4-9 September 2022, Bonn, Germany

UP1.2

UP1
Atmospheric boundary-layer processes, turbulence and land-atmosphere interactions 

Atmospheric boundary-layer (ABL) processes and their interactions with the underlying surface are crucial for weather, climate, air-quality and renewable-energy forecasts. The multitude of interacting processes act on a variety of temporal and spatial scales and include atmospheric turbulence, atmosphere-soil-vegetation interactions, gravity waves, boundary-layer interactions with dry and moist convection, mesoscale flows, submeso motions, etc.

Although significant advances have been achieved during the last decades, an appropriate comprehension of ABL processes and their interactions under different conditions is still a challenge in meteorology. Improving this knowledge will help to correctly represent ABL processes in weather and climate models, allowing to provide more accurate numerical weather prediction (NWP) forecasts and climate scenarios.

This session welcomes conceptual, observational and modeling research related to the physical processes that appear in the ABL, including those devoted to study the interactions with the free atmosphere above and with the surface below. Current contributions evaluating existing models and schemes are also welcome, as well as the presentation of new implementation in numerical modelling.

The following topics are especially encouraged to be submitted to the session:

• Theoretical and experimental studies of the turbulence-closure problem with emphasis on very stable stratification and convection, accounting for interactions between the mean flow, turbulence, internal waves and large-scale self-organized structures.

• Boundary-layer clouds (including fog) and marine, cloud-topped boundary layers: physics and parameterization within NWP and climate models and observational studies.

• Orographic effects: form drag, wave drag and flow blocking, gravity waves.

• Challenges on the surface-exchange processes, including soil-vegetation-atmosphere transfers. Flux aggregation in atmospheric boundary layers over heterogeneous terrain.

• Representation of boundary layers and land-surface interaction in atmospheric models.

• Organization of deep convection across differing atmospheric scales.

• Large-eddy simulation and direct numerical simulation of turbulent flows.

• PBL and surface-layer studies using long-term data (climatology), detailed analysis of case studies and field campaigns presentation.

Including Young Scientist Conference Award
Convener: Gert-Jan Steeneveld | Co-conveners: Carlos Román-Cascón, Nikki Vercauteren, Bert Holtslag
Orals
| Thu, 08 Sep, 14:00–17:15 (CEST)|Room HS 2, Fri, 09 Sep, 09:00–10:30 (CEST), 11:00–15:15 (CEST)|Room HS 2
Posters
| Thu, 08 Sep, 11:00–13:00 (CEST) | Display Thu, 08 Sep, 08:00–Fri, 09 Sep, 14:00|b-IT poster area

Orals: Thu, 08 Sep | Room HS 2

Chairpersons: Gert-Jan Steeneveld, Carlos Román-Cascón, Bert Holtslag
Land-atmosphere interactions from models and observations I
14:00–14:30
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EMS2022-441
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solicited
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Online presentation
Mirjana Sakradzija, Noviana Dewani, Frank Beyrich, Daniel Klocke, Ivan Bastak Duran, Juerg Schmidli, and Linda Schlemmer

Large-Eddy Simulations (LES) are set up to complement the FESSTVaL Field Experiment on sub-mesoscale spatio-temporal variability in Lindenberg using the Icosahedral Nonhydrostatic (ICON) model in a limited-area mode. The LES represent realistic conditions in the area around the observational site and are forced by the ICON-D2 analysis and forecasts. Four nested domains are used of which the innermost one has a horizontal grid resolution of about 75 m. This presentation will focus on the sensitivity of surface turbulent heat fluxes and their spatial variability on soil moisture to explain the discrepancies between the modeled and observed sensible heat flux. In-situ measurements provide the surface heat flux representative of different land cover conditions, crops and forests, while the area-averaged flux is measured along a scintillometer path of several kilometers. Next, a series of experiments will test the sensitivity of the strength of convective circulations on soil moisture. In these experiments, the initial soil moisture is increased, decreased or perturbed in different ways to find what role the variability of soil moisture plays in setting the strength of the convective circulations. The variance of vertical velocity is then compared to the FESSTVaL measurements derived from Doppler Lidar in vertical stare mode to find how realistic the circulations in LES are. This analysis focuses on several "golden days" that include a clear-sky day, three shallow-convective days of which one developed boundary-layer rolls and two precipitating days in June 2021. The complete set of the LES covers the intensive observational period of FESSTVaL from the 7. June to 4. July 2021.

How to cite: Sakradzija, M., Dewani, N., Beyrich, F., Klocke, D., Bastak Duran, I., Schmidli, J., and Schlemmer, L.: What controls the strength of convective circulations in real-case Large-Eddy Simulations during FESSTVaL?, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-441, https://doi.org/10.5194/ems2022-441, 2022.

14:30–14:45
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EMS2022-638
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Onsite presentation
Lijie Zhang, Stefan Poll, and Stefan Kollet

Land surface heterogeneity affects the surface heat flux distribution and induces secondary circulations at a certain scale. Background wind may significantly influence the effect of surface heterogeneity on secondary circulation. In this study, we investigate how background wind affects the evolution of the atmospheric boundary layer, focusing on the influence of the formation of secondary circulation. We used a coupled ICON-LES (Icosahedral Nonhydrostatic Large Eddy Simulation mode) with a land surface model (TERRA-ML) to simulate the development of the atmospheric boundary layer over a river corridor mimicked by continuously distributed soil moisture under different background wind conditions. The atmospheric domain size is 4.8 km x 4.8 km x 4.2 km in X, Y, and Z directions with a horizontal and vertical spatial grid spacing of 50 m using double-periodic boundary conditions. All simulations have the same initial well-mixed atmospheric conditions and constant incoming radiation of 700 Wm-2   with varying background winds with different wind speeds (0 to 16 ms-1) and directions (cross-valley, parallel-valley, or mixed).

The atmospheric states are decomposed into three parts: ensemble-averaged, mesoscale, and turbulence. We show that wind speed and surface heterogeneity jointly affect the surface energy distribution, independent of the wind direction. The secondary circulation structure persists under the parallel-valley wind regardless of wind speed but is destroyed when the cross-valley wind is stronger than the mesoscale horizontal wind speed. The maximum mesoscale vertical wind variance reflects the secondary circulation strength. We show that the secondary circulation strength positively correlates with the Bowen ratio and stability parameter (-Zi/L)  under cross-valley wind and mixed conditions.

How to cite: Zhang, L., Poll, S., and Kollet, S.: Large Eddy Simulation of Surface Heterogeneity Induced Secondary Circulation with Background Winds, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-638, https://doi.org/10.5194/ems2022-638, 2022.

14:45–15:00
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EMS2022-630
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CC
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Onsite presentation
Dmitrii Mironov and Peter Sullivan

Direct numerical simulations (DNS) at bulk Reynolds number Re=104 and bulk Richardson number Ri=0.25 are performed to analyze the structure and mixing intensity in strongly stable boundary-layer flows over thermally homogeneous and heterogeneous surfaces. An idealized plane Couette flow set-up is used as a proxy for real-world flows. The flow is driven by a fixed velocity at the upper surface, while the lower surface is at rest. The temperature at the horizontal upper and lower surfaces is either homogeneous or varies sinusoidally in the streamwise direction, while the horizontal-mean temperature is the same in the homogeneous and heterogeneous cases. 

The stratification is strong enough to quench turbulence over homogeneous surfaces, resulting in velocity and temperature profiles that vary linearly with height. However, turbulence survives over heterogeneous surfaces. Both the molecular diffusion and the turbulence contribute to the downward, i.e., the down-gradient, transfer of horizontal momentum. The total (diffusive plus turbulent) heat flux is directed downward. However, the turbulent contribution to the heat flux appears to be positive, i.e., up the gradient of the mean temperature. An analysis of the second-order velocity and temperature covariances and of the vertical-velocity and temperature skewness suggests that the counter-gradient heat transport is due to quasi-organized cell-like vortex motions generated by the surface thermal heterogeneity. These motions act to transfer heat upwards similar to quasi-organized cell-like structures that transfer heat upwards in atmospheric convective boundary layers. Thus, the flow over heterogeneous surface features local convective instabilities and upward eddy heat transport, although the overall stratification remains stable and the heat is transported downward in the mean. The DNS results are compared to the results from the large-eddy simulation study of weakly stable boundary layer (Mironov and Sullivan 2016, J. Atmos. Sci., 73, 449-464). The DNS findings corroborate the pivotal role of the temperature variance in setting the structure and transport properties of the stably-stratified flow over heterogeneous surfaces, and the importance of third-order transport of temperature variance.

The analysis is focused on the flow configuration where the crests of the surface temperature waves are normal to the mean flow. Work is underway to analyze the effect of the surface heterogeneity orientation (e.g., temperature-wave crests normal vs. parallel to the mean flow) on the flow structure and mixing intensity. 

Implications for modeling (parameterizing) strongly stable boundary layers in large-scale atmospheric models are discussed. 

How to cite: Mironov, D. and Sullivan, P.: Turbulence Structure and Mixing in Strongly Stable Boundary-Layer Flows over Thermally Heterogeneous Surfaces , EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-630, https://doi.org/10.5194/ems2022-630, 2022.

15:00–15:15
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EMS2022-75
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Onsite presentation
Role of changing vegetation properties on the variability of Indian summer monsoon rainfall 
(withdrawn)
Sachin Budakoti and Subimal Ghosh
15:15–15:30
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EMS2022-238
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Onsite presentation
Maike Ahlgrimm, Bastian Kirsch, and Mirjana Sakradzija

During the Field Experiment on Submesoscale Spatio-Temporal Variability in Lindenberg (FESSTVaL) just under forty distinct cold pool cases were observed within the area covered by the observational network (see contribution by B. Kirsch on observed cold pool morphology in UP1.5). Here, we investigate to what degree the Icosahedral Nonhydrostatic (ICON) model is able to reproduce the observed cold pool properties.
ICON is run on a limited area domain centered around the observational facilities used during FESSTVaL and covers most of eastern Germany and western Poland. Similar to the operational “D2” setup, the horizontal resolution is approximately 2km such that deep convective transport is largely resolved, while shallow convection remains a subgrid-scale, parameterized process. The model is tested with two options for the shallow convection scheme (Tiedtke-Bechtold with/without stochastic perturbations) and an optional cold pool perturbation (CPP) scheme aimed at improving convection triggering along cold pool boundaries.
The cold pools produced by the model share many qualitative similarities with the observations: ICON successfully produces cold pools on days when they are observed. As in the observations, the growth of the cold pool area appears to be directly associated with precipitation: When precipitation ceases, the cold pool area stops growing. The intensity of the cold pool (measured as the 2m temperature decrease) however tends to reach its peak before precipitation and area growth cease. It appears that the model may lose the cold pool signature rather more quickly than is observed.
As per design, the CPP scheme enhances the vertical velocity along cold pool boundaries, which leads to more intense precipitation associated with the cold pool forming convection as well as enhanced cold pool intensity.

