Approximately 57% of the global population lives in urban areas, and this proportion is expected to increase in the coming decades. Consequently, cities are where a large and growing share of the population experiences the impacts of climate change. Urban environments can also intensify local warming because of their morphology and physical characteristics, including building density, anthropogenic heat emissions, impervious surfaces, and the thermal inertia of urban materials. Urban green spaces, such as parks, can partly offset these effects through shading, improving human thermal comfort. However, the thermal effect of urban parks is commonly assessed using satellite-derived surface temperature, which provides only instantaneous observations and does not capture its temporal evolution.In this study, we propose an alternative method to evaluate the thermal effect of urban parks using large-eddy simulations (LES) with the PALM model system. The analysis focuses on Villa Ada, a 160 ha urban park in Rome, during a heatwave event in October 2023. Two nested simulation domains, with isotropic grid resolutions of 20 m and 10 m, are forced by boundary conditions from the MOLOCH mesoscale model. The simulations are used to investigate the diurnal variability of surface temperature and 2 m air temperature. The park thermal effect is quantified using a buffer-based approach with concentric rings extending up to 600 m from the park boundary, enabling the analysis of horizontal temperature gradients and their temporal evolution throughout the day. The framework also enables comparison among three land-cover scenarios: the current park configuration (baseline), a fully tree-covered configuration, and a short-grass configuration.Results reveal a marked diurnal cycle in the park thermal effect, with lower magnitudes in the morning, a distinct maximum in the early afternoon, and a gradual decrease toward the evening. The dense-tree scenario shows the strongest cooling effect, with values reaching up to 8.5 K for surface temperature and 6.1 K for 2 m air temperature, whereas the baseline scenario also produces cooling, though of slightly lower intensity. By contrast, the short-grass scenario leads to higher surface temperatures across all buffers, indicating a warming effect.The spatial extent of the cooling signal remains relatively stable in the baseline and dense-tree scenarios, with values typically between 330 m and 380 m. In contrast, the short-grass scenario does not show a comparable cooling signal. The results also reveal a bimodal distribution of surface temperature in the park surroundings, reflecting the contrast between vegetated and built surfaces. These findings highlight that the park cooling effect is a dynamic process, strongly controlled by diurnal evolution and surface heterogeneity, and cannot be fully assessed by instantaneous satellite observations.

This study presents the first application of a coupled PALM and MOLOCH modeling system over the city of Bologna, Italy, aimed at advancing the understanding of urban-scale meteorology through high-resolution numerical simulations. Focusing on a specific heatwave event in August 2023, which serves as a proxy for increasingly frequent extreme conditions in the Mediterranean basin, we investigate the intricacies of atmospheric dynamics at the micro-scale. A novel software, MOLOCH4PALM, is introduced for preparing initial and time-dependent boundary conditions for PALM by ingesting 3D meteorological fields from the mesoscale MOLOCH model in an offline nesting configuration.This integration significantly improves the representation of urban atmospheric dynamics, enabling more realistic simulations of temperature patterns and energy exchanges across scales. Validation against observational data collected during the heatwave confirms the reliability of MOLOCH in capturing large-scale temporal variability, despite a consistent cold bias. In contrast, the coarse PALM configuration (50 m resolution) showed poor performance, with low correlation and a pronounced warm bias. Increasing PALM’s resolution to 10 m and 2 m corrected these biases, with the 10 m configuration offering the best balance between accuracy and computational efficiency. Additionally, wind speed simulations demonstrated the robustness of the coupled system, particularly under calm wind conditions, highlighting its suitability for urban microclimate studies and heat mitigation planning. Further analysis using multiple linear regression quantified how urban morphology, represented by the Sky View Factor (SVF), and wind speed independently influence 2 m air temperatures across various surface materials. The results revealed distinct diurnal patterns. Higher SVF values increased daytime temperatures due to greater solar exposure but accelerated nighttime cooling through enhanced longwave radiation loss. Conversely, while higher wind speeds promoted cooling during peak insolation, they were linked to warmer nighttime temperatures due to atmospheric mixing. These findings underscore the critical role of urban design in modulating thermal comfort.

