Atmospheric boundary layer (ABL) dynamics in complex terrain are inherently three-dimensional, where microscale turbulence plays a critical role in driving larger-scale flow evolution. With numerical weather prediction models approaching sub-kilometer resolutions, it is increasingly important to challenge and validate the models on the small scales with high-resolution observations. Contributing to the TEAMx goal of understanding scale interactions, we present results from an intensive field experiment conducted at the Nafingalm, a pasture at the valley head of a tributary to the Inn Valley (Austria).

The experimental site, a north-south aligned valley system approximately 2x2 km wide and 500 m deep, was instrumented during the Summer 2025 Extended Observation Period (EOP). The setup included two scanning lidars, a profiling lidar, and a network of ground-based meteorological stations. These continuous observations were augmented by the SWUF-3D fleet of multicopter drones (aka Uncrewed Aircraft Systems, UAS) between 1 and 23 July 2025. Up to 30 UAS were operated simultaneously, reaching heights of 220 m above the valley floor to collect distributed measurements of 3D wind, temperature, humidity, and pressure in regions inaccessible to traditional instrumentation.

While continuous lidar scanning mapped the along- and cross-valley flow, the UAS fleet provided direct in situ validation of the assumptions required to derive turbulence statistics from remote sensing. Furthermore, the spatial distribution of the drones allows for direct measurement of shear contributions, buoyancy, and advective tendencies. We present preliminary analyses of two contrasting Intensive Observation Periods (IOPs): one characterized primarily by thermally driven flow and another with increased mesoscale forcing. These cases highlight the strength of synthesizing remote sensing with distributed UAS measurements to resolve scmall-scale dynamics in complex terrain.

How to cite: Wildmann, N., Alexa, A., Lappin, F., Wiech, A., and Gohm, A.: Synergistic Observations of Flow in Complex Terrain: Integrating Lidar and Drones During the TEAMx Campaign, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4439, https://doi.org/10.5194/egusphere-egu26-4439, 2026.

The summer Extended Observation Period (sEOP) of the 2025 TEAMx observational campaign provided a unique opportunity to investigate multi-scale interactions in the mountain boundary layer (MoBL) and enhance our understanding of its structure and turbulent processes. In this study we present a preliminary analysis of observations from two Doppler wind lidars operated in the Weer Valley (Nafingalm, Austria) from 6 June to 24 July 2025. The focus is on 12 Intensive Observation Periods (IOPs) for which complementary airborne observations are available (June 29 and July 2, 5, 9, 11, 13, 15, 18, 19, 20, 22, and 23), though the latter are not included in this initial study. The aim is to characterize observed events by classifying different flow regimes and turbulent features, primarily using Doppler wind lidar data supported by a weather station network. This classification provides a framework for future in-depth studies and large-eddy simulations. Emphasis is placed on identifying recurring scale interactions between local and regional flows—such as valley winds and cross-mountain flows—and the resulting processes, including flow separation, waves, and shear-flow instabilities. Finally, initial turbulence metrics are calculated to support the event classification.

How to cite: Gohm, A., Wildmann, N., Alexa, A., and Wiech, A.: Lidar-based observations of flow regimes and turbulence in a small Alpine valley during TEAMx, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12279, https://doi.org/10.5194/egusphere-egu26-12279, 2026.

How to cite: Kippenberger, M., Schön, M., Ruhl, M., Wahl, E., Freddi, G., Gohm, A., Lehner, M., Bange, J., and Platis, A.: Measuring Horizontal Shear and Turbulence in Mountain Valleys using UAS and Lidar, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20164, https://doi.org/10.5194/egusphere-egu26-20164, 2026.

During the TEAMx winter EOP, an extensive measurement campaign took place on a steep undulating slope in the Inn Valley, Austria. This six-week long campaign featured a suite of instrumentation, including a network of eight turbulence towers installed at two across-slope and an along-slope transects equipped with two levels of sonic anemometers, nano-barometers, and slow response sensors. In addition, four component radiation at two heights measured radiative flux divergence at a central location on the slope, a short-range Doppler wind lidar (Wind Ranger) at the bottom of the slope recorded wind speed and direction, while a fibre optic array at one along-slope and two-across slope transects, and two vertical sections complemented the set-up. Additional observations during intense observational periods included temperature profile measurements using a drone and wind speed and temperature observations using tethered balloon at the top of the slope.

