Driven by atmospheric turbulence, and integrating surface processes to free atmospheric conditions, the Atmospheric Boundary Layer (ABL) plays a key role not only in weather and climate, but also in air quality and wind/solar energy. It is in this context that this session invites theoretical, numerical and observational studies ranging from fundamental aspects of atmospheric turbulence, to parameterizations of the boundary layer, and to renewable energy or air pollution applications. Below we propose a list of the topics included:
- Observational methods in the Atmospheric Boundary Layer
- Simulation and modelling of ABL: from turbulence to boundary layer schemes
- Stable Boundary Layers, gravity waves and intermittency
- Evening and morning transitions of the ABL
- Convective processes in the ABL
- Boundary Layer Clouds and turbulence-fog interactions
- Micro-Mesoscale interactions
- Micrometeorology in complex terrain
- Agricultural and Forest processes in the ABL
- Diffusion and transport of constituents in the ABL
- Turbulence and Air Quality applications
- Turbulence and Wind Energy applications
- "Surface energy balance closure: the role dispersive fluxes induced by submesoscale secondary circulations", by Dr. Matthias Mauder , Karlsruhe Institute of Technology KIT/IMK-IFU, Institute for Meteorology and Climatology - Atmospheric Environmental Research Germany.
vPICO presentations: Wed, 28 Apr
Quantitative knowledge of the surface energy balance is essential for the prediction of weather and climate. However, a multitude of studies from around the world indicates that the turbulent heat fluxes are generally underestimated using eddy-covariance measurements, and hence, the surface energy balance is not closed. This energy balance closure problem has been heavily covered in the literature for more than 25 years, and as a result, several instrumental and methodological aspects have been reconsidered and partially revised. Nevertheless, a non-negligible energy imbalance remains, and we demonstrate that a major portion of this imbalance can be explained by dispersive fluxes in the surface layer, which are associated with submesoscale secondary circulations. Such large-scale organized structures are a very common phenomenon in the convective boundary layer, and depending on static stability, they can either be roll-like or cell-like and occur even over homogeneous surfaces. Over heterogeneous surfaces, thermally-induced mesoscale circulations can occur in addition to those. Either way, the associated dispersive heat fluxes can inherently not be captured by single-tower measurements, since the ergodicity assumption is violated. As a consequence, energy transported non-turbulently will not be sensed by eddy-covariance systems and a bias towards lower energy fluxes will result. The objective of this research is to develop a model that can be used to correct single-tower eddy-covariance data. As a first step towards this goal, we will present a parametrisation for dispersive fluxes, which was developed based on an idealized high-resolution LES study for homogeneous surfaces, as a function of non-local scaling variables. Secondly, we explore how well this parametrisation works for a number of real-world eddy-covariance sites.
How to cite: Mauder, M.: Surface energy balance closure – the role dispersive fluxes induced by submesoscale secondary circulations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1057, https://doi.org/10.5194/egusphere-egu21-1057, 2021.
A set of Python-based tools, WRF4PALM, has been developed for offline-nesting of the PALM model system 6.0 into the Weather Research and Forecasting (WRF) modelling system. Time-dependent boundary conditions of the atmosphere are critical for accurate representation of microscale meteorological dynamics in high resolution real-data simulations. WRF4PALM generates initial and boundary conditions from WRF outputs to provide time-varying meteorological forcing for PALM. The WRF model has been used across the atmospheric science community for a broad range of multidisciplinary applications. The PALM model system 6.0 is a turbulence-resolving large-eddy simulation model with an additional Reynolds averaged Navier–Stokes (RANS) mode for atmospheric and oceanic boundary layer studies at microscale (Maronga et al., 2020). Currently PALM has the capability to ingest output from the regional scale Consortium for Small-scale Modelling (COSMO) atmospheric prediction model. However, COSMO is not an open source model which requires a licence agreement for operational use or academic research (). This paper describes and validates the new free and open-source WRF4PALM tools (available on ). Two case studies using WRF4PALM are presented for Christchurch, New Zealand, which demonstrate successful PALM simulations driven by meteorological forcing from WRF outputs. The WRF4PALM tools presented here can potentially be used for micro- and mesoscale studies worldwide, for example in boundary layer studies, air pollution dispersion modelling, wildfire emissions and spread, urban weather forecasting, and agricultural meteorology.
How to cite: Lin, D., Khan, B., Katurji, M., Bird, L., Faria, R., and Revell, L.: WRF4PALM v1.0: A Mesoscale Dynamic Driver for the Microscale PALM Model System 6.0, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-194, https://doi.org/10.5194/egusphere-egu21-194, 2020.
Determining the accuracy of a hydrostatic weather forecast model in representing atmospheric phenomena is a complex process involving various considerations and test cases. This study delineates an objective assessment of a planetary boundary layer scheme based on turbulent kinetic energy in a single-column version of the innovative atmospheric general circulation model developed at the University of Tehran, which is called UTGAM. Single-column models provide simple frameworks to investigate the fidelity of the simulated physical processes in the atmospheric models. Dependable parameterization of the boundary layer processes has significant impacts on weather forecasts. Specifically, an ongoing issue for the operational hydrostatic models is their deficiencies in the accurate representation of the unresolved processes in stably stratified conditions.
We have utilized the first GABLS intercomparison experiment set up as a simple tool to evaluate the diffusion scheme in the UTGAM. Two different sigma-theta and sigma-pressure single-column grid staggering combined with 33 and 14 vertical levels below 3 km height have been used for the low- and high-resolution simulations. The GABLS1 Large Eddy Simulation (LES) results have been used as a benchmark for comparison. The diffusion scheme explored here is the same as the one in the ECHAM model which has been adapted for use in the UTGAM.
Results depict subtle nuances between the sigma-theta and sigma-pressure coordinates in intercomparison between the low and high vertical resolutions separately, which are more apparent in the lower vertical resolution. Nevertheless, it seems that the diffusion processes have been simulated a bit more accurately in the high-resolution sigma-pressure vertical coordinate. The boundary layer scheme in the UTGAM analogous with most of the operational models in the GABLS1 intercomparison experiment overestimate the diffusion coefficients of momentum and heat. The wind profile with height depicts maxima that are higher than the corresponding LES profile. It is inferred that the scheme mixed momentum over a deeper layer than the LES, but the simulated wind profile is better compared to the other operational models in GABLS1. Considering the vertical profiles of potential temperature revealed that the amount of heat mixing is not suitable in this experiment and causes a negative bias in the lower part of the simulated boundary layer. The simulated amounts of surface friction velocity have proved significant differences with the LES results in all separate experiments. However, the latter large amounts seem unlikely to have a detrimental effect on forecast scores in the operational model. Moreover, the sensitivity of the scheme to the lowest full-level has been partially explored. Decreasing the lowest full-level height concurrent with increasing the vertical resolution exerts a modest influence on the simulation of the boundary layer processes. All the results confirm notable improvements by increasing the vertical resolution in both sigma-theta and sigma-pressure coordinates.
Keywords: Simulation, GABLS1, stable boundary layer, vertical coordinate, diffusion coefficients, UTGAM
How to cite: Behravesh, M., Mohebalhojeh, A. R., and Mirzaei, M.: Assessing a planetary boundary layer scheme by using the GABLS1 experiment in a single-column version of the global model developed based on potential vorticity , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-476, https://doi.org/10.5194/egusphere-egu21-476, 2021.
Large-eddy simulations are useful tools to study transport processes by mesoscale structures in the atmospheric boundary layer, since in contrast to single-tower eddy covariance measurements, they provide not only temporally but also spatially highly resolved information. Therefore, they are well suited to study the energy balance closure problem, for which the mesoscale transport of latent and sensible heat, triggered by heterogeneous ecosystems, is suspected to be a major cause. However, this requires simulations that are as realistic as possible and thus allow a comparison of real measurements in the field and virtual measurements in the simulation.
During the Chequamegon Heterogeneous Ecosystem Energy-balance Study Enabled by a High-density Extensive Array of Detectors (CHEESEHEAD) experiment in the summer of 2019, a heterogeneous 10x10 square km domain was intensively sampled across scales. This data offers a unique possibility to set up large-eddy simulations with realistic surface heterogeneity. We use PALM to simulate two days and an area of 40 by 40 square kilometers incorporating the CHEESEHEAD site. The large scale atmospheric forcings to inform the boundary conditions are determined from the NCEP HRRR product. As the lower boundary condition, we use a soil and land-surface model coupled with a plant-canopy model, which we adapt to the CHEESEHEAD area based on ground-based and airborne measurements of plant physiological data.
In this study, we investigate how well the simulations match with real measurements by comparing simulated profiles and virtual tower measurements with field measurements from radiosonde ascents, lidar measurements of three-dimensional wind and water vapor, eddy-covariance measurements from the 400 meter tower in the center of the study domain, as well as from typical eddy-covariance stations distributed through the study area. This way, we investigate how realistic the simulations actually are and to what extent the knowledge gained from them concerning the energy balance closure problem can be transferred to field measurements.
