Land–atmosphere exchange is mediated by turbulent fluxes that can be quantified with eddy-covariance (EC) measurements. EC has been widely used to measure ecosystem-scale vertical exchange between atmosphere and vegetation, and to test and refine atmospheric turbulence theories, thereby improving the representation of turbulent fluxes in land surface models. Traditionally, research has focused on idealized, homogeneous and flat surfaces, but recent work increasingly targets turbulent exchange in complex, heterogeneous environments under non-ideal conditions, where challenges include advective fluxes, mesoscale circulations between contrasting surface types, and non-stationary nighttime turbulence. Here, we introduce the open-source R package Reddy, which integrates multiple EC analysis methods into a single modular framework. Reddy enables users to tailor post-processing choices to site-specific conditions by integrating turbulence diagnostics and flux calculations in an iterative re-evaluation spiral, where processing choices are adapted step-by-step. The package supports station management and facilitates detailed scientific analyses, and is accompanied by an extensive documentation and hands-on EC processing tutorials. We demonstrate Reddy using measurements from three Norwegian sites: (1) a morning transition following a strongly stably stratified night at an alpine tundra valley, (2) spectral and ogive analysis before and after ice-cover transition at a boreal lake, and (3) fitting flux–variance relations at a permafrost-affected palsa peatland. Comparison with EddyPro shows close agreement for carbon dioxide fluxes derived from closed-path systems, but increased scatter for near-zero methane fluxes derived from open-path systems. Reddy extends existing EC software and helps bridging micrometeorology and ecology in heterogeneous, real-world environments.
An inverse problem of reconstructing the turbulent exchange tensor from large archives of observed wind profiles within the Åkerblom–Ekman (A-E) model framework is investigated using high-vertical-resolution radiosonde observations. The reconstructed tensor provides improved agreement between modeled and observed wind profiles compared to classical formulations based on real turbulent exchange coefficient. Instead of the classical A-E model, where turbulent exchange is described by a single scalar coefficient, we introduce two independent functional degrees of freedom while preserving rotational invariance of the model in the horizontal plane. This generalization leads to a substantial reduction of model residuals. The observational archives are separated into cohorts according to season, time of day, latitude, boundary-layer height, surface roughness, the Richardson number, and the temperature difference at the boundary layer top and bottom. Such stratification further improves model performance and reveals systematic differences in the vertical structure of turbulent exchange under various atmospheric conditions. The lower boundary condition is optimized within a generalized third-type (Navier, Robin) formulation, replacing the classical no-slip condition. This also substantially improves the agreement between modeled and observed wind profiles in the near-surface layer.
This study examines measurements of the atmospheric boundary layer response to rapid changes in surface forcing during the total solar eclipse of 8 April 2024 using coordinated surface mast, uncrewed aerial vehicle, and radiosonde observations near Bloomington, Indiana, USA. Obscuration at this site was 100
Turbulent kinetic energy (TKE) is a key variable in turbulence parameterization schemes. In the Mellor–Yamada–Nakanishi–Niino (MYNN) level-3 and 2.5 models, the turbulent flux of TKE is diagnosed using a downgradient formulation, yet its applicability to buoyancy-dominated convective boundary layers (CBLs) has not been fully assessed. We construct a new large eddy simulation database for a well-developed dry CBL and use it to evaluate the conventional MYNN closure for TKE flux. The results show that the conventional closure fails to correctly diagnose the sign of TKE turbulent flux in the lower part of buoyancy-dominated CBLs. We propose a modified closure that incorporates an additional term capable of representing a counter-gradient TKE flux. We also develop a closure for this new counter-gradient term so that it can be used within the MYNN framework. The modified closure improves the vertical profile of the diagnosed TKE turbulent flux and reduces its overall error by more than 50
Katabatic flows over sloping terrain exhibit low-level jets, with turbulent transport becoming dominant over local shear production near the jet maximum, challenging the applicability of classical similarity theory developed for horizontal terrain. This study investigates the influence of slope angle on local similarity scaling in katabatic flows using one-dimensional Reynolds-averaged Navier–Stokes (RANS) simulations with first- and second-order turbulence closures. The strengths and limitations of these models are assessed against observations from the Pasterze Glacier, the Vatnajökull ice cap, the MATERHORN experiment, and the Val Ferret field campaigns. The second-order closure reproduces the observed mean jet structure and momentum fluxes more accurately and is therefore used to analyse turbulence dynamics and similarity relations. A characteristic height scale, z_TM , is proposed to identify the region where the recently-proposed slope-adjusted stability parameter of Hang et al. (2021) collapses the local dimensionless momentum gradients across observations and simulations. The observed dimensionless temperature gradient exhibits larger scatter, whereas the simulations show a more consistent trend. Overall results indicate that RANS provides an efficient and viable framework for studying katabatic flows, with the second-order closure resolving processes that the first-order closure does not represent but are essential to katabatic dynamics, and that flux–gradient relations based on the slope-adjusted stability parameter collapse the dimensionless momentum gradient well below z_TM , while non-local transport processes limit flux–gradient relations near and above the jet peak.