How to cite: Ahlgrimm, M., Kirsch, B., and Sakradzija, M.: Simulating cold pools with ICON during the FESSTVaL period, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-238, https://doi.org/10.5194/ems2022-238, 2022.

Coffee break
Chairpersons: Gert-Jan Steeneveld, Carlos Román-Cascón, Bert Holtslag
Land-atmosphere interactions from models and observations II
16:00–16:15
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EMS2022-453
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Onsite presentation
Balázs Szintai, Helga Tóth, and László Kullmann

In current state-of-the-art Numerical Weather Prediction (NWP) models Leaf Area Index (LAI) is considered as an external parameter where monthly values are derived from long-term averages. Such an approach is not capable of describing vegetation anomalies e.g. during severe droughts, when LAI values (especially over non-irrigated grasslands and croplands) could be considerably lower than long-year averages of the selected month. A solution for this inaccuracy is presented in this study, with the aim to use satellite LAI observations in the NWP model. The main difficulty with such an approach is that high resolution (e.g. that of Sentinel) satellite vegetation products have a time lag of 10-15 days. To overcome this the following system was developed: satellite vegetation observations are assimilated in an offline land data assimilation system which is capable to deliver a soil and vegetation state analysis 10 days prior the actual date (T-10d). From T-10d we integrate the offline surface model with prognostic vegetation until the current date; and the resulting vegetation state (at time T) could be merged with the operational analyses of the NWP model.

In the present study the AROME-Hungary NWP system is used, which is run operationally at the Hungarian Meteorological Service (OMSZ) at 2.5 km horizontal resolution. For the simulation of surface processes the offline SURFEX land surface model is applied, which in its present version utilizes the ISBA-Ags simplified photosynthesis scheme to simulate LAI prognostically. In SURFEX-offline an Extended Kalman Filter method is used to assimilate Leaf Area Index satellite measurements, from the OLCI sensor of Sentinel-3.

The capabilities of the system and the impact of LAI change on the weather forecast produced by AROME-Hungary are presented during summer 2021. In 2021 a cold spring was followed by very hot and dry summer in the Carpathian Basin. During July and August a severe drought occurred over Southern Hungary and Northern Serbia, and consequently maize fields over large areas were significantly underdeveloped (irrigation is very limited in the region). This LAI anomaly was well captured by the SURFEX-offline system. Results show that LAI can have an impact on the weather forecast produced by AROME in summer anticyclonic cases. Most affected variables are 2 m temperature and precipitation.

Based on the encouraging results it is planned that this system for the improvement of LAI fields in AROME is going to be implemented in the operational NWP chain of OMSZ in near future.

 

How to cite: Szintai, B., Tóth, H., and Kullmann, L.: Introducing a daily updated Leaf Area Index in a mesoscale Numerical Weather Prediction model, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-453, https://doi.org/10.5194/ems2022-453, 2022.

16:15–16:30
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EMS2022-674
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Online presentation
Mireia Udina, Joan Bech, Eric Peinó, Francesc Polls, and Marta Balagué

In the context of the “Land surface Interactions with the Atmosphere over the Iberian Semi-arid Environment” (LIAISE), the WISE-PreP project was designed to study precipitation processes aiming to characterize possible differences in precipitation induced by surface characteristics (irrigated vs non-irrigated areas). Instrumentation deployed during the 2021 campaign included three sites equipped each with a vertical radar Doppler Micro Rain Radar (MRR) and a laser disdrometer (PARSIVEL), covering both irrigated and non-irrigated sites. Time series of vertical precipitation profiles are recorded to study microphysical processes trough the evolution of raindrop size distributions and related variables including precipitation intensity or convective vs stratiform rainfall regimes.

The main meteorological variables at the surface are analysed during LIAISE 2021 campaign and compared with A reference period (2010-2019) for irrigated and non-irrigated areas to characterize local differences either related with dynamics or land-use influences, and their influence in the boundary layer development and precipitation initiation.

First results show higher accumulated precipitation in the non-irrigated area (eastern area) than those in irrigated area (western area) in summer 2021, a feature also observed in summers for the reference period. Maximum and minimum daily temperatures are higher in irrigated areas than in non-irrigated areas. Both results are consistent with current climatology based on monthly precipitation and temperature that indicate the existence of a zonal gradient that increases semi-arid conditions (drier and warmer) from the east to the west. Sea-breeze regime also influences the two type areas, with different time arrival and a slight shift in wind components.

Funding for this research was provided by “Analysis of Precipitation Processes in the Eastern Ebro Subbasin” (WISE-PreP, RTI2018-098693-B-C32) and the Water Research Institute (IdRA) of the University of Barcelona.

How to cite: Udina, M., Bech, J., Peinó, E., Polls, F., and Balagué, M.: Precipitation characteristics and related boundary-layer processes during LIAISE 2021, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-674, https://doi.org/10.5194/ems2022-674, 2022.

16:30–16:45
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EMS2022-218
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Onsite presentation
Sasu Karttunen, Ewan O'Connor, Antti Hellsten, Carl Fortelius, and Leena Järvi

After a century of rapid urbanization, the majority of the world’s population is now living within urban areas. Despite this, micro and mesoscale meteorological dynamics and processes within the urban boundary layer are not thoroughly understood. Furthermore, a major fraction of these urban areas are located in coastal regions. These urban boundary layers are characterized by a high degree of surface heterogeneity and complex dynamics associated with interactions between land and marine air masses. The broad range of relevant spatial and temporal scales associated with coastal urban boundary layer phenomena makes them notoriously difficult to model.

One example of such phenomenon is the sea-breeze, observed in numerous coastal urban areas around the globe. The general sea-breeze circulation has associated spatial scales ranging from O(10 km) to O(100 km). However, it is directly influenced by surface exchanges, which have spatial scales down to O(1 m) in urban areas. By using novel multi-scale modelling methods capable of explicitly resolving all relevant scales of interactions, our aim is to study the complex balance of boundary layer processes during a realistic springtime sea-breeze case in Helsinki, Finland.

The PALM model system, an open source meteorological modelling system for boundary layer flows, has implemented the capability for multi-scale two-way self-nested large-eddy simulation (LES) setups. Such setups of several self-nested LES domains are especially suited for studying problems such as the interaction between the urban surface and the sea-breeze circulation. This approach differs from the more traditional modelling approaches, where interactions at either micro or mesoscale have been parametrized or given as one-way boundary conditions, effectively preventing the possibility of studying the two-way interactions and feedbacks.

We study both the mechanical and thermal influence of the urban surface on the development of the mesoscale circulation. Furthermore, we investigate how the urban surface affects the development of the internal boundary layer and subsequent convection in the lower branch of the cell, where the stable marine air mass is advected over land. This is achieved by studying the spatial scales associated with the convective turbulent structures as well as exchanges of heat and momentum in various regions of the circulation.

In order to verify the simulation setup and the results obtained, we compare the results with observations from Doppler lidars, a C-band dual-polarization weather radar and an in-situ measurement network being operated in the region. We anticipate that we will gain exciting new insights into the development of a sea-breeze circulation, convective internal boundary layers and turbulent structures in coastal urban boundary layers. Furthermore, we expect to quantify the effect of the urban surface on convective boundary layer development in the region.

How to cite: Karttunen, S., O'Connor, E., Hellsten, A., Fortelius, C., and Järvi, L.: Resolving micro to mesoscale interactions between urban surface and a sea-breeze circulation using high resolution large-eddy simulations, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-218, https://doi.org/10.5194/ems2022-218, 2022.

16:45–17:00
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EMS2022-649
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Onsite presentation
Klara Jurcakova and Radka Kellnerova

Coherent structures play a dominant role in momentum and mass transfer in turbulent boundary layers. It is very difficult to directly monitor those structures in an atmospheric boundary layer because their detection from point measurements is very uncertain and continuous spatial atmospheric measurements are seldom. Wind tunnel modeling of ABL flows offers a well-controlled environment and allows the utilization of state-of-the-art experimental methods such as Particle Image Velocimetry with a high temporal and spatial resolution. We have analyzed flow measurements from the meteorological observatory Kopisty (Czech Republic) where an 80m tall meteorological tower equipped with 3D sonic anemometers at four levels is installed. We have built a wind-tunnel scaled model of the observatory and its vicinity to study spatial coherent structures and their characteristics in neutrally stratified ABL flows. The wind-tunnel measurement revealed large organized structures as low- and high-momentum regions or clusters of correlated longitudinal and vertical velocity fluctuations (e.g., sweep and ejection events). Those structures are not eddies, they are in the shape of irregular waves. They can be a few hundred meters long and up to a hundred meters wide. The structures’ size is increasing with the height above ground and in the lowest 100 m, the majority of them are attached to the ground. Both measurements and model results showed the dependence of the flow structure on the upwind terrain conditions. The coherent structures are smaller and stronger in the flow coming from the hilly forested terrain (southwest wind direction) compared to the flow coming from the lakeside (northeast wind direction).

How to cite: Jurcakova, K. and Kellnerova, R.: Spatial structures in atmospheric boundary-layer flow – wind tunnel modeling, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-649, https://doi.org/10.5194/ems2022-649, 2022.

17:00–17:15
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EMS2022-274
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Onsite presentation
Gaëtan Noual, Yves Brunet, Patrick Le Moigne, and Christine Lac

It has been observed repeatedly that the presence of forests at regional scale can affect cloud formation and precipitation, but the direction and extent of this effect is highly variable. It primarily depends on the differences in albedo and roughness length with the surrounding areas, leading to contrasted water and energy balances, and also depends on the water availability and the possibility for secondary atmospheric circulations. In contrast with Amazonia where cloud enhancement has been reported over deforested areas, recent satellite observations have shown that temperate European forests promote summertime cloud formation. This is notably the case of the Landes forest in South-West France, where the destruction of 40% of the pine trees by storm Klaus in 2009 produced a sharp drop in the positive cloudiness anomaly.

