AS2.1

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, 2021.

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.

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.

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.

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.

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.

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.

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.

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.

References:

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 PM_2.5 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.

Abstract

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 [1] and urban [2] 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.

References

[1] 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

[2] 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 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[1] 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

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.

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.

Reference

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.