Turbulent exchange of mass, momentum and heat at the Earth’s surface is the crucial component of the climate system. Correctly representing this interaction is therefore of fundamental importance in weather, climate and air pollution models. Parametrizations of turbulent exchange between the Earth's surface and the atmosphere in virtually all numerical models of atmospheric flows are based on Monin-Obukhov Similarity Theory (MOST). MOST identifies surface fluxes of momentum and heat, and height above the surface, as key parameters fully describing all turbulence exchange. MOST is, however, strongly limited by its assumptions of horizontally homogeneous and flat terrain, not representative of the majority of the Earth’s surface. In addition, MOST does not recognize the anisotropic nature of turbulence forcing, nor does it cover the strongly stable and unstable stratification, often encountered in real world applications. Thus, the limitations of MOST, and its use beyond its intended range of validity contribute to large uncertainty in weather, climate, and air-pollution models, particularly in polar regions and over complex terrain, both experiencing unprecedented warming.

Here we explore why the directionality of turbulence exchange (anisotropy of the Reynolds stress tensor) is fundamental in understanding surface-layer atmospheric turbulence, and present how its inclusion into MOST can provide a first generalized extension of MOST encompassing a wide range of realistic surface and flow conditions. The novel theory shows that the constants in the classical MOST, are actually functions of anisotropy, allowing a seamless transition between classical MOST and the novel generalization. The new scaling relations, based on a large ensemble of measurement datasets ranging from flat to highly complex mountainous terrain, show substantial improvements in scaling under all stratifications. The results also highlight the role of anisotropy in explaining general characteristics of complex terrain and strongly unstable and stable turbulence, adding to the mounting evidence that anisotropy fully encodes the information on the complexity of the boundary conditions.

How to cite: Stiperski, I., Calf, M., Charrondiere, C., and Mosso, S.: Beyond Monin-Obukhov Similarity Theory and the Implications for Turbulence over Complex Terrain, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-5, https://doi.org/10.5194/ems2023-5, 2023.

Accurate estimations of the surface temperature are crucial to determine the exchange of energy and moisture between the surface and atmosphere. Temperature profiles in the atmospheric surface layer (ASL) are generally represented by the Monin-Obukov Similarity Theory (MOST). The logarithmic behavior in the lowest part of the ASL requires the introduction of the thermal roughness length. However, existing models for the thermal roughness length, such as the ratio to the roughness length for momentum, are not well-defined and may be based on incorrect physical assumptions. Therefore, it remains a challenge to predict the surface temperature from standardized temperature measurements at higher levels.

A reason for this uncertainty is the high vertical measurement resolution that would be required to validate the models near the surface. In this study, we designed a Distributed Temperature Sensing (DTS) structure that measures the air temperature vertically up to 1 m above the surface with a resolution of 2 cm and from 1-8 m with a 30 cm resolution. We placed this structure at a grassland site in the Netherlands in May 2022. We saw how the conventional MOST model accurately describes the logarithmic layer. However, we noticed a strong misalignment with the observed temperatures in the lowest 1 to 2 m.

We have introduced an alternative length scale into the existing model concept, omitting the need for the thermal roughness length. We show how one month of temperature profile observations under stable conditions takes one universal form by normalizing the temperature observations with θ∗ and normalizing height by the new length scale. We can describe this form, from directly above the vegetation up to and including the logarithmic layer, using a fully physically based model. Comparisons are also made against alternative models, including a tall canopy model and an adapted MOST model.

How to cite: Boekee, J., Verouden, I., Nollen, P., Dai, Y., ten Veldhuis, M.-C., van der Linden, S., and van de Wiel, B.: Towards uniformity in the near-surface temperature profile, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-88, https://doi.org/10.5194/ems2023-88, 2023.

