrainment. The analysis of the mean velocity fields also allows us to quantify the turbulent entrainemnt of ambient fluid within the buoyant plume, and discuss the role of rotation in this process. [ABSTRACT FROM AUTHOR]

Mountain regions have been recognized to be more sensitive to climate and environmental changes, and in particular to global warming. Several studies report on elevation-dependent warming (EDW), i.e., when warming rates are different in different altitude ranges, particularly focusing on the enhancement of warming rates with elevation. The Andean chain proved to be a relevant climate change hot-spot with positive temperature trends and a widespread glacier retreat over the recent decades. To assess and to better understand elevation dependent warming in this mountain region and to identify its possible dependence on latitude, the Andean Cordillera was split into five domains, three pertaining to the tropical zone and two pertaining to the Subtropics. Further, for each area the eastern and western faces of the mountain range were separately analyzed. An ensemble of regional climate model (RCM) simulations participating in the Coordinated Regional Climate Downscaling Experiment (CORDEX), consisting of one RCM nested into eight different global climate models from the CMIP5 ensemble was considered in this study. EDW was assessed by calculating the temperature difference between the end of the century (2071–2100) and the period 1976–2005 and relating it to the elevation. Future projections refer to the RCP 8.5 high-emission scenario. Possible differences in EDW mechanisms were identified using correlation analyses between temperature changes and all the variables identified as possible EDW drivers. For the maximum temperatures, a positive EDW signal (i.e. enhancement of warming rates with elevation) was identified in each side of both the tropical and subtropical Andes and in all seasons. For the minimum temperatures, on the contrary, while a positive EDW was identified in the Subtropics (particularly evident in the western side of the chain), the Tropics are characterized by a negative EDW throughout the year. Therefore, the tropical boundary marks a transition between discordant EDW behaviours in the minimum temperature. In the Tropics and particularly in the inner Tropics, different EDW drivers were identified for the minimum temperature, whose changes are mostly associated with changes in downward longwave radiation, and for the maximum temperature, whose changes are mainly driven by changes in downward shortwave radiation. This might explain the opposite EDW signal found in the tropical Andes during daytime and nighttime. Changes in albedo are an ubiquitous driver for positive EDW in the Subtropics, for both the minimum and the maximum temperature. Changes in longwave radiation and humidity are also EDW drivers in the Subtropics but with different relevance throughout the seasons and during daytime and nighttime. Also, the western and eastern sides of the Cordillera might be influenced by different EDW drivers. [ABSTRACT FROM AUTHOR]

Five planetary-boundary-layer parametrizations of the Weather Research and Forecasting model are compared with respect to their ability to simulate the very stable and the weakly stable regimes of the stable boundary layer. This is performed for single column models where the large-scale mechanical forcing is represented by geostrophic wind speeds ranging from 0.5 to 12 m s - 1 . The performance of the models is assessed by contrasting the relationships they produce between the turbulence velocity scale and the mean wind speed, between potential temperature gradient and the mean wind speed, and between the flux and gradient Richardson numbers. The level-2.5 Mellor–Yamada–Nakanishi–Niino parametrization simulates the very stable regime the best, mainly because its heat eddy diffusivity decreases with respect to the momentum eddy diffusivity as the stability increases, while the same is not true for the other parametrizations considered. [ABSTRACT FROM AUTHOR]

A sonic anemometer is employed to investigate turbulent statistical characteristics and meteorological properties associated with a local windstorm known as the Vento Norte (Portuguese for north wind), which is observed in central Rio Grande do Sul state situated in southern Brazil. The Vento Norte consists of persistent and warm northerly gusts of moderate to strong magnitude that occur just downstream from a modest zonally-oriented downslope. This flow, which generates a shear-dominated atmospheric boundary layer, is investigated through analysis of turbulent properties of the velocity components and temperature measured over two years from a meteorological tower. The overall micrometeorological analysis highlights the influence of the Vento Norte phenomenon on the daily cycle of turbulent energy and fluxes and on the surface energy budget. A more detailed investigation of Vento Norte characteristics is performed by analyzing an extended Vento Norte event that lasted for approximately 35 h. In this episode, a strong increase in turbulence activity is observed on the vertical velocity component and in the absolute turbulent heat flux, as well as a reduction in the relative humidity. Further, the cospectra of the heat-flux wavelet suggest that the Vento Norte flow perturbs the boundary-layer turbulence in both daytime and night-time conditions. The surface energy budget shows a positive night-time soil heat flux and an enhanced positive latent heat flux, induced by the low relative humidity and intense turbulent activity during the Vento Norte event. The onset and demise of the Vento Norte event are characterized by sub-mesoscale activity in the velocity components and temperature field. [ABSTRACT FROM AUTHOR]

