How to cite: Mei, S.-J.: GUST: A GPU-Accelerated 3D Urban Surface Temperature Model Using Monte Carlo Ray Tracing and Random Walk Simulation , 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-3, https://doi.org/10.5194/icuc12-3, 2025.

Evaluating building air ventilation performance is critical in Hong Kong’s densely populated, high-rise urban environments. Traditional methods, such as wind tunnel testing and Computational Fluid Dynamics (CFD) simulations, are often time-intensive and impractical during the early design stages. Geometric parametrization methods, including Frontal Area Density (FAD) and Permeability (P), offer simpler alternatives by quantifying porosity, but they fail to account for airflow dynamics shaped by building configurations.

This study proposes an innovative approach that integrates the Least Cost Path (LCP) method into building permeability parametrization for building ventilation assessment. By treating urban environments as porous media, the LCP method uses two cost-based metrics: Friction Cost (FC), representing airflow resistance, and Turning Cost (TC), reflecting path configuration complexities. These metrics are applied on a 3D grid system, aligning with the Hong Kong Sustainable Building Design Guidelines (APP152). The methodology employs CFD simulations to validate the LCP method across various random building configurations, ensuring robust comparisons with traditional CFD results.

Results demonstrate that the LCP-derived FC and TC metrics correlate strongly with CFD wind speed simulations (R² > 0.9). Moreover, scenarios meeting APP152 requirements consistently show higher ventilation efficiency, confirming the method’s alignment with current regulatory frameworks. The study also identifies threshold LCP costs that differentiate “good” from “poor” ventilation designs, providing actionable insights for architects and urban planners.

By enabling efficient, geometry-based evaluations of ventilation performance, the LCP method bridges the gap between early-stage design assessments and complex simulations. Its integration within Building Information Modeling (BIM) frameworks further enhances its applicability, offering a scalable, data-driven tool to improve building ventilation strategies in Hong Kong and similar high-density cities.

How to cite: He, Y., Gao, K., Leung, M. K., Poon, D., Zu, Y., Huang, T., Pang, C., Leung, K., Zhao, J., Wong, K. S., Yue, C. S., Cheung, Y., and Ng, E.: A new building ventilation assessment tool for high-density urban areas: integrating the least cost path method into building permeability parametrization, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-6, https://doi.org/10.5194/icuc12-6, 2025.

Vegetation is suggested to mitigate urban heat by shading and increasing evapotranspiration (ET). Understanding ET is crucial for effective urban water resources management and optimized cooling from vegetation. The most direct method to observe and study ET over larger areas is eddy-covariance (EC). EC systems measure the ET from a time-dependent source area called the footprint. The footprint depends on the meteorological conditions including wind conditions and atmospheric stability, and the sensor location and height. Although the time-dependent footprint can be modeled analytically, these analytical footprint descriptions neglect the heterogeneous urban flow field. The Large-Eddy Simulations (LES) technique explicitly resolves turbulent transport incorporating footprint dynamics in the modeled ET. Here we explore the temporal and spatial sensitivity of EC observations to the footprint at two sites in Berlin. For this purpose, we apply analytically derived footprints to analyze half-hourly footprints over a full year and LES to capture the spatial detail in footprints over one day. The LES results show realistic ET rates while differences with the observations seem to be driven by the mesoscale forcing. Our findings suggest that within a day the footprint's time dependency causes as much variation in observed ET as within a year, even when the day has relatively steady large-scale meteorological conditions. Virtual EC systems in the LES frequently show no correlation while being less than 280 m apart demonstrating how EC systems represent only their specific location. Our study shows the urgency of considering footprints when analyzing instantaneous ET observations from EC systems and the ineffectiveness of representing an EC system as measuring its average surroundings.

How to cite: Jongen, H., Teuling, R., Li, D., Akinlabi, E., Gadde, S., and Steeneveld, G.-J.: Modeling urban evapotranspiration with large-eddy simulations: The spatial and temporal influence of eddy-covariance footprints, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-38, https://doi.org/10.5194/icuc12-38, 2025.