The widespread deployment of wind-energy systems locally alters near-surface turbulent transport processes of energy, affecting temperatures and soil moisture. This study, for the first time, systematically investigates the influence of wind turbines on micro-meteorological conditions using large-eddy simulations across a range of atmospheric stability regimes. Simulations were conducted with and without wind turbines to systematically quantify differences in air temperature, surface temperature, sensible heat flux, latent heat flux, and soil moisture. The results show that turbine effects are strongly influenced by atmospheric stability. Under daytime strongly unstable conditions, turbines induced 0.06 K of modest air temperature cooling and reduced sensible heat flux by 6.97 W m−2. In contrast, strongly stable night-time conditions exhibited pronounced air temperature warming of 0.695 K, widespread increases in surface temperature of 0.76 K, and enhanced soil moisture depletion of around 2.09 × 10–3
Aerodynamic roughness parameters, namely the roughness length ( z_0 ) and zero-plane displacement height ( d ), are crucial for representing turbulent momentum exchange in urban boundary layer models. Numerous morphometric methods have been developed to estimate these parameters from wind-tunnel experiments and computational fluid dynamics (CFD) simulations. However, their quantitative performance evaluation for heterogeneous, real urban surfaces has been limited by a scarcity of observational turbulence data. This study addresses this gap by developing a comprehensive turbulence database from 852 large-eddy simulations (LES) over a wide range of real urban morphologies. This database, which includes explicitly resolved surface momentum fluxes ( τ_0 ) and corresponding urban morphometric parameters, is used to evaluate six widely used morphometric methods. Specifically, we focus on evaluating the performance of these formulations in reproducing the surface friction velocity ( u_* ), which governs the momentum exchange in Monin–Obukhov Similarity Theory (MOST). Results show that formulations accounting for building height variability (i.e., heterogeneous methods) generally outperform those based on homogeneous assumptions in reproducing τ_0 . For instance, some heterogeneous formulations achieve high correlations (R > 0.8) but tend to overestimate momentum flux due to an over-representation of pressure drag. We find that the performance of all formulations degrades at both high and low extremes of heterogeneity, with none performing adequately in friction-dominated regimes. By analysing this extensive database, our study delineates the conditions under which urban morphology governs momentum exchange and identifies the critical limitations of existing methods. These findings provide a clear directive for developing improved parameterizations capable of accurately representing momentum fluxes over diverse urban landscapes.