Turbulent mixing processes in the atmospheric boundary layer are usually described using Monin-Obukhov similarity theory (MOST), which combines the mean flow properties in the dimensionless stability parameter. Originally, this theory was derived for steady flows over flat surfaces and fails to describe turbulence under unsteady conditions or in complex terrain. However, recent research showed that the inclusion of geometric properties of the Reynolds stress tensor can help to overcome this problem. From the eigenvalues of the Reynolds stress tensor, three topological limiting cases (one-, two- and three-component) can be derived, where the three-component limiting state describes isotropic turbulence. Additionally including a measure of the deviation from isotropy, i.e. anisotropy, to MOST reduces the scatter of observations around the parametrized stability correction function and thereby improves MOST. However, general climatologies of anisotropy based on eddy-covariance measurements have not been studied so far.
Based on long-term eddy-covariance measurements in Finse, an alpine valley in southern central Norway, the seasonality and diurnal variation of anisotropy is investigated. It is shown that the flow is more anisotropic in winter than in summer. In winter, the wave-dominated one-component limiting state dominates at night and the two-component limiting state dominates during the day. In summer, the flow becomes more isotropic during the day. Correlation analysis shows that anisotropy correlates mainly with wind speed shear. Compared to the two-component limiting state, the one-component limiting state occurs more frequently with a high intermittency factor.
In addition, flux tower measurements from the FLOSS2 experiment are used to study climatologies of the vertical profiles of anisotropy. It is shown that turbulence is strictly anisotropic near the ground and becomes more isotropic with height. During the day, the flow tends towards the two-component limiting state near the ground and from about 10 m altitude towards the three-component limiting state. At night, the one-component limiting state dominates, especially at higher altitudes.
Such climatologies of anisotropy and their relation to large-scale meteorological variables can help in the development of new parametrizations for numerical weather prediction models.

How to cite: Mack, L., Pirk, N., and Vercauteren, N.: Climatologies of Anisotropy in the Reynolds Stress Tensor, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-368, https://doi.org/10.5194/ems2023-368, 2023.

Limited computer resources lead to a simplified representation of unresolved small-scale processes in weather and climate models, through parameterisation schemes. Among the parameterised processes, turbulent fluxes exert a critical impact on the exchange of heat, water and carbon between the land and the atmosphere. Turbulence theory was, however, developed for homogeneous and flat terrain, with stationary conditions. The theory fails in unsteady flow contexts or with heterogeneous landscapes, but no alternative, viable theory is available. This is not only a source of error in forecasts or climate scenarios, but also a source of model uncertainty which should be characterised and considered when using weather and climate models.

Modelling turbulence is particularly challenging in conditions of stable stratification, which can be associated with intermittent or unsteady turbulence leading to model uncertainty. The study develops a modeling approach to represent unsteady mixing possibly associated with turbulence intermittency and with non-turbulent, sub-mesoscale motions that are typically unresolved by numerical weather prediction or climate models. The approach introduces a stochastic parametrisation of turbulence by randomising the stability correction used in the classical Monin Obhukov similarity theory. This randomisation alters the turbulent momentum diffusion and accounts for sporadic events that cause unsteady mixing. A data-driven stability correction equation is developed, parametrised and validated with the goal to be modular and easily combined with existing Reynolds-averaged Navier-Stokes (RANS) models. Field measurements are processed using a statistical model-based clustering technique, which simultaneously models and classifies the nonstationary stable boundary layer. The obtained stochastic stability correction includes the effect of the static stability of the flow on the resolved flow variables, and additionally includes random perturbations that account for localised intermittent bursts of turbulence. The approach is general and effectively accounts for the stochastic mixing effects of unresolved processes of possibly unknown origin. Its implementation in a single-column RANS model is tested for several numerical examples, demonstrating the effectivity to simulate intermittent mixing, without altering the mean behaviour when compared to a standard RANS model.

How to cite: Vercauteren, N. and Boyko, V.: A stochastic stability equation for unsteady turbulence in the stable boundary layer, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-548, https://doi.org/10.5194/ems2023-548, 2023.

Trace gas simulations and inverse modelling on the regional scale require an accurate representation of atmospheric boundary layer (ABL) processes. Thus, complex terrain, such as the Swiss Midlands, poses challenges like uncertainties in local turbulent transport, advection, and diffusion. Current operational numerical weather prediction models still provide limited accuracy for the simulation of vertical mixing processes over such regions. Horizontal transport and vertical mixing in the ABL induced by local circulations directly impacts the vertical profiles of trace gases emitted at the surface. A bias in these processes may introduce significant errors in the estimate of trace gas concentrations, especially for the accumulation of greenhouse gases (GHG) during nighttime stable conditions. Since the potential sources of error in representation of stable boundary layers (SBL) include the boundary layer scheme and numerical mixing, an accurate representation of vertical mixing of trace gases in all types of boundary layers is needed. Therefore, in the present work, high-resolution simulations using Cloud Model 1 (CM1) in Large Eddy Simulation (LES) configuration and with idealized topography, representative of the rolling terrain around the tall tower at Beromünster, are performed. This study contributes to an improved understanding of the relevant storage and transport processes of accumulation of passive tracer in nocturnal cold pools, and their morning depletion, along with sensitivities to initial atmospheric stratifications and upper-level wind. The model setup also includes virtual towers located at valley floor, over the slope and the mountain ridge. Together with the sensitivity experiments, along-valley, time averaged model outputs will be presented. 

