Cold-air pools pose a significant challenge for numerical weather prediction models, primarily because they are often characterized by very stable stratification. Traditional surface-layer parameterizations, which rely on Monin-Obukhov similarity theory, tend to be ineffective in these conditions. Additionally, the processes involved are highly localized and occur on small spatial scales, necessitating models with high horizontal and vertical resolutions. Within the TEAMx research programme, a model intercomparison study is being undertaken. The goal is to assess how well various numerical weather prediction models, each with a horizontal grid spacing of 1 km, can simulate a nocturnal cold-air pool within an Alpine valley. Five models are currently participating in the intercomparison study, including both operational and research models, specifically AROME, ICON, Meso-NH, the Unified Model, and WRF. For the intercomparison, a case study was selected from a multi-day undisturbed period during the PIANO (Penetration and Interruption of Alpine Foehn) field campaign conducted in the Inn Valley, Austria, in fall 2017.

The presentation will show first results from the model intercomparison study. An extensive dataset is available for model evaluation from the PIANO measurement campaign, including vertical profiles of wind, temperature, and humidity from multiple Doppler wind lidars and a microwave temperature and humidity profiler, surface observations including surface-energy fluxes from multiple automatic weather stations and eddy-covariance stations, and spatially distributed temperature measurements from a dense network of temperature sensors. The model evaluation and intercomparison covers the entire lifecycle of the cold-air pool, from its initial formation in the afternoon to its dissipation the following day. The analysis addresses specifically the question of how well the simulations represent the cold-air pool's characteristics, such as its intensity, vertical structure, and spatial heterogeneity, and the processes contributing to the temporal evolution and spatial structure of the cold-air pool.

How to cite: Lehner, M., Goger, B., Quimbayo Duarte, J., Rodier, Q., Schmidli, J., Sheridan, P., Staquet, C., Wastl, C., Westerhuis, S., Wibmer, B., Wieser, H., and Wittmann, C.: TEAMx cold-air pool model intercomparison study, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-504, https://doi.org/10.5194/ems2024-504, 2024.

in the atmosphere over mountains – programme and experiment (http://www.teamx-programme.org/)  are also presented.

How to cite: Farina, S. and Zardi, D.: Surface-layer scaling of velocity, temperature and transport processes in thermally-driven slope winds, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-975, https://doi.org/10.5194/ems2024-975, 2024.

Katabatic winds are gravity flows generated at night by radiative cooling at the ground surface. They are mainly observed in winter during anticyclonic weather episodes associated with stratification and temperature inversion in the lower troposphere. Although their intensity is relatively low, they play a major role in areas with complex terrain, as they systematically contribute to the generation of cold air pools in the valley. There are a large number of in situ observations of the katabatic process along gentle slopes such as those reported in valleys or glaciers but far fewer along steep alpine slopes in the mountains. The katabatic wind consists of a turbulent wall jet along the slope coupled to a turbulent thermal boundary layer cooled at the surface, both subject to the effects of gravity. Little is known about the flow region below the maximum jet velocity, very close to the surface. It is aimed here to deepen general knowledge of the turbulent properties of katabatic winds in this region, which is rarely observed in situ for technical reasons. We further propose and validate a set of laws of the wall necessary for the definition of surface boundary conditions for the appropriate use of regional models over complex terrain. A recent campaign of in situ measurements has been carried out on 3-15 February 2023 in the French Alps near Grenoble. A continuous set of measurements is available from the ground up to a height of 1m, i.e. close to the maximum height of the jet, using a multi-hole 3D velocity probe mounted on a high-precision vertical displacement system. Measurements were taken from z=2 mm close to the snow surface (z+ = 20) to z=1 m high (z+ = 10000). In addition, a thermocouple and an infrared radiation probe were used to determine the temperature profile at the same vertical positions. Firstly, direct measurement of the w velocity component normal to the slope shows a significant contribution towards the ground of the order of 10% of the maximum upslope velocity of the katabatic jet. The trend is well described by a simplified analytical model derived from the Navier-Stokes equations. This model reveals the main role of buoyancy close to the surface, quantified by the strong cooling at ground level and the steep slope. Secondly, measured temperature profile deviates from the logarithmic distribution expected on flat ground. This study will be transposed to a more comprehensive planned as part of the European alpine observation campaign TeamX in 2024-2026 in the Austrian Alps near Innsbruck. A pre-installation was carried out this winter 2024 on the slopes overlooking the Inn valley at InnBox station NF27.