Urban boundary layer (UBL) processes are important to understand the complex interactions between the surface and atmosphere. Increased surface roughness and heat storage are typically attributed to the larger fraction of built surfaces in urban areas. Heatwaves lead to stronger heat storage and to extreme convective UBLs, which is one of many applications where large-eddy simulations (LES) offer an accurate modelling methodology of complex turbulent characteristics in urban flow studies. This study investigates the structure, turbulence characteristics and measurement implications of UBL in Paris, France, during a heatwave in July 2022. High-resolution LES simulations (4–16 m) produced with PALM model system are evaluated against observations from an obervation campaign, including eddy-covariance fluxes (EC), Doppler wind lidar (DWL) profiles, and near-surface measurements from the Météo-France observation network. In addition, wavelet analysis is used to examine resolved turbulent scales and vertical coupling at typical urban measurement heights. PALM reproduces the temporal variability of sensible heat fluxes at the SIRTA observatory with a root mean squared error (RMSE) of 73.4 W m^-2 and a bias of 45.0 W m^-2 , while momentum-related quantities show larger deviations, particularly during weak nighttime stratification. Near-surface temperatures are well captured across the Météo-France network, with an RMSE of 1.18 ^∘C and mean bias of - 0.63 ^∘C . DWL comparisons show that PALM captures the growth and decay of the UBL at urban and suburban sites, with maximum UBL heights reaching 2.4 km in the city centre, approximately 10
Atmospheric boundary layer (ABL) turbulence is often characterized by large time-scale fluctuations, such as diurnal and seasonal variations, which may affect the estimation of the statistical properties of wind fields and related quantities. Superstatistical approaches exploit the scale separation between slow and fast dynamical processes to model the resulting velocity fluctuations. All the results support the validity of a log-normal superstatistical description of near ground ABL turbulence, reproducing both intermittency features and the main scaling properties observed in the data, in a range of temporal scales enclosed in the interval 15 minutes up to 1 day. The probability density functions of wind velocity fluctuations are found to be in good agreement with the predictions of the superstatistical model, consistently with results reported for laboratory flows and microscale atmospheric turbulence. Overall, the combination of Complete Ensemble Empirical Mode Decomposition with Adaptive Noise, and superstatistics offers a robust framework for investigating multiscale and non-stationary wind fluctuations in the atmospheric boundary layer.
Heterogeneous forest canopies can generate complex turbulent structures, but in the presence of a fire plume, these interactions are not fully understood. This study investigates the influence of forest canopy heterogeneity on buoyant plume dynamics resulting from surface thermal anomalies representing wildland fires, utilizing Large Eddy Simulation (LES). The Parallelized Large-Eddy Simulation Model (PALM) was employed to simulate six canopy configurations: no canopy, homogeneous canopy, external plume-edge canopy, internal plume-edge canopy, 100 m gap canopy, and 200 m gap canopy. Each configuration was analyzed with and without a static surface heat flux patch of 5000 W · m^-2 , resulting in a resting buoyant plume. Simulations were conducted under three crosswind speeds: 0, 5, and 10 m · s^-1 . Results show that canopy structure significantly modifies plume behavior, mean flow, and turbulent kinetic energy (TKE) budgets. Plume updraft speed and tilt varied with canopy configuration and crosswind speed. Horizontal pressure gradients associated with plume-atmosphere interaction were modified based on the canopy configuration, resulting in varying crosswind speed reductions at the plume region. Strong momentum absorption was observed above the canopy for the crosswind cases, with the greatest enhancement in the gap canopies. Momentum injection from below the canopy due to the heat source was also observed, resulting in plume structure modulation based on canopy configuration. TKE was found to be the largest in the gap canopy configurations. TKE budget analysis revealed that buoyant production dominated over shear production. At the center of the heat patch, the gap canopy configurations showed enhanced buoyancy within the gap. These results improve our knowledge of fire-canopy-atmosphere interactions that can inform fire models on the impacts of canopy heterogeneity on plume dynamics and ember ejections.
Cellular structures in the atmospheric convective boundary layer (CBL) play a critical role in mass and energy exchange. In this study, large-eddy simulation data of a typical CBL were utilized to investigate the dynamical equilibrium and maintenance mechanisms of the cellular structures near the surface. A novel geometry-based conditional sampling method was proposed to identify and track near-surface downdrafts, which constitute a prominent component of the cellular structures. Dynamical and thermal fields of these structures and their temporal evolution were analysed. Statistics show that smaller-scale downdrafts exhibit lower pressure and weaker horizontal velocities throughout their life cycles. Based on this finding, “expanding-squeezing” and “self-limiting” mechanisms were proposed to explain the scale-selection process: pressure differences between adjacent cells drive the squeezing and dissipation of smaller cells, while continuous surface heating restricts the further growth of larger cells. The surface heating process is pivotal to the self-limiting mechanism. A simplified thermal dispersion model was utilized to quantify the surface heating process. It revealed a simple truth: thermal heterogeneity can be generated continuously by homogeneous surface heating, in the presence of horizontal motions. This thermal heterogeneity, in combination with the pressure forcing in the downdrafts, limited the scale of larger cells. These findings provide new insights into the turbulent coherent structures in the CBL.