How to cite: Bašić, I., Singh, S., and Schmidli, J.: Storage and transport processes in an idealized valley with a LES reference, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-299, https://doi.org/10.5194/ems2023-299, 2023.

Over the last 40 years enormous progress has been made in modelling urban surface to atmosphere exchanges with huge advances in the understanding of the weather and climate of cities. This has been stimulated by greatly increased attention on urban areas as places where people live and sites of some of the most extreme modifications of the environment; by improved instrumentation and measurement campaigns which have enhanced our understanding of key processes and their controls; enhanced conceptual and theoretical frameworks to make sense of what we measure and to underpin numerical models; and computer power which has allowed us to analyse and visualise high resolution data and to model at higher spatial and temporal resolutions. As we move from forecasting to delivering services to a broad array of end-users and  communities, the level of detail needed (about the urban surface and the urban atmosphere), both spatially (across and between cities) and temporally (from events to long term simulations) also has increased and has refocused attention on how best to captured the details of the dynamics of urban environments and the inherent trade-offs between complexity and simplicity of approaches. In this talk, I will address some of these changes, highlighting the  challenges we are trying to address today.

How to cite: Grimmond, S.: Simplicity and  complexity trade-offs in modelling urban-atmosphere, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-630, https://doi.org/10.5194/ems2023-630, 2023.

We investigate the departure from mixed-layer similarity during the afternoon decay of turbulence. More specifically, we aim to characterize the time-development of the departure of the velocity-variances profiles from their mixed-layer similarity reference state, for vastly-different idealistic shapes and time scales of the prescribed surface heat flux decay. For that purpose, we carry out idealized large-eddy simulations of the homogeneous free-convective boundary layer, where the prescribed surface kinematic heat flux (H) follows a sinusoidal or an exponential decay. The duration between the maximum and the zero surface heat flux, τ_f, is taken equal to 6 h or 2 h. A reference simulation with prescribed constant surface heat flux is also performed in order to derive the mixed-layer similarity profiles (self-similar profiles) for the vertical and horizontal velocity variances. The methodology is based on analyzing the collapse of the normalized velocity-variances profiles from different runs, while they depart from the self-similar profiles. Within the descriptive frames where the time is tracked solely by either one of the forcing time scales τ_f or τ_f _tilde = (1/H.dH/dt)^-1, we find that the velocity-variances profiles from different runs do not collapse while they depart from the self-similar profiles, suggesting that the departure is dependent on the shape of the surface heat flux decay. As the mixed-layer similarity relies on the assumption that the CBL is in a quasi-equilibrium, which is maintained as long as the adjustment time scale of the largest eddies (i.e. the convective eddy-turnover time scale, t_*) is much smaller than the characteristic time scale of the surface heat flux decay, we successively consider the ratios r = τ_f / t_* and r_tilde  = τ_f _tilde / t_* (instead of the forcing time scale alone) for tracking the time and characterizing the departure from mixed-layer similarity. As the velocity-variances profiles from different runs depart from mixed-layer similarity, we find them to collapse in the only case where the parameter r_tilde is used for tracking the time, supporting the independence of the departure from the shape of the surface heat flux decay. As a consequence of this result, the knowledge of r_tilde is sufficient to predict the velocity variances and evaluate their departure from the quasi-steady state, irrespective of the shape of the surface heat flux decay.

How to cite: Elguernaoui, O., Reuder, J., Li, D., Maronga, B., Bakhoday Paskyabi, M., Wolf, T., and Esau, I.: The departure from mixed-layer similarity during the afternoon decay of turbulence in the free-convective boundary layer: results from Large-Eddy Simulations, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-90, https://doi.org/10.5194/ems2023-90, 2023.

The study of wind flow characteristics within urban environments is of prior interest for climate, meteorology and air pollution issues. Previous works have shown that within the urban areas the turbulent transfers of momentum, heat and mass between the Atmospheric Boundary Layer (ABL) and the urban canopy are governed by the interactions between large- and small-scale turbulent structures. Hence, to accurately compute flow characteristics at the neighbourhood scale, it is required to compute canopy-generated turbulence, as well as the turbulent structures prevailing in the ABL. This wide range of scales presents a major issue when performing unsteady simulations of turbulent flows such as Large-Eddy Simulations (LES), since the flow field specified at the inlet conditions the downstream development of the flow. In ABL numerical models, the most common method is to use a multi-scale approach based on successive grid-nesting from mesoscale to the local scale. When performing multi-scale nesting to study urban canopy flows, the urban obstacles are generally explicitly taken into account in the smallest high-resolution domain only, larger domains being not refined enough for a realistic representation of the canopy elements. However, it has been shown that adding roughness elements in the larger domain had a noticeable impact on the flow dynamics and statistics within the small domain (Wiersema et al., Mon. Wea. Rev., 2020).