Two contrasting flow regimes exist in the stable boundary layer (SBL), as evidenced from both observational and modeling studies. In general, numerical schemes such as those used in numerical weather prediction and climate models (NWPCs) reproduce a transition between SBL regimes. However, the characteristics of such a transition depend on the turbulence parameterizations and stability functions used to represent the eddy diffusivity in the models. The main goal of the present study is to detail how the two SBL regimes occur in single-column models (SCMs) by analyzing the SBL structure and its dependence on external parameters. Two different turbulence closure orders (first order and an E-l model) and two types of stability functions (short and long tail) are considered. The control exerted by the geostrophic wind and the surface cooling rate on the model SBL regimes is addressed. The model flow presents a three-layer structure: a fully turbulent, weakly stable layer (WSL) next to the surface; a very stable layer (VSL) above that; and a laminar layer above the other two and toward the domain top. It is shown that theWSLand VSL are related to both SBL regimes, respectively. Furthermore, the numerically simulated SBL presents the two-layer structure regardless of the turbulence parameterization order and stability function used. The models also reproduce other features reported in recent observational studies: an S-shaped dependence of the thermal gradient on the mean wind speed and an independence of the vertical gradient of friction velocity du* on the mean wind speed. [ABSTRACT FROM AUTHOR]

In this paper, a review of the Lagrangian stochastic models developed in the last decades for the simulation of the concentration–fluctuation dispersion is presented. The main approaches available in the literature are described and their ability in reproducing the higher order moments of the probability density function is discussed. Then, the Lagrangian approaches for evaluating of the odor annoyance are presented. It is worth to notice that, while Lagrangian stochastic models for mean concentrations are well-known and their ability in correctly reproducing the observation is well assessed, concerning concentration fluctuations the approaches are often new and unknown for most of the scientific community. [ABSTRACT FROM AUTHOR]

The presence of wave‐like structures in the planetary boundary layer and their influence on the scalar fluxes and on the surface energy balance were investigated analyzing one year of continuous measurements collected in southern Brazil. Submeso oscillating patterns in the wind velocity components, temperature and scalar (CO2, H2O) concentrations were isolated using their auto‐correlation functions. The analysis showed that low wind speeds are necessary to trigger wavy motions. During night‐time, in the presence of large vertical temperature gradients, horizontal meandering and internal gravity waves are dominant features of the stable boundary layer. Furthermore, a significant number of meandering cases were identified also during daytime in neutral conditions associated with low values of net radiation. One case‐study showed how, during daytime, the wave‐like patterns may be triggered by variations in the net radiation. Spectral analysis on the whole dataset showed that oscillations in the wind velocity and temperature field are frequently associated with CO2 and H2O wavy patterns with similar time‐scales. These non‐turbulent oscillations produce unpredictable large‐scale contributions to vertical fluxes of temperature and scalar concentrations. The energy budget analysis showed how the choice of a proper averaging time filters out these contributions and improves the energy budget closure, as well as the estimation of the net ecosystem exchange. The results confirm the influence of submeso motions in scalar dispersion, flux patterns and surface energy balance during low wind speed conditions and stable stratification. [ABSTRACT FROM AUTHOR]

Observations of the vertical and temporal structure of the nocturnal boundary layer before and after a transition from the weakly to the very stable regime have been presented in Part I. Here, similar transitions are investigated using a one‐dimensional second‐order closure numerical model, with an energy budget solved at the surface. The transition is driven by a decreasing mean wind at the top of the domain, and simulations with different cloud covers and surface thermal properties are considered. The time of the transition depends on the wind speed at the top of the domain and on the "coupling strength" between the surface and the atmosphere, which is affected by the cloud cover and surface thermal properties. The vertical profiles and temporal evolutions of the terms of the budgets of turbulent kinetic energy (TKE), heat flux and temperature variance are presented. Of these, only TKE budget presents the same dominant terms in both regimes. Absolute heat flux in the model is proportional to the cube of the wind speed in the very stable regime. [ABSTRACT FROM AUTHOR]

