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In summertime and during heat events the urban heat island can negatively impact public health in urban areas. In the context of climate change, climate adaptation receives more attention in urban planning. Microscale urban climate modelling can identify risk areas and evaluate adaptation strategies. Concurrently, evaluating the model results with observational data is essential. So far, model evaluation is mostly limited to short-term field campaigns or a small number of stations. This study uses novel crowdsourcing data from Netatmo citizen weather stations (CWS) to evaluate the urban microscale model PALM for a hot day (Tmax ≥ 30°C) in Bochum in western Germany with anticyclonic atmospheric conditions. Urban-rural air temperature differences are represented by the model. A quality control procedure is applied to the crowdsourced data prior to evaluation. The comparison between the model and the crowdsourced air temperature data reveals a good model performance with a high coefficient of determination (R2) of 0.86 to 0.88 and a root mean squared error (RMSE) around 2K. Model accuracy shows a temporal pattern and night-time air temperatures during the night are underestimated by the model, likely due to unresolved cloud cover. The crowdsourced air temperature data proved valuable for model evaluation due to the high number of stations within urban areas. Nevertheless, weaknesses related to data quality such as radiation errors must be considered during model evaluation and only the information derived from multiple stations is suitable for model evaluation. The procedure presented here can easily be transferred to planning processes as the model and the crowdsourced air temperature data are freely available. This can contribute to making informed decisions for climate adaptation in urban areas.

The Innovative Strategies for Observations in the Arctic Atmospheric Boundary Layer Program (ISOBAR) is a research project investigating stable atmospheric boundary layer (SBL) processes, whose representation still poses significant challenges in state-of-the-art numerical weather prediction (NWP) models. In ISOBAR ground-based flux and profile observations are combined with boundary layer remote sensing methods and the extensive usage of different unmanned aircraft systems (UAS). During February 2017 and 2018 we carried out two major field campaigns over the sea ice of the northern Baltic Sea, close to the Finnish island of Hailuoto at 65°N. In total 14 intensive observational periods (IOPs) resulted in extensive SBL datasets with unprecedented spatiotemporal resolution, which will form the basis for various numerical modeling experiments. First results from the campaigns indicate numerous very stable boundary layer (VSBL) cases, characterized by strong stratification, weak winds, and clear skies, and give detailed insight in the temporal evolution and vertical structure of the entire SBL. The SBL is subject to rapid changes in its vertical structure, responding to a variety of different processes. In particular, we study cases involving a shear instability associated with a low-level jet, a rapid strong cooling event observed a few meters above ground, and a strong wave-breaking event that triggers intensive near-surface turbulence. Furthermore, we use observations from one IOP to validate three different atmospheric models. The unique finescale observations resulting from the ISOBAR observational approach will aid future research activities, focusing on a better understanding of the SBL and its implementation in numerical models.

We have identified certain fundamental limitations of a mixing-length parametrization used in a popular turbulent kinetic energy-based subgrid-scale model. Replacing this parametrization with a more physically realistic one significantly improves the overall quality of the large-eddy simulation (LES) of stable boundary layers. For the range of grid sizes considered here (specifically, 1 m–12.5 m), the revision dramatically reduces the grid-size sensitivity of the simulations. Most importantly, the revised scheme allows us to reliably estimate the first- and second-order statistics of a well-known LES intercomparison case, even with a coarse grid size of O(10 m).

This contribution describes the urban climate model system PALM 6.0 for applications in urban areas (Palm4U). PALM4U contains several components to enable the model to simulate the atmospheric processes in urban areas on a building resolving scale. These components include the following modules: a land- and urban surface model, a chemistry model, a wind turbine model, a plant canopy model, a human biometeorology model, an indoor and building energy model, and a radiation model. A study case representing a realistic urban domain is designed and simulated to show the model capabilities. The chosen domain is located in the Charlottenburg neighbourhood in Berlin, Germany. In this study, we show the flow (wind field) as well as the thermal (radiation and temperature) properties as an example of a typical model application. The study emphasises the potential of using such an urban climate model as a design tool for urban planning strategies, planning of renewable energy systems, and the investigation of implications of climate change.

The potential effect of failing to predict nocturnal deep radiation fog on the development of the daytime convective boundary layer (CBL) is studied using large-eddy simulations. Typical spring and autumn conditions for the mid-latitudes are used to perform simulations in pairs. Fog formation is allowed in one simulation of each pair (nocturnal fog [NF]) and is suppressed in the other (clear sky [CS]). This allows for the identification of properties (temperature, humidity, boundary-layer depth), conditions, and processes in CBL development that are affected by fog. Mixing-layer temperatures and boundary-layer depths immediately after fog dissipation in CSs are shown to be up to 2.5 K warmer and 200 m higher, respectively, than the NF counterparts. Additionally, greater water vapor mixing ratios are found in the CSs. However, owing to greater temperatures, relative humidities at the CBL top are found to be less in CSs than in the corresponding NFs. This relative humidity difference might be an indication that cloud formation is suppressed to some extent. The magnitude of the differences between CSs and NFs during the day is mainly correlated to the fog depth (in terms of duration and liquid water path), whereas the key processes responsible for differences are the atmospheric long-wave cooling of the fog layer (for temperature development) and droplet deposition (for water vapor mixing ratio development). publishedVersion