A longstanding challenge in CO_2 flux measurements above vegetation is the unclosure of the flux balance in night-time stable boundary layers. In recent years, the impact of surface temperature heterogeneity in stable boundary layers on momentum and heat-flux balances as a result of secondary motions has received increased attention. In the current work, we set up a series of idealized large-eddy simulations in stable boundary layers and look at the effect of such surface temperature heterogeneity on the CO_2 flux balance problem, while keeping the surface Rossby number and the background-flow stability fixed. To reflect differences in crops and vegetation, heterogeneous boundary conditions for potential temperature and CO_2 flux are prescribed by introducing a patch in the centre of the domain, having higher temperature or CO_2 flux than the surroundings. In the classical homogeneous temperature setting, increased CO_2 flux in the patch leads to the development of a (shallow) internal CO_2 boundary layer (IBL) over the patch, with a classical decoupling between the ground flux and the flux above the IBL. The introduction of locally higher temperature in the patch leads to increased CO_2 fluxes. Even in case of a homogeneous CO_2 flux distribution, the vertical turbulent flux increases by up to 50 CO_2 profile by improved turbulent mixing. When both heterogeneous temperature and CO_2 fluxes are combined, we find that both effects compete. We further find that the introduction of surface temperature heterogeneity leads to the emergence of strong secondary motions at the spanwise patch edges. However, a detailed CO_2 budget analysis reveals that these motions are only important for flux balances that include the patch edges. Closer to the centre of the patch the dominant mechanism relates to the development of an internal temperature and CO_2 boundary layer over the patch.
The dissimilarity in turbulent transport among physical variables and the role of turbulent coherent structures within the unstable surface layer are investigated via the urban–rural contrasts using one-month observations in Beijing, China. The turbulent transport of heat and momentum is similar under weakly unstable conditions, while the turbulent transport dissimilarity increases as the atmospheric instability strengthens at both sites. This variation is closely linked to the transformation of coherent structures, i.e., mechanically generated structures are replaced by buoyancy-driven ones (e.g., thermal plumes and convective circulations). Meanwhile, the turbulent transport dissimilarity between the heat and passive scalars (i.e., H2O and CO2) is regulated by surface characteristics. In homogenized rural areas, the turbulent transport of heat and passive scalars remains similar under unstable conditions. However, in urban areas, passive scalars are transported differently from heat due to the heterogeneous distribution of their respective sources/sinks. Furthermore, it is also found that the transformation of turbulent coherent structures depends on the balance between mechanical and buoyant effects. For example, stronger shear in urban areas delays the formation of thermal plumes, while stronger surface heating enhances their final intensity. Overall, the transformation of coherent structures and the heterogeneity in source/sink distribution govern turbulent transport dissimilarity, for heat-momentum and heat-passive scalar transport, respectively. The results advance our understanding of turbulent transport dissimilarity in the actual atmosphere, which is helpful to improve the parameterization of the boundary-layer in mesoscale weather models.
