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 (Rn = 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 (Rn, 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.
The Yarlung Tsangbo Grand Canyon (YGC), one of the world’s deepest canyons, is located in the southeastern Tibetan Plateau (SETP). The YGC exhibits the highest frequency of convective activity in China. Due to frequent rainstorms in the wet season, natural disasters such as landslides and debris flows frequently occur, and often block traffic corridors. Thus, understanding the relationship between water vapor changes, convective cloud activity, and extreme rainfall events in the YGC is critical. A comprehensive observation network for water vapor variations, cloud activity, local circulation, and land-air interactions in the YGC was installed to help us to determine the relationship between the water vapor transport and heavy precipitation in the YGC and the physical process that determines the precipitation intensity, especially for cases of strong precipitation.
We analyzed 35 years observation data of daily precipitation to objectively classify the weather systems responsible for the heavy precipitation. Hierarchical clustering method divided the atmospheric circulation of the regional heavy precipitation into two representative patterns: the Tibetan Plateau vortex type (TPVT, accounting for 56.6% of the heavy precipitation events) and the mid-latitude trough type (MLTT，43.4%). The comprehensive analysis of the two patterns shows a clear connection between the heavy precipitation and positive vorticity anomaly, moisture convergence and the southeastward shift of the westerly jet core. Specifically, TPVT heavy precipitation events are caused by potential vorticity dry-to-wet processes during its eastward movement, while MLTT events are associated with the intrusion of deeply extratropical trough-ridge circulations into the SETP.
We used the Weather Research and Forecasting (WRF) model to simulate the water vapor flux during extreme rainfall events. The general shortcoming of the WRF precipitation simulation nudged with the European Centre for Medium-Range Weather Forecasts’ reanalysis dataset version 5 (ERA5), is that it cannot capture strong rainfall period. We tested many WRF parameterization schemes at a 1 km grid resolution. The Multiscale Kain-Fritsch (MSKF) scheme outperforms explicit calculations (NO_CU) in capturing heavy precipitation events accurately, thanks to its enhanced treatment of cloud-radiation interactions. The incorporation of the Turbulence Orographic Form Drag (TOFD) scheme significantly improves precipitation simulation accuracy. This is particularly evident in the better representation of local circulation. A noticeable improvement in near surface wind speeds and the vertical profile of horizontal winds was presented by the TOFD scheme. It was found that when an optimized combination of parameterization schemes in WRF can better capture the variations in the wind and water vapor concentration in the YGC channel, the model produced the best simulation results for extreme rainfall in the YGC.
These analyses have help us understanding the impacts of YGC valley on the water vapor transport and extreme rainfall outbreak mechanism.
How to cite: Chen, X., Cao, D., Zhang, Q., and Xu, X.: Simulations of extreme rainfall in the Yarlung Tsangbo Grand Canyon, China, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-3, https://doi.org/10.5194/ems2024-3, 2024.
The behaviour of pollutants in urbanized mountainous areas is complex due to the interactions between the atmosphere and the orography, particularly noticeable in winter due to frequent thermal inversions inside the valleys. This stable Mountain Boundary Layer, present during anticyclonic situations, can lead to critical air pollution episodes that are detrimental to human health and the environment. Another growing concern is the impact of climatic extremes such as increased heat waves and their impact on air quality.
This study aims to characterize the thermal inversion episodes in a complex orography area like the Central Valley of Andorra (ACV), answering questions like: (i) Which are the frequency and duration of winter inversion episodes in the central Valley of Andorra? (ii) How these characteristics correlate with pollutant concentrations and meteorological variables? Complementary, we explore the limitations of the current observation strategy for monitoring thermal inversions in the valley using low-cost sensors of temperature: (iii) Could the use of a larger number of stations on the slopes (pseudo-profile) overcome these limitations?
Findings indicate an increasing frequency and duration of thermal inversion episodes over the last decades, as well as the heatwaves on mountain areas, predominantly influenced by synoptic high-pressure conditions as daily synoptic classification centred on the Pyrenees has shown. These episodes significantly impact NO2 concentrations, nearly doubling their average levels, while PM10 and O3 did not show a direct correlation. In contrast, during the warm months, exceedances of critical O3 thresholds have been increased in last decades. Furthermore, results show that the effectiveness of low-cost sensors is notably dependent on their placement, particularly in terms of altitude and orientation relative to solar radiation.
How to cite: Trapero, L., Crespillo, A., and Udina, M.: Thermal inversion analysis in the Andorra Central Valley and its relationship with pollutants and meteorological variables, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-722, https://doi.org/10.5194/ems2024-722, 2024.