Landoberflächenmodelle (engl. land surface model, LSM) werden heutzutage nicht nur in numerischen Wetter- und Klimavorhersagemodellen eingesetzt, sondern finden auch zunehmend in Modellen für mikroskalige atmosphärische Phänomene Anwendung. Das LSM übernimmt dabei die wichtige Kopplung zwischen Atmosphäre und Boden und ermöglicht es, dass viele unterschiedliche Land- und Bodentypen abgebildet werden. Das Simulationsmodell PALM wird seit mehr als zwanzig Jahren für atmosphärische Grenzschichtprozesse eingesetzt und seit einigen Jahren immer häufiger für komplexe Anwendungen, beispielsweise in Städten, mit integriertem LSM eingesetzt. In der aktuellen Studie wird das LSM in PALM beschrieben und mit in-situ Beobachtungsdaten aus Cabauw (Niederlande) evaluiert. Dazu werden zwei aufeinander folgende Strahlungstage simuliert und Temperatur, spezifische Feuchte, Horizontalwind sowie Komponenten der Energiebilanz mit Beobachtungsdaten verglichen. Dabei zeigen wir, dass die modellierten Komponenten der Energiebilanz im Analysezeitraum konsistent mit den Beobachtungen sind. Allerdings wird die stabile Schichtung der nächtlichen Grenzschicht von dem Modell überschätzt. Am Tag sind keine signifikanten Unterschiede in der bodennahen Temperatur und Feuchte zu beobachten. In einer Sensitivitätsanalyse wird der Einfluss verschiedener Parameter des LSMs auf den Simulationsverlauf getestet, da die Wahl dieser durch den Nutzer eine erhebliche Unsicherheit des Modellsetups darstellt. Insbesondere eine ungenaue Abschätzung des Blattflächenindexes, der Albedo oder des initialen Feuchtegehalts der Grenzschicht führen zu einem signifikanten Fehler in der Simulation der turbulenten fühlbaren und latenten Wärmeströme am Tag. Die nächtliche Grenzschicht wird hingegen hauptsächlich durch die Rauigkeitslänge und das gewählte Strahlungsmodell beeinflusst.

Stürme können extreme Kräfte entwickeln, die ganze Bäume beschädigen können. Bei solchen Starkwind-Wetterlagen können Baumschäden auftreten, die von einzelnen abgebrochenen Ästen bis hin zum Abknicken ganzer Baumstämme oder der Entwurzelung des ganzen Baumes reichen. Besonders im urbanen Umfeld kann durch diesen sogenannten Windwurf, neben dem eigentlichen Holzschaden, auch erheblicher Sachschaden oder sogar Lebensgefahr für die Stadtbevölkerung entstehen. Daraus resultiert ein besonderes Interesse von Stadtverwaltungen und Stadtplanern, möglichst realistisch vorhersagen zu können, welche Stadtbäume der größten Gefahr durch Windwurf ausgesetzt sind. Computersimulationen sind ein gutes Hilfsmittel um Windwurf vorherzusagen und entsprechende präventive Maßnahmen zu planen. Neben möglichst genauen Informationen über die Beschaffenheit der Bäume selbst, ist eine möglichst gute Vorhersage der Windgeschwindigkeitsspitzen unbedingte Voraussetzung um Sturmschäden an Stadtbäumen vorhersagen zu können. Während Starkwind-Wetterlagen sind Windböen als lokales turbulentes Phänomen die Ereignisse mit den höchsten Windgeschwindigkeiten. Um die lokale Turbulenz in der Größenordnung einzelner Bäume und typischer urbaner Strukturen aufzulösen, wird heutzutage die hochauflösende Large-Eddy-Simulation (LES) als ein angemessenerer und physikalischer Ansatz betrachtet. Das Stadtklimamodell PALM ist ein hochauflösendes LES-Modell, das in der Lage ist, komplexe urbane Umgebungen zu simulieren. Wir haben PALM um ein Windwurf-Modell erweitert und anschließend untersucht, wie sich Windböen, die durch die lokale urbane Umgebung verursacht werden, auf die Windwurf-Gefährdung einzelner Stadtbäume auswirken. Dabei haben wir besonderen Fokus darauf gelegt, wie sich die Windwurf-Gefährdung einzelner Bäume in Baumgruppen während anhaltender Starkwind-Wetterlagen verändert, nachdem Nachbarbäume bereits gefallen sind.

Das Large-Eddy Simulationsmodell PALM-4U wurde in den letzten Jahren als Stadtklimamodell ausgebaut, mit dem Ziel die für stadtklimatologische Fragestellungen relevanten physikalischen Prozesse auf der Mesoskala sowie Mikroskala zu berücksichtigen. Um die Leistungsfähigkeit von PALM-4U zu demonstrieren, präsentieren wir in diesem Beitrag zwei simulierte Szenarien, ein Winter- und ein Sommerszenario, welche die gesamte Bandbreite der implementierten Stadtklimakomponenten ausnutzen. In beiden Szenarien wird die Stadt Berlin simuliert, deren gesamte Stadtfläche vom Simulationsgebiet abgedeckt wird. Der simulierte Bereich erstreckt sich über ein Gebiet von etwa 50 km x 50 km und umfasst somit das vollständige Stadtgebiet von Berlin und die nähere Umgebung. Die räumliche Auflösung variiert im Simulationsgebiet zwischen 16 m und 2 m. Durch die gewählte Gebietsgröße, Gitterweite und die Anzahl der explizit dargestellten Wechselwirkungen stellen die gezeigten Simulationen nach unserer Kenntnis die bisher weltweit größten und umfangreichsten Stadtklimasimulationen auf LES Basis dar. Momentan dienen die Ergebnisse der Simulationen als Grundlage zur Evaluierung des Modells. Im Beitrag zeigen wir wie die Eingangsdaten in die Simulationen einfließen und sich auf die Modellergebnisse auswirken. Dabei wird das Augenmerk sowohl auf die allgemeine Bodenbeschaffenheit, die explizit aufgelöste Vegetation und die Gebäude gelegt, als auch auf die seitlichen Randbedingungen welche aus dem mesoskaligen Modell COSMO stammen um Änderungen auf der Mesoskala abzubilden. Tagesgänge der meteorologischen Größen wie Lufttemperatur, Feuchte und Wind können sowohl im Stadtzentrum als auch in der ländlichen Umgebung im Außenbereich der Stadt wiedergegeben werden und stimmen mit Messungen überein. Durch die hohe räumliche Auflösung und die explizite Berücksichtigung von Hindernissen können flächendeckend für ganz Berlin aktuelle Fragestellungen der Stadtplanung beantwortet werden. Der thermische Komfort kann beispielsweise mittels der direkt von PALM-4U berechneten Größen UTCI oder PET bestimmt werden. "Hot-Spot" Analysen können durch die gesamt-städtische Betrachtung lückenlos durchgeführt werden. Insbesondere kann bei der Betrachtung eines vollständigen Stadtgebiets der urbane Wärmeinseleffekt abgebildet und dank der feinen räumlichen Auflösung im Detail bewertet werden. Die Möglichkeit auch chemische Stoffe und Feinstaub mit PALM-4U zu betrachten, bietet darüber hinaus auch das Potential PALM-4U für den Bereich der Luftqualität anzuwenden. Für die Evaluierung wurden im Vorfeld intensive Messkampagnen durchgeführt, welche mit Hilfe des in PALM-4U vorhandenen virtuellen Messmoduls in den Simulationen nachgebildet wurden. Erste Eindrücke dieser Messungen werden wir im Vortrag präsentieren. Abschließend geben wir einen Ausblick auf die aktuellen Fortschritte der Evaluierungen und gehen auf noch geplante und aktuell durchgeführte Vorhaben für die Evaluierung ein.