To better understand these effects, the atmospheric Meso-NH model coupled with the SURFEX platform has been used at 500 m horizontal resolution at the scale of the region with advanced physical parametrizations. Two complementary study cases were selected, with significantly different soil moisture conditions. Both present summer situations of cloud cover over the Landes forest in contrast to its surroundings. After a validation step using satellite observations, near surface measurements and radiosoundings, simulated fields were compared with pre-Klaus and post-Klaus surface conditions.

In both cases, deforestation tends to decrease cloud top height or reduce the occurrence of the highest clouds. The surface flux partitioning appears as a key factor: the wet case shows a significant decrease in cloud cover, while the dry one shows a slight increase in cloud lifetime. This is in agreement with previous research using satellite data, showing that deforestation generally decreases cloud cover in temperate regions, while having a somewhat contrasting role. The atmospheric budgets of wind velocity components, TKE, temperature and air moisture have been analysed to understand the physical processes at play. A further study with a larger number of simulated case studies and a variety of meteorological situations will allow us to move towards a climatological approach.

How to cite: Noual, G., Brunet, Y., Le Moigne, P., and Lac, C.: Simulating the effects of regional forest cover on mid-latitude boundary-layer clouds, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-274, https://doi.org/10.5194/ems2022-274, 2022.

Orals: Fri, 09 Sep | Room HS 2

Chairpersons: Gert-Jan Steeneveld, Carlos Román-Cascón, Bert Holtslag
PBL studies using OBSERVATIONAL data
09:00–09:15
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EMS2022-306
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Onsite presentation
Elsa Dieudonné, Natalie Brett, Gilberto J. Fochesatto, Jean-Christophe Raut, Barbara D'Anna, Brice Temime-Roussel, Julia Schmale, Roman Pohorsky, Andrea Baccarini, Brice Barret, Stefano Decesari, Antonio Donateo, Gianluca Pappaccogli, Federico Scoto, Maurizio Busetto, Hervé Delbarre, Slimane Bekki, François Ravetta, and Kathy S. Law

A seven-week field campaign took place in Fairbanks, Interior of Alaska, during January and February 2022, in the framework of the Alaskan Layered Pollution And Chemical Analysis (ALPACA) international project. A compact Doppler Lidar was deployed (Leosphere WindCube v2) to profile winds in the atmospheric boundary layer from 40 to 290 m above ground level (a.g.l.). The first objective of this project was to study the influence of strong atmospheric stratification and temperature inversion layering on air pollution. For this purpose, the Lidar was first installed in the urban canopy downtown Fairbanks city (~3.5 weeks). The second objective was to study the influence of shallow cold flows (SCF) on surface-based temperature inversions and the energy budget. Therefore, the Lidar was then re-deployed in a suburban site located nearby the foothills bordering the city, at the outlet of a small valley. Measurements on this site also included surface turbulence by eddy-covariance, radiative fluxes and temperature profiles, both in-situ during tethered balloon launches and remote sensing using a Microwave Radiometer.

The conditions for operating a near-infrared Doppler Lidar were very stringent, as Arctic environments are generally pristine, except for urban areas in wintertime. However, the wind dataset availability up to 100 m a.g.l. was ~47% at the urban site and ~30% at the suburban site. The relationship between Lidar performance and aerosol size distribution is under evaluation at both sites, using observations from Scanning Mobility Particle Sizers. At the suburban site, the SCF descending from the nearby valley was regularly visible, with wind speeds reaching 5-6 m.s-1 at 40 m a.g.l. Cases of continuous flow lasting for more than 24 hours were observed, during which the SCF depth remained stable and very shallow (40 to 60 m a.gl.). Pulsated episodes also occurred, that could be as short as 2 hours, and were characterized by a more variable SCF depth (up to 100 m a.g.l.). The local and regional conditions triggering the onset, variability and characteristics of the SCF are under investigation. Another unexpected observation was made: at both sites, snow precipitations were always found to be associated with ascending winds (up to 1.2 m.s-1). 

To improve our understanding of the dynamical processes highlighted by the Lidar wind observations, the Polar-WRF (Weather Research and Forecasting) model will be run at a very high resolution (~1 km). The focus will be to investigate, and potentially improve, the ability of the Polar-WRF surface layer and land-surface model (via the roughness length for momentum and the stability functions) to reproduce the surface energy budget, the very and weakly stable regimes and the transition between the two at the SCF onset in a continental, high-latitude context. 

How to cite: Dieudonné, E., Brett, N., Fochesatto, G. J., Raut, J.-C., D'Anna, B., Temime-Roussel, B., Schmale, J., Pohorsky, R., Baccarini, A., Barret, B., Decesari, S., Donateo, A., Pappaccogli, G., Scoto, F., Busetto, M., Delbarre, H., Bekki, S., Ravetta, F., and Law, K. S.: Doppler Lidar Wind Profiling in Fairbanks (Interior of Alaska) During the 2022 ALPACA Field Campaign, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-306, https://doi.org/10.5194/ems2022-306, 2022.

09:15–09:30
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EMS2022-615
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Online presentation
Gabin Urbancic

Monin-Obukhov Similarity Theory (MOST) is the canonical theory of boundary layer meteorology and describes the structure of the Atmospheric Surface Layer (ASL). The theory results in the stability functions which relate the non-dimensional local gradient to the Monin-Obukhov stability parameter. In the near-neutral limit MOST converges to the logarithmic wind profile of von Karman and Prandtl. Although MOST is still widely used, limitations have been clear for decades. In the very stable conditions, the surface scaling regime of MOST is replaced by the local scaling regime of Nieuwstadt (1984). More recently, it has been clear that non-local, finite-sized eddies have a large influence on the surface fluxes, even during near-neutral and stably stratified conditions. A local theory, which uses the local gradients, cannot describe all the properties of the ASL. The most significant work on the topic comes from the Hockey-Stick Transition (HOST), developed by Sun et al. (2012). HOST demonstrates two regimes where turbulence first scales with the local gradient until a threshold wind speed is reached, after which the turbulence scales with the wind speed. These features cannot be derived from MOST. Multiple studies have considered how to merge MOST and HOST with some success (e.g. Sun et al. 2020; Grisogono et al. 2020)

In this talk, I will develop the regimes of validity of MOST and HOST using a multitude of datasets measuring the ASL from near-neutral to very stable conditions. I will describe how the different theories differ and in which way they are consistent. The role of bulk shear and finite eddies are discussed illustrating the non-local structure of the ASL and its influence on the associated surface fluxes. I will consider for which scales a finite eddy is active or inactive in the surface fluxes and how it relates to Townsends attached-eddy hypothesis. The talk will end with a prognostic of where improvements and progress is likely to occur.

How to cite: Urbancic, G.: Beyond Monin-Obukhov Similarity Theory and the Hockey-Stick Transition, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-615, https://doi.org/10.5194/ems2022-615, 2022.

09:30–09:45
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EMS2022-569
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Onsite presentation
Victoria Sinclair, Jenna Ritvanen, Gabin Urbancic, Yurii Batrak, Irina Statnaia, Dmitri Moisseev, and Mona Kurpaa

The boundary-layer height and stability are two key parameters that control the exchange of energy between the surface and the atmosphere and strongly influence air quality and pollution dispersion. In this study, we investigate the boundary layer (BL) height and stability at the long-term measurement station of Hyytiälä, located in the Boreal forest of southern Finland at a latitude of 61.85N. A 41-year climatology of the annual and diurnal cycle of BL height was created based on ERA5 reanalysis data. The ability of ERA5 to correctly capture the BL height was determined by comparing ERA5 to BL heights derived from 847 radiosondes that were released from Hyytiälä as part of the 7.5 month “Biogenic Aerosols - Effects on Clouds and Climate” (BAECC) campaign in 2014. Four different methods to estimate the BL height were applied to the radiosondes. A 25-year climatology of surface-layer stability was created based on eddy covariance measurements and was used to identified under which conditions ERA5 can best capture the BL height and to better understand the annual and diurnal cycle of the BL height. The climatology results show that the shallowest (353~m) monthly median BL height occurs in February and the deepest (576~m) in June. The largest variability in BL height, and the largest diurnal range, was found in April and May. Notably, the shallowest BLs were found to occur at night in spring and summer which is also when very stable conditions were most likely to occur. Between November and February, there was no diurnal cycle in BL height due to the limited solar radiation at this time of year. Unstable conditions were rare during the cold season but so were very stable BLs. The absence of very stable conditions in winter is related to the stronger winds, and hence more shear-driven turbulence, compared to either spring of summer. Very shallow and stable BLs are also prevented from developing in autumn and winter due to the increased amount of cloud compared to spring and summer. Good agreement was found between the BL height in ERA5 and the BL height diagnosed from the radiosondes for almost all stability classes but ERA5 does overestimates the BL height in very stable conditions. In addition, ERA5 BL height differs most from the radiosondes at 18 UTC. These results suggest that ERA5 has adequate vertical resolution to correctly resolve the BL height in most conditions, that ERA5 struggles to correctly simulated stable BLs, and lastly that ERA5 does not resolve the evening transition correctly.

 

 

 

 

How to cite: Sinclair, V., Ritvanen, J., Urbancic, G., Batrak, Y., Statnaia, I., Moisseev, D., and Kurpaa, M.: A long-term climatology of boundary-layer height and stability at Hyytiälä in southern Finland., EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-569, https://doi.org/10.5194/ems2022-569, 2022.

09:45–10:00
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EMS2022-142
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CC
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Online presentation
Dorita Rostkier-Edelstein, Eyal Agassi, Pavel Kunin, Tamir Tzadok, Rong-Shyang Sheu, Adam Pitrkowski, and Ayala Ronen

This study describes a regional-scale fog event during 4 consecutive nights in January 2021, in Israel. During the event, we performed a first-of-its kind measurements campaign in Israel, with a  variety of instruments, including both, in-situ measurements of droplets-size distribution, visibility range and meteorological parameters; and ground-based remote sensing with a Doppler Lidar and a thermal IR Whole Sky Imager. In addition, we analyzed the event with IR and visible EUMETSAT imagery, mesoscale WRF-RTFDDA  forecasts and NCEP/NCAR reanalyses.

Reanalyses showed that the event developed under dry Red Sea Trough synoptic conditions at the surface, without cyclonic upper-air circulation, suitable for radiation fog development.