How to cite: Wanner, L., Paleri, S., Speidel, J., Desai, A., Sühring, M., Vogelmann, H., Wagner, T., Oncley, S., Brown, W., and Mauder, M.: On the way to realistic large eddy simulations – A comparison of virtual measurements with CHEESEHEAD19 field measurements, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2132, https://doi.org/10.5194/egusphere-egu21-2132, 2021.
The mass balance of mountain glaciers needs correct assessment for several applications, e. g. sea level rise estimates, catchment hydrology, and natural hazard warnings. It results, at any point on a glacier, from energy, mass, and momentum fluxes at the glacier-atmosphere interface. However, surface fluxes on glaciers are highly heterogeneous in space and time.
To learn more about the processes leading to the spatial surface flux structure over a glacier surface, we employ large-eddy simulations with the WRF model at a horizontal grid mesh size of 48 m over the Hintereisferner, an approximately 6 km long valley glacier in the Austrian Alps. For model evaluation purposes, we use, besides our permanent measurement framework, four turbulence flux towers located on along- and across-glacier transects which were maintained in August 2018 on the glacier surface. Simulations were conducted for two case studies, namely one day with synoptic flow from the South-West (SW), and a day with synoptic flow from the North-West (NW). Comparison with the observations suggests that the model is able to reproduce the larger-scale flow structure and the local processes over the ice surface.
On the SW day, thermally-induced flows dominate the near-surface wind patterns and a stable boundary layer forms above the ice surface due to the alignment of the katabatic glacier wind with the larger-scale flow. Under these conditions, the glacier surface is exposed to horizontal cold-air advection. However, on the NW day, the local terrain leads to the formation of a gravity wave with severe turbulence. The resulting cross-glacier flow erodes the glacier boundary layer, and the glacier ice experiences horizontal warm-air advection. In both cases, the model simulates the complex flow structure on different length scales that affect the vertical and horizontal exchange processes over the glacier surface and the local heat advection during the daytime. The spatial sensible heat flux pattern is strongly connected to the horizontal wind speed, wind direction, and TKE. The experiment suggests a major impact of the large-scale flow structure and the flow modification by the underlying terrain. Our model setup is able to resolve the relevant scales and is therefore a valuable tool to gain insight on the surface fluxes over truly complex, heterogeneous terrain.
How to cite: Goger, B. and Stiperski, I. and the SCHISM Team: The Impact of Large-scale Flow Direction on the Formation of a Glacier Boundary Layer: Two LES Case Studies , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2620, https://doi.org/10.5194/egusphere-egu21-2620, 2021.
Sea breezes are common and recurrent thermally-driven wind circulations formed in coastal areas under conditions of weak synoptic forcing. The different heat capacity between the land and the sea causes the thermal contrast needed for their formation. Therefore, the temperature changes at the surface of both the sea and the land influence the breezes characteristics. In this work, we investigate how sensitive are the sea breezes to changes in land cover and soil moisture, which may have a direct impact on the surface temperature inland. This is done through the design of different sensitivity experiments performed with the Weather Research and Forecasting (WRF) model, where we tested the effect of the land use and soil moisture modification. This was done through the simulation of a typical sea-breeze case study in the coastal area of the southwest of the Iberian Peninsula (Gulf of Cádiz). The differences among the experiments are compared spatially and confronted with observations from different meteorological towers at the coast and inland. A special emphasis is made on the changes observed in the area of the National Park of Doñana. This area is characterised by large shallow marshes with varying seasonal status and extensive rice crops. Thus, contrasting conditions of the surface are typically observed, which also depend on the previous hydrological conditions. Preliminary results highlight the importance of the correct representation of the surface inland to obtain a proper simulation of the sea-breeze system. Besides, new lines of research emerge to analyse the impacts caused by other potential modifications in the surface conditions of the land and the ocean (e.g., global change, urbanization, crop modification, changes in precipitation regimes or sea surface temperature, etc).
How to cite: Mulero-Martinez, R., Román-Cascón, C., Lothon, M., Lohou, F., Yagüe, C., Álvarez, Ó., Bruno, M., Gómez-Enri, J., Izquierdo, A., Mañanes, R., and Adame, J. A.: How are the coastal breezes affected by changes in the land surface? Analysis from a case study using WRF, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4377, https://doi.org/10.5194/egusphere-egu21-4377, 2021.
Atmospheric vortex streets are one of the widely studied dynamical effects of isolated islands. However, the study of vortex shedding is still limited by the availability of observational wind ﬁelds of high spatial and temporal resolutions. Although the geometry, kinematics, and dynamics of vortex streets have been intensively investigated in numerous theoretical, numerical, and observational studies, our understanding of vortex shedding in the real atmosphere and atmospheric models is still insufficient.
Using the non-hydrostatic limited-area COSMO model driven by the ERA-Interim reanalysis, we simulated a mesoscale domain in high spatial (grid spacing 1 km) and temporal resolutions over one decade. This enabled us to investigate vortex streets within the planetary boundary layer despite limited observations. The basic properties of vortex streets are analyzed and validated through a 6-day-long case study in the lee of the Madeira island. The simulation compares well with satellite and aerial observations, and with the existing literature on idealized simulations.
Our results show a strong dependency of vortex shedding on local and synoptic flow conditions, which are to a large extent governed by the location, shape, and magnitude of the Azores high, which represents one pole of the North Atlantic Oscillation. As part of the case study, we have developed a vortex identification algorithm, consisting of a wavelet analysis using a set of objective criteria. The algorithm shows good performance in terms of false-positive rate and enables us to develop a climatology of vortex shedding in this region for the 10-year simulation period. Based on the long term analysis, we can identify an increasing vortex shedding rate from April to August and a sudden decrease in September, which can be well explained by the large-scale wind conditions.
How to cite: Gao, Q., Zeman, C., Vergara Temprado, J., Molnar, P., and Schär, C.: Vortex streets to the lee of Madeira in a km-resolution regional climate model, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5302, https://doi.org/10.5194/egusphere-egu21-5302, 2021.
The similarity theory equations relate the vertical turbulent flux of a variable with its vertical gradient in the surface layer. They were derived from 16-m towers (or higher) with the first measurement typically at 1 or 2 m above the surface, using pairs of values or adjusting functions to the profiles. The resulting expressions are of widespread use for multiple applications although they are supposed to be only valid over flat homogeneous terrain.
The current work applies the standard functions to a site in the centre of an east-west oriented valley, locally flat and at approximately 2 km from the mountain slopes at both sides. The area is surrounded by rain-fed agricultural fields with the upper soil layer getting dry during Summer. Momentum and sensible heat fluxes are derived with the standard similarity functions considering the Obukhov length as the stability parameter, taking measurements of wind and temperature at 2 m and a supplementary temperature observation at 0.3 m, just above the roughness sub-layer. These results are compared against the turbulent fluxes observed with an eddy-covariance system located at the same site during 8 consecutive months in 2018.
The estimated friction velocity differs less than a 20% respect to the observation for the 74% of cases under unstable conditions (61% for the stable regime). For the sensible heat flux, its goodness depends on the soil moisture. Again, a 74% of cases have a relative error below 20% for dry soils, when the observed latent heat flux is small. When soil moisture is significant, only a 24% of cases provide a sensible heat flux that differs less than a 20% from the observation. In addition, this error is positive and grows with the observed latent heat flux. For the stable regime, the number of cases with a relative error below 20% decreases to 31% and 19% for dry and moist soils, respectively.
These results show that similarity theory provides a good performance for the momentum flux over a moderately heterogeneous terrain with sloping surfaces relatively close and with observations below 2 m above the surface. For the sensible heat flux, estimations are similarly good under unstable conditions over a dry soil, while it gets over-estimated when soil moisture and, consequently, the latent heat flux are important. At night, the sensible heat flux is much smaller and thus ill estimated under the aforementioned conditions.
How to cite: Martí, B., Martínez-Villagrasa, D., and Cuxart, J.: Assessment of similarity theory under 2m in a semi-arid environment over moderately complex terrain, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6363, https://doi.org/10.5194/egusphere-egu21-6363, 2021.
Ideally, numerical weather prediction (NWP) and climate models should include a proper representation of the land surface to correctly simulate the surface energy fluxes and, ultimately, provide successful forecasts of atmospheric variables of common interest for the humans (2-m temperature, 10-m wind speed, relative humidity, etc.). However, in some cases, the issues begin in the first link of this chain, i.e., the surface characteristics included in the model do not represent appropriately the real surface ones in certain areas.
This work investigates how the simulated surface energy fluxes change when the land cover (LC) of an area is improved using a more realistic and higher-resolution dataset. We evaluate the Weather Research and Forecasting (WRF) model simulating a fair-weather day in a heterogeneous area of southern France. Firstly, we use the default LC database in WRF, which differed significantly from the real LC in the area. Secondly, we improve the LC representation of the studied area using a more realistic 1-km dataset prepared by the CESBIO research laboratory. The simulated fluxes were evaluated in a 19x19 km area with gridded area-averaged fluxes computed using measurements from five eddy-covariance towers deployed over different vegetation types during the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) field campaign. The evaluation is done using four land-surface models (LSM) available in WRF (Noah, Noah-MP, CLM4 and RUC).