An intercomparison between 10 single-column (SCM) and 5 large-eddy simulation (LES) models is presented for a radiation fog case study inspired by the LANFEX field campaign. 7 of the SCMs represent single-column equivalents of operational numerical weather prediction (NWP) models, whilst 3 are research-grade SCMs designed for fog simulation, and the LES are designed to reproduce in the best manner currently possible the underlying physical processes governing fog formation. The LES model results are of variable quality, and do not provide a consistent baseline against which to compare the NWP models, particularly under high aerosol or cloud droplet number (CDNC) conditions. The main SCM bias appears to be toward over-development of fog, i.e. fog which is too thick, although the inter-model variability is large. In reality there is a subtle balance between water lost to the surface and water condensed into fog, and the ability of a model to accurately simulate this process strongly determines the quality of its forecast. Some NWP-SCMs do not represent fundamental components of this process (e.g. cloud droplet sedimentation) and therefore are naturally hampered in their ability to deliver accurate simulations. Finally, we show that modelled fog development is as sensitive to the shape of the cloud droplet size distribution, a rarely studied or modified part of the microphysical parametrization, as it is to the underlying aerosol or CDNC.

Sensitivity of the PALM model 6.0 with respect to land-surface and building properties is tested in a real urban environment in the vicinity of a typical crossroads in a densely built-up residential area in Prague, Czech Republic. The turbulence-resolving PALM is able to simulate the urban boundary layer flow for realistic setups. Besides an accurate representation of the relevant physical processes, the model performance also depends on the input data describing the urban setup, namely the building and land-surface properties. Two types of scenario are employed. The first one is the synthetic scenarios altering mainly surface and material parameters such as albedo, emissivity or wall conductivity, testing sensitivity of the model simulations to potentially erroneous input data. Second, urbanistic-type scenarios are analysed, in which commonly considered urban heat island mitigation measures such as greening of the streets or changing surface materials are applied in order to assess the limits of the effects of a particular type of scenario. For the synthetic scenarios, surface parameters used in radiation balance equations are found to be the most sensitive overall followed by the volumetric heat capacity and thermal conductivity of walls. Other parameters show a limited average effect; however, some can still be significant during some parts of the day, such as surface roughness in the morning hours. The second type, the urbanistic scenarios, shows urban vegetation to be the most effective measure, especially when considering both physical and biophysical temperature indicators. The influence of both types of scenario was also tested for air quality, specifically PM2.5 dispersion, which generally shows opposite behaviour to that of thermal indicators; i.e. improved thermal comfort brings deterioration of PM2.5 concentrations.

Vehicle-induced effects (VIE) and exhaust fumes interacting with turbulent flow has become known to be a critical factor when investigating the wind flow and the transport of pollutants in urban street canyons. Up to now, mainly the Reynolds-Averaged-Navier-Stokes (RANS) technique has been applied for CFD studies of the processes within urban street canyons; research studies using turbulence-resolving Large-Eddy Simulations (LES), however, were rather rare. As LES models explicitly resolve the dominant turbulent motions, whose knowledge is needed to fully understand the processes, the incorporation of moving objects into a turbulence-resolving model is essential for the accurate simulation of pollutant dispersion in urban environments. In this paper we outline our effort to account for VIE in the LES model PALM. For this purpose, an innovative and easy to implement method was realised to represent a common car shape within the environmental LES setup: the so-called air-block method. Its concept is based on an object (representing the vehicle) in which a fixed velocity is prescribed to the objects grid volumes that equals the driving speed of the vehicle. Control of its movement, however, is achieved via a Lagrangian particle located at its center of gravity. This approach is significantly different from conventional consideration of solid objects as obstacles, since the air-block representation assumes that frictional drag is much smaller (and can thus be neglected) than form drag. By the same token the implementation is much easier to achieve in a complex LES model such as PALM.In this talk we will outline the newly-developed simplistic method to represent driving vehicles in an LES model and show its performance based on a validation study for the turbulent wake flow and dispersion of exhaust fumes. For this purpose we employ existing wind tunnel data and comparative PALM simulations using the conventional solid-obstacle approach (Carpentieri et al. 2012, Atmospheric Environment, 62:9-25, DOI:10.1016/j.atmosenv.2012.08.019; Kastner-Klein et al. 2001, J. Wind Eng Ind Aerodyn, 89:849-861, DOI:10.1016/S0167-6105(01)00074-5).

