Although significant advances have been achieved during the last decades, an appropriate comprehension of ABL processes and their interactions under different conditions is still a challenge in meteorology. Improving this knowledge will help to correctly represent ABL processes in weather and climate models, allowing to provide more accurate numerical weather prediction (NWP) forecasts and climate scenarios.
This session welcomes conceptual, observational and modeling research related to the physical processes that appear in the ABL, including those devoted to study the interactions with the free atmosphere. Current contributions evaluating existing models and schemes are also welcome, as well as the presentation of new implementation in numerical modelling.
The following topics are especially encouraged to be submitted to the session:
• Theoretical and experimental studies of the turbulence-closure problem with emphasis on very stable stratification and convection, accounting for interactions between the mean flow, turbulence, internal waves and large-scale self-organized structures.
• Boundary-layer clouds (including fog) and marine, cloud-topped boundary layers: physics and parameterization within NWP and climate models and observational studies.
• Orographic effects: form drag, wave drag and flow blocking, gravity waves.
• Challenges on the surface energy balance and flux aggregation in atmospheric boundary layers over heterogeneous terrain.
• Representation of boundary layers and land-surface interaction in atmospheric models.
• Organization of deep convection across differing atmospheric scales.
• Large-eddy simulation and direct numerical simulation of turbulent flows.
• PBL and surface-layer studies using long-term data (climatology), detailed analysis of case studies and field campaigns presentation.
Mounting evidence indicates concurrent changes in forest carbon uptake and cloudiness across tropical, temperate, and boreal biomes, pointing to emerging shifts in forest–atmosphere coupling with the potential to amplify climate feedbacks. At the same time, a growing body of research highlights the critical role of within-canopy microclimate, where forests generate strong vertical gradients in radiation, temperature, humidity, and turbulence. These gradients buffer climatic extremes, regulate ecosystem functioning, and control phenological responses that differ markedly between overstory and understory environments. Together, these findings demonstrate that forests actively regulate their internal microclimate while interacting dynamically with the atmosphere above.
Despite these advances, forests and clouds are still largely studied and modelled as separate components, with land treated primarily as a lower boundary condition rather than as an active, three-dimensional driver of atmospheric dynamics. This conceptual separation limits our ability to understand and predict coupled carbon–cloud–climate feedbacks, particularly under ongoing climate change where both carbon uptake and cloud regimes are shifting.
Here, we propose a conceptual and methodological shift towards treating forests and clouds as an integrated, dynamically coupled system. We outline a first-principles framework that bridges biological, chemical, and physical processes across spatiotemporal scales, explicitly linking radiative perturbations, stomatal responses, turbulence, atmospheric chemistry, and cloud formation. In the talk, I will present observations from comprehensive field campaigns, including LIAISE and CloudRoots-Amazon22, integrated with large-eddy simulations to resolve canopy–boundary layer–cloud interactions. This combined observational–modelling approach enables a process-based understanding of how forest structure and function feed back on atmospheric dynamics and cloud development.
Embedding such observationally constrained, canopy-resolving representations into Earth System Models offers a pathway to reduce uncertainties in projections of carbon uptake, cloud dynamics, and their combined influence on climate, ultimately improving our capacity to predict biosphere–atmosphere feedbacks in a changing world.
How to cite: Vila-Guerau de Arellano, J., Moonen, R., Gonzalez-Armas, R., Mangan, M.-R., de Feiter, V., Huitema, A., Hartogensis, O., de Boer, H., and Roeckman, T.: Forests and clouds as a coupled system: moving beyond boundary conditions, EMS Annual Meeting 2026, Utrecht, Netherlands, 6–11 Sep 2026, EMS2026-74, https://doi.org/10.5194/ems2026-74, 2026.
Atmospheric turbulence is a key factor when assessing the performance of earth-to-satellite optical communication links (Free Space Optical Communications, FSOC) as it impacts beam wander and intensity fluctuations of the laser beam. Models that allow to estimate vertical profiles of optical turbulence intensity, expressed as the structure parameters of the refractive index, Cn2, either require costly simulations or are based on crude, empirical relations. In this work, we propose an efficient, physics-based method for estimating Cn2 timeseries and vertical profiles in the convective boundary layer. We do this using the mixed layer column model CLASS (Chemistry Land-surface Atmosphere Soil Slab), which allows us to simulate surface fluxes of sensible and latent heat and the boundary layer height. Assuming linear profiles of the heat fluxes over the boundary layer, in line with mixed layer formulation, we derive profiles of CT2 and Cq2, the structure parameter of temperature and humidity respectively, which together define the Cn2 profile.
We will validate our approach for a reference case at the Ruisdael Observatory of Cabauw, the Netherlands (https://ruisdael-observatory.nl/cabauw/) against measurements of the surface energy balance terms, Cn2 estimated from sonic anemometers and scintillometers, and the boundary layer height estimates from radiosondes and ceilometer. Next, we will present a sensitivity study of Cn2 to surface and boundary layer properties and from these identify some typical cases representative for extremely favourable or unfavourable conditions for FSOC. The main conclusions from this work is that our approach fills a niche in Cn2 profile modelling that better optimises model complexity versus model physics than the currently used approaches.
How to cite: Huerta-Viso, A., Hartogensis, O., Van Eijk, L., Ronda, R., and Vilà-Guerau de Arellano, J.: A simple approach to model optical turbulence for the unstable boundary layer at Cabauw (NL), EMS Annual Meeting 2026, Utrecht, Netherlands, 6–11 Sep 2026, EMS2026-79, https://doi.org/10.5194/ems2026-79, 2026.
The vertical fluxes of sensible and latent heat represent a major contribution to the exchange of energy between the land surface and the atmosphere. Their adequate description in numerical weather prediction and climate models is essential to realistically simulate near-surface weather conditions. Traditionally, these heat fluxes are parameterized relying on the Monin-Obukhov Similarity Theory (MOST) or the use of the Bulk-Richardson number. These parameterizations are based on differences in wind speed, air temperature, and humidity between adjacent measurement or model levels.
Wulfmeyer et al. (2023) estimated the heat fluxes with machine learning approaches and achieved a higher accuracy compared to MOST. Additionally, the analysis revealed radiation as a key predictor. However, their analysis is based on a rather short data period in August 2017 at three nearby locations in Oklahoma, USA, which limits the generalizability of the results.
In our study we replicate and expand the findings from Wulfmeyer et al. (2023) using a multilayer perceptron model (MLP) on a dataset from the boundary layer field site (GM) Falkenberg of the German Meteorological Service. The dataset consists of soil and meteorological variables over a period of twenty years, covering various seasons, synoptic weather situations and extreme weather events.
Our preliminary findings support the role of radiation as a dominant predictor for both the latent and sensible heat fluxes. We further studied the performance of the MLP for datasets of different lengths (e.g., one month as in Wulfmeyer et al., 2023, the same month over twenty years, or complete twenty-year data sets). Additionally, we tested the impact of removing redundancy in the selection of the predictor variables and the performance of the model under extreme conditions.
How to cite: Winkelmann, A., Traub, M., Karlbauer, M., Beyrich, F., Butz, M., and Wulffmeyer, V.: Sensible and Latent Heat Flux Prediction with Deep Learning using twenty years of Falkenberg micrometeorological data, EMS Annual Meeting 2026, Utrecht, Netherlands, 6–11 Sep 2026, EMS2026-536, https://doi.org/10.5194/ems2026-536, 2026.
Despite its omnipresence in atmospheric models, the Penman-Monteith (PM) equation often fails to represent the latent heat (LE) flux accurately. Deviations of several tens of % between modelled and observed LE flux are not an exception. The original PM equation assumed a constant stomatal resistance in time, but most current atmospheric models implement a varying resistance that depends on atmospheric conditions such as radiation, temperature and vapor pressure, while more recent models account for plant physiological stomata control to some degree.
In this study, we present a diagnosis of LE fluxes modelled based on the PM equation combined with a fixed, an environmentally driven and a plant physiology driven stomatal conductance model versus observed LE fluxes by Eddy-Covariance. The analysis covers a decade of observations for a grass and three years for a forest site in the Netherlands. We identify atmospheric conditions where the model and observations match and most strongly disagree and evaluate the contribution of stomatal resistance models in reproducing flux observations. In this study, we demonstrate that implementing models that account for varying stomatal conductance in response to atmospheric and soil conditions does not help to improve LE model estimates for these two datasets. We investigate whether eliminating some of the assumptions underlying the PM equation improves flux estimates and zoom in on the role of aerodynamic versus stomatal conductance in controlling LE flux. The aim is to provide suggestions for conceptual improvements that can help resolve some of the shortcomings in the PM-based LE flux estimation .
How to cite: Jongen-Boekee, J., van de Wiel, B., Romijn Neeteson, N., and ten Veldhuis, M.-C.: Diagnosing LE fluxes, models, sensors and other culprits, EMS Annual Meeting 2026, Utrecht, Netherlands, 6–11 Sep 2026, EMS2026-802, https://doi.org/10.5194/ems2026-802, 2026.
We present work of the Land-Atmosphere Feedback Initiative (LAFI) [1], which aims to quantify and understand land-atmosphere feedback by utilizing synergetic observations and turbulence-permitting simulations. One focus of LAFI is the quantification of entrainment, i.e., the exchange of energy and humidity between the boundary layer and the free atmosphere above.
As a first step, we investigated a proposed similarity relationship of the latent heat entrainment flux at the top of a convective boundary layer. This relation is expressed as the vertical velocity scale times the ratio of the water vapor mixing ratio gradient to the Brunt-Väisälä frequency (summarized in [2]). We studied this equation using several months of turbulence-resolving observations of humidity and vertical wind from Raman and Doppler lidar systems as well as radiosonde temperature measurements from the Atmospheric Radiation Measurement Program's Southern Great Plains site in the US. The analysis, conducted at the lower boundary of the interfacial layer, did not support the proposed formulation. In the search for potential driving variables of the latent heat entrainment flux at this height level, we found a high correlation with the square root of the variances of vertical wind and water vapor mixing ratio.
Further exploration of this connection led to the conclusion that the currently most accurate description of the latent heat entrainment flux is the product of the vertical wind variance and the ratio of the water vapor mixing ratio gradient to the Brunt-Väisälä frequency, confirming recent results over Europe [3].
Within LAFI, these results will be validated through new measurements conducted during a specific field campaign by LAFI in 2025. Here, the profiling of sensible heat fluxes and potential temperature variance is also possible. First results and analog analysis regarding the proposed theoretical descriptions in [2] will be presented at the conference.
References:
[1] https://www.lafi-dfg.de/
[2] Wulfmeyer, Volker et al. (2016): Determination of Convective Boundary Layer Entrainment Fluxes, Dissipation Rates, and the Molecular Destruction of Variances: Theoretical Description and a Strategy for Its Confirmation with a Novel Lidar System Synergy. In Journal of the Atmospheric Sciences 73 (2), pp. 667–692. DOI: 10.1175/JAS-D-14-0392.1
[3] Gibert, Fabien; Edouart, Dimitri; Monnier, Paul; Collignan, Julie; Lopez, Julio; Cénac, Claire (2025): Scalar turbulent fluxes and variances in the interfacial layer from lidar observations and assessment of Lagrangian Stochastic Models. In Quart J Royal Meteoro Soc, Article e70016. DOI: 10.1002/qj.70016.
How to cite: von Klitzing, L., Turner, D. D., Lange, D., Senff, C., and Wulfmeyer, V.: Study of Latent and Sensible Heat Entrainment Fluxes in the Convective Boundary Layer with Lidar Synergy, EMS Annual Meeting 2026, Utrecht, Netherlands, 6–11 Sep 2026, EMS2026-82, https://doi.org/10.5194/ems2026-82, 2026.