How to cite: Brun, C., Bessem, W., Dublanche, A., Lagauzère, M., and Pioz, S.: Turbulent boundary layer measurements near the surface below the jet maximum in a katabatic wind along alpine slope., EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-616, https://doi.org/10.5194/ems2024-616, 2024.

The presence of Low-Level Jets (LLJs) in the nocturnal boundary layer over complex terrain has proven to generate complex interaction with turbulence. Of notice, LLJs represent one of the most common mechanisms triggering the occurrence of the so-called “up-side down” configuration of the nocturnal boundary, where turbulence kinetic energy (TKE) is transported downward to the surface instead of upwards as in a traditional boundary-layer configuration. In this context, we combined eddy-covariance measurements from flux towers with tethered balloon soundings collected in an open valley to investigate turbulence regimes and characteristics in the stably stratified layer between the nose of the nocturnal LLJ and the ground. Data for the analysis are taken from the existing database of the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program, considering measurements collected within the valley floor at the Dugway Proving Ground (Utah, USA) during nights of weak synoptic forcing. The theoretical frame is given by the budget equations of the second-order moments (SOMs), taking into account explicitly the effects of the unsteadiness and the divergence of third-order moments (TOMs), under the simplifying assumption of horizontal homogeneity. The focus of this presentation is on 'simple' cases, characterized by a well-defined shape of the LLJ nose and negligible rotation of the wind with height. Although unsteady, such cases exhibit time derivatives of the normalized SOMs negligible in the relevant budget equations. The traditional resistance, or defect law is rephrased in terms of the velocity and the height characterizing the nose and the surface stress. The relationship shows a dependence on stability and quantifies the interaction between the LLJ dynamic and the turbulence below. The TKE budget turns out to be not satisfied, especially in the lower levels where the residual (i.e., the divergence of TOMs normalized on the dissipation of TKE) is often positive, suggesting a transport of TKE from the considered layer. This observation links the local budget to the vertical structure of the whole turbulent layer and is broadly connected to the vertical profile of TKE, in the flux-gradient closure frame. The ratio between the vertical velocity variance and the TKE is investigated as a function of the flux Richardson number to highlight specific features of local turbulence and their representation in closure models.

How to cite: Leo, L. S., Barbano, F., Brogno, L., Tampieri, F., and Di Sabatino, S.: Near-Surface Turbulence Structure in the Presence of a Nocturnal Low-Level Jet: Results from the MATERHORN Field Experiment, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-409, https://doi.org/10.5194/ems2024-409, 2024.

Accurate representation of atmospheric boundary layer (ABL) processes is crucial for trace gas simulations and inverse modeling on a regional scale, particularly in areas with complex terrain such as the Swiss Midlands. Challenges arise from uncertainties in local turbulent transport, advection, and diffusion. Local circulations in the ABL, influenced by complex terrain, directly impact the vertical profiles of trace gases emitted at the surface. Biases in these processes can lead to significant errors, especially during nighttime stable conditions, affecting the estimation of trace gas concentrations, notably greenhouse gases accumulation. By conducting high-resolution idealized simulations using Cloud Model 1 in Large Eddy Simulation configuration, we aim to provide insights into the trace gas dispersion over the Swiss Midlands, characterised by rolling terrain. This study investigates sensitivities of trace gas simulations to large-scale background wind and valley depth, within stable boundary layers. We investigate how these factors influence the storage and transport processes of passive tracers in nocturnal cold pools, as well as their morning depletion. The main focus of our study is on the temporal evolution of the passive tracer concentration at virtual towers located at the valley center, eastern and western slopes, and the ridge top. The results show that at the valley center the stable regime leads to the build-up of the passive tracer near the valley floor. Furthermore, it is found that the peak values are reduced and the timing of the peak is delayed with increasing inlet height. This means that despite the low vertical turbulent mixing, the local re-circulation within the valley transports the tracer to higher elevations, resulting in stratified tracer accumulation.

How to cite: Bašić, I., Schmidli, J., and Singh, S.: Passive tracer under stable conditions: impact of background wind and valley geometry in an idealized setup, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-683, https://doi.org/10.5194/ems2024-683, 2024.