Comprehensive atmospheric measurements were conducted in the East River Valley in Colorado for a nearly 2-year period from 2021 through 2023 in the framework of the NOAA Study of Precipitation, the Lower Atmosphere, and Surface for Hydrometeorology (SPLASH) and the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) program Surface Atmosphere Integrated Field Laboratory (SAIL) campaigns. The main focus of these research initiatives is to enhance weather and water prediction capabilities by measuring, evaluating, and understanding integrated atmospheric and hydrologic processes relevant to water resources. The East River Valley is embedded in the East River Watershed which is a representative mountainous headwater catchment of the Colorado River Basin and is a primary source of water for much of the southwestern United States. The valley floor is located at more than 2500 m above mean sea level and the surrounding ridges extend above 4000 m.
In this study, we used temperature, humidity, and wind profiles from ground-based remote sensing instruments and radiosondes in the upper part of the valley, as well as near-surface meteorological observations from 5 sites distributed along the valley axis. Temperature and humidity profiles with high temporal resolution were retrieved from infrared spectrometer radiances with the optimal estimation physical retrieval TROPoe. The data set allows one to investigate the seasonal and diurnal cycle of the boundary layer and to investigate the impact of varying spatial snow coverage including the melt period. We show that the diurnal cycle of the boundary layer conditions on many days is very different from a typical thermally driven wind system, especially when snow coverage is low, and we discuss possible factors contributing to the boundary evolution.
How to cite: Adler, B., Cox, C. J., Intrieri, J., Bianco, L., Butterworth, B., de Boer, G., Gallagher, M. R., Gutmann, E., Meyers, T., Sedlar, J., Turner, D. D., and Wilczak, J. M.: Multi-season analysis of the boundary layer structure and evolution in a high-altitude valley in Colorado, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-198, https://doi.org/10.5194/ems2024-198, 2024.
A multiscale modeling study of a real case has been conducted to explore the capability of WRF-LES over the Xiaohaituo Mountain (a game zone for the 24th Beijing-2022-Winter-Olympic-Games). Comparing WRF-LES results with observations collected during the MOUNTAOM (MOUNtain Terrain Atmospheric Observations and Modeling) field campaign, it is found that at 37 m resolution with LES settings, the model can reasonably capture both large-scale events and microscale atmospheric circulation characteristics. Employing SRTM1 (≈30 m) high resolution topographic dataset instead of traditional USGS_30s (≈900 m) dataset effectively improves the model capability for reproducing fluctuations and turbulent features of surface winds. Five sensitivity experiments are conducted to investigate the impact of different PBL treatments, including YSU and SH PBL schemes and LES with 1.5TKE, SMAG and NBA subgrid-scale (SGS) stress models. In this case, at gray zone scales, differences between YSU and SH are negligible. LES outperform two PBL schemes which generate smaller turbulence kinetic energy, and increase the model errors for mean wind speed, energy spectra and probability density function of velocity. Another key finding is that wind field features in the boundary layer over complex terrain are more sensitive to the choice of SGS models than above the boundary layer. With the increase of model resolution, the effects of SGS model become more significant, especially for the statistical characteristics of turbulence. Among these three SGS models, NBA has the best performance compared to observation. Overall, this study demonstrates that WRF-LES is a promising tool for simulating real weather flows over complex terrain.
How to cite: Liu, Y., Liu, Y., Munoz-esparza, D., and Miao, S.: Simulation of Flow Fields in Complex Terrain with WRF-LES: Sensitivity Assessment of Different PBL Treatments, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-208, https://doi.org/10.5194/ems2024-208, 2024.
High-impact convective events are a common weather phenomenon around the world and can pose significant threats to people and property. However, our understanding of the governing processes of these events still remains incomplete, in particular their initiation and spatio-temporal evolution in regions with complex topography. Systematic validation of the quality of analysis and forecast data over complex terrain is often limited by poor availability of detailed observations. Together, these issues constitute a challenge for understanding and predicting convective events.
We aim at contributing to improved forecast quality and process understanding of convective summer-time conditions by leveraging extensive field campaign observations from the KITcube, the mobile atmospheric measurement platform of IMKTRO at KIT. To achieve this, we focus on the 'Swabian MOSES campaign' which took place in the Black Forest region, Germany, during the summer 2023 and provides, among others, observations from a large network of 12 Doppler wind lidars. From each Doppler wind lidar, we retrieve vertical profiles of the wind speed and direction. These observations of the dynamic structure of the lower troposphere are used to characterize the mesoscale (pre-)convective environment and are compared to the operational high-resolution analysis data of the German Weather Service. The three months of observations provide a comprehensive independent data set for the validation of the analysis data across the Black Forest. We present initial results from the field campaign, emphasizing convective conditions, and illustrate the comparison between high-resolution analysis and campaign observations for vertical profiles of wind speed and direction. First results show that the analysis bias differs between different sites, and at different altitudes, with a larger bias in the lowest 2 km. The comparison further reveals that the zonal component of the wind more strongly contributes to the bias. The campaign observations facilitate a better understanding of the complex interactions between the terrain and the overlying atmospheric processes as well as the evaluation of the performance and limitations of a convective-scale numerical weather prediction system.