Convective cold pools (CCPs) are a key driver of transient turbulence and rapid reorganization of the planetary boundary layer (PBL), strongly modulating surface–atmosphere exchanges. At the Amazon Tall Tower Observatory (ATTO) in central Amazonia, CCPs occur nearly daily and represent a dominant mechanism of boundary-layer disturbance. Using the detection framework of Oliveira et al.(2026), which identified 410 CCP events between August 2021 and December 2023, we investigate how CCP-induced changes in CO2, sensible heat, and momentum fluxes depend on cold-pool intensity and pre-existing PBL stability.
CCPs are first classified by intensity based on their minimum temperature anomaly, and subsequently by pre-CCP stability, which controls the background thermodynamic structure and CO2 storage within the forest canopy. Composite time–height analyses reveal a coherent dynamical structure during CCP passage, characterized by a narrow leading-edge updraft followed by a broader region of postfrontal subsidence and enhanced turbulence, consistent with a rotor-like circulation at the cold-pool front.
The scalar response is dominated by the postfrontal phase. After CCP arrival, cold-air advection and turbulent mixing lead to strong negative sensible heat fluxes, indicating downward transport of heat. At the same time, CO2 concentrations increase near the surface and extend upward, accompanied by positive CO2 fluxes, demonstrating the upward transport of CO2-rich air from within and below the canopy. These results suggest that ventilation is not dominated by the brief frontal ascent, but is primarily associated with enhanced turbulent mixing in the postfrontal wake.
The magnitude of these responses strongly depends on pre-existing stability. Under stable conditions, where CO2 accumulates near the surface due to weak mixing and respiration dominates, CCP-induced turbulence leads to the largest increases in CO2 concentration and flux. In contrast, under unstable conditions, the pre-existing mixed state reduces the impact of CCPs on scalar redistribution.
Overall, our results show that CCPs act as an efficient mechanism for redistributing scalars in the Amazon PBL, primarily through postfrontal turbulent mixing rather than frontal lifting. This highlights the importance of cold-pool dynamics in modulating biosphere–atmosphere exchange and provides new insights into the coupling between convective processes and ecosystem fluxes in tropical forests.
Oliveira, M. I., Mortarini, L., Acevedo, O. C., Cely-Toro, I. M., Grosso, M., Reis, L. I., Junior, C. Q. D.,
Quesada, C. A., Ribeiro, B. Z., and Brondani, D. V. (2026). Convective cold pools and attendant
turbulence at the amazon tall tower observatory (atto). Atmospheric Research, 327:108351.
How to cite: Reis, L. I., Oliveira, M. I., Ferreira, V., de Vargas Brondani, D., Acevedo, O., van Asperen, H., Q. Dias-Junior, C., and Mortarini, L.: The Role of Convective Cold Pools in the Transport of Carbon Dioxide over the Amazon Tall Tower Observatory (ATTO), EMS Annual Meeting 2026, Utrecht, Netherlands, 6–11 Sep 2026, EMS2026-420, https://doi.org/10.5194/ems2026-420, 2026.
The exchange of CO2 in the lower troposphere is governed by surface and atmospheric processes operating across a wide range of spatial and temporal scales. Over tropical regions such as the Amazon rainforest, daily occurring shallow-to-deep convective clouds are anticipated to significantly modulate the diurnal cycle and spatial variability of CO2. Yet, the physical processes controlling the exchange at the surface and across the interface of the boundary layer and free troposphere remain poorly understood and insufficiently quantified, limiting accurate estimates of CO2-exchange over the Amazon rainforest under a changing climate.
To address this research gap, we examine the lower tropical troposphere CO2-exchange across shallow-to-deep convective regimes over the Amazon rainforest. More specifically, we develop an analytical framework that adopts well-mixed conditions, assuming vertical uniformity of state variables and greenhouse gases within the atmospheric boundary layer. Utilising this framework, we reconstruct and attribute the diurnal (from sunrise to sunset), day-to-day and spatial lower tropospheric CO2-exchange into seven physical components: (I) free tropospheric background mole fraction, (II) entrainment and detrainment during the night-to-day transition, (III) surface fluxes (including plant assimilation and soil respiration), (IV) entrainment and detrainment from convective boundary layer development as determined by the lapse rate of CO2, (V) boundary layer dilution associated with clouds, (VI) cloud mass flux and (VII) advection. Moreover, by inverting the expression we obtain a first-order, physics-based estimate of net ecosystem exchange that helps interpret the sensitivity of existing inverse modelling estimates. The framework is applied to the global storm-resolving Integrated Forecasting System (IFS) of the ECMWF across three horizontal resolutions (25 km, 9 km, and 4.4 km) and evaluated using comprehensive observations and turbulence and cloud resolving large-eddy simulations from the CloudRoots-Amazon22 campaign (dry season 2022).
Our initial findings indicate that we satisfactorily reproduce and attribute the relative importance of each physical component to the diurnal CO2-exchange in the lower tropical troposphere under clear, shallow convective, and deep convective conditions in ECMWF-IFS. However, larger uncertainties persist in the early morning and late afternoon. Ongoing work aims to further disentangle the role of the physical processes in controlling the CO2-exchange over the Amazon rainforest across time, space and model resolutions.
How to cite: de Feiter, V. S., Janssens, M., Savazzi, A., Agusti-Panareda, A., and Vilà-Guerau de Arellano, J.: Which Processes Govern the Lower Tropical Troposphere CO2-Exchange in ECMWF-IFS?, EMS Annual Meeting 2026, Utrecht, Netherlands, 6–11 Sep 2026, EMS2026-391, https://doi.org/10.5194/ems2026-391, 2026.
Cities strongly modify the surface energy balance, increase aerodynamic roughness, and release anthropogenic emissions, thereby significantly affecting atmospheric boundary layer (ABL) processes from local to regional scales. For cities located in complex terrain, these effects are further modulated by orography: thermal forcing depends on slope and shading, while mechanical forcing strongly deflects, accelerates or decelerates the wind field. High-resolution observations of the ABL wind profile over urban areas in complex terrain are therefore essential to provide evaluation data to test if models are able to represent the interaction between ABL flow and heterogeneous surfaces, with implications for urban heat, air quality, and numerical weather prediction.
Within the ASSURE-urbisphere observational campaign in Bristol, UK, a network of five Doppler wind lidars (DWLs) was deployed across the urban–rural interface. This mid-sized city is located in complex terrain, providing a testbed for investigating coupled urban–terrain effects. A key orographic feature of Bristol is the Avon Gorge, located in the northwest of the city, with a length of approximately 3 km, a typical width of 100 m and an average depth of 80 m.
The five DWLs were operated with different scanning strategies and were deployed across six sites, providing continuous wind and turbulence profiles. This unique dataset enables characterization of how enhanced urban buoyancy and surface roughness modify wind and turbulence, as well as how terrain-induced thermal structures control near-surface atmospheric stability.
We have found that the synoptic wind direction is the primary driver of the formation of a valley-exit jet from the Avon Gorge, with direction determining whether the jet flows into or exits the city. Near-surface atmospheric stability, largely influenced by urban buoyancy, modulates the jet intensity. Under anticyclonic conditions, a mesoscale nocturnal low-level jet (LLJ) is formed. These stable conditions, together with cold-air pooling in the urban basin, also favour the development of a weak and partially decoupled near-surface flow that drains downslope within the gorge. The LLJ core characteristics (intensity, direction, and height) influence the formation (or suppression) and intensity of the valley-exit jet.
These results provide new insights into how coupled urban–terrain interactions modulate ABL dynamics, with direct implications for urban air quality, and human thermal comfort.
How to cite: Céspedes, J., Barlow, J., Escobar-Ruiz, V., McGregor, J., Price, J., Looschelders, D., Christen, A., and Grimmond, S.: Drivers of wind profile variability in Bristol, UK: the role of synoptic flow, complex terrain, and the urban atmosphere, EMS Annual Meeting 2026, Utrecht, Netherlands, 6–11 Sep 2026, EMS2026-413, https://doi.org/10.5194/ems2026-413, 2026.
For NWP modelling, increasing model resolution at the hectometre scale aims to improve the representation of small-scale variability for high-impact weather events. In complex terrain over orography, the increased resolution is expected to be beneficial for capturing complex, and often coupled, fluid flow processes including slope circulations and mountain waves. In urban areas, high-resolution forecasts including the detailed heterogeneous surface characteristics would allow to describe weather conditions at the district level. In the context of climate change, this would aid in the projection, planning, and adaptation of cities toward key health, environmental, or economic objectives. To these ends, however, one key challenge for current operational models is to adapt physical schemes for operations at high resolution. Indeed, at these resolutions, subgrid parameterizations can have a significant impact on the numerically represented scales, a characteristic feature of the so-called “gray-zone” issue for weather forecast models (Honnert et al. 2020).
In this context, this work focuses on the validation and improvement of AROME NWP hectometric forecasts over complex terrain. Following the work of Rogel et al. (2025), the impact of a so-called "pseudo-3D" approach for the Horizontal Shear Production (HSP) is assessed at high resolution, in complex terrain over the Alps. This parameterization extends approaches from Göger et al (2018) and Goecke et Machulskaya (2021), in order to include horizontal effects in the turbulence scheme. The sensitivity to length scale formulations on the horizontal and vertical directions is studied. Other aspects of the analysis include the surface fluxes coupling with the turbulence scheme and the impact of their parameterization toward the representation of slope circulations. Observation data from the TEAMx campaign is used to assess the performance of the model. In a second part, observations from the PANAME observation campaign (Lemonsu et al 2026) are used to evaluate AROME forecasts at 200m horizontal resolution on a heatwave case over Paris. Sensitivity with respect to surface characteristics is investigated. In particular, following the work of Bazile et al (2025), the sensitivity of the HSP parameterization to the explicit inclusion of buildings inside the orography of the model is analyzed. Results will be discussed within the context of the UrbanAIR project, and with the aim of providing accurate forcing data for Large Eddy simulation (LES) models.
How to cite: Rogel, L., Bazile, E., and Voitus, F.: Parameterized Horizontal Shear Production in AROME at hectometric scale: evaluation over complex terrain with the TEAMx data and within the UrbanAIR project, EMS Annual Meeting 2026, Utrecht, Netherlands, 6–11 Sep 2026, EMS2026-762, https://doi.org/10.5194/ems2026-762, 2026.
In the safety assessment for the construction of nuclear facilities in Japan, wind tunnel experiments or computational fluid dynamics (CFD) are required to estimate spatial distribution of air concentrations of a plume emitted from a stack (Nuclear Safety Commission of Japan, 1982). The experimental or CFD results are used to derive effective stack height, which is applied for long-term assessment using a Gaussian plume model. The effective stack height is often found to be lower than the actual height of the stack, considering terrain and building effects in a way that provides a conservative evaluation. Although reliable data on wind velocity and material concentrations are obtained, the effective stack height is estimated under the assumption of neutral stability.
In the atmosphere, heating and cooling within a boundary layer due to solar cycle during a day result in temperature differences, which introduce buoyancy forcing. Plume dispersion within the atmospheric boundary layer is also influenced by roughness elements, terrain, and thermal stability. In terms of thermal stability, atmospheric boundary layers are in general classified into three types; neutral boundary layer (NBL), stable boundary layer (SBL), and convective boundary layer (CBL). In an NBL, turbulence is generated and maintained by wind shear, while in an SBL turbulence is not only maintained by wind shear but also constrained by negative buoyancy. In a CBL, turbulence is mainly produced by shear and/or buoyancy. The most common stability classification scheme is the Pasquill-Gifford (P-G) (Turner, 1970), which defines six stability classes namely A (highly unstable), B (moderately unstable), C (slightly unstable), D (neutral), E (moderately stable), and F (extremely stable). The plume spreads over a flat ground surface in the typical meteorological conditions are determined by the P-G chart. Since atmospheric dispersion behaviors of a plume released from a tall stack are sensitively influenced by atmospheric stability, thermal effects should be incorporated into the effective stack height.