The measurements of droplets-size distribution and visibility range enabled the calculation of liquid-water content and fog-droplets effective radius. The fog event was characterized by a visibility range as low as 90 m. The droplets-diameter main mode was 1-2 micrometers, followed by another around 6 micrometers. Typical liquid-water content  values were 0.01-0.025 g/m3. These parameters are typical of small-droplets fog. A similar identification as small droplets fog was provided by EUMETSAT imagery. 

Analysis of the fog event with the thermal IR Whole Sky Imager, as monitored by the sensor’s field of view, revealed three distinctive properties that make it possible to identify it. First, it exhibits an azimuthal symmetrical shape during the buildup phase. Second, the zenith brightness temperature is very close to the ground-level air temperature. Lastly, the rate of increase in cloud cover up to a completely overcast sky is very fast. Additionally, we validated the use of a Doppler Lidar as a tool for monitoring fog by proving that the measured backscatter-attenuation vertical profile agrees with the calculation of the Lidar equation fed with data measured by in-situ instruments. It was shown that fog can be monitored by those two off-the-shelf stand-off-sensing technologies not originally designed for fog purposes. 

The EUMETSAT imagery was useful to analyze the two-dimensional temporal evolution of the regional fog. The fog evolved from the southern to the central coastal area following the temperature differences from south to north.  Clear patches were observed over coastal-urban areas due to their urban heat-island effect, as previously reported in studies around the world.

High resolution WRF-RTFDDA forecasts (1.1-km grid size), together with an algorithm for fog detection based on simultaneous thresholds of wind speed, dew-point temperature and relative humidity, succeeded in predicting the temporal and spatial development of the dense fog and its disipation. Moreover, they proved useful in distinguishing between near-surface fog and elevated fog/low clouds, a distinction not possible from satellite imagery only. Clear patches at coastal areas, partially covered by urban landuse, were observed in model forecasts, too. WRF-RTFDDA forecasts proved useful in forecasting this massive fog and low-clouds event and in providing warnings to users.

How to cite: Rostkier-Edelstein, D., Agassi, E., Kunin, P., Tzadok, T., Sheu, R.-S., Pitrkowski, A., and Ronen, A.: Study of a unique fog event in Israel during January 2021: from measured microphysics, ground-remote sensing and satellite imagery to mesoscale forecasts and synoptic analysis, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-142, https://doi.org/10.5194/ems2022-142, 2022.

10:00–10:15
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EMS2022-92
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Onsite presentation
Jaemyeong Mango Seo, Cathy Hohenegger, Nima Shokri, and Hannes Nevermann

Convective cold pool is an air mass that is relatively colder than its surrounding air, and is mainly generated by latent cooling of precipitating hydrometeor in convective clouds. As it spreads out, it interacts with the surrounding air through entrainment and with the surface/soil through latent/sensible heat fluxes. In this study, we investigate how the surface/soil thermodynamic properties are affected by the passage of convective cold pools. To that aim, we use observational network data including atmospheric and surface/soil measurements, which were obtained from the intensive measurement campaign FESSTVaL that took place in Lindenberg, Germany in the summer of 2021. For this study, three cold pool cases are selected for analysis. All the observations show a clear decrease in air temperature as well as in surface temperature associated with the passage of the cold pools, but only some of the observations also show a decrease in soil temperature. Analysis reveals that the soil is more likely to cool when the soil is also wettened by precipitation. To understand the mechanism by which the increase in soil moisture affects the decrease in soil temperature, we conduct a simple ideal thermal diffusion model that mimics the passage of a cold pool and the propagation of the signal into the soil. The ideal model results reproduce well the observed soil temperature changes, and shows that the increase in soil moisture due to precipitation affects the heat capacity and the thermal conductivity of the soil, allowing the cold signal to propagate into the soil.

How to cite: Seo, J. M., Hohenegger, C., Shokri, N., and Nevermann, H.: A Case Study on the Soil Temperature Cooling Mechanism by Convective Cold Pools using Observation Network Data, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-92, https://doi.org/10.5194/ems2022-92, 2022.

10:15–10:30
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EMS2022-194
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CC
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Onsite presentation
Carlos Román-Cascón, Carlos Yagüe, Pablo Ortiz, Mariano Sastre, Gregorio Maqueda, Encarna Serrano, Begoña Artiñano, Francisco J. Gómez-Moreno, Elias Díaz-Ramiro, Elisabeth Alonso, Javier Fernández, Rafael Borge, Adolfo Narros, Jose M. Cordero, Ana M. García, and Andrés Núñez

Nitrogen dioxide (NO2) is one of the main concerns regarding air quality due to its impacts on human health in cities, where the main emissions come from the combustion of the vehicle’s engines. However, it is also well known that, apart from variations in emissions, meteorology plays a very important role on the evolution of ground-level concentrations in urban areas.

In this work, we compute correlations between NO2 concentration with different meteorological and turbulent variables during two contrasting field campaigns in winter 2020 and summer 2021 in the city of Madrid, within the framework of the AIRTEC-CM project (*). Specifically, we compare the correlation of NO2 with wind speed and with turbulent variables which include the vertical velocity component of the wind: the turbulent kinetic energy and the friction velocity, computed from two sonic anemometers installed at two different heights, the first one close to the street and the second one on the terrace of a tall building. Hence, the overall objective is to determine the variable and the emplacement that shows better correlation under specific conditions (summer, winter, and a stable period in winter). To this aim, we also investigate the usefulness of using some threshold values of these variables to be associated with high levels of pollutant concentration.

In our study, we highlight the importance of the different atmospheric processes observed during the diurnal cycle, which affect the NO2 levels reached. The highest NO2 concentrations are observed during the evening and the initial part of the night with fair-weather conditions in winter due to the incipient atmospheric-boundary-layer (ABL) stabilization. Nonetheless, the levels reached are very sensitive to small variations in the afternoon and evening wind speed and turbulence. Besides, these days are also characterized by the arrival of nocturnal thermally-driven flows, which are formed only a few hours after the aforementioned stabilization. These wind values are found to play a crucial role on the rapid reduction of the concentrations, becoming very important phenomena for the pollutant reduction in areas of complex terrain and heterogeneous surfaces, as Madrid region is. This result contrasts with the pollutant evolution observed in cities located also in complex terrain, but with different features, like cities in cold pools (i.e., valleys). Finally, we show the importance of some short-lived -and difficult to predict- wind events that generate intermittent turbulence and affect the evolution of the pollutant concentration during the first hours of the stable ABL.

(*) AIRTEC-CM project (S2018/EMT-4329), funded by The Regional Government of Madrid.

How to cite: Román-Cascón, C., Yagüe, C., Ortiz, P., Sastre, M., Maqueda, G., Serrano, E., Artiñano, B., Gómez-Moreno, F. J., Díaz-Ramiro, E., Alonso, E., Fernández, J., Borge, R., Narros, A., Cordero, J. M., García, A. M., and Núñez, A.: Observational analysis of the wind speed and turbulence relationship with NO2 concentration, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-194, https://doi.org/10.5194/ems2022-194, 2022.

Coffee break
Chairpersons: Gert-Jan Steeneveld, Carlos Román-Cascón, Bert Holtslag
11:00–11:15
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EMS2022-116
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Onsite presentation
Stanislaw Krol and Szymon Malinowski

Clouds are the source of the biggest uncertainty in weather and climate models. One cannot fully understand clouds without understanding turbulence and microphysical processes in clouds. EUREC4A [1] experiment conducted during January and February focused on studying stratocumulus clouds (the most common marine clouds on earth) in atmoshperic boundary layer near Barbados. One of many measurements conducted during the campaign, was one using Twin-Otter aircraft, on which an Ultra Fast Thermometer 2b [2] was mounted. This thermometer, developed at Institute of Geophysics at the University of Warsaw is able to measure temperature at a 20 kHz frequency. After averaging to a frequency of 2 kHz, and assuming that the plane is moving at an average speed of 60 m/s, the spatial resolution of the thermometer is of the order of centimeters. The aircraft performed cloud penetrations together with measurements of temperature and other parameters such as the three components of wind velocity, pressure and humidity. The proposed method to study temperature time series is the time-dependent Recurrence Quantification Analysis (RQA) [3].

Recurrence Plots (RPs) are a visualization of a square matrix, which elements correspond to the moments when the system recurs, or when phase space trajectory of a system visits the same area in the phase space. RQA is a technique of data analysis based on the analysis of structures present in RPs. This method is used to study non-linear behaviors in the system, and to study determinism and chaos of the system, and transitions between them. In this study, portions of temperature records corresponding to cloud penetration were analyzed.

The analyzed clouds were divided into groups based on the angle of the penetration (windward, leeward, or from the side), as well as the variability of the wind during the penetration. Preliminary results suggest that the sharp edges of either the windward side of a dissipative cloud or both sides of a developing cloud are the places where the behavior of temperature is the most stochastic. Some clouds exhibited a behavior where the temperature behavior shifted from more stochastic near the windward side to more deterministic near the leeward side of the cloud. There are also side penetrations that, based on the RQA, contain information about the structure of the cloud and its dynamical properties.

We acknowledge funding by Poland’s National Science Centre grant no. UMO-2018/30/M/ST10/00674.

[1] Stevens B. et. al. 2021: EUREC4A, Earth System Science Data, vol. 13(8), pp. 4067-4119, 10.5194/essd-13-4067-2021
[2] Kumala, W. et. al. 2013: Modified ultrafast thermometer UFT-M and temperature measurements during Physics of Stratocumulus Top (POST), Atmos. Meas. Tech., vol. 6, pp. 2043–2054, 10.5194/amt-6-2043-2013
[3] N. Marwan, et. al. 2007: Recurrence Plots for the Analysis of Complex Systems, Physics Reports, vol. 438(5–6), pp. 237–329, 10.1016/j.physrep.2006.11.001

How to cite: Krol, S. and Malinowski, S.: Recurrence quantification analysis of high-resolution cloud temperature data from EUREC4A, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-116, https://doi.org/10.5194/ems2022-116, 2022.

11:15–11:30
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EMS2022-177
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Onsite presentation
Jakub L. Nowak, Holger Siebert, Kai-Erik Szodry, and Szymon P. Malinowski

Wide-spread presence, persistence and high albedo of marine stratocumulus clouds makes them important for the energy balance of the planet, hence also in model-based climate predictions. Typically, stratocumulus clouds occupy upper few hundred meters of the atmospheric boundary layer which is commonly referred as stratocumulus-topped boundary layer (STBL). The transport of moisture from the ocean surface maintains the cloud against entrainment drying from the free toposphere. When the STBL grows in depth, the drivers of the circulation weaken or the subcloud layer stabilizes, then the mixing of air volumes across the entire STBL depth may become impossible to sustain - the boundary layer decouples.