How to cite: Nguyen, D., Thomas, J., Gasch, P., and Oertel, A.: Comparison of wind profiles in the operational ICON-D2 analysis with Doppler wind lidar observations in complex topography from the Swabian MOSES 2023 campaign, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-486, https://doi.org/10.5194/ems2024-486, 2024.
Accurate representation of turbulent exchange in the mountain boundary layer is particularly challenging for numerical weather prediction models. The use of common planetary boundary layer schemes, which invariably assume flat and homogeneous terrain, results in significant model errors and can even decrease the forecast skill over hills and mountains.
We seek to improve the accuracy of planetary boundary layer parameterization schemes over heterogeneous terrain using ensemble-based parameter estimation. Parameter estimation within a data assimilation framework offers a way to reduce model errors by constraining model parameters with atmospheric observations. We consider an idealized modelling environment structured as Observing System Simulation Experiments, consisting of a large-eddy simulation, providing a simulated truth, and a single column model ensemble, where the only model error source is the planetary boundary layer parameterization. This way, we eliminate additional error sources, such as initial-condition error, which could have a detrimental impact on parameter estimation. We attempt to estimate parameters in planetary boundary layer schemes affecting vertical turbulent mixing by assimilating appropriate synthetic surface observations and vertical profiles from the large-eddy simulation run. We demonstrate that, by appropriately configuring the data assimilation system, parameter estimation drives the estimated parameters to converge toward optimal values, and at the same time reduces systematic errors in atmospheric state simulations. This approach holds promise for improving the accuracy of numerical weather prediction models, especially over heterogeneous terrain. It will also make it possible to assimilate many meteorological observations made over mountains, which are often rejected by operational assimilation systems due to large discrepancies of the observed and modelled climatology.
How to cite: Fritz, M., Serafin, S., and Weissmann, M.: Parameter estimation for boundary-layer turbulence parameterizations over heterogeneous terrain, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-522, https://doi.org/10.5194/ems2024-522, 2024.
The Cerdanya Cold Pool Experiment 2015 (CCP15) aimed to study the air thermal structure and wind circulation within the Cerdanya valley (eastern Pyrenees, Catalonia) under fair-weather conditions. The field campaign took place between 6 and 15 October 2015, focusing on the vertical column of the first hundred meters above the valley together with the surface energy balance at its centre. The Cerdanya valley, east-west oriented, is the largest of the Pyrenees mountain range with a wide bottom at 1000 m above sea level (asl) and bounded by high mountain ranges both to the north and south (peaks above 2500 m asl). Its topographical configuration favours the development of a diurnal valley wind system with the formation of a cold-air pool at night. During the campaign, four of a total of six intensive observation periods (IOPs) were affected by the intrusion of a strong wind channelised down valley during the afternoon and evening periods, followed by the development of a strong surface-based thermal inversion at night. The analysis of the observations leads to the following findings, also supported by high-resolution numerical simulations:
- The wind channelling observed in the valley centre requires the presence of a large pressure gradient in the north-south direction, i.e. perpendicular to the mountain range (orographic dipole).
- The wind channelling has a diurnal evolution, occurring during the afternoon and evening periods. The exact onset time and endurance depends on the particular synoptical configuration and on the valley wind dynamics.
- The presence of the wind channelling affects the evolution of the thermodynamic variables at the surface layer and the exchange fluxes at the surface interface, modifying the initial conditions of the nocturnal boundary layer within the valley.
- A strong surface-based thermal inversion develops over the valley under both fair-weather conditions or the presence of such channelled wind. The latter fosters the vertical growth of such inversion and dumps its intensity. The thermal inversion separates the lowest levels close to the ground from the wind channelling flow influence.
How to cite: Martínez-Villagrasa, D., Cuxart, J., Jiménez, M. A., Conangla, L., and Miró, J. R.: Valley wind dynamics and large-scale airflow interaction within the Cerdanya valley (eastern Pyrenees), EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-539, https://doi.org/10.5194/ems2024-539, 2024.