High frequency turbulent data from sonic anemometers and other ancillary information collected at multiple levels on several meteorological flux towers located at two coastal sites are used to study velocity and scalar (temperature and humidity) structure–function parameters over heterogeneous surfaces in an area of complex coastal terrains. Understanding of such parameters has important applications for studies on wave propagation in a turbulent atmosphere. One observational site was located on the Outer Banks near the town of Duck, North Carolina, with data from the CASPER-East Program (October–November 2015). The second site was located near the town of Ferryland, Newfoundland, Canada, with data from the C-FOG (Coastal-Fog) field campaign (September–October 2018). These measurements allowed studies of structure parameters and other statistics for different footprints, including relatively smooth sea surface conditions and aerodynamically rough dry inland areas for both stable and unstable stratifications. The coastal land cover discontinuity for onshore and offshore winds leads to a thin internal boundary layer (IBL) that could often be resolved by the instrumented towers. The drag coefficient, Bowen ratio, and diurnal variation of sensible heat flux were found to be coastal IBL markers. Our study shows that lower values of the dimensional structure parameters are associated with higher altitudes, smooth surfaces (i.e., over water, outside onshore IBL) and stable stratification (e.g., nocturnal boundary layer), ceteris paribus. In particular, observations showed that the structure parameters over land footprint areas (inside the onshore IBL) can be an order of magnitude larger than over the sea surface due to the change of aerodynamic and thermal properties of the surface. This study also discusses the applicability of Monin–Obukhov similarity theory (MOST) for the velocity and scalar structure parameters in the coastal environment for both stable and unstable conditions. While surface inhomogeneities and the complexity of the coastal landforms nominally violate assumptions underlying MOST, our observations show that the nondimensional structure parameters obey MOST reasonably well for all measurement levels, stability condition, and wind direction. In addition, we suggest a new variance-based hybrid scaling for the structure parameters that overcomes some of the shortcomings of the traditional approach (e.g., self-correlation and ambiguity of the dimensionless temperature structure parameter for near-neutral conditions).
Stratocumulus (Sc) clouds that develop over coastal land are strongly influenced by sea-breeze advection. This study investigates the dissipation and fragmentation processes for a coastal cloud based on the DYCOMS II RF01 case, using Large-Eddy Simulations (LES) and a Mixed-Layer Model (MLM) to analyze its day evolution under idealized sea-breeze profile variations. These variations consist of weak versus strong speeds and different starting times. Our results show that the MLM closely follows the LES main cloud evolution with a similar liquid water path, but it overestimates cloud base height near dissipation due to its inability to describe cloud breaking. Of the studied sea-breeze variations, the influence of the breeze starting time on cloud dissipation is negligible compared to the maximum breeze speed. A close examination of the cloud thinning budget reveals that an earlier, stronger sea breeze delays fragmentation by enhancing moisture advection, whereas a weaker, later breeze accelerates it. Since there is no standard definition for cloud fragmentation, we compare four metrics and find that only those based on averaged cloud fraction or liquid water path thresholds yield consistent results. Finally, a critical cloud thickness is found to trigger cloud thinning for all studied cases, while afternoon thickening occurs for a different critical value, exhibiting hysteresis behaviour.
Accurately predicting the diurnal cycles of temperature and wind over mountainous terrain is critical for many applications, yet it remains a challenge. This study evaluates the Weather Research and Forecasting (WRF) model’s ability to capture the diurnal cycles of temperature and wind over the Eastern Snake River Plain (ESRP) in Idaho, USA. The baseline WRF simulation reproduces the general diurnal cycles of temperature and wind but shows a significant nighttime warm bias, which is associated with weaker nocturnal drainage flows and an earlier morning transition. Sensitivity tests show a persistent nighttime warm bias and trade-offs in model tuning. We also investigate whether large-eddy simulations (LES) can reduce nighttime temperature biases. Compared with mesoscale runs at 1 km, LES runs at 125 m and 25 m with the Nonlinear Backscatter and Anisotropy (NBA) model exhibit smaller nighttime temperature biases. Analysis of the potential temperature budget reveals compensating effects of dynamics and radiation in controlling the nocturnal temperature evolution. These results underscore the challenges of modeling heat transfer over complex terrain and suggest that improved subgrid-scale turbulence parameterisations can help address nighttime temperature biases.
The momentum exchange within the atmospheric boundary layer is predominantly governed by turbulence, while the dissipation rate of turbulent kinetic energy, ϵ , emerges as a pivotal parameter for computational models. Ensuring the accuracy and reliability of the estimates of this parameter requires rigorous experimental validation. This challenge is further exacerbated in the offshore environment due to the interactions between the sea surface and the marine atmospheric boundary layer (MABL) in terms of wave modulation and effects associated with atmospheric stability. In this study, we use measurements collected with a scanning Doppler LiDAR for a site located on the coast of Texas, USA, to quantify the variability of ϵ with atmospheric stability, wind speed, and wave conditions. The results indicate that colder offshore winds blowing over warmer seawater associated with unstable atmospheric conditions lead to higher ϵ compared to milder onshore winds associated with stable atmospheric conditions. Empirical models are proposed to predict the variability of ϵ with the cube of wind speed and wave age. Moreover, higher values of ϵ are observed for developing waves occurring predominantly for offshore wind conditions. The present findings contribute to advancing our understanding of turbulence dynamics in complex MABL environments.