The results differed depending on the LSM and displayed a high dependency of the simulated fluxes on the specific LC definition within each grid cell. The simulated fluxes improved when a more realistic LC dataset is used except for some LSMs that considered extreme surface parameters for some LC categories (coniferous forest and urban surfaces). Therefore, our findings encourage to check (and improve if needed) the surface representation in the model over the area of analysis, as well as to update surface parameters for some vegetation types.
How to cite: Román-Cascón, C., Lothon, M., Lohou, F., Hartogensis, O., Vila-Guerau de Arellano, J., Pino, D., Yagüe, C., and Pardyjak, E.: How do the surface energy fluxes change when a more realistic land cover is included in the WRF model? An evaluation using BLLAST data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6519, https://doi.org/10.5194/egusphere-egu21-6519, 2021.
During monsoon season in southern West Africa (SWA), nocturnal stratiform low-level clouds (LLSC) frequently form over a region extending from Guinean coast to several hundred kilometers inland. The cloud deck persists at least until sunrise next day, affecting surface-energy budget and related processes. However, LLSC lifetime is underestimated by numerical weather prediction and climate models.
The DACCIWA (Dynamics-Aerosol-Chemistry-Cloud-Interactions-over-West-Africa) field campaign, in June-July 2016, paved the way for studying LLSC over SWA based on high-quality-observational dataset. The first analyzes of this data highlighted that the LLSC diurnal cycle consists of four main stages: the stable, jet, stratus and convective phases. Unlike the first three, the convective phase, which starts after sunrise and ends when LLSC breaks up, has not been well documented yet.
This study analyzes the LLSC evolution during stratus and convective phases, specifically addressing the LLSC transition toward other low-cloud types during sunlight hours. It is based on comprehensive dataset acquired during twenty-two precipitation-free LLSC occurrences at Savè (Benin) during the DACCIWA fiel campaign. The cloud-characteristics are deduced from ceilometer and cloud-radar measurements. The associated atmospheric conditions are provided by surface meteorological and energy balance stations, radiosoundings and an Ultra-High-Frequency wind profiler.
The LLSC forms (beginning of the stratus phase) decoupled from surface. In thirteen cases, the LLSC remains decoupled until the convective phase (case D). Conversely, in the other nine cases, the cloud gets coupled with surface before sunrise, within the four hours after cloud formation (case C). The coupling is accompanied by cloud base lowering and near-neutral thermal stability in subcloud-layer. Almost all cases C are observed during a period with well-established monsoon-flow over SWA. But, the weak differences of thermodynamical conditions between cases C and D suggest that, contributions of both mesoscale and local processes are crucial for coupling LLSC to the surface before sunrise. In early morning, the macrophysical and thermodynamical characteristics of the LLSC in case C are slightly different from the case D, suggesting that, even during night, the coupling with surface impacts the cloud characteristics.
The LLSC evolution during convective phase depends upon the coupling at initial stage. In cases C, the evolution pattern is quite similar, the cloud base rises up under solar heating and shallow cumulus form when the cloud deck breaks up, around 11:30 UTC or later. For some of cases D, the LLSC couples with surface as the convective atmospheric boundary-layer grows and reaches the cloud base. The subsequent evolution and breakup time are then similar to case C. For most of cases D, LLSC remains decoupled from surface, and shallow cumulus form at the convective mixed layer top, under the LLSC deck. In this scenario, the LLSC breakup-time mostly occurs before 11:30 UTC. Thus, the coupling between LLSC and surface is a key factor for its evolution and maintenance after sunrise. Correct simulation of this feature may improve models performance over SWA. The impacts of LLSC on surface-energy budget and verical development of boundary-layer are also quantified.
How to cite: Zouzoua, M., Lohou, F., Lothon, M., Assamoi, P., Yoboue, V., Dione, C., Kalthoff, N., Adler, B., Babić, K., Pedruzo-Bagazgoitia, X., and Derrien, S.: Breakup of nocturnal low-level stratiform clouds during the southern West African monsoon season, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6538, https://doi.org/10.5194/egusphere-egu21-6538, 2021.
Tai-An city located near the southern foothill of Mont Tai (117.105 °E, 36.256 °N, 1526 m a.s.l.) is known for severe ozone air pollution, frequent nocturnal surface ozone enhancement events, and especially the non-negligible contribution of ozone region transport, owing to diurnal thermally driven circulations induced by steep conical isolated topography. Therefore, In this study, mesoscale wind and temperature structure around Mont Tai region in summer 2018 is predicted by the Regional Atmospheric Modeling System (RAMS). After rigorous model validation, viz. the De Ridder's interpolation technique within the roughness sublayer and the statistical performance metrics, objectively ensuring the credibility of the simulation results, the a priori selection of Valley-wind days identifies are expected to be dominated by the thermally driven flow. We focus on the wind dynamic in the morning and evening transition periods on the valley-wind days. RAMS model not only reproduced the temporal sequence of the flow reversal between different above-ground heights, various local aspects and upstream/downstream positions but also captured the majority of energy transfer mechanisms during transition periods. Besides, we developed the code simulation of direct shortwave radiation included the topographic shadowing effect to repair RAMS missing module.
How to cite: Tsai, I. and Zhang, M.: Simulation Study of the Wind Dynamics over Mont Tai during the Transition Periods, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7053, https://doi.org/10.5194/egusphere-egu21-7053, 2021.
We apply the WRF-NOAHMP model system in a nested configuration from the mesoscale down to the turbulence-permitting resolution of 100 m over the Southern Great Plains. Driven by the ECMWF operational analysis, this setup allows simulations with realistic lower boundary and meteorological forcing. A consistent set of physical parameterizations is applied through the whole chain of domains.
Using this setup, the evolution of the planetary boundary layer and land-atmosphere (L-A) feedback were investigated for selected days during the Land Atmosphere Feedback Experiment (LAFE) performed in August 2017 at the ARM SGP site. The model performance in representing the boundary layer evolution at different horizontal resolutions is presented. Also, detailed comparisons of turbulence parameters derived from the parameterized and turbulence-permitting simulations with observations are presented. The latter provides insight into the performance of turbulence parameterizations and potential improvements.
First comparisons with observations revealed that only the turbulence-permitting simulations realistically represent the temporal evolution and the internal structure of the daytime convective boundary layer as well as the morning and evening transitions. Statistical comparisons with lidar observations revealed differences in details as in the representation of vertical gradients or the variability in the boundary layer.
The results demonstrate that this model configuration is a valuable tool complementing high-resolution observations for the investigation of turbulent processes as well as the test and development of turbulence parameterizations.
How to cite: Bauer, H.-S., Späth, F., Behrendt, A., Wulfmeyer, V., and Lange, D.: A nested configuration of WRF-NOAHMP for process studies and the development of turbulence parameterizations over the SGP site, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8340, https://doi.org/10.5194/egusphere-egu21-8340, 2021.
Land surface-atmosphere interactions determine the atmospheric boundary layer (ABL) features, and in the case of semi-arid regions the water availability in the upper ground strongly conditions the surface energy balance and in general the observed dominant processes. LIAISE (Land surface Interactions with the Atmosphere over the Iberian Semi-arid Environment, eastern Ebro sub-basin) is an observational campaign planned between spring and fall 2021 designed to study the land/atmosphere interactions and the effect of the surface heterogeneities on the ABL in a semi-arid environment enclosing a large irrigated area in summer.
The combined analysis of the ground-based observations and ABL atmospheric measurements, including aircraft and remote-sensing data, is expected to improve the understanding of processes affecting exchange fluxes between the surface and the atmosphere, especially evapotranspiration, and to allow exploring the local and mesoscale circulations induced by the surface heterogeneities. In this sense, mesoscale simulations will be performed over the eastern Ebro sub-basin to contribute to this understanding while evaluating the representation of the surface features in the numerical models and its impact in the organisation of the flow at lower levels.
A first mesoscale modelling inter-comparison for a 2016 summer event in the LIAISE area, is under progress, intended to evaluate the performance of the participating models compared to the observations and explore the differences between them, trying to understand the reasons behind them. In this initial phase the models are run at their standard configurations and the comparison is expected to allow improvements in the definitions of the setup of each model for a later phase.
Four models participate in the inter-comparison: MesoNH, WRF, UKMO Unified Model and MOLOCH. They are run with similar horizontal (2km x 2km and 400m x 400m for the outer and inner domains) and vertical (2m at lower levels and stretched above) grid meshes and, in this first phase, using their default setup. A 48-h integration is made between 16 and 18 July 2016 for a case under a high-pressure system centred over NW France, with well developed thermally-driven circulations in the Ebro Basin. Sea breezes are found at the coast and seem to reach the basin after surmounting the mountain coastal range.
Preliminary results show that each model has a different representation of the surface heterogeneities affecting the grid values of the surface fluxes. Nevertheless, the mesoscale circulations generated by them do not differ significantly between models, the differences lying mostly at smaller scales, namely the ABL characteristics, the values of the exchange fluxes at the surface or the state of the surface and the soil. The challenge at this point is to relate the observed differences to the particularities of the parameterisations and of the physiographic data bases used by each model.