Urban areas, home to about half of the world’s population, are increasingly adopting cool roofs to mitigate heat. These roofs reflect more sunlight and absorb less solar energy compared to conventional roofs, thereby reducing sensible heat fluxes from building rooftops. Many previous studies on the effects of cool roofs on near-surface air temperature were performed with traditional weather and climate models (e.g., the Weather Research and Forecasting or WRF model), where the near-surface air temperature is parameterized based on schemes designed for non-urban environments (such as Monin-Obukhov similarity theory) and does not represent the outdoor air temperature felt by urban residents. To quantify the effects of cool roofs on air temperature within the urban canyon, we perform building-resolving large-eddy simulations across different but idealized urban canyons that mimic different local climate zones. Our findings reveal that the cooling sensitivity, which characterizes the air temperature change (K) per unit amount of forcing (W/m²), increases with the canyon aspect ratio (AR) up to AR = 1, after which it decreases. We further compare the LES results to the simulated results by the single-layer urban canopy model in WRF. This study contributes to the understanding of how cool roofs impact within-canyon air temperatures in different urban settings.

How to cite: Akinlabi, E. O. and Li, D.: Effects of cool roofs on air temperature within the urban canyon: A Large-Eddy Simulation study, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-55, https://doi.org/10.5194/icuc12-55, 2025.

While street trees enhance microclimatic conditions and mitigates climate change, their impact on air quality remains debated. Computational fluid dynamics (CFD) simulations are powerful tools for analyzing the interactions between airflow, street canyons, and vegetation. However, their validation through experimental data remains an ongoing challenge.

This study compares high-resolution wind tunnel experiments and Large-Eddy Simulations (LES) to examine the flow field and pollutant dispersion within a street canyon oriented perpendicular to the prevailing wind. The configuration includes two rows of trees placed along the street sides, vehicle emissions at street level, and an incident flow representative of an urban atmospheric boundary layer.

Experiments were conducted in the wind tunnel at École Centrale de Lyon, where pollutant concentrations and velocity fields were measured using a Flame Ionization Detector (FID) and Laser Doppler Anemometry (LDA), respectively. Additionally, the FID and LDA systems were synchronized to quantify turbulent mass fluxes. Simulations were performed using uDALES, a high-resolution, building-resolving LES code for urban microclimate and air quality modeling.

The results validate numerical simulations against experimental data, highlighting key factors for aligning CFD models with wind tunnel experiments. In particular, the study emphasizes the need to accurately reproduce the boundary layer above buildings and select proper dimensional scaling for consistency. Simulations provide a detailed view of flow and concentration fields, showing that trees significantly alter pollutant distribution. Instead of a uniform two-dimensional pattern, concentrations vary three-dimensionally with tree density, creating alternating high- and low-pollutant regions. However, average pollution levels and overall ventilation efficiency show no clear trend with tree density.

By demonstrating the synergy between numerical simulations and experimental measurements, this study provides valuable insights into vegetation-airflow-pollutant interactions, contributing to more effective urban design strategies aimed at improving air quality in street canyons.

How to cite: Fellini, S., Majumdar, D., Salizzoni, P., and van Reeuwijk, M.: Comparative analysis of wind tunnel experiments and uDALES simulations for pollutant dispersion in tree-lined street canyons, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-741, https://doi.org/10.5194/icuc12-741, 2025.

This study utilizes Large Eddy Simulation (LES) to parameterize urban turbulence in the framework of Local Climate Zones (LCZ). The LES approach's dependability for simulating urban turbulence was confirmed by thorough validation against the Architectural Institute of Japan's (AIJ) high-standard wind tunnel tests. This validation was followed by a systematic investigation of the turbulence dynamics and three-dimensional flow properties across different types of LCZ. The impact of urban shape and surface roughness on turbulence behavior was captured by quantifying and parameterizing key turbulence metrics, such as the turbulence kinetic energy (TKE) dissipation rate, drag coefficients, and characteristic length scales. Considerable degree of heterogeneity in turbulence characteristics among LCZ variants. Because of increased surface roughness and thermal storage effects, compact high-rise zones showed lower characteristic length scales, increased turbulence intensity, and higher TKE dissipation rates. On the other hand, open low-rise zones showed bigger characteristic length scales, lower dissipation rates, and weaker turbulence intensities. These results demonstrate the close relationship between urban morphology and atmospheric processes, as well as the crucial role that LCZ-specific urban layouts play in forming turbulence regimes. The parameterization of urban turbulence within the LCZ framework is advanced in this study, offering a strong methodological basis for air quality prediction and urban climate modeling. This study provides important insights into the intricate relationship between turbulence dynamics and urban structure by combining high-fidelity numerical simulations with conventional urban classifications. It is anticipated that the results will help design climate-resilient cities and guide the creation of sustainable urban planning policies.