The atmospheric response to the solar eclipse of 8 April 2024 in North America is investigated with a specific focus on the marine atmospheric boundary layer (MABL). We leverage measurements collected during the Third Wind Forecast Improvement Project (WFIP3), including Doppler lidars, sonic anemometers, and thermodynamic profiler data to investigate the atmospheric response across sites that experienced partial eclipse conditions with nearly 90 1.2^∘C to 1.4^∘C in coastal regions and from 0.3^∘C to 0.5^∘C over the ocean. This analysis suggests that the MABL’s higher thermal inertia compared to coastal regions moderates the temperature decrease during the eclipse. Wind speed exhibits a more complex behavior, as it is influenced by both the MABL and preexisting synoptic conditions. Although a reduction in wind speed is observable up to approximately 140 m above ground level (AGL) at more inland sites, at other locations closer to the coast, this reduction is constrained to the lowest 100 m AGL. Turbulence parameters retrieved from sonic anemometers, such as turbulence kinetic energy, turbulent heat flux, and friction velocity, decrease during the eclipse at coastal sites, accompanied by a brief transition of atmospheric stability from unstable to neutral or weakly stable conditions. For the open-ocean sites, the variability in turbulence statistics and atmospheric stability is minimal during the occurrence of the eclipse.
Understanding the turbulent and microphysical dynamics of radiation fog is critical for forecasting low-visibility events over the Indo-Gangetic Plain. Here, we combine high-frequency field observations from the Winter Fog Experiment with high-resolution WRF Large-Eddy Simulations (LES) to examine the vertical structure and evolution of a dense fog episode over Indira Gandhi International Airport, New Delhi, during January 6–7, 2023. Turbulence diagnostics reveal that fog initiation was not purely surface-driven but was characterized by volumetric condensation within a shallow stratified layer, predominantly spanning 20–130 m above ground level (a.g.l.), whose depth varied over time and persisted throughout the fog life cycle, triggered by intermittent shear-induced turbulence in the residual layer. The LES reproduces this non-classical, aloft-assisted saturation with realistic cloud water fields and visibility evolution. During the mature phase, a persistent inversion maintained a semi-decoupled boundary layer, characterised by weak near-surface turbulence and enhanced turbulent kinetic energy near the fog top. This sustained aloft saturation suppressed surface mixing and extended fog duration. Dissipation was governed by progressive turbulence deepening and radiative–mechanical coupling: after sunrise, buoyant turbulence near the surface and shear-driven mixing aloft eroded the inversion, promoting vertical recoupling and rapid fog clearance.
The roughness length ( z_0 ) and displacement height ( z_d ) are essential surface-layer parameters in numerical models (e.g., weather, climate, wall-modeled LES, etc.). This work evaluates the consistency of z_0 and z_d estimates from morphometric and anemometric methods using data from two eddy-covariance flux towers (AmeriFlux US-INg and US-INc) in Indianapolis, IN. Results show inconsistencies in estimated z_0 and z_d values depending on the chosen method. The two evaluated anemometric methods estimate non-physical values of z_d when compared to roughness elements surrounding both towers. Additionally, predictions of mean wind speed using surface-layer similarity theory with morphometric estimates exhibit a bias during near-neutral and stable conditions relative to observations. The overestimation of mean wind speed by surface layer similarity theory is consistent with previous observational and modeling studies in urban areas, suggesting that the application of similarity theories to urban environments may have limitations. Differentiation of vegetation from built structures appears to impact morphometric z_0 and z_d estimates, particularly where vegetation is abundant; however, it has little impact on correcting biases in the similarity theory. Specifically, we find that existing similarity theories using morphometric estimates underestimate integral velocity and length scales, and the degree of underestimation depends on the stability conditions. Accounting for the degree of anisotropy in surface-layer turbulence helps reduce the biases between similarity theories and observations during unstable conditions, but not in near-neutral cases. Future work is needed to identify the cause of such biases for near-neutral conditions.