Within the present study, the stratification and turbulence properties in coupled and decoupled marine STBL are compared using high resolution in situ measurements performed by the helicopter-borne platform ACTOS in the region of the Eastern North Atlantic. Particular attention is given to small-scale turbulence.

The coupled STBL was characterized by a comparable latent heat flux at the surface and in the cloud top region, and substantially smaller sensible heat flux in the entire depth. Turbulence kinetic energy (TKE) was efficiently generated by buoyancy in the cloud and at the surface, and dissipated with comparable rate across the entire depth. Structure functions and power spectra of velocity fluctuations in the inertial range were reasonably consistent with the predictions of Kolmogorov theory. The turbulence was close to isotropic.

In the decoupled STBL, decoupling was most obvious in humidity profiles. Heat fluxes and buoyant TKE production at the surface were similar to the coupled case. Around the transition level, latent heat flux decreased to zero and TKE was consumed by weak static stability. In the cloud top region, heat fluxes almost vanished and buoyancy production was significantly smaller than for the coupled case. TKE dissipation rate inside the decoupled STBL varied between its sublayers. Structure functions and power spectra in the inertial range deviated from Kolmogorov scaling. This was more pronounced in the cloud and subcloud layer in comparison to the surface mixed layer. The turbulence was more anisotropic than in the coupled STBL, with horizontal fluctuations dominating. The degree of anisotropy was largest in the cloud and subcloud layer of the decoupled STBL.

Integral length scales were of the order of 100 m in both cases which is smaller than the depth of the coupled STBL or of the sublayers of the decoupled STBL. It may be speculated that turbulence produced in the cloud or close to the surface is redistributed across the entire coupled STBL but rather only inside the sublayers where it was generated in the case of the decoupled STBL.

How to cite: Nowak, J. L., Siebert, H., Szodry, K.-E., and Malinowski, S. P.: Observations of turbulence properties in coupled and decoupled stratocumulus-topped marine boundary layers, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-177, https://doi.org/10.5194/ems2022-177, 2022.

11:30–11:45
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EMS2022-405
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Onsite presentation
Leonie Esters, Anna Rutgersson, Erik Nilsson, and Erik Shalée

Inland freshwater bodies such as lakes provide the largest contribution of natural carbon to the atmosphere. To study this contribution to the atmospheric carbon cycle, eddy-covariance flux measurements at lake sites have become increasingly popular. This method allows to estimate the fluxes from local source processes. The difficulty of such local observations is that large-scale non-local processes as for example entrainment or advection can add erroneous contributions to the eddy covariance flux estimations. Scalar quantities from the free-atmosphere above the measurement tower can be entrained into the boundary layer and to the measurement site. Also, the lake is surrounded by land, from where scalar characteristics can be horizontally advected to the measurement site.

During four years of eddy-covariance measurements of carbon dioxide from Lake Erken, a freshwater lake in mid-Sweden, we found unexpected fluxes of carbon dioxide when the lake was entirely covered with ice. We investigated these unexpected fluxes using a statistical approach, which is based on surface-layer data (van de Boer et al., 2014). This approach reveals that non-local processes produce these erroneous fluxes. Additionally, we found that the strength and occurrence of the non-local processes depend on the fetch (distance between the instrumented tower and upwind shore) and the prevailing wind speed. The combination of fetch and wind speed was defined as the time over water. The smaller the time over water, the higher the contribution of the non-local processes.

We present how we corrected the contribution of the non-local processes to the estimations from the eddy covariance flux tower, not only for the ice-covered periods, but also the periods when the lake was free of ice. We present this analysis approach as an example for Lake Erken and propose that it can be extended to other lakes. This can potentially increase our understanding of the carbon exchange between lakes and the atmosphere.

How to cite: Esters, L., Rutgersson, A., Nilsson, E., and Shalée, E.: Investigation of local carbon fluxes from lakes to the atmosphere from eddy covariance observations and the erroneous contribution from non-local processes, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-405, https://doi.org/10.5194/ems2022-405, 2022.

Models and schemes: development and evaluation
11:45–12:00
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EMS2022-27
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CC
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Online presentation
Surface air temperature bias in meteorological models due to misrepresentation of the atmospheric boundary layer thickness
(withdrawn)
Igor Esau and Marvin Kähnert
12:00–12:15
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EMS2022-454
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Onsite presentation
Michal Belda, Resler Jaroslav, Geletič Jan, Krč Pavel, Maronga Bjorn, Suhring Matthias, Kurppa Mona, and Fuka Vladimír

Numerical models using the large-eddy simulation technique (LES) are particularly useful in the urban environment in which their high resolution allows for capturing small-scale features of built-up areas such as buildings, roads, pavements or urban greenery. However, good model performance is not only the result of an accurate representation of the relevant physical processes. It also largely depends on the availability of proper input data describing the urban setup, namely the building and land-surface properties. In this study an LES-based modeling system PALM 6.0 featuring an improved urban surface model (USM) was used to assess sensitivity with respect to land-surface and building properties in a densely built-up residential area in Prague, Czech Republic, particularly in the vicinity of a typical crossroads. Two types of scenario simulations were employed. First, a set of synthetic scenarios changing surface and material parameters such as albedo or emissivity, by which the sensitivity of the model simulations to potentially erroneous input data was tested. These showed the highest sensitivity to the correct setting of surface parameters used in radiation balance equations. Second, a set of urbanistic scenarios was designed to assess the limits of effects of commonly considered urban-heat-island mitigation measures such as greening of the streets or altering surface materials. In this case, urban greenery is confirmed to be the most effective measure, especially when considering both physical and biophysical temperature indicators. On the other hand, analysis of air quality, specifically with respect to PM2.5 dispersion, confirmed the opposite behavior to that of thermal indicators; i.e. improved thermal comfort brings deterioration of PM2.5 concentrations.

How to cite: Belda, M., Jaroslav, R., Jan, G., Pavel, K., Bjorn, M., Matthias, S., Mona, K., and Vladimír, F.: Sensitivity of the LES model PALM in the urban environment: a case study in Prague, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-454, https://doi.org/10.5194/ems2022-454, 2022.

12:15–12:30
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EMS2022-159
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CC
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Onsite presentation
Jonathan Kostelecky and Cedrick Ansorge

Direct numerical simulation (DNS) of the atmospheric boundary layer (ABL) is becoming more and more popular for its conceptual simplicity and increasing degree of realism: domain sizes and simulation durations can be attained that allow for extrapolation of results to the geophysical limit. Geophysical flows predominantly occur over rough surfaces, which significantly affects drag, mixing and transport properties of the flow. At the same time, the representation of roughness surface-layer modelling is generally based on bulk representations of roughness related to the roughness parameters for scalar and momentum exchange z0H and z0M. Here, we circumvent surface-layer similarity by directly imposing the intricate mechanical boundary condition resulting from a rough wall, while maintaining the efficient and tuned numerical methods for Cartesian meshes by an immersed boundary method (IBM); three-dimensional roughness elements are fully resolved at the bottom wall of a direct numerical simulation. We follow a spline-based approach for a partially staggered arrangement that was introduced by Laizet and Lamballais (J. Comp. Phys 2009, Vol 228, p.5989-6015). By this approach, the flow boundary conditions at roughness objects are fulfilled exactly which allows for a straightforward treatment of roughness effects in the scalar field. We apply this implementation to investigate the effect of fully resolved three-dimensional roughness elements in a turbulent Ekman boundary layer and obtain a priori estimations of scalar and momentum roughness parameters for canonical roughness configurations.

* This work is funded by the ERC Starting Grant ”Turbulence-Resolving Approaches of the Intermittently Turbulent Atmospheric Boundary Layer [trainABL]” of the European Research Council (funding ID 851347).

How to cite: Kostelecky, J. and Ansorge, C.: Direct Numerical Simulation of the Aerodynamically Rough Atmospheric Boundary Layer, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-159, https://doi.org/10.5194/ems2022-159, 2022.

12:30–12:45
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EMS2022-87
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Onsite presentation
Volker Küll and Andreas Bott

In numerical weather prediction (NWP) models atmospheric processes are usually split into grid scale and subgrid scale ones with the latter to be parameterized. Traditionally, all subgrid scale dynamics is classified as turbulence which is mainly active in mixing the planetary boundary layer and the coupling to the surface. Modeled turbulence is essential for the proper representation of the fluxes of tracers, energy and momentum in the lower atmosphere and, therefore, for the quality of weather forecasts.

However, in modern numerical weather prediction models grid sizes keep on decreasing as affordable with increasing computational power. Thus, originally subgrid scale physical processes such as turbulence have become partially resolved on the model grid. After the classical K approaches, this first lead to the introduction of vertically non-local turbulence parameterization schemes to explicitly represent the vertical extent of the largest eddies in the PBL.

Meanwhile also horizontal grid sizes in the order of 1 km have become small enough to also resolve at least the larger eddies extending over the whole PBL on the model grid. This is accounted for in the NLT3D scheme. It employs a transilient matrix formulation which has proven to be a flexible way to extent the non-local representation of
turbulent fluxes also to the horizontal direction. NLT3D has already been tested in the framework of the WRF model in several short term simulations over the North American prairie and Appalachian mountain range. It has now been implemented into the ICON model of Deutscher Wetterdienst to be analyzed in longer term simulations over Europe. Additionally, the impact of different grid size of the hosting model is analyzed over more inhomogeneous terrain.

We will present results from ICON simulations including NLT3D and discuss comparisons with the results from the classical ICON model setup including its operational turbulence scheme. For validation also comparisons with observational data from field campaigns and with data of the operational station network of DWD will be shown.

How to cite: Küll, V. and Bott, A.: Testing the Nonlocal Three-dimensional Transilient Turbulence (NLT3D) scheme in the ICON model, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-87, https://doi.org/10.5194/ems2022-87, 2022.