Here we present the measurement campaign design and focus on the first results that highlight the spatio-temporal variability of the flow on the slope, tightly coupled with the synoptic forcing. During conditions of low synoptic forcing, persistent katabatic flows developed on the slope with acceleration down the slope and warmer conditions towards one side of this non-uniform slope. On the other hand, during foehn conditions, very large differences in the mean and turbulence characteristics can be observed between the upper across-slope transect and lower stations that are more exposed to foehn. These differences translate to distinct behaviour of similarity scaling relations, as well as the importance of different terms in the momentum and TKE budgets. 

How to cite: Stiperski, I., Brun, C., Ghirardelli, M., Gohm, A., Rotach, M., and Lehner, M.: Flow Regimes and Turbulence Structure on a Steep Slope in Winter: Findings from the TEAMx wEOP, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9768, https://doi.org/10.5194/egusphere-egu26-9768, 2026.

We present the intensive field campaign conducted from mid-June to mid-October 2025 on Monte Baldo (Italian Pre-Alps) within the TEAMx programme, aimed at improving process understanding and model representation of mountain boundary-layer exchanges. This effort was driven by the DECIPHER project, which aims at disentangling mechanisms controlling atmospheric transport and mixing processes over mountain areas at different space- and timescales.

Measurements targeted a steep (~25°), east-facing, grass-covered slope in the southern Monte Baldo range, selected for its regular topography and pronounced diurnal cycle of thermally driven slope winds. The setting also enables investigation of coupling at the mountain–plain interface, linking local slope circulations to the adjacent lowland atmosphere in the Po Valley.

A coordinated suite of instruments captured processes from the surface layer to the lower troposphere and their interactions across scales. Near-surface thermodynamic variability and turbulent exchange were monitored using multi-level flux towers and a slope-wide network of thermohygrometers. Variability in aerosol and particulate matter was measured using co-located mass and optical sensors. Along- and cross-slope winds were observed with multiple wind lidars, while boundary-layer and lower-tropospheric profiles were obtained with a tethered balloon system and a Raman lidar. The surface heat budget was characterized using radiation measurements together with soil temperature and moisture observations. Complementary observations included high-frequency near-surface turbulence profiling and distributed soil-moisture monitoring using a cosmic-ray neutron sensor.

This contribution details the observational setup, characterizes the regional setting, and illustrates the potential of the dataset for evaluating slope-wind structure and the associated surface fluxes, boundary-layer mixing, and exchange pathways between mountains and adjacent plains.

How to cite: Doglioni, G., Carpentari, S., Giovannini, L., and Zardi, D. and the Monte Baldo partners: The observational effort for characterising turbulence and transport processes on the pre-Alpine range of Monte Baldo during the TEAMx Observational Campaign. , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18348, https://doi.org/10.5194/egusphere-egu26-18348, 2026.

Quantifying the exchange of mass, momentum and energy between the earth’s surface and the atmosphere is pivotal for the understanding and prediction of weather and climate processes. Due to the superposition of horizontal and vertical transport, exchange processes in complex terrain are especially efficient and important.

This contribution presents a new project embedded in the international TEAMx campaign. The LIVAVERT(EX)² project - linking valley flow and vertical exchange in complex terrain - focuses on observing exchange processes in the Sarntal Alps region, a local hotspot of convection initiation in the Alps. As part of the project, a novel airborne Doppler lidar (ADL) is deployed for the first extended measurements in complex terrain. Thereby, 3D wind observations are available at 100 m along-track and vertical resolution, providing spatially resolved insight into valley wind systems and vertical exchange. The variability observed across repeated flights enables the differentiation between recurring and transient features.

TEAMx also encompasses a KITcube deployment in the Sarntal Alps region, which establishes an extensive meso-scale ground-based Doppler lidar (GDL) network. Through the comparison of ADL and GDL observations of valley flow, the Doppler lidar wind profiling accuracy and representativeness can be validated. Additionally, new ways to validate existing GDL-based volume flux estimation methods are created. Combining volume flux budget and direct vertical exchange observations allows a more quantitative insight into valley flow and its relation to convective initiation over the surrounding mountains than ever before. Overall, the LIVAVERT(EX)² project aims to improve our understanding and the prediction of atmospheric processes in complex terrain.

How to cite: Gasch, P. and Schaeffler, L.: Linking valley flow and vertical exchange in complex terrain – the LIVAVERT(EX)2 project, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4800, https://doi.org/10.5194/egusphere-egu26-4800, 2026.