Urban heat islands intensify heat stress and degrade air quality in densely built areas, yet the physical processes governing near-surface thermal variability remain poorly quantified. This study applies the coupled MOLOCH and PALM model system 6.0 (PALM-4U) over Bologna (Italy) during a summer 2023 heatwave to resolve meter-scale atmospheric dynamics within the Urban Canopy Layer and Roughness Sublayer at 2 m horizontal resolution. The coupled configuration was validated against in situ meteorological observations and Landsat-8 LST data, showing improved agreement in air temperature and wind speed compared to standalone mesoscale simulations. Results reveal pronounced diurnal and vertical variability of wind speed, turbulent kinetic energy, and friction velocity, with maxima between two/three times the median building height (hc). Distinct surface-dependent contrasts emerge: asphalt and roofs act as strong daytime heat sources (Bowen ratio beta asphalt approximate to 4.8) and nocturnal heat reservoirs at pedestrian level (z approximate to 0.07 hc), while vegetation sustains daytime latent heat fluxes (beta vegetation approximate to 0.6 divided by 0.8) and cooler surface and near-surface air (Temperature anomaly of surface Delta Ts approximate to -9 degrees C and air Delta Tair approximate to -0.3 degrees C). Thermal anomalies decay with height, vanishing above z approximate to 2.5 hc due to turbulent mixing. These findings provide insight into fine-scale energy exchanges driving intra-urban thermal heterogeneity and support climate-resilient urban design.

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.

Modification to the law of the wall represented by a dimensionless correction function ϕRSL(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 Eww(k) (k is the longitudinal wavenumber) and ϕRSL is then established using a co-spectral budget (CSB) model interpreted by the moving-equilibrium hypothesis. The key finding is that ϕRSL is determined by the ratio of two turbulent viscosities and is given as νt,BL/νt,RSL, where νt,RSL=(1/A)∫0∞τ(k)Eww(k)dk, νt,BL=kv(z−d)u*, τ(k) is a scale-dependent decorrelation time scale between velocity components, A=CR/(1−CI)=4.5 is predicted from the Rotta constant CR=1.8, and the isotropization of production constant CI=3/5 given by rapid distortion theory, kv 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 Eww(k) and is determined by the turbulent kinetic energy dissipation rate (ε), the CSB model also predicts that ϕRSL scales with LBL/Ld, where LBL is the length scale of attached eddies to z=d, and Ld=u*3/ε is a macro-scale dissipation length.

The influence of coherent vortices on the turbulence structure above and below a dense plant canopy is investigated using turbulence measurements collected at five levels on top and inside a coniferous forest on an Alpine Plateau. Five different stability regimes, from free convection to very stable stratification, were identified and considered in the analysis. Coherent structures are detected by fitting the vertical velocity auto-correlation functions with a theoretical oscillating function. This allowed to evaluate the coherent vortices characteristic timescales and to discriminate between turbulent data subsets characterized by fine-scale turbulence and subsets in which turbulence is dominated by coherent structures generated at the canopy top. An original methodology to fit cross-correlation function and to single out the most energetic frequency in presence of periodic behaviour is presented and applied to the turbulent momentum flux. The analysis shows how not far from neutral conditions the dominant time scale of the momentum flux is mainly determined by the longitudinal wind velocity component, rather than vertical one, while in free convection and very stable conditions coherent vortices do not seem to influence the momentum transport, dominated by large-scale structures. This is confirmed by the fine-scale turbulence and coherent structures efficiency in turbulence transport throughout the stability regimes.

<p>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, L<sub>s</sub>. φ measures the ratio of the eddy viscosity from attached eddies to a zero-plane displacement (<em>d</em>) and the actual eddy viscosity in the RSL at <em>(z-d)</em>, for <em>(z-d)/L<sub>s</sub>>>1</em>, 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 <em>z</em>-dependent mean pressure gradients that then lead to variability in second-order flow velocity statistics with <em>z.</em>  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, L<sub>d</sub>=u<sub>*</sub><sup>3</sup>/ε.The friction velocity, u<sub>*</sub>, 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, L<sub>d</sub> 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.</p>

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 are presented. The shear length scale at the canopy top together with the coherent turbulent structures time and separation length scale were evaluated to determine influence of stability on the inception and development of the roughness sublayer. Five stability regimes were identified. The definition of an intense table 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 roughness sublayer. The shear length scale increases with decreasing stability, presenting two asymptotes for large unstable and stable stratification and a linear behaviour close to neutral stratification. The coherent structure time and length scales are detected using 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, while the separation length scale at the canopy top presents a linear dependence on the shear length scale, whose slope is maximum in neutral conditions and decreases departing from neutrality. A new parameterization describing the dependence of the coherent eddies’ separation length scale on the h/L stability parameter is presented.