Severe wind gusts associated with strong-wind events like winter storms are regarded as the biggest thread to aviation safety during takeoff and landing. In order to get forecasts for gust hazards during strong-wind conditions, airport operators currently still rely on empirical wind gust estimation (WGE) methods that are driven solely by mesoscale and regional numerical weather prediction (NWP) model data. These models currently have grid resolutions on the order of 1 kilometer and do not adequately resolve local topography and vegetation. This is especially problematic, as these local obstacles are a primary source of near surface turbulence. This turbulence is responsible for wind gusts, but it can also no be resolved by mesoscale and regional NWP models and has to be parameterized entirely. Consequently the empirical WGE methods completely neglect the real local obstacles around a specific airport, which leads to significant uncertainties in the wind gust hazard forecasts for airspace that is downwind from those obstacles. In case this airspace is used for takeoff and landing, airport operators have no access to reliable wind gust hazard forecasts. A more adequate and physical approach to resolving local obstacles and the turbulence they induce, is high-resolution large-eddy simulation (LES). We use the high-res LES model PALM to investigate, how wind gusts induced by local obstacles impact airspace used for takeoff and landing at the airport in Frankfurt, Germany. Moreover, we evaluate how accurate these LES results are compared to high-res wind measurements from multiple sites throughout the airport and we quantify the forecast quality of the LES approach vs. the traditional empirical WGE approach. Finally, we outline a path to apply PALM as a magnifying glass nested into mesoscale models with the goal of providing a new and improved forecasting system for gust hazards at airports. This new forecasting system can be used to improve existing empirical WGE approaches and it can be applied in an operational setting to produce new gust forecast products.

In this paper we study the influence of the cloud microphysical parameterization, namely the effect of different methods for calculating the supersaturation and aerosol activation, on the structure and life cycle of radiation fog in large-eddy simulations. For this purpose we investigate a well-documented deep fog case as observed at Cabauw (the Netherlands) using high-resolution large-eddy simulations with a comprehensive bulk cloud microphysics scheme. By comparing saturation adjustment with a diagnostic and a prognostic method for calculating supersaturation (while neglecting the activation process), we find that, even though assumptions for saturation adjustment are violated, the expected overestimation of the liquid water mixing ratio is negligible. By additionally considering activation, however, our results indicate that saturation adjustment, due to approximating the underlying supersaturation, leads to a higher droplet concentration and hence significantly higher liquid water content in the fog layer, while diagnostic and prognostic methods yield comparable results. Furthermore, the effect of different droplet number concentrations is investigated, induced by using different common activation schemes. We find, in line with previous studies, a positive feedback between the droplet number concentration (as a consequence of the applied activation schemes) and strength of the fog layer (defined by its vertical extent and amount of liquid water). Furthermore, we perform an explicit analysis of the budgets of condensation, evaporation, sedimentation and advection in order to assess the height-dependent contribution of the individual processes on the development phases.

In this article we describe the implementation of an online-coupled gas-phase chemistry model in the turbulence-resolving PALM model system 6.0 (formerly an abbreviation for Parallelized Large-eddy Simulation Model and now an independent name). The new chemistry model is implemented in the PALM model as part of the PALM-4U (PALM for urban applications) components, which are designed for application of the PALM model in the urban environment (Maronga et al., 2020). The latest version of the Kinetic PreProcessor (KPP, 2.2.3) has been utilized for the numerical integration of gas-phase chemical reactions. A number of tropospheric gas-phase chemistry mechanisms of different complexity have been implemented ranging from the photostationary state (PHSTAT) to mechanisms with a strongly simplified volatile organic compound (VOC) chemistry (e.g. the SMOG mechanism from KPP) and the Carbon Bond Mechanism 4 (CBM4; Gery et al., 1989), which includes a more comprehensive, but still simplified VOC chemistry. Further mechanisms can also be easily added by the user. In this work, we provide a detailed description of the chemistry model, its structure and input requirements along with its various features and limitations. A case study is presented to demonstrate the application of the new chemistry model in the urban environment. The computation domain of the case study comprises part of Berlin, Germany. Emissions are considered using street-type-dependent emission factors from traffic sources. Three chemical mechanisms of varying complexity and one no-reaction (passive) case have been applied, and results are compared with observations from two permanent air quality stations in Berlin that fall within the computation domain. Even though the feedback of the model's aerosol concentrations on meteorology is not yet considered in the current version of the model, the results show the importance of online photochemistry and dispersion of air pollutants in the urban boundary layer for high spatial and temporal resolutions. The simulated NOx and O3 species show reasonable agreement with observations. The agreement is better during midday and poorest during the evening transition hours and at night. The CBM4 and SMOG mechanisms show better agreement with observations than the steady-state PHSTAT mechanism. © 2021 Copernicus GmbH. All rights reserved.

We revisit the longstanding problem of grid sensitivity, i.e., the lack of grid convergence in large-eddy simulations (LES) of the stable boundary layer. We use a comprehensive set of LES of the well-known Global Energy and Water Cycle Experiment Atmospheric Boundary Layer Study 1 (GABLS1) case with varying grid spacings between 12.5 m and 1 m to investigate several physical processes and numerical features that are possible causes of grid sensitivity. Our results demonstrate that there are two resolution regimes in which grid sensitivity manifests differently. We find that changing the numerical advection schemes and the subgrid-scale models alters the simulation results, but the options tested do not fully address the grid-sensitivity issue. Moreover, sensitivity runs suggest that the surface boundary condition and the interaction of the surface with the near-surface flow, as well as the mixing with the free atmosphere, are unlikely to be the causes of the observed grid sensitivity. One interesting finding is that the grid sensitivity in the fine grid-spacing regime (grid spacings $$\le 2\,\mathrm{m}$$ ≤ 2 m ) is closely related to the reduction in the energy content of large-scale turbulence, leading to less turbulence kinetic energy and hence lower boundary-layer heights. The present work demonstrates that there is still an urgent need to address this grid-sensitivity issue in order to perform reliable LES of the stable boundary layer.