How to cite: Banyard, T., Hindley, N., Orr, A., Wright, C., Gumber, S., and Ross, A.: Dual Radiosonde Soundings of Gravity Wave Breaking over the Alps during the 2025 Winter TEAMx Observational Campaign, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20048, https://doi.org/10.5194/egusphere-egu26-20048, 2026.

This contribution presents an overview of the activities and results of the INTERFACE project, which aims to quantify the non-closure of the surface energy balance across various Alpine sites, where processes related to the lack of closure, i.e., advection due to the development of thermally-driven circulations, are expected to be particularly significant. This objective is addressed by combining flux station and unmanned aerial system (UAS) measurements. The UAS provides spatially distributed observations around eddy-covariance sites, which are essential for estimating advection.

The analysis of eddy-covariance data from various sites representing diverse Alpine contexts (e.g., valley floor, slope, and mountain top) and climatic settings (North vs. South of the main Alpine crest) allows a systematic quantification and comparison of the characteristics of the surface energy balance, including the lack of closure. Particular emphasis is placed on the evaluation of the role of thermally-driven circulations in the non-closure of the surface energy balance, utilizing objective criteria to select days with well-developed slope and valley winds.

The INTERFACE project contributes to the TEAMx international research programme, which aims to improve our understanding of exchange processes in the atmosphere over mountains.

How to cite: Giovannini, L., Carpentari, S., Destro, M., Di Santo, D., Lehner, M., Monsorno, R., Rotach, M. W., Sankar, M. S., Saunders, B., Soltaninezhad, M., Tondini, S., Vendrame, N., and Zardi, D.: Investigating the surface energy balance closure over mountain areas: results from the INTERFACE project, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13553, https://doi.org/10.5194/egusphere-egu26-13553, 2026.

In the greater TEAMx framework and following HEFEX in 2018 and HEFEXII in 2023, the third HinterEisFerner EXperiment (HEFEXIII) took place in August and September 2025 on Hintereisferner glacier, Tyrol, Austria. During one month, a 9 m tower was deployed on the glacier, equipped with high-frequency measurements of three-dimensional wind components and temperature at five levels (0.5 m, 1 m, 3 m, 5 m and 9 m), as well as low-frequency measurements of two-dimensional wind components, temperature, and relative humidity at three additional levels (2 m, 4 m and 7 m). Furthermore, two lidar systems and a swarm of drones strategically measuring at various locations along the glacier axis, were used to assess the flow conditions in the atmosphere above the glacier. This dataset enables an evaluation of boundary-layer characteristics close to the surface while relating them to the flow in the larger mountain boundary layer above.
Here, we describe the general meteorological conditions observed during the campaign, as well as the turbulence characteristics. We contrast periods with an undisturbed boundary layer with conditions characterised by external disturbances, such as thermally driven upvalley winds or mountain waves on the glacier boundary layer and their influence on the katabatic flow. We focus specifically on vertical profiles of temperature and wind obtained during one of the drone IOPs, as well as the average turbulent fluxes of momentum and heat, and the budget terms of the turbulent kinetic energy at the different measurement heights. 

How to cite: Schlagbauer, L., Stiperski, I., Georgi, A., Sauter, T., and Nicholson, L.: First Results from HEFEXIII - current state of the Hintereisferner boundary layer, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5662, https://doi.org/10.5194/egusphere-egu26-5662, 2026.

Glacier boundary layers present an ideal atmospheric laboratory for studying persistently stable boundary-layer dynamics over inclined surfaces. On glaciers in summer, turbulence is strongly controlled by the katabatic flow dynamics that is intimately coupled with very stable stratification at the glacier surface and the slope angle. In these kind of conditions, the basic assumptions of Monin–Obukhov Similarity Theory (MOST) are rarely met, due to the significant flux divergence, and the imposition of an alternative limiting scale. Still, bulk approaches based on MOST have shown good agreement under very stringent conditions, while alternative scaling approaches that add the slope angle into the scaling parameter, or use jet maximum height have shown promise in providing scaling frameworks for such flows.