This study addresses the problem of the generation of realistic unsteady atmospheric inflow conditions on an idealised urban-like canopy consisting of a staggered array of cubes of constant height and packing density. Here we investigate the performance of using drag-porosity based simulations as precursor calculations for obstacle-resolving LES. The objective is to show that by using a precursor simulation with a less-costly canopy modelling method as inlet, LES of ABL flows within and above a canopy of explicitly resolved obstacles is possible with minimum flow adjustment in the inlet region.

The LES atmospheric model ARPS (Advanced Regional Prediction System) is used with an Immersed Boundary Method (IBM) to model the flow over building-like obstacles. In order to assess the proposed approach, three inflow conditions are explored and compared : (1) a periodic precursor calculation over a large domain covered by cube array modelled with IBM, (2) a synthetic turbulence inflow obtained from purposely deteriorating flow information of the periodic case (without modifying moments and spectra), and (3) a drag-porosity precursor calculation (in which the urban canopy is modelled as a porous media depending on canopy averaged morphological characteristics). To the authors’ knowledge, the use of an urban-designed drag model as precursor calculation for an obstacle-resolving simulation is the first to be performed within a single atmospheric solver.

How to cite: Bucquet, Q., Calmet, I., and Perret, L.: Benefits of precursor simulations with a drag-porosity model as inlet conditions for LES of atmospheric flow within urban canopy, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-210, https://doi.org/10.5194/ems2023-210, 2023.

Atmospheric Boundary Layer (ABL) processes directly influence the transport, diffusion and storage of passive tracers, such as greenhouse gases (GHGs). On a regional scale, local advection and mixing can modulate the vertical distribution of these trace gases. To test and optimise the representation of vertical mixing in numerical models, ICON hindcast simulations with a model grid spacing of 1 km have been performed over the Swiss Plateau for selected weather situations. These simulations are further evaluated against surface station and profiler observations and tower measurements at two the sites on Swiss Plateau namely Payerne and Beromüsnter, the latter is a tall tower (with a height of 212m) with measurements at five inlet heights. The simulations have been done with different configurations of the operational turbulence scheme in ICON, based on a 1D Turbulent Kinetic Energy equation (NWP turbulence), and with a newly developed turbulence scheme, based on prognostic equations for two energies turbulence (2TE+APDF). Results for the stable wintertime situation show that with the 2TE+APDF scheme, the fog becomes more persistent in the model and agrees better with the observations. The better skill of the 2TE+APDF scheme is partly attributed to an improved turbulence length scale formulation leading to a better representation of boundary layer top entrainment. This improved simulation of the stable boundary layer profiles will also be relevant for an improved simulation of periods with high air pollution and GHG concentrations. The results show that the 2TE+APDF scheme performs similar to or better than the combination of operational turbulence and shallow convection scheme in ICON for stable boundary layers.

How to cite: Singh, S. and Schmidli, J.: Evaluation of ABL parameterization methods in ICON over the Swiss Plateau, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-279, https://doi.org/10.5194/ems2023-279, 2023.

To correctly reproduce the dispersion of pollutants the plume rise modeling is fundamental to obtain the injection height, i.e. the altitude at which the majority of the smoke burden is released into the surrounding atmosphere, because from this depends how the plume from fire emissions are ultimately transported In 2013 Alessandrini et al. proposed a method for the buoyant plume rise computation in which a scalar transported by the particles and representing the temperature difference between the plume and the environment air is introduced. As a consequence, no more particles than those inside the plume have to be released to simulate the entrainment of the background air temperature. A second scalar, the vertical plume velocity, is assigned to each particle. In this way the entrainment is properly simulated and the plume rise is calculated from the local property of the flow. However, the method may have some drawbacks when two plumes from different sources cross, as in the case of two neighbouring chimneys or scattered fires. In this case, the scheme can lead to non-physical situations that lead to an increase in temperature. To avoid this problem, the scheme has been modified. In the new model, the particles do not carry the temperature difference but the temperature and mass so that the transport of the heat quantity can be simulated. 
The model is  tested against data from two laboratory experiments in neutral and stable stratified flows. The comparison shows a good agreement. Then, we teste the model in the case  of two plumes from adjacent stacks combining during the rising stage. The results are presented in terms of statistical indeces and plots.