In this study, we perform LESs of a diurnal cycle of atmospheric boundary layer (ABL) flows based on the similar computational conditions to Kumar et al. (2006). As a first step, our objective is to investigate the turbulence characteristics of various thermal-stratified ABL flows and classify them based on the P-G chart.
How to cite: Nakayama, H., Sato, T., and Takemi, T.: Large-eddy simulation analysis of turbulence characteristics of atmospheric boundary layers during a diurnal cycle, EMS Annual Meeting 2026, Utrecht, Netherlands, 6–11 Sep 2026, EMS2026-38, https://doi.org/10.5194/ems2026-38, 2026.
Eddy-covariance (EC) fluxes collected on urban rooftops are commonly interpreted as representative of turbulent fluxes above the roughness sublayer when sensors are installed well above the mean building height. However, extended EC observations from a tall rooftop in Enschede, the Netherlands, reveal vertical divergence in sensible heat flux (SHF), with measurements at 10 m above the roof sometimes substantially differing from those at 5 m. To investigate the mechanisms behind this behaviour, we integrate dual-level EC measurements with building-resolved large-eddy simulations (LES) performed using the PALM4U model over a realistic urban environment and synoptic conditions. Two clear-sky case studies are examined: a windy spring episode dominated by mechanical shear and organized convective rolls, and a calm , hot summer event characterized by buoyancy-driven convective cells. The LES captures both the diurnal cycle of SHF and the enhanced vertical divergence observed during the calm conditions. Analysis of the flow structure indicates that the windy case dominated by mechanical shear with nearly aligned streamlines, weak flow deflection, and predominantly local turbulent transport, while the calm case is associated with pronounced streamline deformation, inclined thermal structures, and stronger non-local transport contributions. Evaluation of flux-gradient relationships based on K-theory suggests that local gradient-based parameterizations perform well under windy conditions, whereas they break down under calm, convective regimes, indicating that horizontal advection due to flow distortion and non-local turbulent transport dominates under convective conditions. Double-averaging further shows that dispersive flux contributions are largely restricted to the immediate canopy layer and remain negligible at the EC measurement heights. Overall, the findings demonstrate that vertical divergence of SHF above urban rooftops is mainly controlled by flow distortion and non-local transport processes due to thermal stratification. Our results demonstrate that commonly used height-based criteria (i.e. 2–5 times the mean building height) are insufficient to ensure representative EC measurements in complex urban settings and LES can serve as a powerful tool to understand the flow in complex urban settings before installing EC instruments for flux measurements.
How to cite: Gadde, S., Steeneveld, G.-J., and Timmermans, W.: Mechanisms of Vertical Sensible Heat Flux Divergence in the Urban Roughness Sublayer: Insights from Observations and LES, EMS Annual Meeting 2026, Utrecht, Netherlands, 6–11 Sep 2026, EMS2026-483, https://doi.org/10.5194/ems2026-483, 2026.
Trees located within the urban canopy are known to mitigate the urban heat island effect and heat waves by lowering the canopy air temperature via evapotranspiration and surface shading. They are also invoked as a nature-based solution to improve air quality through the absorption and deposition of anthropogenic pollutants. However, urban trees can have the adverse effect of reducing ventilation within the canopy as well as the dispersion of pollutants. Heat and mass exchanges between the urban canopy and the overlying atmosphere play a key role in air temperature and air quality at the pedestrian level. These exchanges are driven by turbulent motions developing at different scales, within the urban canopy, in the overlying atmospheric boundary layer, and at their interface. Quantifying the role of urban trees in these exchanges and their links to ventilation or dispersion processes within the canopy is challenging due to the complexity of urban surfaces and the various processes involved in surface-atmosphere interactions.
The CITRY field campaign was carried out to characterize turbulent exchanges of momentum, heat, and mass as well as fine particles concentrations in an urban environment from canopy to boundary-layer scales, under various stability conditions and seasonal tree characteristics, specifically the presence of leaves. The experimental set-up was installed in a residential area of the city of Nantes (France) over a period of 10 months (March-December 2025). A wide range of instruments was deployed at different heights from the roof of a 14m-tall building and within the urban canopy. Within the urban canopy, four ultrasonic anemometers were installed on two masts, at 6 m and 10 m above ground level (a.g.l.). Two ultrasonic anemometers and one Doppler LiDAR wind profiler were positioned on the rooftop to measure wind and turbulence at 20 m and 24 m a.g.l., and between 55 m and 400 m, respectively. Within and above the canopy, gas analyzers (LI‐COR) were also installed at 10 and 24 m a.g.l., close to the ultrasonic anemometers, to deduce water vapor and carbon dioxide turbulent fluxes. Particulate matter sensors, covering particle diameters from 0.2 to 40 micrometers, were deployed across the site, on streetlight poles and on masts at several heights within and above the urban canopy, to capture the spatial variability of particle concentration.
We will present a preliminary analysis of this observational dataset, including a statistical assessment of the effects of wind sector, wind speed, atmospheric stability, and leaf state on water vapor and carbon dioxide fluxes and particulate matter concentrations. Additionally, for selected periods, the main characteristics of the turbulent structures responsible for canopy–atmosphere exchanges will be shown, as well as their size dependency to the thermal stability.
How to cite: Maynard, P., Calmet, I., Conan, B., Keravec, P., Perret, L., Shahu, K., Bonnefond, J.-M., Dupont, S., Irvine, M., Cuny-Guirriec, K., and Rebours, A.: From canopy to boundary-layer scales: in-situ quantification of urban-trees effects on turbulent exchanges, ventilation, and pollutant dispersion, EMS Annual Meeting 2026, Utrecht, Netherlands, 6–11 Sep 2026, EMS2026-93, https://doi.org/10.5194/ems2026-93, 2026.
Urban street canyons are a critical part of the urban morphology, where building-induced flow structures and atmospheric turbulence jointly control pollutant dispersion and ventilation efficiency. While most existing studies rely on simplified, idealized canyon geometries, the complex spatial heterogeneity of real urban morphology remains largely underexplored. This study addresses this gap by numerically analysing wind dynamics and pollutant transport through high-fidelity Large-Eddy Simulation (LES) of a real neighborhood of the historical centre of the city of Bologna (Italy). This has been selected because a well-recognised hotspot for urban pollution and an archetypal case study of European cities, characterised by a dense, asymmetric urban configuration.
The LES is first validated against an experimental dataset obtained from a water channel experiment specifically designed to reproduce the same real city neighborhood. Also, numerical results are compared with RANS simulations previously carried out. The analysis pointed out the main processes driving the ventilation dynamics within the urban canopy layer: LES resolves the dominant turbulent structures governing momentum and scalar transport, including the primary canyon recirculation vortex and the rooftop shear layer originating at the upstream building. Turbulence generation is concentrated within this shear layer, which drives momentum exchange between the outer atmospheric flow and the canyon interior. Inside the canyon, turbulence remains persistently anisotropic, with streamwise and spanwise fluctuations dominating over vertical motions, while vertical mixing intensifies toward the taller downstream building. Velocity variaces distributions reveal spatially heterogeneous momentum transport, with deeper penetration of high-momentum fluid on the downstream side, a feature directly attributed to the building height asymmetry. Pollutant dispersion is governed by the interplay between recirculation-driven trapping and shear-layer-induced dispersion, producing non-uniform concentration fields with pronounced near-ground accumulation. Turbulent scalar flux analysis further uncovers counter-gradient transport upstream of the emission source and alternating ventilation and re-entrainment patterns. Comparison with RANS predictions highlights LES superiority in capturing vertical velocity and scalar transport, particularly within the rooftop shear layer.
These findings highlight that real urban geometry produces fundamentally three-dimensional, asymmetric flow and pollutant trapping mechanisms that idealized canyon models cannot reproduce. Resolving unsteady turbulent structures via LES is essential, since mean-flow approaches miss the vertical mixing and counter-gradient transport that ultimately determine pedestrian-level air quality.
How to cite: Cintolesi, C., Sengupta, B., Romano, S., Petronio, A., Monti, P., and Di Sabatino, S.: Wind Field Characterization and Pollutant Transport in a Real Urban Canyon: A Large-Eddy Simulation Study in Bologna, Italy, EMS Annual Meeting 2026, Utrecht, Netherlands, 6–11 Sep 2026, EMS2026-620, https://doi.org/10.5194/ems2026-620, 2026.
Air quality represents a key objective of urban governance, particularly in historically grown cities exhibiting complex topography. For this purpose, municipal authorities or engineering offices working on their behalf frequently utilise atmospheric models to assess the urban climate or the dispersion of pollutants. Due to the limited financial resources available, these tasks are unable to utilise research-grade high-resolution models such as PALM or WRF-LES, which require significant computing power for long-term simulations. Instead, the focus is either on the simulation of specific, limited periods under designated synoptic conditions. This approach is employed, for instance, in the context of urban climate assessment. Or simplified or analytical models are utilised, for example for the assessment of pollution dispersion . The latter are deemed to be "bad" in terms of the fact that they do not consider a significant number of processes included in a high-resolution, coupled physical model. The central question is how "bad" the results are when compared to the actual wind field.
The measurement of the wind profile above an city over a prolonged period is challenging. Nevertheless, their feasibility has increased significantly in recent years due to the advent of commercially available compact wind Lidar systems. In the present study, Lidar measurements obtained above the city centre of Trier, Germany, are utilised as ground truth, and are compared with simulations conducted using the two wind-filed models employed by the German regulatory pollutant dispersion model, AUSTAL. One of these is TALdia3, an analytical wind field model, and the other is METRAS-PCL, a prognostic wind field model derived from Uni Hamburg's mesocale model METRAS. In order to assess the resulting uncertainties in pollutant dispersion prognoses, hypothetical reference sources are simulated and the results are analysed.
How to cite: Woth, A. and Drüe, C.: Verification of simple Wind Field Models for Air Pollution Simulation over the City of Trier, EMS Annual Meeting 2026, Utrecht, Netherlands, 6–11 Sep 2026, EMS2026-791, https://doi.org/10.5194/ems2026-791, 2026.
The near-surface air temperature (Ta) is largely driven by surface temperature (TS), which depends on the surface energy balance and is strongly influenced by surface material properties. While TS can be estimated globally from satellite observations at high spatial resolution, Ta is typically measured only at weather stations, resulting in limited spatial coverage. Consequently, many studies use TS in place of Ta when a high spatial resolution is needed (e.g., at the city scale), although their spatial patterns differ and their physical relationships are not fully characterized. Previous studies using Landsat-derived TS (30 m resolution) have shown that surface changes can affect the surrounding thermal environment, with significant TS anomalies extending up to 168 m into adjacent undisturbed areas. One notable case is a highway constructed in 2013 across the dense Atlantic Rainforest in southeastern Brazil, where TS increases extended asymmetrically into the forest, reaching up to 128 m on one side of the road and 64 m on the other. This reach and asymmetry were hypothesized to result from warm air advection from the highway into the forest canopy, modulated by the prevailing sea-breeze circulation. In this study, we use the PALM large-eddy simulation model to isolate and test this hypothesis by explicitly simulating the Atlantic Rainforest with and without the highway. Idealized simulations are performed under a mean wind speeds perpendicular to the road axis to assess how the highway modifies air temperature at canopy height and how these effects propagate into the forest on both sides. The simulations show an increase in Ta of more than 1.0°C reaching 50m inside the forest canopy and of 0.5°C reaching 100m. The reach can occur on both sides and are not just driven by the mean wind but also by the increase in soil and leafs exposure to the sun radiation because of vegetation removal. The results provide a mechanistic understanding of how this type of infrastructure can alter the microclimate of protected forested areas and help clarify the relationship between TS-based observations and Ta responses, also providing guidance for the use of Ts when Ta observations are unavailable.