This study provides a detailed analysis of the influence of atmospheric stratification on the flow dynamics above and within a dense forest for a 19-days campaign at the Amazon Tall Tower Observatory (ATTO) site. Observations taken at seven levels within and above the forest along an 81-meter and a 325-meter towers allow a unique investigation of the vertical evolution of the turbulent field in the roughness sublayer and in the surface layer above it. Five different stability classes were defined on the basis of the behavior of turbulent heat, momentum and CO2 fluxes and variance ratio as a function of h/L stability parameter (where h is the canopy height and L is the Obukhov length). The novelty is the identification of a ‘super-stable’ (SS) regime (h/L>3) characterized by extremely low wind speeds, the almost completely suppression of turbulence and a clear dominance of submeso motions both above and within the forest. The obtained data classification was used to study the influence of atmospheric stratification on the vertical profiles of turbulent statistics. The spectral characteristics of coherent structures and of submeso motions (that may influence the energy and mass exchange above the Amazon forest) have been analyzed by wavelet analyses. The role of the main structures in momentum, heat and CO2 transport at the different levels inside and above the forest and in different diabatic conditions was thoroughly investigated through multiresolution and quadrant analyses. In unstable and neutral stability, the flow above the canopy appears modulated by ejections, whereas downward and intermittent sweeps dominate the transport inside the canopy. In the roughness sublayer (z £ 2h) the coherent structures dominating the transport within and above the canopy have a characteristic temporal scale of about 100 sec, whereas above this layer the transport is mainly driven by larger scale convection (temporal scale of about 15 min). In stable conditions the height of roughness sublayer progressively decreases with increasing stability reaching the minimum value (z<1.35h) in the SS regime. Above the canopy the flow is clearly dominated by ejections but characterized by a higher intermittency mainly in SS conditions. On the other hand, the rapid shear stress absorption in the highest part of the vegetation produces a less clear dominance of sweeps and a less defined role of odd and even quadrants inside the canopy in the transport of momentum, heat and CO2. In the weakly stable regime (0.152 transport.

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.

The Wavelet and the Multiresolution analysis are applied to ten nocturnal hours of observations of 3-D wind velocity taken within and above a forest canopy in Central Amazonia. Data from the ATTO Project, consisting in 7 levels of turbulence observations along both 81 and 325-meter towers, are used. The presented night is dynamically rich presenting three distinct periods. In the first one the boundary layer is characterized by canopy waves and coherent structures generated at the canopy top. In the second period an intense orographic gravity wave generated at around 150 m strongly influences the boundary layer structure, both above and below the canopy. In the third period, a very stable stratification at the canopy top enables the development of a low-level jet that interferes and disrupts the vertical orographic wave. During the night the wavelet cospectra identified turbulent and non-turbulent structures with different length and time-scales that are generated at different levels above the canopy and propagated inside it. The contributions of the different temporal scales of the flow above and within the canopy were identified using Wavelet and Multiresolution two-point cospectra. The analysis showed how turbulent and wave-like structures propagates in different ways and, further, the ability of low-frequency processes to penetrate within the canopy and to influence the transport of energy and scalar in the roughness sublayer and within canopy. Keywords: Coherent structures, Canopy Waves, Gravity Waves, Stable Boundary Layer, Low-Level Jet, wave-turbulence interaction.

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.

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.

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.

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.

Oscillations in the horizontal components of the wind velocity associated with oscillations in air temperature during low–wind speed episodes are ubiquitous in the stable boundary layer and are labeled as wind meandering. The meandering structure is recognizable by a clear negative lobe in the Eulerian autocorrelation functions of the horizontal wind velocity components and of the sonic temperature and by a corresponding peak at low frequency in the velocity components and temperature spectra. These distinctive features are used to isolate meandering occurrences and to study its properties in relation to the classical description of the planetary stable boundary layer. It is shown 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 meandering and nonmeandering events and divides the nocturnal boundary layer in two different regimes characterized by different turbulent properties. The data comparison with a turbulence model based on Rotta return to isotropy showed that meandering and nonmeandering cases may have similar dynamics. This suggests that meandering may not be connected to a laminarization of the flow and shows that the Rotta scheme may still describe the energetic transfer between wind velocity components in the very stable boundary layer if the Rotta similarity constant c depends on the flux Richardson number. The data confirm a c value of 2.2 for Rif = 0 compatible with its conventional value. The analysis presented refers to one year of continuous measurements on 10 levels carried out at a coastal site in southeastern Brazil.