UP1.2

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.

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.

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.

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.

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.

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.

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.

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.

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., Erner, 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.

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.

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.

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.

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 z_0H and z_0M. 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.

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 NLT_3D 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. NLT_3D 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 NLT_3D 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.

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.

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.

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.

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.

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.