How to cite: Lustosa, R., Camponogara, L. F., da Rocha, H., Maronga, B., Schwenkel, J., and Costa, F.: Modeling the Thermal Impact of a Highway on Its Surroundings Using PALM: A Case Study in the Atlantic Rainforest, Brazil, EMS Annual Meeting 2026, Utrecht, Netherlands, 6–11 Sep 2026, EMS2026-780, https://doi.org/10.5194/ems2026-780, 2026.
Secondary circulations, either cell-like or roll-like, develop frequently in convective conditions. They often fill the entire boundary layer and do not move with the mean wind, thereby violating the ergodic hypothesis. Nevertheless, they contribute to the overall flux of sensible and latent heat and the associated transport can be quantified through the dispersive flux, which is the spatial covariance of the temporal mean of the vertical velocity and the transported scalar. Since this dispersive flux inherently cannot be measured by single-tower eddy covariance, neglecting this flux leads to a systematic underestimation, which is one major contributor to the energy-balance closure problem. It is not the only reason for this problem, but one of the most important ones that generally exists at all sites around the world and that acts in the same direction as the small-scale turbulent flux. We show findings from dedicated measurement campaigns, such as CHEESEHEAD19, aiming at quantifying dispersive fluxes. The results are compared against systematic large-eddy simulations (LES) under different stability regimes. Since it is not feasible to conduct such expensive simulations for all eddy-covariance measurement to quantify the additional, otherwise neglected dispersive flux, similarity relationships are derived from the LES results and their universal applicability is evaluated for several long-term flux measurement sites. In addition, an outlook is provided on the use of machine learning models informed by LES to integrate the effects of surface thermal heterogeneity for the prediction of dispersive fluxes. This analysis demonstrates that dispersive fluxes, being one of the main reasons for the energy balance closure problem, can be readily quantified with a limited amount of additional data, most of all the boundary-layer height, to correct recent and historical eddy-covariance measurements.
How to cite: Mauder, M.: The role of secondary circulations and dispersive fluxes for the energy-balance closure problem, EMS Annual Meeting 2026, Utrecht, Netherlands, 6–11 Sep 2026, EMS2026-572, https://doi.org/10.5194/ems2026-572, 2026.
The BRISA project focuses on sea breezes developed in three Spanish semi-arid areas: the coast of the Gulf of Cadiz, the island of Mallorca and the eastern part of the Ebro Valley. The breezes in these regions interact with processes of different scales, such as the mesoscale thermal-low pressure systems formed over the Iberian Peninsula in summer, the secondary circulations generated in inhomogeneous surfaces characterised by wetlands and irrigated/non-irrigated agricultural patches, or the thermally-driven flows favoured in complex-terrain regions. These interactions affect the formation and the characteristics of the sea breezes, with numerous impacts that affect society, highlighting their thermoregulatory role during extreme temperatures, or their importance for inland and offshore wind energy resources, among others.
The methodology of the BRISA project (PID2024-159841OA-I00) combines the use of long-term in situ observations and experimental field campaigns, as well as the use of numerical weather prediction models. In this work, we present the BRISA-Cádiz field campaign (July 2026), which will bring together national and international boundary-layer researchers and oceanographers to characterise the horizontal and vertical extension of the breezes. Among the planned activities are the installation of a WindCube LIDAR just at the shoreline, the launching of frequent radiosondes inland and at sea, the profiling of the atmosphere with tethered balloons, the use of drones with instrumentation to characterise the horizontal boundary-layer meteorology and turbulence contrasts between the land and the sea, the use of marine instrumentation to monitor the sea surface conditions, and the use of Distributed Temperature Sensing (DTS) systems together with surface energy balance stations to monitor the near-surface temperature and turbulent fluxes.
In this work, we will present the instrumentation deployed during the campaign as well as some first analyses performed after it. The high amount of data expected to be gathered during the intensive observational periods will allow us to study how breezes form, evolve, and impact the meteorological conditions in coastal sites. The next step following the observational characterisation of the breezes will be the evaluation of high-resolution numerical models, to ultimately improve how these processes should be represented within them.
How to cite: Román-Cascón, C., Jiménez, M. A., Martínez-Villagrasa, D., Mañanes, R., Mercader, J., Ramón Miró, J., Luján-Amoraga, E., Bolado-Penagos, M., Cuxart, J., Le Moigne, P., Boone, A., Pardyjak, E., Kral, S., Reuder, J., Sophie-Möller, F., Bouhlal, N., Wrenger, B., Groh, J., and Serra, A.: Improving the understanding and prediction of sea breezes and their effects on the meteorological conditions of complex semi-arid regions: The BRISA field campaign on the coast of Cádiz, EMS Annual Meeting 2026, Utrecht, Netherlands, 6–11 Sep 2026, EMS2026-339, https://doi.org/10.5194/ems2026-339, 2026.
Foehn winds are warm, dry downslope flows that occur on the leeward side of mountain ranges. In the Alps, the descent of foehn winds is often restricted to specific hotspots, where the interaction between complex terrain, mountain-induced gravity waves, and flow separation concentrates the descending air. These descent hotspots are associated with localized warming and drying, significantly affecting weather conditions, forecast uncertainty, and have implications for ecosystems and human activities in the region. Previous studies using the COSMO model—run at 1 km resolution—have visualized these hotspots and linked them to gravity wave dynamics. However, whether a 1 km resolution is sufficient to accurately resolve near-surface flow separation—a key factor in foehn dynamics and their predictability—remains an open question.
To investigate this question, we conducted large-eddy simulations for a case study in the Rhine Valley (February 2017). Simulations were performed with the Icosahedral Nonhydrostatic (ICON) model using three nested domains with horizontal grid spacings of 520 m, 260 m, and 130 m, respectively. Turbulence at all three domains was represented using a 3D Smagorinsky closure. Additionally, we used offline Lagrangian trajectories to identify the descent pathways of foehn air parcels, enabling a detailed evaluation of how model resolution affects the spatial distribution of descent hotspots in the Alps. This study is the first to combine trajectory analysis with LES in the context of foehn research, allowing for a detailed visualization of foehn descent pathways.
Preliminary results are consistent with previously observed hotspots of foehn descent in the Alps, confirming the Rhine Valley as a key pathway for flow descent. The results show that while some flow descents are indeed channelled along the predominantly north–south oriented valley, the main hotspots of foehn descent tend to accumulate along the east–west segments. This indicates that the most frequent descent pathways are controlled not by valley-parallel channelling, but by direct downslope acceleration on the north-facing slopes. The main features of the descent—particularly the typical origin height of trajectories between 500 m above ground level and the mean Alpine crest height, and the descent travelled distance—are preserved as grid spacing is refined from 500 m to 250 m. However, notable differences emerge at higher resolution, including a 30% increase in the number of descending trajectories in the valley and the development of faster and more intense descent events associated with stronger foehn jets. These findings indicate that while the essential foehn dynamics are captured at mesoscale resolutions, higher-resolution configurations are required to represent the finer-scale features and intensity of foehn descent in the Alps.
How to cite: Quimbayo-Duarte, J., Tian, Y., Sprenger, M., and Schmidli, J.: Investigating Foehn Descent Hotspots in the Swiss Alps Using LES-Based Trajectory Analysis, EMS Annual Meeting 2026, Utrecht, Netherlands, 6–11 Sep 2026, EMS2026-618, https://doi.org/10.5194/ems2026-618, 2026.
Storage and transport processes over complex terrain remain a major source of uncertainty in atmospheric models, particularly under stable boundary layer (SBL) conditions. This work builds upon previous idealized studies and presents a high-resolution large-eddy simulation (LES) case study of passive tracer evolution in the Beromünster region of the Swiss Midlands using realistic terrain, land use, and surface tracer fluxes.
The simulations are designed to capture both stable and convective boundary layer regimes over a 48-hour period, allowing investigation of the full diurnal cycle of tracer evolution. Emphasis is placed on nighttime tracer accumulation within the SBL and its subsequent depletion during the morning transition. In addition, the sensitivity of tracer transport to horizontal grid spacing is assessed to better understand resolution requirements in complex terrain.
Spatially heterogeneous and temporally varying surface tracer fluxes representative of CO₂ exchange, together with realistic topography and land use, enable a more physically consistent representation of lower boundary conditions compared to previous idealized studies. The simulations are evaluated against CO₂ observations from the Beromünster tall tower, which provides measurements at multiple heights (2–212 m), offering detailed insight into the vertical structure and temporal evolution of tracer concentrations.
The results indicate that cold-air pooling and terrain-driven drainage flows lead to pronounced spatial variability in near-surface tracer concentrations and strong stratification during nighttime conditions. During the morning transition, tracer concentrations rapidly decrease as boundary layer growth and slope and valley flows promote vertical export. Initial comparisons suggest that the simulations reproduce the general temporal evolution, while showing a tendency toward stronger stratification during SBL conditions. It suggests that the diurnal evolution of tracer concentrations is primarily controlled by cold-air pooling and local circulation patterns.
Overall, this study extends process understanding from idealized configurations to a more realistic setting and provides new insight into passive tracer dynamics over the Swiss Midlands, with implications for the evaluation and development of atmospheric models such as ICON and ICON-ART.
How to cite: Bašić, I. and Schmidli, J.: Passive Tracer Evolution over Complex Terrain: A LES Case Study of the Beromünster Region, EMS Annual Meeting 2026, Utrecht, Netherlands, 6–11 Sep 2026, EMS2026-642, https://doi.org/10.5194/ems2026-642, 2026.
Thermally driven down-valley flows play a crucial role in the local meteorology of mountainous regions, governing the transport of mass, energy, and pollutants. Accurately representing these phenomena requires resolving the interactions between the atmospheric flow, large-scale winds, thermal forcing, and steep underlying orography.
This study presents an in-depth investigation of down-valley flow dynamics in the Aure valley (French Pyrenees) using high-resolution simulations (200 m horizontal grid spacing) with the Weather Research and Forecasting (WRF) model. The analysis focuses on the physical processes and multiscale interactions occurring within the valley atmosphere.
To explicitly resolve the physical processes governing the generation and evolution of valley flows, an offline one-way nesting technique is implemented to bridge the gap between large-scale atmospheric forcing and sub-kilometer scales. While the accurate simulation of turbulence in the atmospheric boundary layer at these scales remains a complex issue, it offers significant advantages under stable conditions and over heterogeneous surfaces, as is the case of the valley considered here, where resolving small-scale surface features is desirable. This framework explicitly represents the development of the nocturnal surface inversion and topographically channeled flows, avoiding using the traditional planetary boundary layer parameterizations.
The numerical setup is evaluated against a comprehensive observational dataset from a field campaign conducted in summer 2023, including surface meteorological stations distributed across key valley locations and in situ radio soundings launched at different stages of nocturnal flow development. These vertical profiles are used to assess the model’s ability to reproduce the depth, thermal stratification, and kinematic structure of the nocturnal down-valley flow.