Here we use a dense network of atmospheric turbulence observations during the HEFEX II campaign, that took place on the Hintereisferner Glacier, Austria in 2023. The campaign features ten turbulence towers with multi-level observations, distributed across the entire glacier surface (from the accumulation area to downstream of the glacier tongue) and therefore experiencing different flow conditions (katabatic flow depth) or slope angles. We focus on the mathematical invariant representing turbulence anisotropy that has recently been used to extend MOST to more realistic terrain conditions. Focusing on the flux-variance relations we show that katabatic flows over glaciated terrain display distinct turbulence characteristics at varying degrees of anisotropy that differ considerably to the previous studies over non-glaciated terrain. These peculiarities are further examined to isolate the difference between katabatic and canonical flows in terms of their flow anisotropy. We also test alternative scaling approaches, including those based on the katabatic jet height, local terrain slope, and formulations designed to avoid the self-correlation that is shown to be an issue in very stable stratification.

How to cite: Mosso, S. and Stiperski, I.: Anisotropy scaling of a sloping glacier boundary layer, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9639, https://doi.org/10.5194/egusphere-egu26-9639, 2026.

High-resolution numerical weather prediction (NWP) models are increasingly being used to study the interactions between the atmosphere and glaciers in complex alpine terrain. However, their performance under these conditions has not been sufficiently confirmed by observations, especially at a dekameter scale. This study comprehensively validates the Large-Eddy Simulation (LES) configuration of the ICOsahedral Nonhydrostatic (ICON) model using observations from the HEFEX II (2023) and HEFEX III (2025) field campaigns. Both campaigns included four weeks of intensive observations at Hintereisferner in the Ötztal Alps and were part of the international TEAMx research program, which studies multi-scale transport and exchange processes in mountainous environments.

HEFEX II focused on characterizing the spatial gradients and temporal variability of surface-layer variables, such as temperature, humidity, and wind. HEFEX III utilized coordinated UAV-based vertical profiling in combination with multiple on-glacier lidar systems to resolve atmospheric flow fields and wind patterns within the valley. Together, the two campaigns provide a unique and unprecedented observational dataset in complex glacierized terrain, offering an exceptional basis for model evaluation.

ICON-LES was applied in a one-way nested configuration, achieving a target horizontal resolution of 51 meters over the study area. We assessed model performance using qualitative and quantitative validation approaches, particularly emphasizing the model’s ability to reproduce the spatio-temporal variability of key atmospheric parameters across surface and boundary-layer scales. The results demonstrate strong agreement between ICON-LES simulations and multi-platform observations, indicating that the model realistically captures flow structures and variability in a high alpine glacier environment.

These findings support the use of ICON-LES as a reliable tool for studying atmosphere-glacier interactions and lay the groundwork for future climate impact and feedback studies in complex terrain. At the same time, the analysis highlights the current limitations of high-resolution numerical modeling and emphasizes the importance of using advanced observational techniques and large-eddy simulations together to improve our understanding of processes in mountainous regions.

How to cite: Georgi, A., Sauter, T., and Schlagbauer, L.: Validation of High-Resolution ICON-LES Using Observations from HEFEX II and HEFEX III Field Campaigns, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11044, https://doi.org/10.5194/egusphere-egu26-11044, 2026.

Traditional planetary boundary layer (PBL) parametrisations in numerical weather prediction (NWP) models assume horizontally homogeneous conditions. Under this assumption, one-dimensional (1D) PBL parametrisations are used, which only consider vertical mixing and neglect horizontal shear production in the prognostic turbulent kinetic energy (TKE) equation used by 1.5-order parametrisations. However, as high-performance computing capabilities continue to improve, NWP model resolutions are reaching the hectometre scale, resolving more surface features and smaller atmospheric processes, thus increasingly violating the 1D PBL assumption. This is especially true in complex terrain, where, for example, thermally driven circulations create persistent slope and valley winds characterised by intense shear in both horizontal speed and direction.

We set up nested simulations with the Weather Research and Forecasting (WRF) model for the Inn Valley, Austria, down to a hectometre-scale resolution using a modified PBL parametrisation that introduces an additional tendency for horizontal shear production into the TKE equation. This helps to account for horizontal heterogeneity in the atmosphere induced by local flow processes and acts as an intermediate step towards a complete representation of horizontal wind shear.

During the TEAMx 2025 summer Extended Observation Period (sEOP), uncrewed aircraft systems (UAS) measured vertical profiles – including TKE and turbulent fluxes – at multiple locations along a transect across the Inn Valley. Complementary radiosoundings and remote-sensing measurements captured mean wind and temperature profiles at various locations along the valley. These observations allow us to evaluate modelled vertical and horizontal wind shear, as well as turbulent properties. Results using the modified PBL parametrisation are compared with those using the traditional PBL parametrisation, which does not take into account horizontal wind shear.