Large-eddy simulation (LES) provides a physically sound approach to study complex turbulent processes within the atmospheric boundary layer including urban boundary layer flows. However, such flow problems often involve a large separation of turbulent scales, requiring a large computational domain and very high grid resolution near the surface features, leading to prohibitive computational costs. To overcome this problem, an online LES–LES nesting scheme is implemented into the PALM model system 6.0. The hereby documented and evaluated nesting method is capable of supporting multiple child domains, which can be nested within their parent domain either in a parallel or recursively cascading configuration. The nesting system is evaluated by first simulating a purely convective boundary layer flow system and then three different neutrally stratified flow scenarios with increasing order of topographic complexity. The results of the nested runs are compared with corresponding non-nested high- and low-resolution results. The results reveal that the solution accuracy within the high-resolution nest domain is clearly improved as the solutions approach the non-nested high-resolution reference results. In obstacle-resolving LES, the two-way coupling becomes problematic as anterpolation introduces a regional discrepancy within the obstacle canopy of the parent domain. This is remedied by introducing canopy-restricted anterpolation where the operation is only performed above the obstacle canopy. The test simulations make evident that this approach is the most suitable coupling strategy for obstacle-resolving LES. The performed simulations testify that nesting can reduce the CPU time up to 80 % compared to the fine-resolution reference runs, while the computational overhead from the nesting operations remained below 16 % for the two-way coupling approach and significantly less for the one-way alternative.

There is a strong interaction between the urban atmospheric canopy layer and the building energy balance. The urban atmospheric conditions affect the heat transfer through exterior walls, the long-wave heat transfer between the building surfaces and the surroundings, the short-wave solar heat gains, and the heat transport by ventilation. Considering also the internal heat gains and the heat capacity of the building structure, the energy demand for heating and cooling and the indoor thermal environment can be calculated based on the urban microclimatic conditions. According to the building energy concept, the energy demand results in an (anthropogenic) waste heat; this is directly transferred to the urban environment. Furthermore, the indoor temperature is re-coupled via the building envelope to the urban environment and affects indirectly the urban microclimate with a temporally lagged and damped temperature fluctuation. We developed a holistic building model for the combined calculation of indoor climate and energy demand based on an analytic solution of Fourier's equation and implemented this model into the PALM model.

In this paper the land surface model embedded in the PALM model system is described and evaluated against in situ measurements at Cabauw, Netherlands. A total of 2 consecutive clear-sky days are simulated, and the components of surface energy balance, potential temperature, humidity, and horizontal wind speed are compared to observations. For the simulated period, components of the energy balance are consistent with daytime and nighttime observations, and the daytime Bowen ratio also agrees fairly well with observations. The model simulates a more stably stratified nocturnal boundary layer than the observations, and near-surface potential temperature and humidity agree fairly well during the day. Moreover, a sensitivity analysis is performed to investigate dependence of the model on land surface and soil specifications, as well as atmospheric initial conditions, because they represent a major source of uncertainty in the simulation setup. It is found that an inaccurate estimation of leaf area index, albedo, or initial humidity causes a significant misrepresentation of the daytime turbulent sensible and latent heat fluxes. During the night, the boundary-layer characteristics are primarily affected by surface roughness and the applied radiation schemes.

The PALM model system 6.0 is designed to simulate micro- and mesoscale flow dynamics in realistic urban environments. The simulation results can be very valuable for various urban applications, for example to develop and improve mitigation strategies related to heat stress or air pollution. For the accurate modelling of urban environments, realistic boundary conditions need to be considered for the atmosphere, the local environment, and the soil. The local environment with its geospatial components is described in the static driver of the model and follows a standardized, hereafter called PALM input data standard. The main input parameters describe surface type, buildings and vegetation. Depending on the desired simulation scenario and the available data, the local environment can be described at different levels of detail. To compile a complete static driver describing a whole city, various data sources are used, including remote sensing, municipal data collections and open data such as OpenStreetMap. This manuscript shows how input data sets for three German cities can be derived. Based on these data sets, the static driver for PALM can be generated. As the collection and preparation of input data sets is tedious, prospective research aims at the development of a semi-automated processing chain to support users in formatting their geospatial data.

There is a strong interaction between the urban and the building energy balance. The urban climate affects the heat transfer through exterior walls, the longwave heat transfer between the building surfaces and the surroundings, the shortwave solar heat gains and the heat transport by ventilation. Considering also the internal heat gains and the heat capacity of the building structure, the energy demand for heating and cooling and the indoor thermal environment can be calculated based on the urban climate. According to the building energy concept, the energy demand results in an (anthropogenic) waste heat, this is directly transferred to the urban environment. Furthermore, the indoor temperature is re-coupled via the building envelope to the urban environment and affects indirectly the urban climate with a time shifted and damped temperature fluctuation. We developed and implemented a holistic building model for the combined calculation of indoor climate and energy demand based on an analytic solution of Fourier’s equation. The building model is integrated via an urban surface model into the urban climate model.

We present the capability of PALM 6.0, the latest version of the PALM model system, to simulate neutrally stratified urban boundary layers. The studied situation includes a real-case building setup of the HafenCity area in Hamburg, Germany. Simulation results are validated against wind-tunnel measurements of the same building layout utilizing PALM's virtual measurement module. The comparison reveals an overall very high agreement between simulation results and wind-tunnel measurements not only for mean wind speed and direction but also for turbulence statistics. However, differences between measurements and simulation arise within close vicinity of surfaces where the resolution prevents good representation of the inertial sub-range of turbulence. In the end, we give guidance how these differences can be reduced using already implemented features of PALM.