12:45–13:00
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EMS2022-385
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Onsite presentation
Stephanie Reilly, Ivan Bastak Duran, Anurose Theethai-Jacob, and Juerg Schmidli

The most frequently used turbulence parameterizations in numerical weather prediction (NWP) and general circulation (GC) models are turbulence kinetic energy (TKE) schemes. These turbulence schemes are strongly dependent on a key component, the turbulence length scale. The turbulence length scale is used to parameterize the molecular dissipation of TKE and is also required for calculating the turbulence coefficients. Traditionally, the turbulence length scale formulations do not take into account the transfer of TKE across scales, as they are designed for scales above the energy production range of the turbulence spectra. However, with computational power growing, it has become increasingly possible to run numerical models at scales that are within the gray zone of turbulence. At resolutions within this gray zone, the cross-scale transfer of TKE needs to be taken into account in order to accurately represent the turbulence. For this purpose, a turbulence length scale diagnostic was developed. This is achieved by calculating the turbulence length scale from the so-called effective dissipation rate, which is a combination of the cross-scale TKE transfer and the dissipation rate. The effective dissipation rate is estimated from the budget of the TKE using large-eddy simulation (LES) data. A similar approach is used to calculate the turbulence length scale from the budgets of the scalar variances. This study thus makes use of three different turbulence length scale diagnostics based on: the TKE, the variance of the total specific water content, and the variance of the liquid water potential temperature. Using the turbulence length scale diagnostics as a reference, a series of five exisiting algebraic turbulence length scale formulas are evaluated. The objective evaluation is carried out in terms of a local root mean square error and a non-local three-component technique. The algebraic formulations are evaluated for a set of five idealized LES cases, simulated using the MicroHH model. These cases represent different boundary layer conditions. Based on the evaluation, the length scales proposed by Nakanishi and Niino and Honnert et al. are found to be most representative of the turbulence length scale diagnostics.

How to cite: Reilly, S., Bastak Duran, I., Theethai-Jacob, A., and Schmidli, J.: An Evaluation of Algebraic Turbulence Length Scale Formulations, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-385, https://doi.org/10.5194/ems2022-385, 2022.

Lunch break
Chairpersons: Gert-Jan Steeneveld, Carlos Román-Cascón, Bert Holtslag
14:00–14:15
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EMS2022-566
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Onsite presentation
Juerg Schmidli, Ivan Bašták Ďurán, and Mirjana Sakradzija

The unified parameterization of turbulence and clouds in the atmospheric boundary layer is one of the challenges in current weather prediction and climate models. An update of the two-energy turbulence scheme is presented, the 2TE+APDF scheme. The original version of the two-energy scheme is able to successfully model shallow convection without the need of an additional parameterization for non-local fluxes. However, the performance of the two-energy scheme is worse in stratocumulus cases, where it tends to overestimate the erosion of the stable layers. To alleviate this problem, we propose several modifications: an update of the stability parameter to consider local stratification, a more flexible computation of the turbulence length scale, and the introduction of the entropy potential temperature to distinguish between a shallow convection and a stratocumulus regime. In addition, the two-energy scheme is coupled to a simplified assumed PDF (probability density function) method in order to achieve a more universal representation of the cloudy regimes. The updated turbulence scheme is evaluated for several idealized cases and one selected real case in the ICON modeling framework. The results show that the updated scheme corrects the overmixing problem in the stratocumulus cases. The performance of the updated scheme is comparable to the operational setup, and could be thus used instead of the operational turbulence and shallow convection scheme in ICON. Additionally, the updated scheme improves the coupling with dynamics, which is beneficial for the modeling of coherent flow structures in the ABL, such as, for example, cloud streets.

How to cite: Schmidli, J., Bašták Ďurán, I., and Sakradzija, M.: Unified parameterization of turbulence and boundary layer clouds using the updated two-energies turbulence scheme , EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-566, https://doi.org/10.5194/ems2022-566, 2022.

14:15–14:30
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EMS2022-617
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Onsite presentation
Marten Klein and Heiko Schmidt

Key challenges in the modeling and simulation of Ekman boundary layers (EBLs) in the atmosphere and in the ocean are related to the interplay of Coriolis, viscous, and inertial forces that act on all relevant scales of the turbulent flow. The interplay of these forces can lead to nonuniversal flow properties such as intermittency, boundary-layer resonance, and ‘eruption’ of the surface layer thickness, which can locally excite waves, mean flows, and turbulence (e.g., [1,2,3]). Advanced wall models should sufficiently capture the dynamical complexity in order to improve upon parameterization schemes that describe the mean effects but do not directly resolve the small-scale processes (e.g. [4]).

The difficulties associated with such a cross-regime modeling of EBL flows is addressed in the present study by utilizing the stochastic one-dimensional turbulence (ODT) model formulated for atmospheric boundary layers [5] using an adaptive grid [6]. ODT is as self-contained flow model that aims to resolve all relevant flow scales but only for a single vertical column. Turbulent eddy events are sampled from a stochastic process and punctuate the deterministic flow evolution. The stand-alone model resolves vertical viscous and nongeostrophic horizontal Coriolis forces along the wall-normal one-dimensional domain.

Here, we investigate a periodically forced EBL flow over a smooth and flat surface. The configuration aims to model the effects of wind-shear and tidal forcing in the ocean and coastal environment, respectively, and is mathematically related also to alternating up- and down-slope winds in the atmosphere. In this study, the local wall-shear stress is the result of the forcing surplus laminar Ekman dynamics and turbulent mixing. We show that ODT is able to capture the regime transition from the laminar to an intermittently turbulent flow with fixed model parameters when the Reynolds number is increased. ODT furthermore captures the boundary-layer resonance and the dependence of the turbulence intensity on the wall oscillation phase. Preliminary model predictions are in agreement with theory, direct numerical simulations, and reference measurements. It is concluded that ODT has reasonable capabilities for regime-independent fluctuation modeling yielding improvements of the bulk–surface coupling by capturing transient small-scale processes.

References

[1] M. Klein, T. Seelig, M. V. Kurgansky, A. Ghasemi, I. D. Borcia, A. Will, E. Schaller, C. Egbers, U. Harlander (2014). J. Fluid Mech., 751:255–297.

[2] A. Ghasemi, M. Klein, A. Will, U. Harlander (2018). J. Fluid Mech., 853:111–149.

[3] M. Vincze, N. Fenyvesi, M. Klein, J. Sommeria, S. Viboud, Y. Ashkenazy (2019). EPL, 125:44001.

[4] M. Klein, H. Schmidt, D. O. Lignell (2022). Int. J. Heat Fluid Flow, 93:108889.

[5] A. R. Kerstein, S. Wunsch (2006). Boundary-Layer Meteorol., 118:325–356.

[6] D. O. Lignell, A. R. Kerstein, G. Sun, E. I. Monson (2013). Theor. Comp. Fluid Dyn., 27(3):273–295.

How to cite: Klein, M. and Schmidt, H.: Stochastic modeling of transient Ekman flow at arbitrary Reynolds number driven by horizontal bottom wall oscillation, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-617, https://doi.org/10.5194/ems2022-617, 2022.

14:30–14:45
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EMS2022-693
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CC
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Online presentation
Sofia Farina, Dino Zardi, and Andrea Bisignano

Thermally-driven slope winds regularly occur over simple inclines and mountain valley sidewalls under clear-sky, calm-wind synoptic situations. As typical features of mountainous terrains these flows contribute to the transport of momentum, heat, and mass, including pollutants and other passive tracers.

A simple model for the dispersion of a passive tracer in a thermally driven wind over a slope is presented. The source of the tracer is pointwise and the emission continuous. Dispersion results from the combination of along-slope advection by a pure anabatic/katabatic flow and slope-normal turbulent diffusion. The mean wind is modeled following Prandtl’s (1942) steady state solution assuming a K-closure for momentum and heat fluxes, where the eddy viscosity and diffusivity are constant. The advection-diffusion equation for the passive tracer is solved with a constant eddy diffusivity as well. Results are compared with those that are obtained neglecting the dynamical structure of the wind, i. e.  adopting a classical Gaussian model with uniform wind velocity. A sensitivity analysis of resulting concentrations on the height of the source above ground and on the wind strenght is proposed. Two different regimes are identified, depending on the relative position of the source and the velocity maximum. Moreover, mathematical relationships between the position and the intensity of the ground concentration field, and their dependence on environmental parameters are outlined.

Reference

Prandtl L. 1942. Führer durch die Strömungslehre, Chapter 5. Vieweg und Sohn: Braunschweig, Germany. [English translation: Prandtl L. 1952. Mountain and valley winds in stratified air, in Essentials of Fluid Dynamics: 422–425. Hafner Publishing Company: New York, NY]

 

How to cite: Farina, S., Zardi, D., and Bisignano, A.: Modelling the dispersion of a passive tracer from a continuous poit source in a steady thermally-driven slope wind, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-693, https://doi.org/10.5194/ems2022-693, 2022.

14:45–15:00
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EMS2022-34
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Onsite presentation
Kr Sreenivas, Shaurya Kaushal, and Dhiraj Kumar Singh

After the sunset, under calm and clear sky conditions, aerosol laden surface air-layer, cools rapidly due to radiative cooling[1, 2, & 3].  Radiative cooling extends to several 100 meters from the surface and results in the development of a stable inversion layer. However, ground surface, owing to its high thermal inertia, lags in the cooling process and about a meter thick air layer just above the ground can be 2-60 C cooler than the ground[1,2]. Thus, at the surface about a meter thick unstable convective layer is present which capped by a stable inversion layer that extends up to about 300 meters. This configuration involving a convective mixed layer topped by a stably stratified inversion layer is a classic case of penetrative convection[4 & 5].  Here, we present a computational study of this penetrative convection in the nocturnal atmospheric boundary layer (see Fig 1) and show its relevance to fog-layer dynamics. Field and laboratory measurements of aerosol number density is used to model the strength of the cooling radiative flux term. Vertical profiles of horizontally averaged temperature, density and heat flux are presented. Dynamics of penetrative motion of the fluid from the mixed layer into the stable inversion layer across the interface, results in entrainment and growth of the mixed layer. Here we show, correct length scale to be used in the Richardson number correlation[6], to estimate the entrainment rate and to model the mixed layer growth. Analysis of the mixed layer and the entrainment zone, shows a good agreement with the previously reported laboratory experiments on penetrative convection[4]. We show how aerosol number density impacts the growth or decay of the mixed layer (see Fig 2).  Our study also indicates that occurrence of fog near the ground surface could induce a large-scale vertical mixing, which is observed in the field experiments.

References:

1. Mukund, V., et. al., (2014). Field and laboratory experiments on aerosol‐induced cooling in the nocturnal boundary layer. Quarterly Journal of the Royal Meteorological Society, 140(678), 151-169.