How to cite: Ortiz, P., Carbone, J., Roman-Cascón, C., Lohou, F., Lothon, M., and Yagüe, C.: High-Resolution Modelling of Down-Valley Flows in the Pyrenees, EMS Annual Meeting 2026, Utrecht, Netherlands, 6–11 Sep 2026, EMS2026-417, https://doi.org/10.5194/ems2026-417, 2026.
In the western Mediterranean island of Mallorca, sea breezes develop on roughly half of the days during the warm season. These wind circulations are characterized by low-level jets that penetrate the island from its two main basins and converge over its central area, a process strongly influenced by the synoptic environment and the island’s complex orography. This study characterizes the spatial and temporal structure of sea-breeze convergence under different synoptic conditions, with particular attention to the evolution, scales, variability, and governing physical processes of convergence events.
A six-day high-resolution numerical simulation was conducted with the Meso-NH 5.7 model, using three nested domains with horizontal resolutions of 5 km, 1 km, and 250 m. The simulation was validated against observations from automatic weather stations operated by the Spanish National Meteorological Agency (AEMET). In addition to reproducing the observed flow, the model provides access to budget terms for key variables, including turbulent kinetic energy, vertical and horizontal velocity, and potential temperature, enabling a process-based interpretation of their interactions. These diagnostics allow a precise assessment of the wind structure and the forcings that drive sea breeze initiation, inland propagation, convergence, and the associated updrafts.
The analysis identifies three types of sea breeze events, each associated with different synoptic conditions and associated patterns of convergence. Two dominant wind regimes emerge during these events: a coastal stationary regime, in which the front retains a nearly constant vertical structure at the shoreline; and an interior quasi-stationary phase associated with convergence and updraft development. This classification is reinforced by the behavior of the budget terms, which highlight the individual role of each physical process. Inland penetration is primarily driven by the pressure gradient forcing, while vertical turbulent mixing acts as the main opposing mechanism, weakening and disorganizing the sea breeze front. Together, these results clarify how mesoscale circulations evolve over complex terrain and highlight the interplay between local forcing and synoptic scale conditions in shaping sea breeze convergence over Mallorca.
How to cite: Serra, A., Maimó-Far, A., and Jiménez, M. A.: A sea breeze convergence zone in the center of a complex terrain island, EMS Annual Meeting 2026, Utrecht, Netherlands, 6–11 Sep 2026, EMS2026-409, https://doi.org/10.5194/ems2026-409, 2026.
Sea breezes are mesoscale atmospheric circulations that develop in coastal areas due to the thermal contrast between land and sea, with relevant environmental and socio-economic impacts, such as pollutant dispersion or thermal comfort, among others. While their dependence on synoptic conditions and land–sea thermal gradients has been widely studied, the potential influence of tidal variability has received much less attention.
This study focuses on the role of tidal variations in the formation and modulation of sea breeze events using observational data from a coastal site. Measurements from a shoreline weather station equipped with a 3D sonic anemometer are combined with tidal forecasts obtained using the Tidex2020 software.
Sea breeze events are identified using an objective algorithm adapted from previous studies and subsequently classified into two main groups: those initiating during the rising tide phase (flood) and those beginning during the falling tide phase (ebb). This classification ensures that flood events are preceded by low-tide conditions and ebb events by high-tide conditions.
Results show a higher frequency of flood events than ebb ones. As expected, sonic anemometer data demonstrate that flood events exhibit higher sensible heat flux (SH) values in the hours preceding breeze onset due to the increased intertidal area exposed during the preceding ebb tides, which could favour the needed conditions for sea breezes development. Regarding wind speed, breezes developing under high-tide conditions (flood events) tend to reach higher wind speeds than those developing under low-tide conditions (ebb events), which could be associated with the different surface roughness and land–sea thermal contrast near the shoreline in both types of events. In this presentation, we will discuss all these factors influencing the breeze development, as well as the additional effect of the current direction in comparison to the wind direction, which also affects the surface roughness of the sea surface (waves formed).
How to cite: Luján-Amoraga, E., Román-Cascón, C., Jiménez-Rincón, J. A., Yagüe, C., Ortiz-Corral, P., Carbone, J., Izquierdo, A., and Bolado-Penagos, M.: Tidal variations as a factor influencing sea breezes, EMS Annual Meeting 2026, Utrecht, Netherlands, 6–11 Sep 2026, EMS2026-418, https://doi.org/10.5194/ems2026-418, 2026.
Numerical weather prediction and climate models typically rely on parameterisations developed for mid-latitude conditions and apply those also over the Arctic. These parameterisations do not adequately capture the intermittent and non-stationary nature of turbulence under stable stratification, as they frequently occur over the Arctic or during nighttime over land. Yet adapting the physical representation of Arctic air–sea–ice–ocean interactions may not only affect the variability and long-term changes of Arctic atmospheric circulation, but also have the potential to influence mid-latitude atmospheric circulation.
As part of the WarmWorld project consortium, which develops a storm- and eddy-resolving Earth system model based on the German community model ICON, we aim to improve the representation of turbulence under stably stratified conditions. Motivated by previous work by Boyko and Vercauteren (2023), who developed a data-driven stochastic generalization of traditional Monin–Obukhov similarity theory, we aim to develop and implement an improved parameterisation of turbulent surface fluxes that captures variability and the net effects of non-turbulent, small-scale processes on the mean flow.
As a first step, we run the ICON model in a limited-area setup at a horizontal resolution of 5km, centred on the research vessel Polarstern during the MOSAiC expedition in winter 2019/20. This setup allows for an evaluation of the model’s default performance under Arctic winter conditions and facilitates the identification of pronounced yet common model biases, such as cold surface temperatures and excessive near-surface stability. In particular, we investigate how changes in model resolution and adaptations to the turbulent surface-flux parameterization over sea ice under stably stratified Arctic conditions affect the lower Arctic boundary layer and may help to mitigate model biases.
Alongside this, and based on MOSAiC turbulence measurements, initial attempts are made to develop a data-driven stochastic model that accounts for unsteady mixing and uncertainty around classical surface stability functions under stable conditions. This is achieved by identifying a scaling of the parameters of a stochastic differential equation (SDE) with a flow stability parameter, such as the bulk Richardson number. The resulting stochastic formulation of the surface stability functions can then be incorporated into a bulk parameterization of turbulent surface fluxes within the model.
How to cite: Vercauteren, N., Riebold, J., Kuttikulangara, A., Krumscheid, S., Panton, J., and Handorf, D.: Turbulence intermittency and parameterisations for stably stratified conditions in the ICON-WarmWorld model, EMS Annual Meeting 2026, Utrecht, Netherlands, 6–11 Sep 2026, EMS2026-761, https://doi.org/10.5194/ems2026-761, 2026.
The characteristics of the planetary boundary layer drastically depend on its density stratification. In the stable boundary layer (SBL), strong stratification can lead to the absence of turbulence. Quasi-laminar patches develop locally and can extend across the entire boundary layer, leading to a globally intermittent flow. Multiple studies have also shown that small-scale turbulence becomes more anisotropic with increasing stratification, with frequent occurrence of a one-component Reynolds stress tensor (i.e., kinetic energy along a main direction) that also characterizes the large scales, such as gravity waves or other submeso motions. Turbulence intermittency and anisotropy challenge traditional boundary-layer theories, which are based on homogeneous, isotropic turbulence.
Direct numerical simulations (DNSs) of stably stratified turbulent Ekman flows over a smooth wall have shown that globally intermittent flows are well organized at both the large and the small scales. Quasi-laminar and turbulent patches, even near the surface, are aligned along a distinct direction comparable with the orientation of large-scale coherent structures that develop further above in the outer layer. Within the turbulent patches, small-scale hairpin vortices are oriented along a similar direction. Whether the flow organization contributes to the occurrence in the SBL of one-component states of anisotropy at both large and small scales is the object of the present contribution.
We analyze the spectra of kinetic energy and the anisotropy of the stress tensor at multiple heights and across scales from the previously mentioned DNSs. Comparison with the better-known setup of a neutrally stratified Ekman flow, where continuous and isotropic turbulence develops, is performed.
Large-scale coherent structures in the outer layer are found to be one-component and to influence the spectra of kinetic energy down to the surface. While in the neutral case only one peak of kinetic energy is observed, occurring at small scales, a second peak at large scales is found in the stably stratified case, with an energy-containing length scale consistent across the entire boundary layer. This suggests that the large-scale organization near the surface is related to the one-component structures further above. The anisotropy of the large scales near the surface, however, has a two-component signature (i.e., kinetic energy along two main directions), typical of sheared flow, instead of a one-component signature. This feature is observed only in the stable case, characterized by a shallower spiral than in the neutral case, pointing to directional wind shear as the driver of large-scale anisotropy. Patches of organized hairpins do not have a one-component contribution to the flow anisotropy, suggesting that the one-component structures in the outer layer do not influence the small-scale anisotropy near the surface. Instead, directional wind shear appears to influence the hairpins’ orientation, which is height-dependent.
How to cite: Gucci, F., Vercauteren, N., and Harikrishnan, A.: Anisotropy of globally intermittent flows in the Ekman boundary Layer, EMS Annual Meeting 2026, Utrecht, Netherlands, 6–11 Sep 2026, EMS2026-471, https://doi.org/10.5194/ems2026-471, 2026.
Analysing 1 minute data gathered at the Baseline Surface Radiation Network (BSRN) station at Cabauw, the Netherlands, for approximately 50 cloudless sky days, it is found that the apparent effective emission coefficient ε of the atmosphere, defined via Rin = εσT24 , with Rin the incoming longwave radiation, T2 the air temperature at 2 m (in K) and the Stephan-Boltzmann constant, dips just after sunrise. The objective our study is to show that this feature is a diagnostic for various relatively little-described aspects of land-atmosphere interactions during the morning transition. The observed dip is caused by the fact that just after sunrise T2rises faster than Rin .This occurs during two so-called cross-over periods. Firstly, a stable cross-over period, earlier found by Bosveld et al. (2014) and Angevine et al. (2001) when the atmosphere just above the surface is stably stratified, and, secondly, an unstable cross-over in which this feature continues to exist, when a convective boundary layer (CBL) develops. These findings imply that during the morning transition a decoupling takes place between the air layer near the ground, in which T2is increasingand an air layer higher in the atmosphere determining Rin. By rewriting Eq. 1 in terms of an effective temperature, Teff, to which an effective height, zeff, can be assigned, it is made plausible that the height of latter air layer is higher than 200 m. The consequence of this finding is that, during the morning transition of clear days,all empirical formulas for ε found in the literature can hold only under the locally dependent environmental conditions for which these are trained. These formulas cannot be applied universally. Furthermore, because the conditions just after sunrise are non-stationary, it is to be expected that radiation algorithms in numerical models in which time steps of several hours are used, will fail to determine accurately Rin,during the morning transition. This might explains the relatively large systematic errors in Rin in climate models as reported in the literature.It will be shown that formulas for ε derived for stationary conditions using sophisticated radiation transfer models such a MODTRAN, also do not work during the highly non-stationary circumstances in the morning transition.
How to cite: De Bruin, H. and Verhoef, A.: On how BSRN data reveal poorly understood features of land-atmosphere interaction during the morning transition. A tribute to Cabauw Ruisdael Observatory, EMS Annual Meeting 2026, Utrecht, Netherlands, 6–11 Sep 2026, EMS2026-760, https://doi.org/10.5194/ems2026-760, 2026.
Understanding stably stratified boundary layers over land is of importance for the prediction of nocturnal frost extremes and fog events. With increasing stability, the role of turbulent fluxes diminishes in the surface heat budget, while the relative importance of the radiative contributions and the surface heat transport increase. Indeed, it is known from model intercomparison studies like GABLS that the ability of models to model cold extremes is very sensitive role of surface coupling: models with different formulations and/or surface-coupling parameters show a large variety of surface flux and temperature predictions.