Including radiative transfer processes within the urban canopy layer into microscale urban climate models (UCMs) is essential to obtain realistic model results. These processes include the interaction of buildings and vegetation with shortwave and longwave radiation, thermal emission, and radiation reflections. They contribute differently to the radiation budget of urban surfaces. Each process requires different computational resources and physical data for the urban elements. This study investigates how much detail modellers should include to parameterise radiative transfer in microscale building resolving UCMs. To that end, we introduce a stepwise parameterization method to the the PALM model system 6.0 to quantify individually the effects of the main radiative transfer processes on the radiation budget and on the flow field. We quantify numerical simulations of both simple and realistic urban configurations to identify the radiative transfer processes which have major effects on the radiation budget, such as surface and vegetation interaction with short wave and long wave radiation, and those which have minor effects, such as multiple reflections. The study also shows that radiative transfer processes within the canopy layer implicitly affect the incoming radiation since the radiative transfer model is coupled to the radiation model. The flow field changes considerably in response to the radiative transfer processes included in the model. The study highlights those processes which are essentially needed to assure acceptable quality of the flow field. Omitting any of these processes may lead to high uncertainties in the model results.

In a world with increasing extreme weather events, such as dry or extreme rain periods, due to climate change and an ever growing population specifically in urban areas, a forsighted planning and adaption of cities and their urban surroundings is becoming more and more important. Here, particularly health and comfort of the urban population, such as thermal comfort, air quality, ventilation or UV exposure, but also other aspects like safety and environmental sustainability play an important role. In order to create the cities of tomorrow that meet the real requirements to host healthy and firendly living conditions, city planners are relying on scientific models where they can simulate how changes in the urban environment can effect its climate. The PALM-4U (Parallelised Large-Eddy Simulation Model for Urban Applications) model was specifically developed to be able to simulate a large variety of parameters on short timescales and at the high resolution that is required to resolve single buildings or obstacles like trees within the city.In September 2019, the second phase of the German research project MOSAIK (model-based city planning and application in climate change), a module within the large over-arching project [UC]² (Urban Climate Under Change) that focusses on the further development of the model, has started.In this overview, we will present the PALM-4U‘s current capabilities and outline the planned future development in the coming years like windbreak modelling, coupling with traffic flow models, including biogenic volatile organic compounds in urban air quality modelling. Furthermore, our PALM-4U community model strategy will be explained.

Large-Eddy Simulation (LES) allow to simulate pollutant dispersion at a fine-scale turbulence-resolving scale with explicitly resolved turbulent transport around building structures and in street canyons. The microscale urban climate model with atmospheric chemistry PALM-4U (i.e. PALM for Urban applications; Maronga et al., 2019, Met. Z., https://doi.org/10.1127/metz/2019/0909) has been developed within the collaborative project MOSAIK (Model-based city planning and application in climate change). With such a large-eddy simulation (LES) model, pollutant dispersion around buildings and within street canyons can be simulated, with explicitly resolving the turbulent transport in urban environments.Cyclic boundaries are frequently applied in LES in order to obtain lateral boundary conditions for the turbulent quantities. In addition to the default cyclic boundary conditions, PALM-4U allows also time-dependent boundary conditions from regional models to account for variable weather conditions and regional scale pollutant transport. Turbulent fluctuations, which are not included in the boundary conditions from the regional simulation but are needed as additional boundary conditions for the LES model are produced by a turbulence generator (Maronga et al, 2019, GMDD, https://doi.org/10.5194/gmd-2019-103).PALM-4U simulations with and without time dependent boundary conditions from regional simulations with WRF-Chem are performed for different setups in order to test the impact of the domain configuration. The simulations indicate that cyclic boundary conditions can lead to unrealistic accumulation of pollutants over urban areas with strong sources, which is not the case when time-dependent boundary conditions are applied. However, even though a turbulence generator is applied, explicit setting of time-dependent boundary conditions requires large model domains, in order to obtain fully developed turbulence within the domain of interest, increasing the computational demand of the simulation.

International audience; Cities are particularly vulnerable to meteorological hazards because of the concentration of population, goods, capital stock and infrastructure. Urban climate services require multi-disciplinary and multi-sectorial approaches and new paradigms in urban climate modelling. This paper classifies the required urban input data for both mesoscale state-of-the-art Urban Canopy Models (UCMs) and microscale Obstacle Resolving Models (ORM) into five categories and reviews the ways in which they can be obtained. The first two categories are (1) land cover, and (2) building morphology. These govern the main interactions between the city and the urban climate and the Urban Heat Island. Interdependence between morphological parameters and UCM geometric hypotheses are discussed. Building height, plan and wall area densities are recommended as the main input variables for UCMs, whereas ORMs require 3D building data. Recently, three other categories of urban data became relevant for finer urban studies and adaptation to climate change: (3) building design and architecture, (4) building use, anthropogenic heat and socio-economic data, and (5) urban vegetation data. Several methods for acquiring spatial information are reviewed, including remote sensing, geographic information system (GIS) processing from administrative cadasters, expert knowledge and crowdsourcing. Data availability, data harmonization, costs/efficiency trade-offs and future challenges are then discussed.

In polar regions, where the boundary layer is often stably stratified, atmospheric models produce large biases depending on the boundary-layer parametrizations and the parametrization of the exchange of energy at the surface. This model intercomparison focuses on the very stable stratification encountered over the Antarctic Plateau in 2009. Here, we analyze results from 10 large-eddy-simulation (LES) codes for different spatial resolutions over 24 consecutive hours, and compare them with observations acquired at the Concordia Research Station during summer. This is a challenging exercise for such simulations since they need to reproduce both the 300-m-deep convective boundary layer and the very thin stable boundary layer characterized by a strong vertical temperature gradient (10 K difference over the lowest 20 m) when the sun is low over the horizon. A large variability in surface fluxes among the different models is highlighted. The LES models correctly reproduce the convective boundary layer in terms of mean profiles and turbulent characteristics but display more spread during stable conditions, which is largely reduced by increasing the horizontal and vertical resolutions in additional simulations focusing only on the stable period. This highlights the fact that very fine resolution is needed to represent such conditions. Complementary sensitivity studies are conducted regarding the roughness length, the subgrid-scale turbulence closure as well as the resolution and domain size. While we find little dependence on the surface-flux parametrization, the results indicate a pronounced sensitivity to both the roughness length and the turbulence closure.