The surface energy balance (SEB) is a physical principle that represents how the energy is distributed between the lowest layer of the atmosphere and the Earth's surface. Its understanding is pivotal not only for determining the meteorological and climatological conditions in the atmospheric boundary layer, but also for applications such as weather and climate modeling, agricultural and forest management, air pollution modeling, ecosystem carbon budget, and biological modeling. Indeed, all the mentioned applications assume a balance between the energy fluxes. Yet, even over flat and horizontally homogeneous terrain, the SEB closure is rarely observed. For an ideal massless layer at the surface, the SEB formulation states that net radiation is balanced by the sum of the turbulent fluxes of sensible and latent heat, and the soil heat flux (R_n = H + LE + G). Previous research suggests that the persistent lack of closure either results from measurements limitations or from omitted processes. In fact, measurements are usually not taken at the interface between the atmosphere and the surface, but at a certain distance above (R_n, H and LE) or below the ground (G). Consequently, fluxes should be referred to a volume rather than a surface. SEB studies identified the storage of heat in the volume of air/soil above/below the measurement point as a contributing factor to the SEB residual. Nowadays, it is widely recognized that non-turbulent, advective fluxes derived from surface heterogeneities are the main reason behind the SEB non-closure. 
We systematically assessed this ideal formulation using data collected from the sites of the i-Box network, a long-term measurement setup in the Inn Valley (Austria). These stations represent different topographic categories (valley floor, mountain top, and slopes) and present different slope angle, orientation and land-use characteristics. Different to assessing SEB closure (or non-closure) over ideal surfaces, the goal of the present study is to characterize the SEB residual in complex, mountainous terrain. In other words it is investigated which processes contribute to what degree to the expected non-closure of the SEB in this type of environment. Specifically, we determined the relation between the SEB imbalance and different flow regimes (thermally-driven valley-slope wind circulations and foehn events), stability classes, seasons and topographic categories. Furthermore, the effects of specific post-processing procedures on the magnitude of the SEB residual were assessed for a valley-floor and a north-facing slope site. We compared the turbulent fluxes using double rotation, planar-fit and sectorial-planar-fit coordinate rotation methods. In general, these procedures lead to small differences among fluxes such that they do not impact the relative importance of different processes to explain the SEB non-closure. However, this comparison allowed us to evaluate the uncertainty of the magnitudes of the turbulent fluxes and therefore of the SEB residual. 

How to cite: Destro, M., Rotach, M. W., and Lehner, M.: Characterization of the surface energy balance in complex terrain, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-240, https://doi.org/10.5194/ems2024-240, 2024.

Boundary layer processes and turbulence in a complex terrain are influenced by thermally driven flows, as well as dynamical or non-local winds. We investigate the variability in turbulence kinetic energy (TKE) with elevation, and topography in a shallow high mountain valley in the Canadian Rockies. The Fortress Mountain Research Basin in the Kananaskis Valley, Alberta, was chosen for this study. Data from three high-frequency eddy-covariance systems at a north-west-facing slope location, and at two ridgetops at the south and north valley side walls were used for the analysis, and combined with large-eddy simulations (LES). The observed data and simulations focused on a sunny summer day when turbulence was well-developed, and cross-ridge flows interacted with thermally driven circulations. The observed TKE time series compared reasonably well with simulations at the north-west-facing slope and southern ridgetop. The model was then used to evaluate the vertical and horizontal TKE budget equation. Analysis of the TKE budget showed that horizontal shear driven by interactions of cross-ridge flows with the up-valley flow could be an important source of TKE production on the north-west-facing slope station in the Fortress Valley. At the northern ridgetop, both model and observations showed no contributions from the vertical production terms, while model showed a significant contribution from the horizontal shear production to TKE at this location. The correlation between the TKE at the valley station and the wind speed at a different location above the valley suggests that both horizontal and vertical exchange processes are an important part of TKE production mechanisms in this high mountain valley.

How to cite: Rohanizadegan, M., Petrone, R., Pomeroy, J., and Kosovic, B.: Analysis of Turbulence Kinetic Energy Dynamics in Complex Terrain, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-512, https://doi.org/10.5194/ems2024-512, 2024.