Here, therefore we dig into the process of atmosphere-vegetation-soil heat exchange, with a focus on cold extremes over short grass. First, novel observational results are presented. Detailed temperature profiles over the atmosphere-vegetation-soil continuum were obtained at the Cabauw national Observatory. Coiled distributed temperature sensing with optical fibers allowed us to probe the vertical temperature profiles with mm to cm resolution, ranging from a few meters above the ground to a couple of decimeters into to the soil.
The observed temperature dynamics are mimicked with various conceptual models to describe the heat transfer through the atmosphere-grass system. It is shown that traditional skin-layer type of formulations are unable to describe the temperature dynamics in a realistic manner. In the future, we therefore need to find more realistic, and yet simple alternatives to mimic natural temperature dynamics near the ground under stratified conditions. Potential alternatives and their limitations will be discussed, as to explore the road to future model implementations to improve the prediction of cold extremes over short grass.
How to cite: Van de Wiel, B., Steenge, J., Romijn Neeteson, N., ter Horst, T., Hornstra, R., van de Linden, S., Vis, G., and Ten Veldhuis, M.-C.: Stable boundary layers, cold extremes and the role of surface coupling, EMS Annual Meeting 2026, Utrecht, Netherlands, 6–11 Sep 2026, EMS2026-792, https://doi.org/10.5194/ems2026-792, 2026.
Land-atmosphere interactions play a key role in the Earth’s climate. The surface temperature is a key parameter in calculating the latent and sensible heat flux and thus important for the closure of the surface energy balance (SEB). Yet vegetated surfaces have different properties compared to bare soil and thus behave differently. Grass-vegetated surfaces are by far the most common type of land cover, covering over 40 % of all land area. Therefore, accurate modelling of soil and grass temperatures is essential for improving numerical weather prediction models.
In current weather models, the surface temperature is often estimated using an empirical skin resistance model, which may lead to significant errors in both the phase and amplitude of the surface temperature, negatively affecting the closure of the SEB. A more refined and physics-based approach is thus needed for accurate modelling of heat transfer processes in the vegetation-soil continuum.
In this research we investigate a new modelling approach for grass-vegetated and topsoil layers, using both analytical and numerical diffusive modelling approaches, building on a previous work, where grass was treated as a homogeneous sponge-layer with a uniform thermal diffusivity. The aim is to capture the temperature dynamics within the grass (and soil) layer and compare these with millimetre-resolution observations using distributed temperature sensing (DTS) measurements.
Results indicate that a purely diffusive model is accurate in describing the temperature dynamics within the soil, but is not fully able to capture the heat transfer within the vegetation layer accurately. Therefore, adjustments are made to the vegetation ‘sponge’-layer, adding a more realistic height-dependent density and a height-dependent (radiative) source term.
First results from a rudimentary analytic model already show promising results for temperature profiles in quasi-steady state, both during night- and daytime. Similar temperature profile shapes to the DTS measurements are achieved, that would not have been possible for a purely diffusive model. Likewise, the same adjustments were made to a numerical grass model using realistic forcing. This also resulted in more realistic temperature profiles within the grass layer, especially during nighttime.
As a next step, we will translate the detailed physics into more rudimentary formulations, to be incorporated in the parameterisations of weather forecasting and large eddy simulation models.
How to cite: Steenge, J., van de Wiel, B., ten Veldhuis, M.-C., van der Linden, S., and Romijn, N.: Earth’s Green Blanket: A study of Heat Transfer through Grass, EMS Annual Meeting 2026, Utrecht, Netherlands, 6–11 Sep 2026, EMS2026-455, https://doi.org/10.5194/ems2026-455, 2026.
Since the last two centuries, gravity waves have been an atmospheric enigma that scientists strive to fundamentally define. These events occur throughout the earth’s troposphere to the mesosphere, but full physical understanding of this phenomenon within the boundary layer remains out of reach. Generally, gravity waves are caused by airflow within stable atmospheric conditions losing stability due to a jump in air density and/or wind speed (Lyulyukin et al., 2015). Because this trigger of instability occurs in hydrostatically stable conditions, the flow becomes dynamically unstable resulting in continuous oscillations in unstable vertical arrangements that can last for long distances and periods of time (Mayor, 2017). Buoyancy and shear induced gravity waves induce vertical transport of heat, momentum, energy, and water vapor. This redistribution of key atmospheric components yields important consequences within the boundary layer. Yet, because they evolve in mostly stable atmospheric conditions and can occur at the sub-meso scale, they are considered less critical for atmospheric models and thus not precisely represented.
Boundary layer measurements of these gravity wave events provide insight for improved parameterizations of numerical solutions. As these events require specific atmospheric conditions to occur, however, it proves rare to obtain continuous and sufficient data when and where these events happen. Thus, most atmospheric observations lack substantial information to derive key wave information (Mahrt, 2014).
The Centre for the Testing of Environmental Sciences Technology (C-TEST) conducted a recent remote sensing measurement campaign in 2025 on the coast of Blyth, where gravity wave events were detected. Four significant gravity wave events were captured between 300-2000m in altitude from its suite of instruments, including a MWR (microwave radiometer) and two scanning lidars. One lidar was continuously measuring vertically, providing consistent information on vertical wind speed and backscatter content. The second lidar alternated every 5 minutes between vertical measurements and DBS (doppler beam swinging), providing horizontal wind speed and direction.
This analysis aims to promote remote sensing measurements to provide insight on gravity wave characterization and evolution throughout the boundary layer. The analysis is still ongoing, but preliminary results show interesting characterization of the events. Buoyancy and shear regimes have been defined by evaluating the backscatter, wind speeds, and potential temperature profiles. For some events, the Taylor Goldstein linearization relationships are applicable as the horizontal and vertical wind components are in phase, whereas the backscatter density is 90˚ out of phase (Nappo, 2012). The combination of continuous temperature, humidity, wind speed and backscatter profiles from all instruments give insight into the complex coastal conditions that lead to the development and dissipation of these gravity wave events. Quantification in momentum flux, turbulent kinetic energy, and other important atmospheric profiles additionally contribute valuable information for improving numerical modeling and physical understanding.
How to cite: Benzo, C. and Gottschall, J.: Coastal boundary layer gravity wave characterization by scanning lidars and a microwave radiometer, EMS Annual Meeting 2026, Utrecht, Netherlands, 6–11 Sep 2026, EMS2026-85, https://doi.org/10.5194/ems2026-85, 2026.
Extreme wind gusts pose significant threats to human safety and infrastructure, yet limited pre-onset observational constraints lead to substantial uncertainties in nowcasting and forecasting. In this study, we investigate two severe wind events: a dry extreme surface gust event (wind speed > 35 m s⁻¹) that occurred in Beijing on 30 May 2024, and a wet convective severe wind event in coastal East China on 30 April 2021. Their dynamical characteristics are examined using a high-density radar wind profiler mesonet, a meteorological tower, automated weather stations, and radar and satellite observations. For the Beijing event, multi-source observations show that a multicellular storm developed ahead of a convergence line formed by the collision between northeasterly cold outflows and environmental southerlies during downhill propagation. Evaporative cooling initiated strong downdrafts, which were further intensified by downward momentum transport and pressure gradient forcing. After reaching the plain, two convective segments merged into a well-organized squall system containing a midlevel mesovortex and an intense rear-inflow jet. Low-level frontogenesis and shearing deformation sustained mesoscale convection and enhanced small-scale turbulent processes. Turbulent inverse energy cascades intensified markedly as wind speeds increased. In the coastal event, cloud clusters developed ahead of a baroclinic midlevel trough and organized into a comma-shaped squall system that moved offshore. A mesovortex embedded within the stratiform region generated strong northerly rear inflows that descended to the surface, producing damaging gusts. These cases highlight the distinct but complementary roles of multiscale dynamical processes in accelerating surface winds to extreme intensity. These findings demonstrates the value of radar wind profiler mesonet observations for resolving multiscale convective dynamics and provides important observational support for improving numerical weather prediction of extreme gust events.
How to cite: Guo, J., Guo, X., and Chen, T.: On the Fine-scale Boundary-layer Dynamic Processes Leading to Extreme Gust Wind Events in East China: Insights Gained From Radar Wind Profiler Mesonet Observations, EMS Annual Meeting 2026, Utrecht, Netherlands, 6–11 Sep 2026, EMS2026-17, https://doi.org/10.5194/ems2026-17, 2026.
Chamoli district in the Garhwal Himalaya of Uttarakhand serves as an important ecological transition zone linking high-altitude mountain systems to downstream fluvial environments. The region’s rugged topography, rich biodiversity, and glacier-fed river networks significantly contribute to regional hydrological sustainability and climate resilience. However, comprehensive assessments of the mechanisms governing land–atmosphere coupling in this climate-sensitive mountainous terrain remain relatively scarce.
In this study, an information-theoretic framework is applied to investigate seasonal interaction networks between land-surface and atmospheric processes. The analysis considers major hydro meteorological variables, including precipitation (P), air temperature (T), latent heat flux (LH), sensible heat flux (SH), wind speed (WS), incoming shortwave radiation (SWR), and relative humidity (RH). Network structures are evaluated across four climatological seasons: pre-monsoon (March–May), monsoon (June–September), post-monsoon (October–November), and winter (December–February). The constructed interaction networks differentiate between instantaneous linkages representing real-time coupling and lagged connections reflecting memory-dependent influences.
Entropy-based metrics reveal pronounced dynamical variability during the pre-monsoon and monsoon periods, whereas winter conditions exhibit relatively stable and subdued interaction patterns. The post-monsoon season emerges as a transitional phase in the regional land–atmosphere system. SH, SWR, and LHF emerge as the dominant driving variables, exhibiting transfer entropy values of 0.3434, 0.1249, and 0.0455, respectively, whereas T acts as the primary receiving variable with a value of −0.5137. Overall, synchronous coupling intensifies during the monsoon, while winter is characterized by comparatively stronger memory-controlled interactions. A comparative assessment of pre- and post-pandemic periods indicates a reduction in entropy deviations around 2019, followed by a noticeable increase after 2021, suggesting altered information flow within the coupled system. These findings enhance the understanding of seasonal land–atmosphere dynamics over Chamoli and establish a baseline for evaluating future shifts associated with natural climate variability and anthropogenic influences.
Keywords: Land–atmosphere interaction; Information theory; Entropy networks; Biosphere–atmosphere coupling; Himalayan ecosystems.
How to cite: Jaiswal, R., Pandey, M. K., and Verma, S.: Decoding Biosphere–Atmosphere Coupling over the Himalayan Ecosystems Using an Entropy-Driven Network Framework, EMS Annual Meeting 2026, Utrecht, Netherlands, 6–11 Sep 2026, EMS2026-155, https://doi.org/10.5194/ems2026-155, 2026.
Deployment of wind turbines has been continuously increasing over the past years and decades. Currently, priority is set to the exploitation of offshore environments, but to reach climate goals, it will be essential to take into account unused onshore potentials. This presents unique challenges due to surface heterogeneity, including elevation, buildings and plant canopies, which can considerably alter the wind field. The influence of complex terrain also varies depending on atmospheric stability. These combined effects create a variable wind field with locally high wind shear and strong turbulence, which must be considered during site assessments due to their potential impact on turbine loads and wake behavior.