High-resolution large-eddy simulations (LESs) are used to investigate the effect of turbulence as well as the interaction between atmosphere and soil on the life cycle of nocturnal radiation fog. The first part of the article focuses on the validation of the LES model system for simulating radiation fog against measurement data from the meteorological super-site in Cabauw (Netherlands). Differences found in the fog life cycle, depth, and liquid water content between LES and observations can, as in former studies, be largely ascribed to the presence of local advection processes in the observational data and uncertainty in the measurement data used to initialise the model. Moreover it is found that the choice of the droplet number concentration within the cloud microphysical representation has a high impact on the liquid water content within the fog layer, but a rather small effect on its life cycle. In the second part of the article, a set of LES runs is analysed with idealised initial conditions in order to study the effect of turbulent mixing as well as the initial state of the soil on the development of radiation fog. As expected, the results show that turbulent mixing has a strong impact on the time of fog formation, which is complicated by the interaction with both radiative cooling and water vapour removal by dew deposition. Furthermore, it is found that the near-surface soil temperature plays a key role for the exact time of fog formation, whereas near-surface soil moisture is decisive for the lifting and dissipation time of the fog layer.

Surface-layer-resolving large-eddy simulations (LESs) of free-convective to near-neutral boundary layers are used to study Monin–Obukhov similarity theory (MOST) functions. The LES dataset, previously used for the analysis of MOST relationships for structure parameters, is extended for the mean vertical gradients and standard deviations of potential temperature, specific humidity, and wind. Also, local-free-convection (LFC) similarity is studied. The LES data suggest that the MOST functions for mean gradients are universal and unique. The data for the mean gradient of the horizontal wind display significant scatter, while the gradients of temperature and humidity vary considerably less. The LES results suggest that this scatter is mostly related to a transition from MOST to LFC scaling when approaching free-convective conditions and that it is associated with a change of the slope of the similarity functions toward the expected value from LFC scaling. Overall, the data show slightly, but consistent, steeper slopes of the similarity functions than suggested in literature. The MOST functions for standard deviations appear to be unique and universal when the entrainment from the free atmosphere into the boundary layer is sufficiently small. If entrainment becomes significant, however, we find that the standard deviation of humidity no longer follows MOST. Under free-convective conditions, the similarity functions should reduce to universal constants (LFC scaling). This is supported by the LES data, showing only little scatter, but displaying a systematic height dependence of these constants. Like for MOST, the LFC similarity constant for the standard deviation of specific humidity becomes nonuniversal when the entrainment of dry air reaches significant levels.

Large-Eddy Simulation (LES) models are so far barely applied to dispersion and chemical transformation of pollutants in urban air quality studies. Within the joint project MOSAIK (Modellbasierte Stadtplanung und Anwendung im Klimawandel/Model-based city planning and application in climate change) a new LES based state-of-the-art microscale urban climate model PALM-4U, has been developed. The new model includes both gas phase and aerosol chemistry. For practical applications, our approach is to go beyond the simulation of single street canyons to chemical transformation, advection and deposition of air pollutants in the larger urban canopy. First LES results of a test case for an urban quarter of Berlin (Germany) are presented.

Urban climate models become increasingly important for academic research and sustainable urban planning. The turbulence-resolving micro climate model PALM-4U is currently being developed to better meet the high demands of urban planning related to the spatial detail of modeling scenarios. The study presented here, shows which data sources and approaches can be used to supply models like PALM-4U with the required highly detailed urban surface layers. It is thereby jointly made use of remote sensing, municipal and open data to generate surface layers at 1m spatial resolution. The surface layers describe buildings, vegetation, terrain, water, pavement, streets and bridges. The heterogeneous data sources and their differing quality and standards require the utilization of various remote sensing and GIS techniques. In this study it is demonstrated that the surface layers can successfully be generated for urban climate simulations. To ensure easy transferability, it should be considered to use thematically and/or coarser but more standardized data, after a sensitivity analysis of PALM-4U.

We investigate the scaling for decaying turbulence kinetic energy (TKE) in the free-convective boundary layer, from the time the surface heat flux starts decaying, until a few hours after it has vanished. We conduct a set of large-eddy simulation experiments, consider various initial convective situations, and prescribe realistic decays of the surface heat flux over a wide range of time scales. We find that the TKE time evolution is dictated by the decaying magnitude of the surface heat flux up to $$0.7 \tau $$ approximately, where $$\tau $$ is the prescribed duration from maximum to zero surface heat flux. During the time period starting at zero surface heat flux, we search for potential power-law scaling by examining the log–log presentation of TKE as a function of time. First, we find that the description of the decay highly depends on whether the time origin is defined as the time when the surface heat flux starts decaying (traditional scaling framework), or the time when it vanishes (proposed new scaling framework). Second, when varying $$\tau $$ , the results plotted in the traditional scaling framework indicate variations in the power-law decay rates over several orders of magnitude. In the new scaling framework, however, we find a unique decay exponent in the order of 1, independent of the initial convective condition, and independent of $$\tau $$ , giving support for the proposed scaling framework.

Large cities and urban regions are confronted with rising pressure by environmental pollution, impacts of climate change, as well as natural and health hazards. They are characterised by heterogeneous mosaics of urban structures, causing modifications of atmospheric processes on different temporal and spatial scales. Planning authorities need reliable, locally relevant information on urban atmospheric processes, providing fine spatial resolutions in city quarters or street canyons, as well as projections of future climates, specifically downscaled to individual cities. Therefore, building-resolving urban climate models for entire city regions are required as a tool for urban development and planning, air quality control, as well as for design of actions for climate change mitigation and adaptation. To date, building-resolving atmospheric models covering entire large cities are mostly missing. The German research programme ""Urban Climate Under Change"" ([UC]2) aims at developing a new urban climate model, acquiring three-dimensional observational data for model testing and validation, and testing its practicability and usability in collaboration with relevant stakeholders to provide a scientifically sound and practicable instrument to address the above mentioned challenges. This article provides an outline of the collaborative activities of the [UC]2 research programme.