The nocturnal boundary‐layer regime transition from weakly stable (strong wind) to very stable (weak wind) is analyzed using 10 levels of turbulence observations made at a 140 m micrometeorological mast near the southeastern Brazilian coast. The combination of synoptic and local flow favors the systematic occurrence of such a transition, typically 5 to 7 h after sunset. The regime transition is marked by decreases in temperature, wind speed, turbulent kinetic energy (TKE) and absolute heat flux. The decrease in temperature is often abrupt and the inflection point in the temperature series marks the regime transition. Absolute heat flux peaks before the transition during the weakly stable period, while temperature variance peaks near the transition. Composites from 36 cases when the cooling rate exceeded 2 °C/h are used to describe the vertical structure of the stable boundary layer (SBL) in both regimes. For these abrupt transitions, dimensionless variables that relate thermal and mechanical properties of the flow are compared as indicators of the SBL regime, and the gradient Richardson number is found to be better for that purpose. The absolute heat flux is shown to be proportional to the cube of the wind speed only in the strong wind limit of the very stable regime. Simulations of similar transitions using a second‐order model are described in Part II. [ABSTRACT FROM AUTHOR]

A 140-m micrometeorological tower has been operating since August 2016 at 4 km from the coastline and 250 m from a thermal power plant that releases heat from its 20-m stacks in southeastern Brazil. The measurements include 11 levels of turbulence observations and 10 levels of slow-response temperature and humidity. The observed atmospheric structure is largely dependent on the wind direction with respect to the power plant. When winds blow from the plant to the tower, the air layer between 20 and 60 m of the atmosphere may be warmed by as much as 2°C. In these circumstances there are events when the emissions pass directly by the tower. They allow the analysis of turbulence structures of thermal plumes generated from the plant's heat release in comparison with those generated by the surface heating. In the more common case of winds blowing from the tower to the plant, the observations allow a detailed description of the local atmospheric boundary layer. During the day, vertical profiles of turbulent quantities and their spectral distributions show a cycle controlled by interactions between the land and oceanic surfaces, such as a thermal internal boundary layer. At night, there is a systematic tendency of progressive stabilization throughout the period, suitable for the analysis of the boundary layer transition from weakly to very stable conditions. The data also grant the inference of detailed vertical profiles of turbulent diffusion coefficients directly from observations. [ABSTRACT FROM AUTHOR]

In a diurnal cycle, distinct thermal and mechanical forcing generates different manifestations of a planetary boundary layer. The stable boundary layer occurs when the soil surface has a lower temperature than the air above. In this layer, wind shear is the main mechanism of turbulence generation. In the present study, a direct numerical simulation of an Ekman layer over a smooth wall is presented to investigate the different turbulent patterns that occur during evolution from a neutral boundary layer to a weakly stable boundary layer. The preliminary study shows the appearance of turbulent structures near the surface, due to the imposition of a stratification. [ABSTRACT FROM AUTHOR]

In a diurnal cycle, distinct thermal and mechanical forcing generates different manifestations of a planetary boundary layer. The stable boundary layer occurs when the soil surface has a lower temperature than the air above. In this layer, wind shear is the main mechanism of turbulence generation. In the present study, a direct numerical simulation of an Ekman layer over a smooth wall is presented to investigate the different turbulent patterns that occur during evolution from a neutral boundary layer to a weakly stable boundary layer. The preliminary study shows the appearance of turbulent structures near the surface, due to the imposition of a stratification. [ABSTRACT FROM AUTHOR]

In this study we analyze natural complex signals employing the Hilbert–Huang spectral analysis. Specifically, low wind meandering meteorological data are decomposed into turbulent and non turbulent components. These non turbulent movements, responsible for the absence of a preferential direction of the horizontal wind, provoke negative lobes in the meandering autocorrelation functions. The meandering characteristic time scales (meandering periods) are determined from the spectral peak provided by the Hilbert–Huang marginal spectrum. The magnitudes of the temperature and horizontal wind meandering period obtained agree with the results found from the best fit of the heuristic meandering autocorrelation functions. Therefore, the new method represents a new procedure to evaluate meandering periods that does not employ mathematical expressions to represent observed meandering autocorrelation functions. [ABSTRACT FROM AUTHOR]

The looping parameter m is the main value to characterize the meandering phenomenon. This parameter is relationship with negative lob in the observed autocorrelation function generated from components of horizontal speed. In this work, we present a study comparing the mean values of the looping parameter between 2 diverse sites in the Brazilian sector. [ABSTRACT FROM AUTHOR]

In this work it's derived a mathematical model to describe the meandering contaminants dispersion in the planetary boundary layer. The model employs a heuristic functional form for the velocity autocorrelation function which describes the negatives lobes observed in meandering data. Furthermore, the model is derived from the linearization of the Langevin equation. The mathematical equations, that represent the longitudinal and lateral wind components describe the bi-dimensional pattern of the meandering flow and can be utilized to calculate the contaminant concentrations occurring in situations in which the meandering phenomenon plays a decisive role. [ABSTRACT FROM AUTHOR]