How to cite: Jiménez, M. A., Cuxart, J., Grau, A., Boone, A., Donier, S., Le Moigne, P., Miró, J. R., More, J., Tiesi, A., Malguzzi, P., Brooke, J., and Best, M.: Land surface Interactions with the Atmosphere over the Iberian Semi-arid Environment (LIAISE): 1st modelling intercomparison, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8492, https://doi.org/10.5194/egusphere-egu21-8492, 2021.
The Eastern Ebro basin is composed of an extensive lower irrigated area, surrounded by dry-fed slopes and wooden mountain ranges to the North, East and South, while to the West is open to the agricultural Western Ebro basin. Previous studies, based on research data or on statistics for one station, indicate that these features determine the local circulations in the area. A network of stations is used here to analyze a period of 15 years, taking representative data for the different units of landscape. A filtering procedure is developed which selects the events with predominance of local circulations, based on detecting stably stratified nights.
The analysis of the filtered data indicates the presence of a valley circulation between the lower plain and the slopes and mountains that reverses its sense of circulation between day and night, which intensity varies in summer due to an increasing thermal contrast between irrigated and rain-fed areas. The presence of sea-breeze in the late afternoon in the warm months is common, bringing cooler and wetter marine air to the area after crossing the mountain range at the South. At night in the centre of the basin, cold air pools are formed, which evolve to persistent fog events in winter, causing the statistics to be very different in that season compared to the rest of the year.
How to cite: Grau Ferrer, A., Jiménez Cortés, M. A., Martínez Villagrasa, D., and Cuxart Rodamilans, J.: Observed mesoscale patterns in the irrigated Eastern Ebro basin, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8659, https://doi.org/10.5194/egusphere-egu21-8659, 2021.
The most frequently used boundary-layer turbulence parameterization in numerical weather prediction (NWP) models are turbulence kinetic energy (TKE) based schemes. However, these parameterizations suffer from a potential weakness, namely the strong dependence on an ad-hoc quantity, the so-called turbulence length scale. The physical interpretation of the turbulence length scale is difficult and hence it cannot be directly related to measurements or large eddy simulation (LES) data. Consequently, formulations for the turbulence length scale in basically all TKE schemes are based on simplified assumptions and are model-dependent. A good reference for the independent evaluation of the turbulence length scale expression for NWP modeling is missing. We propose a new turbulence length scale diagnostic which can be used in the gray zone of turbulence without modifying the underlying TKE turbulence scheme. The new diagnostic is based on the TKE budget: The core idea is to encapsulate the sum of the molecular dissipation and the cross-scale TKE transfer into an effective dissipation, and associate it with the new turbulence length scale. This effective dissipation can then be calculated as a residuum in the TKE budget equation (for horizontal sub-domains of different sizes) using LES data. Estimation of the scale dependence of the diagnosed turbulence length scale using this novel method is presented for several idealized cases.
How to cite: Bastak Duran, I., Schmidli, J., and Reilly, S.: A Budget-Based Turbulence Length Scale Diagnostic, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9160, https://doi.org/10.5194/egusphere-egu21-9160, 2021.
Thermally driven winds observed in complex terrain are characterized by a daily cycle dominated by two main phases: a diurnal phase in which winds blow upslope (anabatic), and a nocturnal one in which they revert their direction and blow down slope (katabatic). This alternating pattern also implies two transition phases, following sunrise and sunset respectively.
Here we study the up-slope component of the slope wind with a focus on the morning transition based on from the MATERHORN experiment, performed in Salt Lake Desert (Utah) between Fall 2012 and Spring 2013.
The analysis develops along three main paths of investigation. The first one is the selection of the suitable conditions for the study of the diurnal component and the characterization of the morning transition. The second one focuses on the deep analysis of the erosion of the nocturnal inversion at the foot of the slope in order to investigate the physical mechanisms driving it. And the third one consists in the comparison between the experimental data and the results of an analytical model (Zardi and Serafin, 2015). The study of the morning transition in the selected case studies allowed its characterization in terms of the relation with the solar radiation cycle, in terms of its seasonality and in terms of its propagation along the slope and along the vertical direction. Most of the results of this investigation are related to the identification of the main mechanisms of erosion of the nocturnal inversion at the foot of the slope and to its role to the beginning of the transition itself. Finally, it is shown how the above model can fairly reproduce the cycle between anabatic and katabatic flow and their intensity.
Zardi, D. and S. Serafin, 2015: An analytic solution for daily-periodic thermally-driven slope flow. Quart. J. Roy. Meteor. Soc., 141, 1968–1974.
How to cite: Farina, S., Zardi, D., Di Sabatino, S., Marchio, M., and Barbano, F.: Characterization of the morning transition from downslope to upslope winds and its connection with the nocturnal inversion breakup at the foot of a gentle slope, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12408, https://doi.org/10.5194/egusphere-egu21-12408, 2021.
Diurnal wind systems typically develop in mountainous areas following the daytime heating and nighttime cooling of sloping surfaces. While down-slope winds have been extensively treated in the literature, up-slope winds have received much less attention. In particular, the physical mechanisms associated with the development of these winds, as well as the search for appropriate parameterization of turbulent fluxes of mass, momentum, and heat over slopes in numerical weather prediction models are still open research topics.
Here we present some preliminary results from the analysis of turbulence data (sonic wind speed, temperature, humidity, and turbulent fluxes) collected at two slope stations which are part of the i-Box initiative. The i-Box project (Rotach et al. 2017) aims at studying turbulent exchange processes in complex terrain areas. The experimental setup is composed of six stations disseminated in the surroundings of the alpine city of Innsbruck, in the Inn Valley. The two stations adopted for the present study are located at different points on the valley sidewalls, one with a slope angle of 27° (labelled NF27) and one with a slope angle of 10° (NF10). Both stations are located over slopes covered by alpine meadow and at an altitude of about 1000 m MSL (400 m above the valley floor). The station NF27 has two measurement points, 1.5 and 6.8 m AGL, while the station NF10 has one measurement point, at 6.2 m AGL.
The analysis shows that criteria proposed in the literature for the selection of valley-wind days may not apply for the identification of slope-wind days. Furthermore, from the analysis of second order moments, scaling relationships are derived for up-slope flow conditions. In addition, measurements representing the evolution of the up-slope flow structure from the early morning to the mid-afternoon are compared with an existing, simplified, analytical model, which provides the evolution of the vertical profiles of temperature and along-slope wind velocity as generated by a sinusoidal forcing representing the daily cycle of surface temperature. An improvement of the existing model, where the surface energy budget is considered as the boundary condition for the surface temperature, is also tested.
How to cite: Marchio, M., Farina, S., and Zardi, D.: Investigating the dynamics of thermally driven up-slope flows: analysis of data from measurements in the Inn Valley (Austria) and comparison with a simple model, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12412, https://doi.org/10.5194/egusphere-egu21-12412, 2021.
This work presents the characterization and comparison of the response of evapotranspiration (ET) to variations in shallow soil moisture (SM) in three years with different precipitation regimes: 2017, 2018 and 2019, through the analysis of tower data from La Herrería site, a forest site in the foothills of the Guadarrama Mountains in Spain. The aim of this work is to improve the comprehension of the relations of these variables (ET and SM) and their dependence on rain regimes in the studied years. To assess this, monthly SM regimes are considered, with three main types: transitional, wet and dry. The study shows the highly variable response of ET to variations in SM, which depends on the three considered SM regimes. In transitional regimes, SM strongly constrains ET variability, in wet regimes, SM does not impact ET variability, and in dry regimes, SM has a small impact in ET variability, due to its small variations. In particular, the months which suit satisfactorily to these regimes are identified, such as July 2018 (transitional, r=0.73), November 2019 (wet, r=-0.27) and August 2018 (dry, r=0.36), being r the coefficient of linear correlation between ET and SM. Some months that do not fit in the proposed scheme are also identified, and they have to be analyzed independently. This research shows the need to take into account different physical processes that affect ET, the complexity in the treatment of observational (tower) data for this type of analysis, and illustrates how the election of the length of the studied period is important for this type of analysis. Hence, it should be carefully chosen, because the interpretation of the results can be different depending on this choice.
How to cite: Valverde, J., Román-Cascón, C., Yagüe, C., and Maqueda, G.: How does the soil moisture influence the surface energy fluxes? An observational study at La Herrería Forest (Central Spain), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13674, https://doi.org/10.5194/egusphere-egu21-13674, 2021.
Mesoscale-to-Large Eddy Simulation (LES) grid nesting is an important tool for many atmospheric model applications, ranging from wind energy to wildfire spread studies. Different techniques are used in such applications to accelerate the development of turbulence in the LES domain. Here, we explore the impact of a simple and computationally efficient Stochastic Cell Perturbation method (SCPM) to accelerate the generation of turbulence in the Weather Research and Forecasting (WRF) LES model on the Turbulence Kinetic Energy (TKE) budget. In a convective boundary layer, we study the variation of TKE budget terms under the initial conditions of the Scaled Wind Farm Technology (SWiFT) facility located in West Texas. In this study, WRF LES is used with a horizontal grid resolution of 12 m, and is one-way nested within an idealized mesoscale domain. It is crucial to understand how forced perturbation shifts the balance between the terms of the TKE budget. Here, we quantify the shear production, and buoyant production in an unstable case. Since additional production terms are introduced in the SCPM method, we investigate the dissipation term of TKE. In addition, we also study the generation of turbulent transport. Generally, it integrates over height to null in a planar homogeneous case without subsidence, indicating it is positive over some heights and negative over other heights. Furthermore, we also study the variation of the TKE transport term after extending the random perturbation up to a certain height. The findings of this study will provide a better understanding of the contribution of different budget terms in a forced LES simulation.