How to cite: Ferrero, E., Tenti, B., and Alessandrini, S.: Plume rise model for merging plumes, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-344, https://doi.org/10.5194/ems2023-344, 2023.

Spring frost events can cause significant economic losses in the agricultural sector. Wind machines have become increasingly popular for mitigating frost damage. During radiative frost nights, wind machines generate strong jets to erode the near-surface thermal inversion through air mixing. This process accelerates both vertical air-air and local plant-air heat exchange, which causes the plants to warm up. However, previous studies have been limited in scope, either focusing on the warming effectiveness of inversion erosion (air-air) or local plant-air heat exchange at specific measurement points. The dynamic interaction between the warm air jet and canopy layer, as well as the heterogeneous plant-air heat exchange over the orchard, are not yet fully understood.

To this end, we introduce a multilayer canopy model (Patton et al., 2016) and a wind machine model (Heusinkveld et al., 2020) in a large-eddy simulation (LES). The wind machine model incorporates a momentum source with an idealized actuator disk. The coupled canopy model parametrizes canopy-induced drag, turbulence dissipation, and plant-air heat exchange (Boekee et al., 2023). This coupled model allows evaluation of the wind machine effectiveness in enhancing plant tissue temperature. We evaluate model performance with two observational cases in different seasons (leafless and full leave periods). Temperature data were obtained using a grid of fiber optic cables with a resolution of 25 cm along the cable over 6.75 ha. The preliminary results indicate that the coupled model is capable to simulate both plant-air heat exchange and in-canopy temperature dynamics. Finally, as to guide optimal frost mitigation, we conducted a sensitivity analysis for various machine operation scenarios in different seasons. This will guide wind machine operation and reduce fruit frost damage.

How to cite: Dai, Y., Boekee, J., van Hooft, A., ten Veldhuis, M.-C., and van de Wiel, B.: Modeling the warming effectiveness of wind machines operation in fruit orchards, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-150, https://doi.org/10.5194/ems2023-150, 2023.

The meterological phenomena draw their energy from the Earth’s surface and dissipate most of their energy close to the surface. The land surface, through its topography, soil moisture, temperature or vegetation activity, impacts the atmosphere from daily to seasonal time scale. The Working Group on Numerical Experimentation survey on systematic errors established that the outstanding errors in the modelling of surface fluxes of momentum and sensible and latent heat is the second most important issue. Therefore to reduce these biases, an accurate assessment of the Land-Atmosphere (L-A) exchanges, and their correct representation, are essential for weather and climate forecasts.

The evaluation of L-A exchanges in numerical weather and climate prediction models in the context of heterogeneous surfaces is the main objective of the Models and Observations for Surface-Atmosphere Interactions (MOSAI) project. The main objective of our study corresponds to the first scientific objective of MOSAI : to investigate and determine the uncertainty and representativeness of L-A exchanges measured over heterogeneous landscapes by reference towers. To achieve these objectives, dedicated field experiments are needed to document the variability of the L-A exchanges within a grid mesh around the ACTRIS sites. A set of a one year-field campaign per site are currently performed at Météopole (Toulouse, 2021), at SIRTA (Palaiseau, 2022) and at P2OA (Lannemezan, 2023).

Two long-term objectives are defined with the aim of completing the measured surface flux at the ACTRIS sites. The first objective concerns the horizontal representativeness of the local measured surface flux in the heterogeneous landscape at the scale of the ESM or NWP grid. The second one is here to quantify the surface flux uncertainties (random and systematic errors) and provide information on the SEB non-closure at each site.

Results concerning the second objective will be presented using the Météopole campaign during which six sites with different surface types were implemented to estimate the Surface Energy Balance (SEB). The arrangements of the different surface patchesis studied to quantify the heterogeneity of our landscape (patches size distribution,...). With that, several indicators that aim to quantify the surface flux heterogeneities are investigated and applied to several domain sizes (from footprint to larger domains). These indicators are cross-analysed with the SEB non-closure to investigate a possible impact of surface heterogeneity and secondary circulations on SEB non-closure.

How to cite: Jomé, M., Lohou, F., Lothon, M., Canut, G., Couvreux, F., Brut, A., Derrien, S., Maurel, W., Etienne, J.-C., Vial, A., and Garrouste, O.: Evaluation of the representativity of reference long-term surface flux measurements in an heterogeneous landscape : the Météopole campaign (MOSAI project), EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-74, https://doi.org/10.5194/ems2023-74, 2023.