In this article we outline the model development planned within the joint project Model-based city planning and application in climate change (MOSAIK). The MOSAIK project is funded by the German Federal Ministry of Education and Research (BMBF) within the framework Urban Climate Under Change ([UC]2) since 2016. The aim of MOSAIK is to develop a highly-efficient, modern, and high-resolution urban climate model that allows to be applied for building-resolving simulations of large cities such as Berlin (Germany). The new urban climate model will be based on the well-established large-eddy simulation code PALM, which already has numerous features related to this goal, such as an option for prescribing Cartesian obstacles. In this article we will outline those components that will be added or modified in the framework of MOSAIK. Moreover, we will discuss the everlasting issue of acquisition of suitable geographical information as input data and the underlying requirements from the model's perspective. publishedVersion

Monin–Obukhov similarity theory is used in large-eddy simulation (LES) models as a surface boundary condition to predict the surface shear stress and scalar fluxes based on the gradients between the surface and the first grid level above the surface. We outline deficiencies of this methodology, such as the systematical underestimation of the surface shear stress, and propose a modified boundary condition to correct for this issue. The proposed boundary condition is applied to a set of LES for both neutral and stable boundary layers with successively decreasing grid spacing. The results indicate that the proposed boundary condition reliably corrects the surface shear stress and the sensible heat flux, and improves grid convergence of these quantities. The LES data indicate improved grid convergence for the surface shear stress, more so than for the surface heat flux. This is either due to a limited performance of the Monin–Obukhov similarity functions or due to problems in the LES model in representing stable conditions. Furthermore, we find that the correction achieved using the proposed boundary condition does not lead to improved grid convergence of the wind-speed and temperature profiles. From this we conclude that the sensitivity of the wind-speed and temperature profiles in the LES model to the grid spacing is more likely related to under-resolved near-surface gradients and turbulent mixing at the boundary-layer top, to the SGS model formulation, and/or to numerical issues, and not to deficiencies due to the use of improper surface boundary conditions.

In many engineering and meteorological applications, atmospheric turbulence within the planetary boundary layer is described in terms of its representative parameters. One such parameter is the structure-function (or structure) parameter that is used to characterize the intensity of turbulent fluctuations of atmospheric flow variables. Structure parameters are derivatives of structure functions, but are used more frequently than the latter ones for practical needs as they do not explicitly include dependence on the separation distance. The structure parameter of potential temperature, which is the subject of this study, describes the spatial variability of the temperature fluctuations. It is broadly represented in theories and models of electromagnetic and acoustic wave propagation in the atmosphere, and forms the basis for the scintillometer measurement concept. The authors consider three methods to compute the potential temperature structure function and structure parameter: the direct method, the true spectral method, and the conventional spectral method. Each method is tested on high-resolution potential temperature datasets generated from large-eddy simulations of a variety of convective boundary layer flow cases reproduced by two representative numerical codes. Results indicate that the popular conventional spectral method routinely exaggerates the potential temperature structure-function parameter, likely due to the unrealistic assumptions underlying the method. The direct method and true spectral method are recommended as the more suitable approaches.

Urban pedestrian-level air quality is a result of an interplay between turbulent dispersion conditions, background concentrations and heterogeneous local emissions of air pollutant and their transformation processes. Still, the complexity of these interactions cannot be resolved by the commonly used air quality models. By embedding the sectional aerosol module SALSA to the large-eddy simulation model PALM, a novel, high-resolution, urban aerosol modelling framework has been developed. The first model evaluation study on the vertical variation of aerosol number concentration and size distribution in a simple street canyon without vegetation in Cambridge, UK, shows excellent agreement with measurements. Dispersion conditions and local emissions govern the pedestrian-level aerosol number concentrations. Out of different aerosol processes, dry deposition is shown to decrease the total number concentration by over 20 %, while condensation and dissolutional increase the total mass by over 10 %. Following the model development, the application of PALM can be extended to local- and neighbourhood-scale air pollution and aerosol studies that require a detailed solution of the ambient flow field.

Urban areas are an important part of the climate system and many aspects of urban climate have direct effects on human health and living conditions. This implies that reliable tools for local urban climate studies supporting sustainable urban planning are needed. However, a realistic implementation of urban canopy processes still poses a serious challenge for weather and climate modelling for the current generation of numerical models. To address this demand, a new urban surface model (USM), describing the surface energy processes for urban environments, was developed and integrated as a module into the PALM large-eddy simulation model. The development of the presented first version of the USM originated from modelling the urban heat island during summer heat wave episodes and thus implements primarily processes important in such conditions. The USM contains a multi-reflection radiation model for shortwave and longwave radiation with an integrated model of absorption of radiation by resolved plant canopy (i.e. trees, shrubs). Furthermore, it consists of an energy balance solver for horizontal and vertical impervious surfaces, and thermal diffusion in ground, wall, and roof materials, and it includes a simple model for the consideration of anthropogenic heat sources. The USM was parallelized using the standard Message Passing Interface and performance testing demonstrates that the computational costs of the USM are reasonable on typical clusters for the tested configurations. The module was fully integrated into PALM and is available via its online repository under the GNU General Public License (GPL). The USM was tested on a summer heat-wave episode for a selected Prague crossroads. The general representation of the urban boundary layer and patterns of surface temperatures of various surface types (walls, pavement) are in good agreement with in situ observations made in Prague. Additional simulations were performed in order to assess the sensitivity of the results to uncertainties in the material parameters, the domain size, and the general effect of the USM itself. The first version of the USM is limited to the processes most relevant to the study of summer heat waves and serves as a basis for ongoing development which will address additional processes of the urban environment and lead to improvements to extend the utilization of the USM to other environments and conditions.