How to cite: Kumar, M., Banerjee, T., Jonko, A., Mirocha, J., and Lassman, W.: Assessing the Turbulence Kinetic Energy Budget in the Boundary Layer Using WRF-LES: Impact of Momentum Perturbation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13794, https://doi.org/10.5194/egusphere-egu21-13794, 2021.
In marine atmospheric boundary layer (MBL), turbulence plays an important role in vertical transport of mass, heat and moisture, which is crucial for the emergence and evolution of stratocumulus clouds. We use high resolution in situ measurements of flow velocity, temperature, humidity and liquid water content performed from the helicopter-borne platform ACTOS in the region of Eastern North Atlantic in the course of ACORES campaign to compare turbulence properties in coupled and decoupled stratocumulus-topped boundary layer. Derived parameters include turbulence kinetic energy, its production and dissipation rates, anisotropy of the inertial range, turbulent fluxes of sensible and latent heat as well as characteristic lengthscales.
Inside the observed coupled MBL, turbulence is intensively produced by buoyancy at the cloud top and at the surface, and dissipated with equal rate across the entire layer depth. Turbulence is close to isotropic and inertial range exhibits scaling relatively close to that predicted by Kolmogorov theory. Inside the decoupled MBL properties of turbulence in the bottom sub-layer (BSL) vary from those in the cloud and sub-cloud layers, which together form upper sub-layer (USL). Transition in between the BSL and the USL is most pronounced in the gradient of specific humidity. The USL is characterized by weak buoyancy production in the cloud, strong anisotropy of turbulence and the scaling deviating from that predicted by Kolmogorov theory. In BSL, fluxes of buoyancy and latent heat decrease with height from the maximum at the surface down to about zero at the transition.
In general, results are consistent with the conceptual explanation of decoupling mechanism involving two separated zones of circulation and mixing: surface driven and cloud top driven. Our observations suggest that contrasting turbulence parameters need to be considered together with convection organization in order to properly quantify the vertical transport between ocean surface and stratocumulus cloud.
How to cite: Nowak, J., Siebert, H., Szodry, K., and Malinowski, S.: Turbulence properties in coupled and decoupled stratocumulus-topped boundary layers, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14366, https://doi.org/10.5194/egusphere-egu21-14366, 2021.
Two types of ABL numerical modeling are compared: three-dimensional large-eddy simulation (LES) models and one-dimensional Reynolds-averaged Navier-Stokes (RANS) models. The evening transition is simulated through facilitating the formation of the convective boundary layer (CBL) by having a constant positive surface heat flux, and the subsequent decay of the CBL when the surface heat flux is decreased.
Several features of this process have been studied in relative depth, in particular the TKE decay rate at different stages of the evening transition, the sensitivity of the results to the domain size, and the dynamics of the large- and small-scale turbulence during the transition period. LES experiments with different setups were performed, and the results were then compared to those obtained through RANS experiments based on the k-epsilon model (a two-equation model for TKE and dissipation rate, where model constants are chosen to allow for correct simulation of SBL main properties , as well as CBL growth rate ).
This study was funded by Russian Foundation of Basic Research within the project N 20-05-00776 and the grant of the RF President within the MK-1867.2020.5 project.
2. Burchard H. Applied Turbulence Modelling in Marine Waters. Berlin, Germany: Springer, 2002. P. 57-59
How to cite: Tkachenko, E., Debolskiy, A., and Mortikov, E.: Numerical simulation of the evening transition in the atmospheric boundary layer using LES and RANS models, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14787, https://doi.org/10.5194/egusphere-egu21-14787, 2021.
The stable boundary layer is typically characterized by weak and sometimes intermittent turbulence, particularly under very stable conditions. In mountain valleys, nocturnal temperature inversions and cold-air pools form frequently under synoptically undisturbed and clear-sky conditions, which will dampen turbulence. On the other hand, thermally driven slope and valley winds form under the same conditions, which interact with each other and are both characterized by jet-like wind profiles, thus resulting in both horizontal and vertical wind shear, which creates a persistent source for turbulence production. Data will be presented from six flux towers in the Austrian Inn Valley, which are part of the i-Box measurement platform, designed to study near-surface turbulence in complex, mountainous terrain. The six sites are located within an approximately 6.5-km long section of the 2-3-km wide valley approximately 20 km east of Innsbruck. The data are analyzed to characterize the strength and intermittency of turbulence kinetic energy and turbulent fluxes across the valley and to determine whether the persistent wind shear associated with thermally driven flows is sufficient to generate continuous turbulence.
How to cite: Lehner, M. and Rotach, M. W.: Characterization of near-surface turbulence in the stable atmosphere of the Alpine Inn Valley, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2009, https://doi.org/10.5194/egusphere-egu21-2009, 2021.
Predictability of the atmospheric boundary layer is impaired by possible rapid transitions between fully turbulent states and quiescent, quasi-laminar states. Such rapid transitions are observed in Polar regions or at night when the atmospheric boundary layer is stably stratified, and they have important consequences in the strength of mixing with the higher levels of the atmosphere.
In some cases, perturbations of the flow can play an important role in triggering transitions. Using different randomised models of the stable boundary layer, we will investigate the role of natural fluctuations of atmospheric processes to trigger regime transitions.
We then apply a combination of methods from dynamical systems, statistical modelling and information theory to study and detect those regime transitions. A statistical-dynamical indicator is developed as an early-warning signal of regime transitions that can be applied to highly non-stationary field data.
How to cite: Vercauteren, N., Kaiser, A., Boyko, V., Faranda, D., and Krumscheid, S.: Detecting regime transitions in the stable boundary layer, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2312, https://doi.org/10.5194/egusphere-egu21-2312, 2021.
Stably stratified atmospheric boundary layers are often characterized by a veering wind profile, in which the wind direction changes clockwise (counterclockwise) with height in the Northern Hemisphere (Southern Hemisphere). Wind-turbine wakes respond to this veer in the incoming wind by stretching from a circular shape into an ellipsoid. Englberger, Dörnbrack and Lundquist (2020) investigate the relationship between this stretching and the direction of the turbine rotation by means of large-eddy simulations (LESs).
The basic physics underlying the interaction process of a rotating wake with a veering inflow can be described with the superposition of a Rankine vortex as representation of the wind-turbine wake with the characteristic hemispheric-dependent nighttime Ekman spiral of the atmospheric wind. In dependence of the rotational direction and the hemisphere, this superposition results in an amplification of the spanwise flow component if a counterclockwise rotating rotor interacts with a northern hemispheric Ekman spiral (a clockwise rotating rotor interacts with a southern hemispheric Ekman spiral). In case of a clockwise rotating rotor interacting with a northern hemispheric Ekman spiral (a counterclockwise rotating rotor interacting with a southern hemispheric Ekman spiral), the superposition leads to a weakening of the spanwise flow component. In case of no veering inflow, the magnitude of the spanwise flow component is independent of the rotational direction.
These theoretical superposition effect of the Ekman layer with the wake vortex occur in nighttime LESs, where the rotational direction dependent magintude of the spanwise flow component further impacts the streamwise flow component in the wake. In particular, there is a rotational direction dependent difference in the wake strength, the extension of the wake, the wake width, and the wake deflection angle. In more detail, a northern hemispheric veering wind in combination with a counterclockwise rotating actuator results in a larger streamwise velocity output, a larger spanwise wake width, and a larger wake deflection angle at the same downwind distance in comparison to a clockwise rotating turbine.
Englberger, Dörnbrack and Lundquist, 2020, Does the rotational direction of a wind turbine impact the wake in a stably stratified atmospheric boundary layer? Wind Energ. Sci. 5, 1359-1374.
How to cite: Englberger, A., Dörnbrack, A., and Lundquist, J. K.: The superposition of a rotating wake with the atmospheric Ekman spiral , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3857, https://doi.org/10.5194/egusphere-egu21-3857, 2021.
Low wind conditions (wind speed < 1-2m/s) are the most critical atmospheric states for the dispersion of a pollutant due to highly non-stationary and inhomogeneous diffusion conditions governed by the meandering, weak, sporadic and intermittent turbulence. These atmospheric conditions coupled with thermal stable conditions remain a challenge for the numerical modelling of turbulent flows and dispersion at local scale.
Numerical simulation of a pollutant dispersion in these atmospheric conditions using the RANS (Reynolds Averaged Navier Stokes) equations is known to be highly dependent on selected turbulence models. On one hand, the modelling of turbulence and dynamic of wind field, by means of first order Eulerian closure models based on the turbulent viscosity hypothesis, lacks an adequate and complete representation of the anisotropic effect. On the other hand, the isotropic aspect attributed to the dispersion of the pollutant, through the simple gradient diffusion model, tends to underestimate the horizontal diffusion of the pollutant, thus overestimating the concentration along the plume axis near the source.