How to cite: Wahl, E., Freddi, G., Gohm, A., Platis, A., Kippenberger, M., and Lehner, M.: The Role of Horizontal Shear Production in Hectometre-Scale WRF Simulations over Alpine Terrain, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7284, https://doi.org/10.5194/egusphere-egu26-7284, 2026.

Mesoscale numerical weather prediction (NWP) models typically employ planetary boundary layer (PBL) schemes to represent subgrid-scale turbulence, including 1.5-order parameterizations that explicitly predict turbulent kinetic energy (TKE). One crucial assumption of these models is that horizontal gradients, such as horizontal shear, can be neglected in the TKE tendency terms. This assumption may not hold in complex terrain, where the interaction between terrain-induced flows and the orography itself can create substantial horizontal mixing.

We investigate this limitation using a high-resolution (Δx=500 m) WRF model simulation employing the traditional 1.5-order Mellor–Yamada–Nakanishi–Niino (MYNN) PBL scheme. We focus on a valley wind case in the Inn Valley, Austria, which occurred on 29 June 2025 during the TEAMx campaign. The event was characterized by clear skies and weak synoptic forcing, favoring the development of a convective boundary layer and thermally driven daytime up-valley winds with substantial mechanical mixing.

The simulations are compared against observational data from four Doppler wind lidars and several ground measurement stations in the valley. The evolution of the wind system is represented reasonably well by the model, but the peak strength of the valley wind is underestimated. Observations from one of the lidars show that the PBL scheme appears to underestimate TKE when the turbulence is dominated by mechanical production. This bias may result from the lower wind speeds or an incomplete representation of TKE production in the PBL scheme, with potential interactions between the two factors. An estimate of the horizontal subgrid-scale diffusion suggests that accounting for the currently neglected horizontal shear production in the TKE equation could lead to an improved TKE representation.

How to cite: Freddi, G., Gohm, A., Wahl, E., and Lehner, M.: Underestimation of Mechanically Generated Turbulence in a Traditional PBL Scheme over Complex Terrain: A TEAMx Case Study for the Inn Valley, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7862, https://doi.org/10.5194/egusphere-egu26-7862, 2026.

The atmospheric boundary layer (ABL) over mountainous terrain plays an important role in modulating the exchange of momentum, heat, and moisture between the surface and the free atmosphere. Unlike flat terrain, where boundary layer dynamics are relatively homogeneous, the mountain boundary layer (MoBL) exhibits pronounced heterogeneity driven by the complex interplay of multiscale orographic features. These interactions generate a broad spectrum of atmospheric motions, from turbulent eddies and coherent thermals to thermally and dynamically induced slope and valley flows. Understanding this complexity is essential for improving weather prediction, climate modeling, and air quality assessment in mountainous regions. This study investigates the structure and dynamics of the convective boundary layer (CBL) over highly complex terrain during a TEAMx test flight on 18 September 2024. Specifically, we address the following questions: What are the dominant characteristics of coherent structures in the CBL? How stationary are these features in space and time? What is their diurnal cycle? How does the model compare to observations?

To address these questions, we employ the ICON model in large-eddy simulation (LES) mode at a horizontal resolution of 65 m, using a nested domain configuration (520 m to 65 m) to capture processes across scales. The simulation domain encompasses a region around the Sarntal Alps, one of the TEAMx target areas. The ICON-LES results are compared with novel airborne wind measurements obtained during a test flight of the AIRflows system aboard the TU Braunschweig Cessna F406 research aircraft. AIRflows delivers high-resolution, three-dimensional wind profile measurements along the aircraft track, providing a unique opportunity to validate and evaluate the LES output in real atmospheric conditions. Preliminary results reveal a complex, spatially variable CBL structure with persistent thermal features and localized regions of enhanced turbulence. The comparison with AIRflows data confirms the presence and spatial organization of key dynamical structures captured by the model, while also highlighting discrepancies that inform model improvement. This work contributes to a deeper understanding of the CBL in mountainous regions and demonstrates the value of combining advanced numerical simulations with targeted airborne observations for model validation and process studies.

How to cite: Schmidli, J. and Gasch, P.: Structure of the convective boundary layer over complex terrain: ICON-LES and high resolution 3D wind observations during a TEAMx test flight, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1856, https://doi.org/10.5194/egusphere-egu26-1856, 2026.