The influence of the topography at the ATTO site (Amazon Tall Tower Observatory) in the formation of Gravity Waves (GW) was investigated in this work. The role of these waves in gas transport on different heights in the nocturnal boundary layer, was also investigated. For this, were analyzed: experimental data of wind speed, temperature, humidity and CO 2 to identify the GW. Furthermore satellite images, ECMWF ERA5 (European Center for Medium-Range Weather Forecasts) reanalysis, and numerical simulations from the mesoscale JULES-CCATT-BRAMS model (Brazilian Regional Atmospheric Modeling System) version 5.3 were used to perform GW simulations. The simulations involved two configurations (Control-With Topography and No Topography). The sattelite images made it clear that the GW was not associated with convective activities. The simulations showed clear ondulatory signs in air temperature, vertical component of the wind, and gases like CO and O 3. Moreover, they also reveal that the wave originated by the presence of the local topography and this orograph gravity wave had and important role in the transport of gases above the ATTO site. • The Amazonian topography plays an important role in the generation of gravity waves. • The thermodynamics and chemistry of the Amazonian atmosphere are strongly influenced by the topography. • Orographic gravity waves are important for the transport of scalars in the Amazon region. [ABSTRACT FROM AUTHOR]

• Lowering of the roughness sublayer depth with the atmospheric stratification increases. • Turbulent flow decoupling above and within the canopy due to the LAD of the forest. • With increasing stability submeso structures propagates downwards. • In super stable conditions submeso structures are present in the whole roughness sublayer. • In super stable conditions submeso motions can modulate the forest exchange. Understanding the processes that govern the mixing and transport of scalars within and above the Amazon Forest is of great importance for many environmental applications. The impact of atmospheric stability on the roughness sublayer (RSL) as well as the influence on it by the processes in the overlying atmosphere are investigated using measurements collected at the Atmospheric Tall Tower Observatory. Five different stabilities are defined according to the turbulent fluxes' behaviour. Ejections dominate the transport in the RSL. In near neutral and unstable conditions coherent structures propagate up to 2–3 times the canopy height (h) and intermittently penetrate in the lowest part of the forest where sweeps drive the transport processes. In the unstable regime a weakening of the wind inflection at the canopy top and a transition to a convective regime above z = 2 h are observed. In stable conditions three regimes were defined characterised by a progressive lowering of the RSL and the weakening of the mixing-layer type coherent structures. In the 'weakly stable' regime the intense momentum and scalar fluxes appear driven by the coherent structures being able to penetrate inside the canopy intermittently coupling the flow above and within the forest. The 'very stable' regime is characterized by weak winds, a weakening of coherent structures and a decrease of the turbulent fluxes inhibited by buoyancy. The definition of a 'super stable' regime allowed the identification of a peculiar condition characterized by low-wind and weak coherent structures confined close to the canopy top and producing negligible transport. Submeso motions dominate the flow dynamics in this regime both above and inside the RSL. Multiresolution analysis highlights the ability of submeso motions to propagate inside the canopy and to modulate the exchange, particularly of scalars, fully driving the large positive CO 2 flux observed inside the forest in the super stable regime. [ABSTRACT FROM AUTHOR]

A statistical characterization for two-point temperature fluctuations in the planetary boundary layer (PBL) is analyzed and its implications on the long-standing closure problem discussed. Despite the non-triviality of the dynamics of temperature fluctuations, our analysis supports the idea that the most relevant statistical properties can be captured solely in terms of two scaling exponents. They turned out to be weakly dependent on the stability properties of the PBL. Its statistics have been investigated by collecting data from a field experiment carried out in the urban area of Turin (Italy) from January 2007 to March 2008. Our results confirm those from a large-eddy simulation (LES) analysis carried out for the convective PBL with different level of convection. We extend the scenario to the stable PBL, a regime much more difficult to simulate when exploiting LES. [ABSTRACT FROM AUTHOR]

The speed of sound in moist air is discussed and a more accurate value for the coefficient of the linear dependence of sonic temperature on specific humidity is proposed. An analysis of speed-of-sound data measured by three sonic anemometers in a climate chamber and in the field shows that the temperature response of each instrument significantly influences not only the determination of sonic temperature, but also its fluctuations. The corresponding relative contribution to the error in the evaluation of the temperature fluctuations and the turbulent heat fluxes can be as high as 40%. The calibration procedure is discussed and a method of correction is proposed. [ABSTRACT FROM AUTHOR]