In continuous regional climate simulations, the equilibrium between land-surface and atmospheric processes plays a key role both in near surface temperature and in precipitation estimations. A too strong coupling can cause the upper soil layer to dry out in larger amounts than it supposed to, which leads to reduced precipitation and temperature overestimation in summer. The amount of available soil moisture is critical both on summer heat wave strength and on precipitation formation. The strength of coupling determines the partition between sensible and latent heat fluxes which in return influences both temperature and precipitation.

In this study we use 1-year long simulations at 50 km and at 5 km horizontal resolution to analyze the coupling strength between the land surface and the atmosphere with the WRF model. Both simulations refer to the same year and the initial and boundary conditions are given based on the ERA5 reanalysis dataset. The simulation is initialized based on ERA5 only at the beginning, then only the boundary conditions are updated to create continuous, regional climate modelling like environment. The 50 km resolution simulation covers most of Europe, while the 5 km simulation covers the Carpathian region.

The coupling strength and the effect of surface is analyzed through changing the land cover from the USGS to the CORINE one. The study will focus on the Carpathian region as local summer precipitation formation is highly dependent on local factors rather and not on large-scale weather systems. Though land-atmosphere coupling mostly affects the surface based latent and sensible heat fluxes, the resolution of the simulations affects the amount of the coupling strength through the advection.

How to cite: Breuer, H. and Varga, Á. J.: Scale dependent land-atmosphere coupling strength in regional climate modelling, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-506, https://doi.org/10.5194/ems2023-506, 2023.

Thermally-driven breezes are diurnal wind circulations initiated by surface temperature gradients in coastal and mountainous areas when fair-weather synoptic conditions dominate. Understanding these breezes is crucial for many other physical processes or human applications: wind energy, human thermal comfort, transport and diffusion of pollutants/humidity, sea surface currents in coastal areas, convection initiation, or local weather differences, among others.

The characteristics of the mountain and coastal breezes depend on the strength of the surface temperature gradient, but also on the interaction with other winds of different spatio-temporal scales, such as synoptical winds. Besides, the thermodynamic vertical profile of the atmospheric boundary layer (ABL) can also impact the breeze characteristics. In this context, some recent modelling experiments have shown how the vertical structure of the pre-existing ABL is a key factor controlling the relative impact that specific surface changes have on the breeze evolution. This motivates further modelling and observational experiments, which is the main objective of the WINDABL project*.

Hence, we plan to characterise the vertical ABL structure and the horizontal heterogeneity of the terrain during favourable conditions for the breeze formation in coastal and mountainous areas. This will be carried out mainly with the launching of meteorological soundings and through the installation of meteorological stations at strategic locations, including surface turbulence measurements. The coastal area of interest corresponds to the northern part of the Gulf of Cádiz (southwestern Iberian Peninsula coast) while the mountain breezes will be investigated at the Vallée d’Aure, on the Northern side of the Pyrenees, taking advantage of the large amount of instrumentation that will be deployed close to this area in 2023 during the MOSAI field campaign**, and in collaboration with the LATMOS-i project***.

This joint observational effort will be complemented with high-resolution numerical simulations of selected cases with the Weather Research and Forecasting (WRF) model. In this sense, the observational data gathered with the new instrumentation will allow for a much more detailed model evaluation, for which a sensitivity study to the selected physical parameterizations will be performed.

In this work we will present the main observational and modelling strategy of WINDABL, as well as some preliminary results obtained from the observational data and numerical experiments.

* The WINDABL project (PR2022-055) is a project to impulse the career of young researchers funded by the University of Cádiz (Spain) (Plan Propio).

 ** MOSAI (Model and Observation for Surface-Atmosphere Interactions, https://mosai.aeris-data.fr/).

*** LATMOS-i (Land-ATMOSphere interactions in a changing environment: How do they impact on atmospheric-boundary-layer processes at the meso, sub-meso and local scales in mountainous and coastal areas?) (PID2020-115321RB-I00, funded by MCIN/AEI/ 10.13039/501100011033).

How to cite: Román-Cascón, C., Jiménez-Rincón, J. A., Ortiz-Corral, P., Yagüe, C., Brnas, T., Fernández-Castillo, P., Izquierdo, A., Bruno, M., Mulero-Martínez, R., Vázquez, R., Sepic, J., Steeneveld, G.-J., Adame, J. A., Vegas-Cañas, C., Carbone, J., Lohou, F., and Lothon, M.: The WINDABL project: How are the surface thermally-driven WINDs influenced by the vertical structure and horizontal inhomogeneities of the Atmospheric Boundary Layer?, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-361, https://doi.org/10.5194/ems2023-361, 2023.