Therefore, the purpose of this study is to investigate the behaviour of anisotropic RANS models for dispersion of a pollutant in low wind stable conditions. The models used to simulate the dynamic field are the second order RSM (Reynolds Stress models), whereas the algebraic models used to model the concentration turbulent flux of concentration are either the AFM (Algebraic Flux model) or the GGDH (Generalized Gradient Diffusion Model). The simulations are performed using a 3-dimensional CFD code, Code_Saturne® (EDF), in which these turbulence models are implemented.
The models are validated with a well-known Idaho Falls experiment (USA) for the dispersion of a passive tracer under low-wind stable conditions. Various inflow boundary conditions for wind profiles and turbulence parameters are applied. In order to assess the predictive capacity of these models, a comparative statistical analysis is performed using standard statistical performance measures. The model results are also compared with the results from a Gaussian plume dispersion model.
How to cite: Alam, B., Feiz, A. A., Ngae, P., Kumar, P., Kouichi, H., and Chpoun, A.: CFD simulation of atmospheric dispersion over a flat field in low-wind stable conditions using anisotropic turbulence models, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5197, https://doi.org/10.5194/egusphere-egu21-5197, 2021.
We present results on the modelling of intermittent turbulence in the nocturnal boundary layer using a data-driven approach. In conditions of high stratification and weak wind, the bulk shear can be too weak to sustain continuous turbulence, and the sporadic submeso motions play an important role for the turbulence production. We show a way to stochastically parametrise the effect of the unresolved submeso scales and include it into a 1.5-order turbulence closure scheme. This is achieved by introducing a stochastic equation, which describes the evolution of the non-dimensional flux-gradient stability correction for momentum ($\phi_m$). The unperturbed equilibrium solution of the equation follows the functional form of the universal similarity function. The stochastic perturbations reflect the instantaneous excursions from its equilibrium state, and the distribution of values covers the scatter found in observations at high stability.
The non-stationary parameters of this equations are estimated from a time-series data of the FLOSS2 experiment using a model-based clustering approach. The clustering analysis of the parameters shows a scaling relationship with the local gradient Ri number, leading to a suggested closed-form model for the stochastic flux-gradient stability correction. The spatial correlation in height of the perturbations is included in the model as well. The resulting equation captures the transition of the stability correction across and beyond the critical Ri up to a value of 10. The out-of-sample prediction shows a valid transient dynamics into and within the regime of strongly-stable stratification.
How to cite: Boyko, V., Krumscheid, S., and Vercauteren, N.: A data-driven stochastic parametrisation of intermittent turbulence in the stably stratified atmospheric boundary layer, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5613, https://doi.org/10.5194/egusphere-egu21-5613, 2021.
The presence of waves in the nocturnal boundary layer has proven to generate complex interaction with turbulence. On complex terrain environments, where turbulence is observed in a weak but continuous state of activity, waves can be a vehicle of additional production/loss of turbulence energy. The common approach based on the Reynolds decomposition is unable to disaggregate turbulence and wave motion, thus revealing impaired to explicit the terms of this additional interaction. In the current investigation, we adopt a triple-decomposition approach to separate mean, wave, and turbulence motions within near-surface boundary-layer flows, with the aim of unveiling the role of wave motion as source and/or sink of turbulence kinetic and potential energies in the respective explicit budgets. This investigation reveals that the waves contribute to the kinetic energy budget where the production is not shear-dominated and the budget equation does not reduce to a shear-dissipation balance (e.g., as it occurs close to a surface). Away from the surface, the buoyancy effects associated with the wave motion become a significant factor in generating a three-terms balance (shear-buoyancy-dissipation). Similar effects can be found in the potential energy budget, as the waves affect for instance the production associated with the vertical heat flux. On this basis, we develop a simple interpretation paradigm to distinguish two layers, namely near-ground and far-ground sublayer, estimating where the turbulence kinetic energy can significantly feed or be fed by the wave. To prove this paradigm and evaluate the explicit contributions of the wave motion on the turbulence kinetic and potential energies, we investigate a nocturnal valley flow observed under weak synoptic forcing in the Dugway Valley (Utah) during the MATERHORN Program. From this dataset, the explicit kinetic and potential energy budgets are calculated, relying on a variance-covariance analysis to further comprehend the balance of energy production/loss in each sublayer. With this investigation, we propose a simple interpretation scheme to capture and interpret the extent of the complex interaction between waves and turbulence in nocturnal stable boundary layers.
How to cite: Barbano, F., Brogno, L., Tampieri, F., and Di Sabatino, S.: Reciprocal Interaction between Waves and Turbulence within the Nocturnal Boundary-Layer, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6379, https://doi.org/10.5194/egusphere-egu21-6379, 2021.
This study describes a method to estimate the nocturnal stable boundary layer height (SBLH) by means of lidar observations. The method permits two approaches which yield independent retrievals through either spatial or temporal variance vertical profiles of the attenuated backscatter. Then, the minimum variance region (MVR) on this profile is identified. Eventually, when multiple MVRs are detected, a temperature-based SBLH estimation derived from radiosonde, launched within the searching time, is used to disambiguate the initial guess. In order to test the method, two study cases employing lidar-ceilometer (Jenoptik CHM 15k Nimbus) measurements are investigated. Temperature-based estimates from a collocated microwave radiometer permitted validation, using either temporal or spatial backscatter variances. The dataset was collected during the HD(CP)2 Observational Prototype Experiment (HOPE) .
 U. Saeed, F. Rocadenbosch, and S. Crewell, “Adaptive Estimation of the Stable Boundary Layer Height Using Combined Lidar and Microwave Radiometer Observations,” IEEE Trans. Geosci. Remote Sens., 54(12), 6895–6906 (2016), DOI: 10.1109/TGRS.2016.2586298.
 U. Löhnert, J. H. Schween, C. Acquistapace, K. Ebell, M. Maahn, M. Barrera-Verdejo, A. Hirsikko, B. Bohn, A. Knaps, E. O’Connor, C. Simmer, A. Wahner, and S. Crewell, “JOYCE: Jülich Observatory for Cloud Evolution,” Bulletin of the American Meteorological Society, 96(7), 1157-1174 (2015). DOI: 10.1175/BAMS-D-14-00105.1
How to cite: Araujo da Silva, M. P., Muñoz-Porcar, C., Saeed, U., Rey, F., Pay, M. T., and Rocadenbosch, F.: On stable boundary-layer height estimation using backscatter lidar data and variance processing, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8765, https://doi.org/10.5194/egusphere-egu21-8765, 2021.
According to the Monin-Obukhov similarity theory (MOST), in the stratified surface layer of the atmosphere, the mean vertical velocity and scalars gradients are related to the turbulent fluxes of these quantities and to the distance z from the surface in a universal manner. The stability parameter ζ=z/L, where L is the Obukhov turbulent length scale, is the only dimensionless parameter that determines the flux-gradient relationships. This imposes a dependency of the dimensionless velocity and buoyancy gradients on ζ in form of universal nondimensional stability functions for the surface layer. Over the decades a number of them were proposed and derived mostly from extensive field campaigns of measurements in the ABL. The stability functions differ from each other by both open coefficients and functional dependence on ζ. They have a limited range of applicability, which is often extended by incorporating the assumption about their asymptotic behavior.
A generalization of MOST by considering the dependence of the dimensionless gradients on the local stability parameter z/Λ in the framework of first order closures allows the extension of the universal stability functions from the surface layer to most of the ABL. However, because of applicability constraints, differences in the asymptotic behavior and in other implied assumptions, it is not immediately obvious, which set of stability functions will perform best. In this study we analyze a set of stability functions which are implemented in a uniform manner into a one-dimensional first-order closure. The latter applies a turbulent mixing length with generalized local MOST scaling which fits to a surface schemes employing corresponding functions for consistency. We use two numerical experiment setups accompanied with LES data for validation which correspond to the weakly stable GABLES1 case and to LES simulations of the very stable ABL based on measurements at the Antarctic station DOME-C (van der Linden et al. 2019). We also focus on the sensitivity of the 1D model results to coarser grids with respect to both the used surface flux schemes and the ABL turbulence closures since their are meant to be used in climate models because of numerical efficiency.
Authors want to aknowledge partial funding by Russian Foundation for Basic Research (RFBR project N 20-05-00776), sensitivity analysis and closure development were performed with support of Russian Science Foundation (RSF No 20-17-00190). Steven van der Linden for providing LES data of DOME-C based experiments.
van der Linden S.J. et al. Large-Eddy Simulations of the Steady Wintertime Antarctic Boundary Layer // Boundary Layer Meteorology 173.2 (2019): 165-192.
How to cite: Debolskiy, A., Mortikov, E., Glazunov, A., and Lüpkes, C.: Evaluating turbulent length scales from local MOST extension with different stability functions in first order closures for stably stratified boundary layer, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14957, https://doi.org/10.5194/egusphere-egu21-14957, 2021.