2. Mukund, V., et. al., (2010). Hyper-cooling in the nocturnal boundary layer: the Ramdas paradox. Physica Scripta, 2010(T142), 014041.

3. Ponnulakshmi, V. K., et. al., (2012). Hypercooling in the nocturnal boundary layer: Broadband emissivity schemes. Journal of the atmospheric sciences, 69(9), 2892-2905.

4. Deardorff, J. W.,  et. al., (1969). Laboratory investigation of non-steady penetrative convection. Journal of Fluid Mechanics, 35(1), 7-31.

5. Kumar, R. (1989). Laboratory studies of thermal convection in the interface under a stable layer. International journal of heat and mass transfer, 32(4), 735-749.

6. Sreenivas, K. R., et. al., (1995). Modeling the dynamics of the mixed layer in solar ponds. Solar energy, 54(3), 193-202.    

How to cite: Sreenivas, K., Kaushal, S., and Singh, D. K.: Penetrative convection in Nocturnal ABL: Numerical Simulations, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-34, https://doi.org/10.5194/ems2022-34, 2022.

15:00–15:15
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EMS2022-595
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CC
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Onsite presentation
Yue Tian, Juerg Schmidli, and Julian Quimbayo-Duarte

This study investigates the skill of the COSMO model (v5.7) at 1.1 km horizontal resolution in simulating the near-surface foehn properties and evolution for five south foehn events and a 5-year-long analysis dataset. A significant cold bias, as well as a moist bias, are found in the major northern foehn valleys, including the Rhine Valley, during foehn hours in all the chosen cases and the 5-year climatological statistics. The model biases in different foehn types are inspected over the 5-year period. Among deep foehn types, the cold bias is larger (smaller) and the moist bias is smaller for the moister (drier) foehn events. Several possible causes of the cold bias are examined with sensitivity experiments for the five foehn cases. The sensitivity experiments include changes to the parameterization of the land-atmosphere interface (i.e. adoption of a skin temperature, a change of the heat resistance in the laminar sublayer, and a new formulation of the bare soil evaporation), to the 1D turbulence parameterization (including horizontal shear production of turbulence as a first step towards 3D effects), and to the horizontal grid spacing (1.1 km versus 550 m). While several of the sensitivity experiments impact the 2-m temperature indifferently during both foehn- and non-foehn hours, only a change in the horizontal grid spacing has a significant impact on the 2-m temperature during foehn hours. The 550-m run shows also an improvement in the simulated foehn duration and northward foehn extent. Possible reasons for the improvements and the remaining bias will be discussed.

How to cite: Tian, Y., Schmidli, J., and Quimbayo-Duarte, J.: A station-based evaluation of south foehn forecasting with COSMO-1, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-595, https://doi.org/10.5194/ems2022-595, 2022.

Display time: Thu, 8 Sep 08:00–Fri, 9 Sep 14:00

Posters: Thu, 8 Sep, 11:00–13:00 | b-IT poster area

Chairpersons: Gert-Jan Steeneveld, Carlos Román-Cascón, Bert Holtslag
ONSITE Poster Session
P26
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EMS2022-18
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Young Scientist Conference Award
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Onsite presentation
Amandine Kaiser and Nikki Vercauteren

Atmospheric boundary layers with thermally stable stratification (SBL) are the least understood type of boundary layer due to suppressed turbulence and the presence of myriads of processes on multiple spatio-temporal scales that modulate the turbulence. Classical approaches to turbulence parameterization fail to reproduce turbulent dissipation in SBL contexts and this is a known source of errors in weather and climate models.

One of the research challenges is to develop an accurate representation of distinct regimes of the SBL and transitions between them. Defining all mechanisms which lead to these sudden regime transitions and predicting or detecting them is still a challenge. Stochastic modeling approaches are a promising framework to analyze the different types of triggers for regime transitions. Therefore, we test the sensitivity of the SBL to intermittent turbulence mixing events by extending a single-column model with physically meaningful randomizations. Noise in the boundary conditions for example represents fluctuating geostrophic wind or cloud cover. This model is used as a numerical tool to systematically investigate noise-induced regime transitions of the SBL.

The overall objective is to improve the representation of the atmospheric boundary layer in numerical weather prediction and climate models by developing stochastic parametrization concepts. It is very useful to identify abrupt regime transitions in order to anticipate air pollution hazards, fog, and frost, all of which coincide with the onset of a very stable boundary layer. In addition, current SBL parametrizations fail at representing mixing in the very stable regime and may need to switch to a parametrization where turbulence stationarity assumptions are relaxed.

How to cite: Kaiser, A. and Vercauteren, N.: Sensitivity of the Nocturnal and Polar Boundary Layer to Transient Phenomena, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-18, https://doi.org/10.5194/ems2022-18, 2022.

P27
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EMS2022-160
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Onsite presentation
Jelena Radovic, Vladimír Fuka, and Štěpán Nosek

This study presents the outcome of a series of different street scale Large Eddy Simulations (LES) performed on a 3D urban array. The configuration and the setup of these simulations are built upon the results previously published by Nosek et al., 2018, BAE 138; Kluková et al., 2021, JWEIA 208. The LES simulations are focused on the turbulent flow, passive scalar transport, and dispersion within and above the previously mentioned 3D urban array which portrays a typical building setup present in many cities across Europe. This study is connected to the project named “The Role of coherent structures’ dynamics on scalar transport and dispersion in the urban canopy layer” and it is backed up by the experimental data obtained in the wind tunnel measurements.

The LES computations are carried out by the open-source computational fluid dynamics (CFD) software called OpenFOAM.  The simulations differ in terms of resolution and refinement of the computational grid within the street canyons of the urban array, the type of solvers used (e.g., pimpleFoam or a solver based on a projection method), and the boundary conditions imposed (periodic and turbulent inlet boundary conditions). The comparison between the LES and Detached Eddy Simulation (DES) approach is investigated and presented as well.

The findings of this study are serving to a better understanding of how well the different setups and configurations of the OpenFOAM LES simulations are capable of representing the turbulent structures in the street canyons and their influence on the passive scalar dispersion. In addition, the results confirm the optimal OpenFOAM simulation setup for the investigation of these types of problems. 

How to cite: Radovic, J., Fuka, V., and Nosek, Š.: Sensitivity analysis of the OpenFOAM LES simulations performed on the 3D urban array structure, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-160, https://doi.org/10.5194/ems2022-160, 2022.

P28
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EMS2022-555
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Onsite presentation
Robert Grosz, Stanisław Król, Jakub Nowak, Wojeciech Kumala, and Szymon Malinowski

UltraFast Thermometers (UFT’s), developed successively at the Institute of Geophysics, University of Warsaw, allow for airborne measurements of temperature fluctuations in turbulent atmosphere with the resolution better than 1 cm, which provides insight into small-scale turbulent mixing in clouds, atmospheric boundary layer and free atmosphere.

In recent years new versions of UFT thermometers (UFT-2 family) were used in two measurement campaigns: ACORES and EUREC4A. In ACORES UFT-2 was deployed on the helicopter-borne measurement platform ACTOS used to sample marine stratocumulus clouds with the resolution reaching 3 mm. During the EUREC4A campaign similar 20 kHz temperature time series have been collected with the UFT-2b deployed onboard the BAS Twin Otter aircraft (average speed of 60 m/s) in the subtropical low atmosphere, in and between trade wind warm cumulus clouds. Data, resolving scales down the dissipation range, allow to estimate directly the temperature dissipation rate (TD) in cloud interiors, cloud shells, air spaces between the clouds, and in the atmospheric boundary layer.

Fig. 1. Example of the UFT-2 0.5 s long measurements from EUREC4A campaign: high-resolution plots of temperature (red) and TD (blue) before and during penetration through a cumulus cloud.

Until now, no experimental data on temperature dissipation in free atmosphere and in clouds from in situ measurements have ever been published. Such data may help to understand cloud microphysical processes with phase changes. Classically, during turbulent mixing of air masses, temperature is considered a passive scalar. In clouds, in the course of condensation/evaporation heat is released/absorbed, which may affect fine-scale fluctuations of temperature. Thus, statistical properties of temperature dissipation should differ from situations in which the temperature is just a passive scalar. Examples of temperature fluctuations and associate TD records (see Fig. 1), characteristic to the various atmospheric conditions, will be presented and discussed.

Acknowledgements: This project has received funding from Polish National Science Center (NCN) under grant agreement 2018/30/M/ST10/00674.

How to cite: Grosz, R., Król, S., Nowak, J., Kumala, W., and Malinowski, S.: Temperature dissipation in convective clouds during ACORES and EUREC4A, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-555, https://doi.org/10.5194/ems2022-555, 2022.

P29
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EMS2022-561
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Onsite presentation
Páll Ágúst Þórarinsson, Gunilla Svensson, and Stefan Wallin

The wind turning in the atmospheric boundary layer (ABL), the angle between the surface friction and wind just above the ABL, is greatly affected by the interaction between turbulence, Coriolis effects and density stratification within the layer. The wind-turning angle can be used as an indicator of cross isobaric mass flow which is notable in the formation and lifespan of synoptic scale cyclones (Holton & Hakim 2013, Beare 2007). It is therefore also good an indicator of how parameterization of sub-grid processes can affect outcome in climate and numerical weather prediction models.

By examining different ways of modelling turbulence in the ABL and boundary conditions we aim to find an explanation for why many state-of-the-art numerical weather prediction and climate models have such different results for the wind turning amongst themselves and compared to observations and large eddy simulations (LES). As a benchmark for our study, we use the first GEWEX Atmospheric Boundary-Layer Study (GABLS1) (Svensson & Holtslag 2009).

We anticipate to find variations in the representation of the turning of the wind by running suites of experiments by exploring parameter variations using a single column model of the ABL. We vary surface boundary conditions and internal turbulence parameterization parameters, and different temporal and vertical resolutions.

In the study we will use LES data (Sullivan et al. 2016), supersite observations, worldwide dataset of radio soundings (the Integrated Global Radiosonde Archive), and climate models.

References

Beare, R. J., 2007, ‘Boundary Layer Mechanism in Extratropical Cyclones’, Quarterly Journal of the Royal Meteorological Society, 133 (623), 503-515.

Holton, J. R. & Hakim, G. J, 2013, An Introduction to Dynamic Meteorology, 5th edn Academic Press, Oxford.