How to cite: Wang, S. and Yang, J.: Turbulence parameterization scheme of Local Climate Zone-Based Urban Morphologies Using Large Eddy Simulation, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-63, https://doi.org/10.5194/icuc12-63, 2025.

Cities account for approximately 2% of the global land area but consume 60 to 85% of the world's energy and produce 70% of the world's carbon dioxide emissions. In recent years, photovoltaic (PV) electricity energy generation has grown exponentially. Roof-top PV deployments offer several advantages, such as local clean energy generation, energy independence as well as efficient space usage. The installation of roof-top PV alters the urban surface properties and thus influences surface-atmosphere interactions. Previous studies considered only the impact of solar panels on urban air temperatures from an energy balance and mesoscale approach. However, these models do not resolve the complex flow around buildings, not to mention PV panels, which plays an important role in momentum and thermal exchange processes. Therefore, we developed a new parametrization in the PALM model system based on the effective albedo method in which PV modules are represented by an change in surface albedo only. This effective albedo is a function of the PV module albedo and its energy conversion efficiency. This method will be refined into a more sophisticated PV representation in the course of our current research project. PALM is a Large-Eddy Simulation (LES) model capable of resolving turbulence and heat transfer processes in resolved street canyons, i.e. in urban microscale simulations. In this study we employ PALM to investigate the impact of roof-top PV on the urban microclimate, outdoor thermal comfort, indoor energy demand, and CO₂-equivalent savings. Furthermore, we conduct sensitivity studies regarding PV deployment height, synoptic forcings, PV roof coverage, and conversion efficiency. All simulations are embedded in a validated framework of representative local climate zones, which enables comparability and generalizability of the results. Our work can contribute to urban planning, as we examine under which conditions PV enhances urban heat or can be used as urban adaptation measure.

How to cite: Anders, J., Zhai, H., Mauder, M., and Maronga, B.: Impact of roof-top photovoltaics on the urban microclimate: High-resolution LES study using the PALM model system, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-86, https://doi.org/10.5194/icuc12-86, 2025.

Materials with specific radiative properties, known as “cool” materials, have been developed to enhance radiative cooling of buildings and urban microclimate. The choice of roof coating also influences the potential for water condensation. In addition, dew formation is a way to collect water, but it also significantly affect building energy consumption. Other studies have highlighted the impact of condensation on the heat flux exchanged between urban surfaces and the atmosphere, yet none have specifically examined its impact on the urban microclimate. Urban microclimate models often neglect the condensation on roof surfaces, as it is the case with SOLENE-microclimat.

This work therefore aims to integrate dew formation on urban surfaces into the SOLENE-microclimat model. For this purpose, condensation and evaporation processes are added to the roof energy balance. This work focuses on an industrial district in Singapore. The effectiveness of SOLENE-microclimat in representing this district is confirmed by comparing the simulated results with on-site measurements. This comparison validates the simulated surface temperatures for cool and conventional roofs. The results also show a significant dew potential at night in the hot and humid climate of Singapore. The height of condensed water is higher for cool roofs than for conventional roofs. In addition, in the morning, the surface temperatures of cool roofs are lower, which favours a more gradual evaporation of the condensed water film. The impact of dew on urban climate and energy consumption is also investigated.

Furthermore, this work highlights the limitations of urban microclimate models to accurately estimate dew potential. Indeed, the prediction of condensed water height is highly sensitive to uncertainties in input data (air temperature and relative humidity), as well as to model errors.