The impacts of different implementations of turbulence and convection in the Regional Climate Model (RegCM, version 4.6) on the ability to predict the particle matter (PM) concentrations in the lower troposphere over selected regions of Europe are presented. PM were simulated by the chemical transport model CAMx (Comprehensive Air quality Model with extensions, version 6.50) driven by RegCM meteorology using offline coupling of these two models. The results from four simulations for a European domain driven by two different PBL parametrizations (marked as Holtslag and UW) and two different convection parameterizations (marked as Grell and Tiedtke) are compared over the four regions of Europe, namely the Alps, Benelux, Po Valley and Central Europe. Spatial differences as well vertical profiles are contrasted with each other from the different configurations. Validation of the overall PM2.5 concentrations based on EMEP data has shown better agreement between simulated and ground measured values for the simulations driven by UW scheme of PBL.
How to cite: Bartík, L., Huszar, P., and Belda, M.: Impact of the model representation of PBL and convection on the PM concentrations over Europe, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2417, https://doi.org/10.5194/egusphere-egu21-2417, 2021.
The statistical characteristics of turbulence inside the urban canopy are analyzed basing on measurements of the eddy covariance tower in Moscow.
The representation of turbulent processes in the urban boundary layer is nowadays a weak point in weather and climate forecast models since no theory describes well the atmospheric boundary layer (ABL) over the surfaces of complex geometry. To contribute to the knowledge of the mechanisms governing turbulent exchange in the complex geometry of the city, the 22-meter eddy covariance tower was installed in the Meteorological Observatory of Moscow State University in 2019. The fluctuations of temperature and three wind speed components were measured using three METEK ultrasonic anemometers at levels 2.2 m, 11.1 m, 18.8 m. We present results based on data obtained from November 2019 to May 2020.
To work with the eddy covariance data, the gap-filling algorithm was developed based on the Gaussian distribution of the variable to be filled before and after the gap, taking into account their covariances. The new method of filling the gaps was compared with linear interpolation and Gaussian distributions neglecting correlation between variables demonstrated fair performance. The three-sigma method was used to filter out spurious peaks.
The data of acoustic measurements were compared with the data from cup anemometers, deployed at similar heights nearby. The main statistical characteristics of the measured series were calculated. Links between turbulent fluxes of heat and momentum with turbulent moments of other orders were obtained. The presence of correlation between the third and second moments in the boundary layer over a complex surface discovered earlier in natural  and urban  landscapes was tested using the data of the new mast. Variances of the velocity components grow with height within the lowest 10 meters. The daily amplitude of the 20-min temperature variance is proportional to the daily amplitude of the 20-min-averaged temperature. The proportionality of TKE to the square of the averaged horizontal velocity, which is strictly valid for homogeneous ABLs, was confirmed for a case of complex surface geometry.
The data analysis was supported by the Moscow Center for Fundamental and Applied Mathematics.
 Barskov K.V., Stepanenko V.M., Repina I.A., Artamonov A.Yu., and Gavrikov A.V. Two regimes of turbulent fluxes above a frozen small lake surrounded by forest. Boundary-Layer Meteorology, 173(3):311–320, 2019. http://dx.doi.org/10.1007/s10546-019-00469-w
 Pashkin A. D., Repina I. A., Stepanenko V. M., Bogomolov V. Y., Smirnov S. V., Telminov A. E. An experimental study of atmospheric turbulence characteristics in an urban canyon. IOP Conference Series: Earth and Environmental Science. Vol. 386. No. 1. IOP Publishing, 2019. http://dx.doi.org/10.1088/1755-1315/386/1/012035
How to cite: Drozd, I., Gavrikov, A., Artamonov, A., Pashkin, A., Repina, I., and Stepanenko, V.: Experimental study of the statistical properties of turbulence inside the urban canopy in Moscow, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10105, https://doi.org/10.5194/egusphere-egu21-10105, 2021.
A swarm of quadrotor UAVs is presented as a system to measure the spatial distribution of atmospheric boundary layer flow. The big advantage of this approach is, that multiple and flexible measurement points in space can be sampled synchronously. The algorithm to obtain horizontal wind speed and direction is designed for hovering flight phases and is based on the principle of aerodynamic drag and the related quadrotor dynamics using only on-board sensors.
During the FESST@MOL campaign at the Boundary Layer Field Site (Grenzschichtmessfeld, GM) Falkenberg of the Lindenberg Meteorological Observatory - Richard-Aßmann-Observatory (MOL-RAO), 76 calibration and validation flights were performed. The 99 m tower equipped with cup and sonic anemometers at the site is used as the reference for the calibration of the wind measurements. The validation with an independent dataset against the tower anemometers reveals that an average accuracy of σrms < 0.3 m s-1 for the wind speed and σrms,Ψ< 8° for the wind direction was achieved.
Furthermore, we compare the spatial distribution of wind measurements with the swarm to the tower vertical profiles and Doppler wind lidar scans. We show that the observed shear in the vertical profiles matches well with the tower and the fluctuations on short time scales agree between the systems. Flow structures that appear in the time series of a line-of-sight measurement and a two-dimensional vertical scan of the lidar can be observed with the swarm and are even sampled with a higher resolution than the deployed lidar can provide.
In addition to the intercomparison of the mean wind velocity measurements, turbulence data of the UAV-swarm measurements are analyzed and a comparison to sonic anemometer measurements is provided.
How to cite: Wetz, T., Wildmann, N., and Beyrich, F.: Distributed wind measurements with multiple quadrotor UAVs in the atmospheric boundary layer, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10239, https://doi.org/10.5194/egusphere-egu21-10239, 2021.
A brown air pollution haze that forms over some international cities during the winter has been found to be associated with negative health outcomes and high surface air pollution levels. Previous research has demonstrated a well-established link between the structure of the atmospheric boundary layer (ABL) and surface air quality; however, the degree to which the structure of the ABL influences for formation of local-scale brown haze is unknown. Using continuous ceilometer data covering seven consecutive winters, we investigate the influence of the structure of the ABL in relation to surface air pollution and brown haze formation over an urban area of complex coastal terrain in the Southern Hemisphere city of Auckland, New Zealand. Our results suggest the depth and evolution of the ABL has a strong influence on severe brown haze formation. When days with severe brown haze are compared with those when brown haze is expected but not observed (based on favorable meteorology and high surface air pollution levels), days with severe brown haze are found to coincide with significantly shallower daytime convective boundary layers (~ 48% lower), and the nights preceding brown haze formation are found to have significantly shallower nocturnal boundary layers (~ 28% lower). On severe brown haze days the growth rate during the morning transition phase from a nocturnal boundary layer to a convective daytime boundary layer is found to be significantly reduced (70 m h-1) compared to days on which brown haze is expected but not observed (170 m h-1). Compared with moderate brown haze, severe brown haze conditions are found to be associated with a significantly higher proportion of days with a distinct residual layer present in the ceilometer profiles, suggesting the entrainment of residual layer pollutants may contribute to the severity of the haze. This study illustrates the complex interaction between the ABL structure, air pollution, and the presence of brown haze, and demonstrates the utility of a ceilometer instrument in understanding and predicting the occurrence of brown haze events.
How to cite: Marley, H., Dirks, K., Neverman, A., McKendry, I., and Salmond, J.: The Relationship Between Atmospheric Boundary Layer Structure, Brown Haze, and Air Pollution in Auckland, New Zealand, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10402, https://doi.org/10.5194/egusphere-egu21-10402, 2021.
This study deals with different inflow conditions on wind-turbines in LES in order to analyse the impact on the wake. The wind turbine regarded in this study has a hub height of 57.19 m while the radius of the blade measures 40m. Furthermore, the blade element momentum method (BEM) is used to calculate the development forces of the wind turbine blades on the flow. First, the syntheticly generated turbulence of a Mann box generator is considered. Second, atmospheric boundary layer simulations from Englberger and Dörnbrack (2018) are applied as inflow conditions for the three wind components and the potential temperature to calculate the wake of the wind turbine. The distribution of turbulent kinetic energy in eddys of different sizes is worked out in their energy spectrum.The inflow conditions represent the -5/3 Kolmogorov spectrum. The wake characteristics are evaluated for both inflow datasets and the arising differences are discussed in this study
 Mann, J. (1994). The spatial structure of neutral atmospheric surface-layer turbulence. Journal of fluid mechanics 273
How to cite: Wrba, L. and Englberger, A.: Comparison of Mann Turbulence and atmospheric turbulence as inflow conditions on a wind turbine in large-eddy simulations (LES), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10683, https://doi.org/10.5194/egusphere-egu21-10683, 2021.
Parametrizations which traditionally are used in atmospheric modeling, energy-balance and biogeochemical calculations are based on the Monin-Obukhov similarity theory (MOST). MOST assumes a uniform horizontal distribution of aerodynamic and temperature roughness of an underlying surface. These conditions are violated in heterogeneous landscapes, what requires special experiments to establish the limits of MOST applicability. Investigation of the atmospheric boundary layer (ABL) turbulent structure within urban area is an important task. The aim of our work is to establish links between statistical characteristics of turbulence in the urban landscape under different regimes of ABL.