The Scintec Acoustic Wind Profiler Sodar MFAS was installed at Split Airport (on the Adriatic coast, Croatia) in December 2022. However, due to power consumption issues, the instrument was not fully operational until June 2023, when it was connected to the regular power grid. Five months of measurements, from June to October 2023 were analysed: during this period, the time step of the measurements was set to 20 minutes, resulting in 20-minute averages of horizontal and vertical wind at heights ranging from 30 m to a maximum of 400 m, with a 10-m vertical step. The top height of the measurements was strongly dependent on the background atmospheric conditions.

From June to October 2023, several characteristic winds were observed: (1) sea and land breezes; (2) sirocco (Cro. "jugo") - strong, steady southeast wind bringing moisture; and (3) bora (Cro. "bura") - cold, strong, and gusty northeast wind. Since the measurements were made during the warmer part of the year, the sea and land breezes were the predominant winds, which were then further analysed. The mean daily vertical profile of the sea and land breezes was estimated: the breezes rotated clockwise during the day, starting as weak easterly to southeasterly winds with a speed of 2-3 m/s in the early morning hours, turning to weak southerly winds around noon, and then to faster southwesterly winds with a speed of ~5 m/s around 1 pm local time. The sea breezes reached their maximum speed around 4 pm local time and maintained their southwesterly direction. After 4 pm local time, the breezes generally began to weaken and change direction, first to westerly and then to northwesterly winds, which ceased around 8 pm local time. Easterly to northeasterly land breezes typically started couple of hours later, reaching up to ~5 m/s. The vertical structure of the sea and land breezes was further analysed: Preliminary analyses indicate that during the daytime hours, the horizontal winds were fairly uniform in the first 400 m and maintained their direction, but gained some speed with height. On the other hand, a reversal of circulation was frequently observed during the nighttime hours, from weaker easterly winds in the lowest 200 m to stronger northwesterly winds at 200-400 m altitude.

The synoptic conditions favourable for the occurrence of sea and land breezes were then extracted using the ERA5 Global Reanalysis dataset.

Additional Sodar MFAS measurements will be used to analyse sirocco and bora, i.e., winds that occur more frequently during the colder part of the year.

How to cite: Jakić, J. and Sepic, J.: Preliminary analysis of the vertical wind profiles measured using Acoustic Wind Profiler Sodar MFAS installed at the Split airport (Croatia), EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-534, https://doi.org/10.5194/ems2023-534, 2023.

During June 2022 surface air temperatures across most of Europe were above the 1991-2020 average and daily maximum temperatures reached over 40 ºC over southern Europe (according to Copernicus.eu). Unusually high temperatures were also reached in Germany, where heat waves took place with over 35 ºC. In the Jülich Observatory for Cloud Evolution (JOYCE), these extreme temperature and humidity conditions were registered. JOYCE combines a rather unique set of ground-based remote sensing instruments that provide information about Boundary Layer thermal structure. The present investigation utilizes some of those measurements to diagnose total mean flux of heat and moisture within the Boundary Layer. In order to analyze the daily evolution of these fluxes during a heat wave, we utilize measurements of temperature and humidity from an Atmospheric Emitted Radiance Interferometer (AERI) and plot them in the mixing diagram approach, i.e.,  in an energy space (Lq versus , where L is the latent heat of vaporization and is the specific heat). Additionally, we quantify, in a 2D vector representation, the contributions of surface, advection and entrainment fluxes to the total mean flux. Estimates of horizontal temperature and humidity advection are obtained from measurements of the 30º elevation scan of a Microwave Radiometer (MWR) and from wind velocities measured by a Doppler Lidar. Additionally, surface flux measurements from the Integrated Carbon Observation System (ICOS) in a near by station in Selhausen are utilized to quantify the contribution of these surface fluxes in the mixing diagram. The total mean flux shows a daytime evolution with both sensible and latent heat components observed in days before the heat wave; whereas a high sensible heat flux dominates during the heat wave and the advective contribution becomes more important when the heat wave ends. We discuss the daily evolution of these fluxes, as well as the implementation of the mixing diagram approach for their study utilizing measured quantities. The present investigation can shed light on the Land-Atmosphere interaction and the closure of the surface energy and water budgets. Furthermore, understanding how the surface conditions can affect the atmospheric variables is valuable for a better characterization, and subsequent prediction, of extreme events such as heat waves in a warming climate.

How to cite: Burgos Cuevas, A., Löhnert, U., Pospichal, B., Marke, T., and Vicencio Veloso, J. M.: Quantifying the atmospheric boundary layer evolution during a 2020 heat wave over western Germany, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-353, https://doi.org/10.5194/ems2023-353, 2023.