In our study we employed PALM to examine how complex terrain and atmospheric stability influence turbine performance, longevity, and wake behavior. We used geospatial data from the test site WINSENT in southern Germany, located on a plateau with a forested escarpment upwind of the main wind direction. For the simulation we focused on three variable elements: 1) plant canopies, 2) additional terrain upwind and 3) atmospheric stability. With a combination of these we created twelve different scenarios. For each of them we examined load distribution over the rotor blades, power output and wake recovery of a single NREL 5 MW turbine.
Additional upstream terrain can substantially modify the incoming flow by generating internal boundary layers and enhanced turbulence, with effects that strongly depend on stability. Under neutral and unstable conditions, added terrain accelerates wake recovery, while also reducing mean power output. In stable conditions, plant canopies play a critical role by increasing turbulence and boundary-layer depth, leading to more even load distributions across the rotor and significantly faster wake recovery. While mean power differences are smaller in stable stratification, load variability and wake deficits are strongly affected.
How to cite: Brassat, J. and Maronga, B.: Large Eddy Simulations investigating the effects of atmospheric stability on wind turbine loads and wakes in complex terrain, EMS Annual Meeting 2026, Utrecht, Netherlands, 6–11 Sep 2026, EMS2026-197, https://doi.org/10.5194/ems2026-197, 2026.
The Planetary Boundary Layer height (PBLH) is a fundamental parameter governing the dispersion of pollutants and, consequently, the accuracy of Air Quality (AQ) modeling and forecasting. As Arpae-SIMC provides daily AQ forecasts for the Emilia-Romagna region and over the entire Italian domain, which are also employed as boundary conditions by regional and national stakeholders, a comprehensive sensitivity analysis was performed to evaluate the performance of the PBLH parametrization.
In the current operational workflow, ICON-2I - the implementation over Italy of the ICON model running at 2.2 km - provides meteorological forcing for the CHIMERE AQ model. This study compares the performance of the native CHIMERE PBL scheme against a novel parametrization developed by DWD for ICON. The novel approach refines the description of the boundary layer across different stability regimes, from stable and near-neutral conditions to unstable convective states. These parametrizations are validated against radiosounding data and Automated Lidar Ceilometers (ALC) as ground truth. The analysis accounts for the diverse Italian topography, ranging from complex mountainous terrain to flat areas characterized by stagnant, near-zero wind speed conditions. The ALC data are provided by the CNR-ISAC, thanks to a measurement campaign in Bologna.
Preliminary results for summer 2024 in downtown Bologna, obtained by comparing the various parametrizations against ALC measurements, indicate that the novel ICON scheme significantly improves PBLH estimation during daytime convection by reducing systematic overestimation. Conversely, the native CHIMERE convective scheme exhibits superior performance during the sunrise transition, as it better captures the early-morning growth of the PBL compared to the ICON scheme. Generally, the daytime PBLH values yielded by the ICON configuration are lower than those produced by the native CHIMERE scheme. Analysis of nocturnal regimes reveals that the native CHIMERE method for stable conditions leads to significant overestimation during periods of atmospheric stability coupled with low-level jets. In contrast, the novel approach integrated in the ICON framework provides more reliable results under these conditions.
These findings highlight the capability of the updated PBL scheme to provide a more accurate and consistent characterization of the boundary layer within the AQ modeling framework.
How to cite: Beltrami, L., Marsigli, C., Minguzzi, E., Cesari, D., Mouji, N., Bracci, A., and Barnaba, F.: Sensitivity analysis of Planetary Boundary Layer height parameterization in the ICON - CHIMERE modeling chain, EMS Annual Meeting 2026, Utrecht, Netherlands, 6–11 Sep 2026, EMS2026-291, https://doi.org/10.5194/ems2026-291, 2026.
Dry intrusions (DIs) are slantwise descending airstreams in extratropical cyclones transporting high momentum from the upper troposphere towards the planetary boundary layer. Near the surface, this momentum transport can result in gale-force wind gusts that contribute to the damage caused by winter storms in Western Europe. The accurate representation of extreme near-surface winds remains a challenge: The underlying processes contributing to the momentum transport to the surface cover multiple scales and are therefore treated differently in models. Especially small-scale to micro-scale processes such as surface fluxes, the surface roughness variability and the sub-grid surface roughness are not explicitly resolved in current operational models. These processes require an accurate representation, as they affect the downward mixing of the cold and dry air of the DI into the atmospheric boundary layer.
In this study, we first present the dynamic and thermodynamic structure of the lower troposphere in the cold sector during DI events using radiosonde observations at three North Atlantic coastal stations. DI events are identified via backward trajectories based on ERA5 reanalysis data. Trajectories descending at least 400 hPa within 48 hours are classified as DI trajectories. Second, we document potential biases and flow-dependent errors in state-of-the-art operational numerical weather prediction models (ICOsahedral Nonhydrostatic (ICON), Model Integrated Forecasting System (IFS)). We compare operational radiosonde measurements with IFS and ICON model data by analyzing background and analysis departures, as well as analysis increments (difference between background and analysis) for vertical profiles of temperature, humidity and wind. Preliminary results suggest a dry bias in the lower troposphere and a temperature dipole structure within the planetary boundary layer during DI events in both models. The temperature dipole introduces a cold bias near the surface and a warm bias at the transition of the planetary boundary layer to the free troposphere. Based on targeted radiosonde observations of cold front passages from the recent NAWDIC campaign, we analyse the temporal evolution of these biases relative to the time of the cold front passage and investigate mechanisms leading to the model bias. For a detailed case study, the radiosonde observations are complemented by ground- and airborne wind lidar and water vapor measurements.
How to cite: Dürlich, V., Gasch, P., Oertel, A., and Quinting, J.: Investigating model biases in the cold sector of extratropical cyclones using ground-based and airborne observations, EMS Annual Meeting 2026, Utrecht, Netherlands, 6–11 Sep 2026, EMS2026-237, https://doi.org/10.5194/ems2026-237, 2026.
The convective velocity scale is a cornerstone for describing vertical-velocity variance in the bulk of the convective boundary layer driven by surface heating, and is widely used in observations, theory, and operational meteorological models. This scaling relies on the quasi-equilibrium assumption that the surface heat flux varies slowly compared to the adjustment time scale of the large scale convective eddies (or the eddy turnover time). This quasi-equilibrium assumption typically holds over land, from late morning to early afternoon, but breaks down in the late afternoon. Recent idealized large-eddy simulations (LES) of free convection reported departure from the classical convective scaling due to quasi-equilibrium breakdown, and identified the relevant parameters to describe the vertical-velocity variance during the late afternoon transition. Both the characteristic time scale of the surface heat flux decay, τ = ΙH-1dH/dtΙ-1 (H is the surface heat flux), and the convective eddy turnover time, t* = zi / w* (zi is the boundary-layer depth and w* is the convective velocity scale) should be taken into account. During the early afternoon transition when the parameter r = τ / t* is larger than 1, the quasi-equilibrium assumption is satisfied. The departure starts during the late afternoon transition when r approaches 1, and a new scaling regime emerges when r is smaller than 1. In this study, we evaluate these scaling predictions using extensive field observations spanning 264 days. Despite substantial day-to-day variability (due to random errors in the estimation of turbulence statistics or the sensitivity of the turbulent flow to unknown local disturbances not included in the analysis), averaging across multiple days reveals a good agreement between observations and the LES-derived scaling, supporting its validity for describing vertical-velocity variance in this regime.
How to cite: El guernaoui, O., Li, D., and Boukharfane, R.: Field observations validate LES-derived scaling of vertical-velocity variance during the afternoon transition of the CBL, EMS Annual Meeting 2026, Utrecht, Netherlands, 6–11 Sep 2026, EMS2026-310, https://doi.org/10.5194/ems2026-310, 2026.
The convective boundary layer (CBL) plays a central role in moist convection and pollutant dispersion, yet its vertical structure remains difficult to diagnose objectively from radiosonde observations. Traditional approaches, often relying on single-variable gradients, emphasize identifying a single boundary-layer top, which can obscure the complexity of coupled thermodynamic transitions. To address this limitation, we extend the application of the Haar discrete wavelet transform (DWT) to simultaneously analyze vertical profiles of potential temperature (θ) and water vapor mixing ratio (r). This framework decomposes profiles into multiscale means and deviations, enabling diagnosis of multiple structural features, including surface-layer height, entrainment-zone depth and intensity, and boundary-layer height. By leveraging the localization properties of wavelets and explicitly quantifying θ–r covariance across scales, the method captures coupled transitions and reveals structural decoupling that may mask layer boundaries in traditional gradient-based approaches. Applications to high-resolution radiosonde data from the International H2O Project and the ARM Southern Great Plains Central Facility demonstrate that the method reliably identifies canonical CBL structures, while also objectively diagnosing atypical profiles characterized by diffuse gradients or θ–r decoupling. Sensitivity experiments further highlight the impact of vertical resolution, showing that coarse operational soundings smooth critical transitions and introduce systematic biases in diagnosed layer depths. These findings underscore the importance of scale-aware diagnostics for both research-grade and operational datasets. Overall, the enhanced Haar wavelet framework advances CBL analysis beyond single-height detection, providing a comprehensive, multiscale structural diagnosis that captures the complexity of thermodynamic coupling and entrainment processes. This approach offers a robust tool for improving understanding of boundary-layer dynamics and their role in atmospheric convection and pollutant transport.
How to cite: Kang, S. L., Choi, C. R., and Ryu, J. H.: Wavelet covariance approach to measure convective boundary-layer structure, EMS Annual Meeting 2026, Utrecht, Netherlands, 6–11 Sep 2026, EMS2026-312, https://doi.org/10.5194/ems2026-312, 2026.
Conservation agricultural practices, such as maintaining spontaneous weed cover in tree alleys, are a useful measure to reduce soil erosion, improve soil fertility and increase the uptake of CO2 by tree plantations. Particularly, when weeds were allowed to grow in olive tree orchards, CO2 uptake was significantly higher as compared to when the weeds were removed, either mechanically or by using pesticides. The eddy covariance (EC) technique is frequently employed to quantify ecosystem-atmosphere exchanges of trace gases and energy, as it provides non-invasive, continuous and direct measurements of atmospheric components (e.g. CO2) and energy flux densities.
One of the main hindrances in EC studies is the lack of spatial replication. At heterogeneous sites, such as olive groves, variability caused by different patches of vegetation, land cover or irregular orography, can be a significant source of uncertainty, as measured gas or energy balances are only representative of the footprint area of the station, which is not uniformly distributed over the vegetation or land cover types. In recent years, there have been developments of lower-cost EC (LC-EC) setups that open the way to improve the spatial representativeness of EC measurements and therefore gaining more information about the heterogeneity of ecosystems and how it influences flux densities.
In this study we present a year of measurements for two different treatments (weed-free and weed-covered) in an irrigated olive grove in SE Spain, performed with conventional EC setups, with above and below-canopy EC stations. Besides, we present the current state of development of an alternative LC-EC setup.
The newer LC-EC setup is based on previous published versions of similar systems, and consists of a combination of two sensors to measure CO2 and H2O molar densities, integrated with an ultrasonic anemometer. With the reference to above and below-canopy conventional EC setups, three replicates of the LC-EC setup will be installed at different olive grove management systems: traditional, intensive, and super-intensive orchards. The first objective is to validate the newer setups as a tool to potentially increase the number of replicates in EC studies, by performing a thorough setup-to-setup comparison. The second objective is to compare how the different treatments affect year-round CO2, water vapour and energy balance and, therefore, to establish which managements are more beneficial in the context of climate change adaptation and mitigation.