Concerns about climate change are growing as extreme events such as heat waves, droughts and storms become more frequent and intense. Together with the reduction of green-house gas emissions to reduce climate change, scenarios for adapting to climate change need to be developed, for instance in cities during heatwaves. In the latter case, the scenarios depend upon the distribution of the air temperature field close to the ground. How does this temperature field depend upon the large-scale variability of the heatwave episodes is a key question to design the scenarios. This question is particularly relevant when the temperature distribution at the bottom of an urbanized alpine valley is considered. This question is addressed in this paper, for the Grenoble valley.

Three 30-year periods are considered, in the past around 2005, and in the future around 2050 and 2070. ERA5 reanalysis are used for the past period and data from the Regional Atmospheric Model (MAR) forced by the global climate model MPI are used for the future periods. WRF simulation data of the Grenoble valley boundary layer for a few past and future heatwaves are also considered.

Heatwaves during the three 30-year periods are first identified and an indicator is sought to characterize their variability. The large-scale wind speed at 700 hPa over the Grenoble valley appears to be a good indicator to analyze the impact of the heatwave variability on the bottom-valley flow.

When the large-scale wind speed is smaller than 10 m/s (this is the case for the majority of heatwaves of this analysis), a decoupling is generally observed between the large-scale wind and the bottom-valley flow. The latter flow is then of thermal origin. The temperature field is therefore controlled by the thermal wind and does not depend upon the heatwave episode. More precisely, during daytime, the main factor influencing the temperature field is the land cover, while at night, the temperature field is sensitive to local mixing induced by the thermal wind. For all heatwave episodes associated with a small large-scale wind speed, the wind comes from the North due to the center of the Omega weather pattern to be located on the west of France.

When the large-scale wind speed is greater than 10 m/s, the coupling with the bottom valley flow occurs through momentum transfer (see Doran and Whiteman 1994). The temperature field still does not depend upon the heatwave episode during the day, being controlled again by the land cover. During nighttime, by contrast, the temperature field depends upon the large-scale wind. However, for all heatwave episodes associated with a high large-scale wind speed,  the wind comes from the South-West implying that the temperature field is eventually the same whatever the episode.

How to cite: Vazquez Ballesta, M., Staquet, C., Gabeiras, J., and Hedde, T.: Impact of heatwave large-scale variability on the temperature field at the Grenoble valley bottom , EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-1072, https://doi.org/10.5194/ems2024-1072, 2024.

The Tibetan Plateau is the highest and most extensive upland region in the world, with an average elevation over 4,000 m above sea level. It is referred to as the “water tower of Asia” because it is the source region of several major rivers in Asia, hence providing water resources to almost two billion people. As near-surface wind speed plays a key role in regulating surface evaporation and thus the hydrological cycle, among other implications, it is crucial to explore its spatio-temporal characteristics over this region. However, due to its vast and complex geographical area with steep terrain, high elevations, and harsh environmental conditions, in-situ measurements are scarce and mostly located in valleys, limiting the understanding of wind speed climate across the Tibetan Plateau.

This study explores the climatology of near-surface wind speed over the Tibetan Plateau by using for the first time homogenized (i.e., variations caused by non-climatic factors have been removed) observations from 104 measuring stations, together with reanalysis products and regional climate model simulations. By exploring wind characteristics with a focus on seasonal cycle through cluster analysis, three regions of distinct wind regimes are identified: (1) the central Tibetan Plateau; (2) the eastern and the peripheral areas of the plateau; and (3) the Qaidam basin, a topographic depression strongly influenced by the blocking effect of the surrounding mountainous terrain. Notably, the ERA5 reanalysis, with its improvements in spatial and temporal resolution, model physics and data assimilation, demonstrates closer agreement to the measured wind conditions than its predecessor ERA-Interim. However, the newest ERA5-Land product does not show improvements compared to ERA5, most likely because they share most of the parametrizations. Furthermore, the two analyzed dynamical downscalings of ERA5 fail to capture the observed wind statistics and exhibit notable biases and discrepancies also when investigating the diurnal variations.

How to cite: Minola, L., Martin, N. P. P., Zhang, G., Ou, T., Kukulies, J., Curio, J., Guijarro, J. A., Deng, K., Azorin-Molina, C., Shen, C., Pezzoli, A., and Chen, D.: Climatology of near-surface wind speed from observational, reanalysis and high-resolution regional climate model data over the Tibetan Plateau, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-14, https://doi.org/10.5194/ems2024-14, 2024.