Svensson G. & A. A. M. Holtslag, 2009, ‘Analysis of model results for the turning of the wind and related momentum fluxes in the stable boundary layer’, Boundary Layer Meteorology, 132 (2), 261-277.

Sullivan, P. P., Weil, J. C., Patton, G., E., Jonker, H., J., J. & Mironov, D., V., 2016, ‘Turbulent Winds and Temperature Fronts in Large-Eddy Simulations of the Stable Atmospheric Boundary Layer’, Journal of the Atmospheric Sciences, 73 (4), 1815-1840.

How to cite: Þórarinsson, P. Á., Svensson, G., and Wallin, S.: Wind Turning in the Atmospheric Boundary Layer from Differences in Turbulence Parameterization and its Impact on Climate Modelling Results , EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-561, https://doi.org/10.5194/ems2022-561, 2022.

P30
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EMS2022-603
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Onsite presentation
Fiona Paulus and Roel Neggers

Moisture and aerosol exchange between the Arctic and the mid-latitudes plays a crucial role in the ongoing rapid warming of the Arctic climate. In particular the downward longwave radiation associated with liquid clouds as embedded in warm-air intrusions has been found to contribute significantly to enhanced melting of the ice sheet. The observed persistence of liquid clouds at high latitudes is not yet fully understood, but the transitions between cloud and cloud-free states are likely to be linked to large-scale dynamics. Recent Lagrangian Large-Eddy-Simulations (LES) following air masses into the Arctic have shown that the evolution of the mixed-layer depth is predominantly controlled by large-scale subsidence, and that relatively abrupt changes in subsidence rates can cause cloud collapse. This subsequently causes a drastic change in the surface energy budget. In order to support this modeling result with observations, simultaneous measurements of the large-scale subsidence and the mixed-phase cloud properties are necessary.

During the recent HALO-(AC)3 field campaign in Spring 2022, we used dropsonde patterns to measure divergence and other gradients across meso-scale areas. This approach has already been tested in the subtropics during the during the NARVAL and EUREC4A field experiments and has now been performed in the Arctic for the first time. Given the considerable differences in atmospheric dynamics between the subtropics and the Arctic, it is not clear a priori if this method can be equally successful. To find out, we present a comparison of independent measurements obtained during different flights and by two airplanes sampling the same area. Preliminary results show for the first time that the dropsonde method yields reliable estimates of mesoscale gradients in the Arctic, including subsidence and vorticity. Ongoing efforts to combine the wealth of HALO-(AC)3 data with targeted Lagrangian high resolution simulations will be briefly discussed.

How to cite: Paulus, F. and Neggers, R.: Measuring meso-scale gradients in the Arctic during HALO-(AC)3, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-603, https://doi.org/10.5194/ems2022-603, 2022.

P31
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EMS2022-472
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Onsite presentation
maurin Zouzoua, Sophie Bastin, Marjolaine Chiriaco, Marie Lothon, Fabienne Lohou, Laurent Barthès, Cécile Mallet, Solène Derrien, Frédérique Cheruy, Eric Bazile, Jan Polcher, Romain Roehrig, and Guylaine Canut

The surface turbulent fluxes, namely sensible and latent heat fluxes, are keys factors governing the boundary layer processes. Therefore, their correct representation in numerical models is crucial for accurate weather forecasts and climate projections. However, the formulation of these fluxes in such models is the second source of uncertainty, leading to incorrect surface-atmosphere interactions in the simulations. Model evaluation is essential to draw development perspectives. Existing methods mostly consist of direct comparison between observed and modelled fluxes, blending other sources of errors such as incoherent grid-scale representation (soil and vegetation types) and inaccurate environmental forcing (radiative fluxes, temperature, moisture and wind speed). Thus, quantifying errors solely due to fluxes formulation is still challenging. This study, within the framework of the French project MOSAI (Model and Observation for Surface-Atmosphere Interactions), aims at proposing a novel evaluation approach to better identify the weakness of numerical models in surface turbulent fluxes formulation. The concept is to freeze the errors due to other sources by using a machine-learning model, notably a multi-layer perceptron, trained to estimate the fluxes from variables describing the conditions in the surface layer. Hourly data collected over several years at three operational instrumented sites of ACTRIS-France research infrastructure (SIRTA in Paris, Météo-pole in Toulouse and P2OA in Lannemezan), are used. Then, after adaptation to the outputs of numerical models involved in the MOSAI project (RegIPSL, LMDZ, AROME and ARPEGE), the trained-perception will be applied to assess their surface turbulent fluxes.

How to cite: Zouzoua, M., Bastin, S., Chiriaco, M., Lothon, M., Lohou, F., Barthès, L., Mallet, C., Derrien, S., Cheruy, F., Bazile, E., Polcher, J., Roehrig, R., and Canut, G.: Using artificial neural network to better evaluate surface turbulent heat fluxes in weather and climate numerical models, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-472, https://doi.org/10.5194/ems2022-472, 2022.

P32
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EMS2022-162
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Onsite presentation
Pablo Fernández-Castillo, Carlos Román-Cascón, and Carlos Yagüe

Storm Filomena, in January 2021, was one of the most extreme snowstorms of the last century in Spain, blanketing wide areas across central Spain in exceptional amounts of snow. Once the storm left the Iberian Peninsula, the snow layer persisted for many days due to stationary stable synoptic conditions, leading to extremely low surface temperatures, with several records being hit. The storm caused major disruption and economic impact.

Despite Filomena being an extreme weather event, not many studies have addressed the cold spell once the low pressure system left the Iberian peninsula. In this work, we analyse the physical mechanisms that explain such low temperatures, studying the influence of different scales (synoptic and microscale) by means of reanalysis data and data analysis of a micrometeorological weather station in a rural area in central Spain, where snow accumulation exceeded 40 cm. Results suggest that the stable synoptic-scale weather conditions led to enhanced influence of the microscale and strong surface-based temperature inversions. In particular, significant changes are observed in the surface energy balance (SEB) due to a reduction in the net radiation available at the surface as a result of a higher albedo and emissivity of the surface, with respect to the period before the arrival of Filomena. The effect of the snow layer is also clearly visible on the soil temperatures and ground heat flux, which remained essentially constant throughout the period of study as a result of its lower thermal conductivity. Thus, all the terms involved in the SEB will be thoroughly analysed to study the influence of the snow layer on them and on the surface temperatures reached.

How to cite: Fernández-Castillo, P., Román-Cascón, C., and Yagüe, C.: Analysis of the surface energy balance and its impacts on the extreme low temperatures in Spain after snowstorm Filomena, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-162, https://doi.org/10.5194/ems2022-162, 2022.

P33
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EMS2022-33
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Onsite presentation
Elsa Dieudonné, Hervé Delbarre, Anton Sokolov, Felix Ebojie, Patrick Augustin, and Marc Fourmentin

Previous observational studies about Low-Level Jet (LLJs) in the North Sea area provided insight about LLJs occurring above offshore and inland sites. The present work completes this corpus by adding a statistical study performed above a coastal site, relying on four years of wind profiles recorded by a small Doppler lidar installed in Dunkerque port (northern coast of France, southernmost North Sea). The instrument, a Leosphere Windcube v2, had a range of 280 m allowing the detection of LLJs up to 244 m above mean sea-level. LLJs were found on 4.74 % (9,649) of the 10-minute average wind profiles, laying the basis for a robust statistical analysis of the jet core height, speed and direction, and for the study of the seasonal, annual and daily cycles of the LLJs occurrence. The altitude of the jet core (wind peak) was typically 114 m above mean sea-level, with a core speed around 7-8 m.s-1. The core wind direction exhibited a dominant mode in the NE direction (onshore coastwise) that occurred mostly during the afternoon (11:00 to 18:00 UTC), and a secondary mode in the ESE direction (offshore cross-coast) that occurred mostly during the late night (00:00 to 07:00 UTC). The months with the highest LLJ frequency were April, May and August, following the annual cycle of the dominant NE mode. The conditions of occurrence and possible formation mechanisms of the LLJs over Dunkerque were investigated using three parameters computed from the ERA5 reanalyzes data: the land-sea thermal contrast, the bulk Richardson number and the geostrophic flow. The dominant NE LLJ mode was associated with anticyclonic conditions (easterly geostrophic flow, warmer land, unstable atmosphere), this mode likely gathering local sea breeze events and regional circulations generated by the wind channeling at the entrance of the English Channel.

How to cite: Dieudonné, E., Delbarre, H., Sokolov, A., Ebojie, F., Augustin, P., and Fourmentin, M.: Characteristics of the Low-Level Jets Observed over Dunkerque (North Sea French coast) using 4 years of wind lidar data, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-33, https://doi.org/10.5194/ems2022-33, 2022.

P34
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EMS2022-605
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Onsite presentation
Lisa Schielicke, Christoph Gatzen, and Daniel Gruhlke

Wind power is highly dependent on the large scale weather situation, but additionally, small-scale variations of the boundary layer can have significant effects on the wind power production. In a joint collaboration with Statkraft, the authors analysed the influence of diurnal variations of the boundary layer on the amount of wind power produced across Germany.

Statkraft maintains wind power plants across the country and monitors the power production. A high-resolution data set of wind power production, broken down to the locations of the power plants, has been analysed with respect to small-scale variations of the wind power production. These sudden and often unexpected drops in wind power production are known to occur during daytime, especially in the morning (“morning dip”).

This work uses the data of 2021 to give the frequency of these morning dips across Germany. Their diurnal and seasonal occurrence is analysed as well as the regional occurrence. The occurrence of morning is analysed with respect to different pattern of meteorological conditions.

We found that morning dips are frequent in the warm season, whereas they are almost missing in the winter. Their average diurnal occurrence is dependent on the time of sun rise, and the relative frequency of days with morning dips was largest in the southern and eastern parts of Germany. Sunny conditions were most likely to be associated with morning dips, even when the large-scale flow was only modest. Cloudy and rainy conditions were not favorable for morning dips, even when the large-scale flow was strong. We suppose that morning dips are associated with sharp changes of the wind speed within the boundary layer, and that the change of boundary-layer depth during the morning causes the rapid drop of wind power production in particular on sunny days in the warm season.

How to cite: Schielicke, L., Gatzen, C., and Gruhlke, D.: Meteorological analysis of "Morning dips" in wind power production across Germany in 2021, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-605, https://doi.org/10.5194/ems2022-605, 2022.

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