This paper presents some results of an experiment in which all-season monitoring of the temporal variability and spatial structure of atmospheric turbulence is carried out under conditions close to those of an urban canyon. Measurements are made on the basis of the Geophysical observatory of the Institute of monitoring of climatic and ecological systems SB RAS,Russia, Tomsk. The measurement system includes five sonic anemometers located at different points and heights. This approach makes it possible to estimate the terms of the balance equations of statistical moments and, accordingly, the value of the contribution of horizontal and vertical transport to the formation of turbulent fluxes.
The possibility of parametrizing the third moment (flux of heat flux) by the type of convective advection for the conditions of an urban canyon has been confirmed. It is experimentally shown that in the inner region of the layer at a height of 10 m this third moment is expressed as the product of the potential temperature flux and the convective advection rate. Near the underlying surface, the third moment is expressed according to the approximation of turbulent diffusion.
The data processing and analysis was supported by RFBR under grant 20-05-00834, and the experimental work was supported by the Ministry of Science and Higher Education of the Russian Federation Project № IX.138.2.3.
How to cite: Repina, I., Pashkin, A., Stepanenko, V., Bogomolov, V., Smirnov, S., and Telminov, A.: Statistical Characteristics of Turbulence in an Urban Canyon, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10948, https://doi.org/10.5194/egusphere-egu21-10948, 2021.
Three-dimensional (3D) sonic anemometers are commonly used to measure 3-D wind in eddy-covariance systems for the fluxes of momentum, sonic temperature, and when integrated with fast-response gas analyzers, the fluxes of CO2/H2O. A 3-D sonic anemometer has three pairs of sonic transducers spatially positioned with optimized geometry for 3-D wind measurements. The three pairs form three individual sonic paths, each of which is between paired transducers mutually emitting and receiving ultrasonic signals. The transmitting time of the signals in reference to the sonic path length is used to calculate air flow speed and sonic temperature at high frequencies, which can be used for flux computations. However, under unfavorable weather conditions the dew, frost, snow, and/or ice often deposit on the transducer signal transmitting surface. The deposition interferes with the transducer emitting and receiving signals, bringing significant uncertainties to the wind and sonic temperature measurements. These uncertainties degrade the quality of flux data and even interrupt the data continuity, especially in climates where this deposition is most frequent. To minimize the uncertainties, and to optimize the data quality and continuity as much as possible, Campbell Scientific developed the weather-condition-regulated, heated 3-D sonic anemometers: CSAT3AH and CSAT3BH. The former is a heated CSAT3A used for Campbell Scientific open-path and closed-path eddy-covariance systems. The latter is a heated CSAT3B, universally configured with any other gas analyzer for eddy-covariance measurements or used as a stand-alone sensor for wind aerodynamic measurements. Both models use the same heating technology equipping a sonic anemometer with an electronic heating controller (CSAT3H) programmatically regulating the power to heat sonic transducers, arms, and the strut. Based on weather conditions (air temperature, relative humidity, wind speed, and atmospheric pressure) and sonic anemometer operation status (diagnosis codes), the controller regulates heat to prevent frozen and liquid deposition from interfering with the sonic signal. The two new models of sonic anemometers were tested and assessed at several locations, including inside an environment-controlled laboratory chamber, over a forest canopy in a cold region, and at a snow-covered field station at a high plateau. This poster addresses working rationale, operating heating algorithm, and sensor performances.
How to cite: Mahan, H., Gao, T., Li, X., Forbush, T., Payne, K., Yang, Q., Li, Y., Zhou, H., Wu, S., Zheng, N., and Zhou, X.: Weather-condition-regulated, heated 3-D sonic anemometers (CSAT3AH and CSAT3BH): Working rationale, operation algorithm, and performance assessment, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13671, https://doi.org/10.5194/egusphere-egu21-13671, 2021.
Detailed observation of small scale perturbation in the atmospheric boundary layer during the first generated cumulus cloud are conducted. Our target is to study this small scale perturbation, especially related to the thermal activity at the first generated cumulus cloud. The observation is performed during the daytime on August 17, 2018, and September 03, 2018. Location is focused in the urban area of Kobe, Japan. High-resolution instruments such as Boundary Layer Radar, Doppler Lidar, and Time Lapse camera are used in this observation. Boundary Layer Radar, and Doppler Lidar are used for clear air observation. Meanwhile Time Lapse Camera are used for cloud existence observation. The atmospheric boundary layer structure is analyzed based on vertical velocity profile, variance, skewness, and estimated atmospheric boundary layer height. Wavelet are used to observe more of the period of the thermal activity. Furthermore, time correlation between vertical velocity time series from height 0.3 to 2 km and image pixel of generated cloud time series are also discussed in this study.
How to cite: Nugroho, G. A., Yamaguchi, K., Nakakita, E., Yamamoto, M. K., Kawamura, S., and Iwai, H.: Study of Thermal Activity in The Mixing Layer During First Generated Single Cloud by Using Combined Observation From Boundary Layer Radar, Doppler Lidar and Time Lapse Camera , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14016, https://doi.org/10.5194/egusphere-egu21-14016, 2021.
Turbulent flows within and above urban and vegetative canopies in the atmospheric boundary layer have profound implications for a variety of important problems in agricultural and urban meteorology, such as the spreading of pollens and pollutants. We study such turbulence via Direct Numerical Simulations (DNSs), by using the code developed in (2019 Mortikov), in which there is a closed channel between two parallel walls and a canopy of constant areal density profile on the lower wall. We impose periodic boundary conditions in the horizontal directions and a no-slip impenetrable boundary condition in the wall-normal direction. For the canopy, we use different formulations of the Forchheimer drag. We assess the role of the canopy on the turbulent flow. In particular, we show the influence of added drag on the mean profiles, balance equations of the second-order moments, and the local anisotropy of the flow.
We observe that the turbulence transport profile undergoes an abrupt transition at the canopy top and transfer of energy from the roughness sublayer above the canopy to inside the canopy.
The pressure-strain correlation removes energy from the wall-normal fluctuations, which has the least share of the turbulent kinetic energy and distributes it among the other components in the bulk of the canopy. In the inertial range, within and above the canopy, the energy spectra for the streamwise component is steeper than the spanwise and the wall-normal components and is closer to the Kolmogorov -5/3 spectrum as observed in the eddy covariance measurements in the roughness sublayer (2020 Bhattacharjee).
We thank the DST, CSIR (India), SERC (IISc) for computational resources, the AtMath Collaboration at the University of Helsinki, and ICOS by University of Helsinki for their support. This study was also partially funded by RFBR project number 20-05-00776.
2019, Mortikov, E. V., Glazunov, A. V., & Lykosov, V. N. Numerical study of plane Couette flow: turbulence statistics and the structure of pressure–strain correlations, Russian Journal of Numerical Analysis and Mathematical Modelling, 34(2), 119-132. doi: https://doi.org/10.1515/rnam-2019-0010.
2020, Bhattacharjee S., Pandit R., Vesala T., Mammarella I., Katul G., and Sahoo G. Anisotropy and multifractal analysis of turbulent velocity and temperature in the roughness sublayer of a forested canopy, arXiv:2010.04194.
How to cite: Sahoo, G., Bhattacharjee, S., Debolsky, A., Kadanstev, E., Mortikov, E., Pandit, R., and Vesala, T.: Direct Numerical Simulation of turbulent canopy flow, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15113, https://doi.org/10.5194/egusphere-egu21-15113, 2021.
The boundary layer is the place of many complex physical processes spanning various time and space scales, part of which need to be parametrised in NWP models. These parametrisations are known sources of uncertainty in the models, due to the difficulty of accurately representing the processes, and the resulting simplifications and approximations that have to be done. Model uncertainty is part of what ensemble prediction systems seek to represent. This can be achieved in particular by using stochastic perturbation methods, where noise is introduced during model computations to change its state and produce different simulations. Well-known and widely used perturbation schemes like the Stochastically Perturbed Parametrisation Tendencies (SPPT) scheme have shown their effectiveness and their interest in building ensembles. However, part of the model uncertainty is not yet well represented in current ensemble systems, while some of the assumptions made by SPPT can be questioned. This argues for a diversity of approaches to represent model errors. In this active research field, alternative perturbation methods are investigated, such as the Stochastically Perturbed Parametrisations (SPP) method, or other methods focusing on the perturbation of particular physical processes. The work presented here focuses on the last ones. Based on two examples of methods published in the literature, perturbations have been applied to the turbulence and shallow convection parametrisation schemes of the mesoscale NWP model Arome from Météo-France. The perturbation of turbulence is based on the use of subgrid-scale variances to regulate the amplitude of an additive noise, while shallow convection is perturbed through a stochastic closure condition of the scheme. A simplified 1D framework has been used, in order to assess the ability of the method to produce an ensemble with sufficient dispersion and to compare its results with those from existing methods like SPPT.
How to cite: Fleury, A. and Bouttier, F.: Representation of model error through process-based perturbations for ensemble prediction : application to turbulence and shallow convection parametrisations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15167, https://doi.org/10.5194/egusphere-egu21-15167, 2021.
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