Land cover changes such as deforestation are known to have an impact on cloudiness and precipitation. However, contradictory results have been obtained depending on the latitude and scale considered, highlighting our limited understanding of the physical processes involved.

This work focuses on summer cloudiness at mesoscale in a temperate region, under the influence of a large forest (the Landes forest in France). It is based on atmosphere-surface mesoscale modeling (Meso-NH coupled to SURFEX).

Based on observed data, the model configuration at 500 m horizontal resolution is first optimized. It successfully simulates the higher summer cloud cover observed over the forest, compared to its surroundings. Then, the physical processes leading to cloud formation are characterized on a representative case. A comparative analysis of diagnostics and budgets over forest and non-forest areas shows that sensible heat flux and roughness length, both higher over the forest, are the main drivers of cloud convection. The difference in heating between the forest and its surroundings modifies the local circulations by significantly strengthening the sea breeze, as well as generating a forest breeze. In a third step, the impact of the 2009 Klaus storm, responsible for the loss of about one third of the trees in the Landes forest, is considered. On fifteen representative convective summer days, the model reproduces the decrease in summer cloud cover reported in a previous study based on satellite observations. As a complementary analysis tool, the mesoscale simulations allow to quantify the impacts of Klaus on the whole diurnal cycle of the boundary layer.

How to cite: Le Moigne, P., Noual, G., Brunet, Y., and Lac, C.: Impact of regional forest cover on boundary-layer clouds in southwest France., EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-429, https://doi.org/10.5194/ems2023-429, 2023.

In parameterization of the bidirectional ammonia exchange models over vegetated surfaces there are some crucial parameters: the stomatal, the soil, and the cuticular compensation point concentrations as the function of [NH4+]/[H+] ratio in the apoplast, soil, and droplets on leaves, as well as the cuticular resistance. These factors determine the direction and the magnitude of the ammonia flux. Two-layer bidirectional exchange models are generally used to partition the measured flux into different parts.  However, the parameterizations are mostly based on empirical relationships involving uncertainties and resulting in disagreements among the applied models in the estimation of the stomatal/soil/cuticular flux ratio. The main reasons for the deviations may be the following: i) Overestimation of the soil compensation-point when calculated from the bulk ammonium content of the soil because a part of ammonium content in the soil is bound in the solid phase. Hence Henry's law for the liquid phase cannot be applied to this fraction. ii) Neglecting the part of soil-derived ammonia recaptured by leaves. For this reason, soil emissions may be underestimated. iii) Lack of bioassay measurement. The models generally use empirical approximations to calculate the stomatal compensation point concentration e.g., by deriving it from the bulk ammonium content of the leaf tissue, which can be a source of bias. iv) Inaccurate or rough estimate of cuticular resistance, which is determined by the ratio of acidic air components and ammonia, besides the temperature and humidity. Models often consider a constant site-specific average for this parameter, even though the ratio of acidic substances to ammonia gas has diurnal and annual variations. Due to these uncertainties, the estimation of the share of fluxes controlled by soil and vegetation is often uncertain. Furthermore, the uncertainty of the parameterization limits the model's applicability and reduces its robustness. As a conception, we aim to simultaneously measure the ammonia flux above the canopy and on the soil. Hence, as the difference between the two fluxes we can determine the amount of ammonia recaptured by the canopy. By means of the soil flux measurement, we can also estimate the bias of empirical soil flux parameterization from soil bulk ammonium content. We also plan performing bioassay measurements to exclude the potential error derived from empirical estimation. Also, we intend calculate the acid/base gas ratio using daily mean concentrations, considering the diurnal variations of humidity-dependent nitric acid/ammonia/ammonium nitrate equilibrium. We will use different conceptional models separately, based on previously developed simulation for all variations of day/night, wet/dry bare soil/vegetation cases in flux estimation at a fertilized crop, making it possible to partition the ratio of ammonia emitted by soil and recaptured either by stomata (built up in plant tissue) or by wet cuticula (net loss). 

How to cite: Horváth, L., Gombi, C., Huszár, H., Nagy, Z., Pintér, K., Szabó, A., Takács, T., Torma, P., Tóth, E., Weidinger, T., Szabó, G., and Bozóki, Z.: Developing a method to estimate the ammonia loss from fertilized cropland and the part of soil emission recaptured by the canopy, applying field scale bidirectional models and flux measurements, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-145, https://doi.org/10.5194/ems2023-145, 2023.