How to cite: Callejas Rodelas, J. Á., Aguirre García, S., Aranda Barranco, S., Serrano Ortiz, P., Kowalski, A. S., and Pérez Sánchez-Cañete, E.: Carbon dioxide and water vapour balance of different management regimes in an olive orchard in SE Spain using conventional and lower-cost eddy covariance measurements, EMS Annual Meeting 2026, Utrecht, Netherlands, 6–11 Sep 2026, EMS2026-394, https://doi.org/10.5194/ems2026-394, 2026.
Persistent low-wind events (LWEs) are a key driver of wind-power variability in Europe, yet their vertical structure is rarely considered explicitly in related studies. Most existing studies quantify LWEs using single-level wind speeds, failing to distinguish between vertically coupled weak-flow regimes and near-surface decoupling within the atmospheric boundary layer (ABL). Because wind turbines operate within the ABL, understanding the coupling between near-surface and hub-height winds is essential for characterizing wind-drought risks.
In this study, we propose a two-level classification of European LWEs using ERA5 reanalysis data for 1979-2024. By comparing 10 m and 100 m wind speeds, we distinguish deep LWEs, in which weak winds extend through both levels, from shallow LWEs, in which near-surface winds are weak while turbine-height winds remain available. Using this framework, we analyze the European climatology of LWEs and their associated boundary-layer structure.
Our results show that shallow and deep LWEs are physically distinct weak-wind regimes. Shallow LWEs are mainly a land phenomenon and are concentrated in the cold season. They are associated with strong vertical shear, weakly mixed boundary-layer conditions, and weak or downward surface sensible heat fluxes, consistent with stable and vertically decoupled flow. Deep LWEs show smaller shear and more vertically coupled weak-flow conditions. Their seasonal behavior depends strongly on surface type: over land they are most common in the cold season, while over the ocean they peak in the warm season.
Sensitivity tests using cut-in thresholds between 3 and 4 m s⁻¹ show that the main spatial and seasonal patterns are robust. The classification provides a useful physical framework for future work on low-wind predictability, atmospheric blocking and wind-energy risk.
How to cite: Zhou, L. and Esau, I.: Low-wind events in Europe: the role of vertical structure and ABL coupling, EMS Annual Meeting 2026, Utrecht, Netherlands, 6–11 Sep 2026, EMS2026-416, https://doi.org/10.5194/ems2026-416, 2026.
Over last few years we have observed tens of atmospheric gravity wave (GW) events on our operational network and research instruments in The Netherlands, including the North Sea. These events are ducted GWs that are trapped in the stable boundary layer and propagate horizontally, characterized by a near-monochromatic wave with vertical velocity amplitudes up to a few m/s, a period of a few minutes, a duration of one hour or more, and a spatial extent on the order of 100 km or more. The origin of those GWs are non-orographic, and likely linked to weather fronts. These GWs can lead to a strong modulation of wind, temperature and humidity in the lower 100m’s of the atmosphere.
We observe these GWs by various instruments, including Doppler lidars, automatic lidar ceilometers, weather radars, (tower) in-situ sensors and microbarometers that are part of the KNMI synoptic observation network, Cabauw atmospheric research station or North Sea wind lidar network. Together they provide 3D information on mesoscale GWs, including vertical profiles, horizontal spatial extent, and evolution during propagation. We have recently highlighted two GW events: in the Netherlands and Belgium in the night and early morning of June 30, 2022 [1], over the North Sea in the night and early morning of May 2, 2025 [2].
Here we give an overview of our research on boundary-layer GW observations. We explore the occurrence, properties and impact of those GW events in the Netherlands, both onshore and offshore. These comprehensive sets of observations may serve as a testbed for high resolution weather models that aim to capture mesoscale GW events and the effect they have on the (lower) atmosphere. The siting of our North Sea wind lidars, i.e., in the middle of large offshore wind farms, provides the possibility to study the effect of GWs on wind farm performance.
[1] Knoop S, Assink J D, Leijnse H, Tijm S, de Haij M J, Bosveld F C, Theeuwes N E, Evers L G, Unal C and Laffineur Q, “High-resolution observations of a mesoscale gravity wave event in the nocturnal boundary-layer over The Netherlands and Belgium”, submitted to Journal of Geophysical Research: Atmospheres (preprint on https://doi.org/10.22541/essoar.176478826.60490095/v1)
[2] Knoop S, Assink J D, de Haij M J, de Jong M, “Doppler lidar gravity wave observations within North Sea wind farms”, to be published in Journal of Physics: Conference Series (Torque 2026)
How to cite: Knoop, S. and Assink, J.: Atmospheric boundary-layer gravity wave observations in the Netherlands, EMS Annual Meeting 2026, Utrecht, Netherlands, 6–11 Sep 2026, EMS2026-456, https://doi.org/10.5194/ems2026-456, 2026.
Accurate forecasting of wind gusts is essential for anticipating severe weather events, given their significant impact on infrastructure, transportation, and public safety. Wind gusts are strongly influenced by atmospheric boundary-layer processes, particularly turbulence and vertical momentum transport, which remain challenging to represent in numerical weather prediction models.
This study evaluates and compares the performance of two wind gust diagnosing methods: (1) an IFS-based scheme, which computes gusts using turbulent velocity scales derived from surface stress and atmospheric stability; and (2) a NOAA-based scheme (UPP method), which estimates maximum gust potential through momentum transport from the top of the boundary layer. Both diagnosing methods are implemented within a consistent high-resolution numerical modelling framework and validated using data from representative stations of the Basque Country automatic weather station network. The modelling setup includes a state-of-the-art turbulence closure scheme (MYNN) and a surface-layer formulation that accounts for thermal roughness as a function of vegetation height, ensuring a coherent representation of boundary-layer processes and surface–atmosphere interactions.
The validation covers approximately two years, focusing on severe wind events that occurred in 2024 and 2025 and had a notable impact in the Basque Country. The analysis distinguishes between exposed (e.g., coastal and elevated) and non-exposed (e.g., urban and valley) stations, reflecting differences in surface characteristics and boundary-layer conditions.
Results show that the NOAA diagnosing method generally performs better for non-exposed stations, while the IFS scheme—despite a general tendency to underestimate wind gusts—captures peak gusts more accurately in exposed locations. The findings also reveal sensitivity to wind direction and background atmospheric conditions, highlighting the influence of boundary-layer structure on gust representation. Although preliminary, this study provides a consistent evaluation of gust-diagnosing approaches under different boundary-layer regimes, supporting future improvements in operational wind forecasting at Basque Meteorology Agency (Euskalmet).
How to cite: Arrillaga, J. A., R. Gelpi, I., Egaña, J., and Gaztelumendi, S.: Assessing Wind Gust Diagnosing Methods over Complex Terrain in the Basque Country, EMS Annual Meeting 2026, Utrecht, Netherlands, 6–11 Sep 2026, EMS2026-578, https://doi.org/10.5194/ems2026-578, 2026.
During Arctic winter conditions, turbulent fluxes play a major role in coupling the sea-ice surface with the atmosphere aloft. Since climate models often struggle to accurately model this turbulent exchange in the predominantly stably stratified Arctic boundary layer (BL) it is still a major source of model uncertainty [1]. Therefore, we implemented a new turbulent flux parametrization [2] into ICON-NWP based on observations from the SHEBA campaign in 1997 and evaluate the simulations with observations from the MOSAiC expedition [3]. Results are shown for Pan-Arctic simulations displaying both local and regional effects on the BL. The new parameterization improves fluxes in very stable situations by strongly reducing turbulent heat fluxes. However, tuning of the diffusion scheme to reduce the flux bias in weakly-stable regime contribute most to the overall model improvement. The reduction of the turbulent heat fluxes led to more realistic BL temperature profiles and stronger sea-ice thickness growth during the winter season. Finally, we assess the impact of the improved turbulence representation in coarser-resolution pan-Arctic ICON simulations over climatological timescales to investigate its relevance for climate applications.
This work was supported by the DFG funded Transregio-project TRR 172 “Arctic Amplification (AC)3“.
References
[1] A. Solomon, M. Shupe, G. Svensson, N.P.Barton, Y.Batrak: „The winter central Arctic surface energy budget: A model evaluation using observations from the MOSAiC campaign” Elementa – Science of the Anthropocene, Vol.11, 1, (2023), DOI 10.1525/elementa.2022.00104.
[2] V. Gryanik u. C.Lüpkes, „New Modified and Extended Stability Functions for the Stable Boundary Layer Based on SHEBA and Parametrizations of Bulk Transfer Coefficients for Climate Models“, Journal of Atmospheric Science Vol.77, 8, (2020), DOI 10.1175/JAS-D-19-0255.1.
[3] Cox J. et al., „Continuous Observations of the Surface Energy Budget and Meteorology over the Arctic Sea Ice during MOSAiC“.
How to cite: Gebhardt, F., Handord, D., and Lüpkes, C.: The effect of improved turbulence parametrization in ICON simulations of Arctic boundary layers , EMS Annual Meeting 2026, Utrecht, Netherlands, 6–11 Sep 2026, EMS2026-661, https://doi.org/10.5194/ems2026-661, 2026.
Sea-ice cover exerts important controls on the Arctic climate and may form horizontally heterogeneous patterns, especially in the marginal ice zone (MIZ). Earth System Models (ESMs) represent the sea-ice heterogeneity within a grid cell as an ice fraction. The heterogeneous sea-ice cover, however, causes complex nonlinear surface-atmosphere interacting processes that cannot be quantified appropriately using solely the ice fraction. Among the nonlinear interacting processes are the secondary circulations in the atmospheric boundary layer (ABL) that are driven by the sea-ice and ocean water surfaces and their thermal contrast. An effective representation of the surface-atmosphere momentum, temperature and moisture exchanges for a grid cell of an ESM should accommodate for the occurrence of secondary circulations. This is of particular relevance when leads evolve in the sea ice. These elongated cracks in the sea-ice cover expose local regions of open ocean water with surface temperatures much higher than the surrounding sea ice. As a result, convective plumes develop above leads. Even if leads occupy a small areal fraction only, their impact on the regional temperature, atmospheric stability over sea ice, and surface-atmosphere fluxes in winter is disproportionally large.
To quantify and parameterise secondary circulations related to leads, we extend a thermal heterogeneity parameter [1], which defines the ratio between buoyancy effects of surface thermal contrasts to the inertia of the mean flow. This extension incorporates factors such as temperature difference between the sea-ice and water surfaces, the angle between geostrophic wind and lead orientation and typical length scales. Data are used from the Boundary layer and Aerosol and Cloud Study in the Arctic II (BACSAM II) flight campaign, where turbulence was measured at two different heights simultaneously: on an aircraft and 60 m below the aircraft using a passive trailing body called T-bird. The aircraft data are analysed with a wavelet transform, enabling a multiscale decomposition to extract a mesoscale contribution to the fluxes. Surface temperature characteristics are obtained from the Modis global Level-2 product (resolution: 1 km). A case study reveals a strong correlation between thermal heterogeneity parameters and mesoscale flux contributions for 20 km subintervals with 1 km rolling steps along the flight legs. The correlation is enhanced for leads oriented normal to wind, and when fetch dependent downstream effects are included.
[1] Margairaz, Fabien & Pardyjak, Eric & Calaf, Marc. (2020). Surface Thermal Heterogeneities and the Atmospheric Boundary Layer: The Thermal Heterogeneity Parameter. Boundary-Layer Meteorology. 177. 1-20. 10.1007/s10546-020-00544-7.
How to cite: Staudinger, I. and Vercauteren, N.: Representing sea-ice heterogeneities and the Arctic boundary-layer using a thermal heterogeneity parameter, EMS Annual Meeting 2026, Utrecht, Netherlands, 6–11 Sep 2026, EMS2026-639, https://doi.org/10.5194/ems2026-639, 2026.
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