Abstract: A diffusion equation limit derived from the Langevin stochastic particle model is used to study the dispersion of scalars caused by an evolving turbulence in the transition process occurring during the sunset period. Therefore, the random displacement equation is employed to simulate the cross-wind concentrations of pollutants released from low and high point sources. Turbulence inputs are parameterized to describe in a continuous manner the dispersion effects produced by a decaying convective elevated and a shear-dominated stable surface turbulence. The simulation results show that for the initial stage of the sunset transition phenomenon, the pollutants are transported rapidly to the surface. On the other hand, for the sunset evolution advanced stages, the dispersion process happens in a deep stable boundary layer (SBL), in which the pollutants can travel long distances practically without reaching the surface. The major progress shown in this analysis is the description of the transport properties associated to decaying convective eddies in the residual layer (RL). The study shows that the diffusion effects associated to these decaying convective eddies strongly influence the dispersion of scalars during the sunset transition period. [Copyright &y& Elsevier]

The parallelized large-eddy simulation model (PALM) was used to understand better the turbulent exchanges of a passive scalar above and within a forested region located in the central Amazon. Weak (2 ms − 1 ) and strong (6 ms − 1 ) wind conditions were simulated. A passive scalar source was introduced to the forest floor for both simulations. The simulations reproduced the main characteristics of the turbulent flow and of the passive scalar transport between the forest and the atmosphere. Noteworthily, strong and weak wind conditions presented different turbulence structures that drove different patterns of scalar exchange both within and above the forest. These results show how passive scalar concentration is influenced by the wind speed at the canopy top. Additionally, higher wind speeds are related to stronger sweep and ejection regimes, generating more intense plumes that are able to reduce the passive scalar concentration inside the forest canopy. This work was the first that used PALM to investigate scalar transport between the Amazon rainforest and the atmosphere. [ABSTRACT FROM AUTHOR]

• Interplay of different flow regimes influences the Amazonian nocturnal boundary layer. • Strong stability in the Amazonia NBL allows the interaction of a gravity wave and LLJ. • During the inception of the gravity wave the turbulent fluxes are very small. • Confining the scalar below the LLJ nose at the forest-atmosphere interface. We investigated the role of turbulent coherent structures (CS), gravity waves (GW) and low-level jet (LLJ) propagation in the flow dynamics of the Nocturnal Boundary Layer (NBL) within and above a forest canopy at the Amazon Tall Tower Observatory (ATTO), in Central Amazon. Seven levels of wind velocity and temperature measurements allowed the study of the flow structure below and above the surface layer. We analyzed one dynamically rich night in 2015, which includes three distinct periods. In the first one, the NBL is characterized by CS generated at the canopy top. In the second period, the change in wind direction triggers the onset of a orographic GW above the roughness sublayer. The wave, suppressing the propagation of CS, strongly influences the boundary layer structure, both above and below the canopy. In the third period, low turbulence intensity at the canopy top enables the development of a LLJ. As the jet shear layer propagates upward, it disrupts the wave oscillations, while LLJ dominates the flow dynamics. The wavelet analyses identified i) turbulent and non-turbulent structures with different length and time-scales; ii) coupling of the flow at different levels and the vertical propagation of turbulent and wave motions; and iii) the ability of turbulent and low frequency processes associated with the orographic GW to penetrate within the canopy. Further, scalar measurements of methane, carbon monoxide and carbon dioxide identified the LLJ nose as upward limit for how far scalars can be transported. [ABSTRACT FROM AUTHOR]

The research purpose of this work is guiding the spatial morphological design of blocks via relevant indicators to realize suitable wind environments. In doing so, it is necessary to find the most suitable indicator types and value ranges for each urban spatial morphology. At present, most of the relevant research has been based on the numerical simulation of ideal block shapes and rarely proposes results based on actual block types, which often tend to be complex environments. Therefore, this paper firstly presents a theoretical speculation on the main factors influencing indicator effectiveness via analyzing physical significance and formulating principles for each indicator. These speculations are verified via wind environment measurement and statistical analysis, indicating that porosity (P0) can be used as an important indicator to guide the design of block wind environments in the case of deep street canyons, while frontal area density (λF) can be used as a supplement in shallow street canyons with no height differences. Finally, computational fluid dynamics (CFD) is used to quantify the impact of block height difference and street canyon depth on λF and P0, thereby finding suitable types of urban form and value ranges for λF and P0. This paper provides a feasible wind environment index system for urban designers. [ABSTRACT FROM AUTHOR]