How to cite: Ruiz, M., Stavropulos-Laffaille, X., Rodler, A., and Musy, M.: Modelling of dew formation on conventional and cool roofs of an industrial district in Singapore: Potential, impact and limitations, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-129, https://doi.org/10.5194/icuc12-129, 2025.

Mean radiant temperature (T_mrt) is one of the most important meteorological variables for human thermal comfort on clear, calm and hot days. It is the sum of all short- and longwave radiation that a human is exposed to. In urban environments, a large part of the radiation received by a person consist of emitted longwave radiation from vertical sunlit facades, especially when standing in close vicinity to such surfaces. This requires accurate estimations of wall surface temperatures. Here, we present a new simple approach based on a step heating equation to calculate wall surface temperatures in the SOlar and LongWave Environmental Irradiance Geometry (SOLWEIG) model. Wall surface temperatures are calculated based on short- and longwave radiation that the wall surface is exposed to together with air temperature and information of thermal conductivity, specific heat capacity, density, wall thickness, albedo and emissivity. Moreover, wall surfaces are divided into unique voxels that enable partial shading of a wall. The approach presented here is fast, as opposed to often commonly used conduction models that require detailed information on different layers of a wall to estimate heat transfer and the resulting surface temperature. Its implementation into the SOLWEIG model enables differentiation of buildings materials into e.g. wood, concrete and brick, which was previously not possible. The minimal input data required on wall surface characteristics also makes it easier to incorporate new wall materials.

How to cite: Wallenberg, N., Holmer, B., Lindberg, F., Lönn, J., Maesel, E., and Rayner, D.: A simple step heating approach for wall surface temperature estimation in the SOlar and LongWave Environmental Irradiance Geometry (SOLWEIG) model, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-132, https://doi.org/10.5194/icuc12-132, 2025.

Precise boundary conditions are crucial for urban microclimate simulations because, unlike macroscale models, their smaller spatial extent makes the model’s prognostic variables more dependent on these conditions. Incoming shortwave and longwave radiation are the most critical parameters for accurate results, serving as the primary drivers interacting with various urban structures and materials within the model domain. Thus, accurately obtaining these parameters through measurements and using them as boundary conditions is essential. They are often neither directly measured nor estimated through proxies, such as cloud cover, or are only partially available (e.g., only global radiation), leading to unnecessary inaccuracies when comparing measurements with simulations.

To address this challenge, we developed a method employing missing data algorithms to deduce cloud cover at three different height levels, from which site-specific radiation data can be derived. This method enhances input accuracy by decomposing global radiation into direct and diffuse components, facilitating the construction of a robust forcing file. When longwave radiation data is unavailable, information about air temperature, humidity and cloud cover can be used to estimate incoming longwave radiation data. Consequently, our approach significantly enhances the simulation of Mean Radiant Temperature (MRT), air temperature, and humidity, aligning closely with in-situ measurements.

In this contribution, we present the methodology and demonstrate its successful application in a model area in Hong Kong. The results prove the effectiveness in generating realistic microclimatic predictions that align with empirical observations, even with incomplete datasets and in complex environments. This approach offers a valuable tool for studies facing challenges with accurate measurements, yielding more precise simulation results, especially in complex urban environments where precise data acquisition is difficult.

How to cite: Schöneberger, P., Sinsel, T., Ouyang, W., Tan, T. Z., Bruse, M., and Simon, H.: Methodological advancements to improve boundary conditions for ENVI-met microclimate simulations based on incomplete measurement data, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-180, https://doi.org/10.5194/icuc12-180, 2025.

The dispersion of traffic pollutants in a street canyon with roadside hedges was investigated in an atmospheric boundary layer wind tunnel. Two types of hedge arrangements were studied: (1) one central hedgerow between the main traffic lanes, and (2) two eccentric hedgerows sidewise of the main traffic lanes. Hedges differing in height and porosity were tested. Pollutant concentrations were sampled at the lower half of the building facades and at pedestrian level on the sidewalks in front of the buildings for perpendicular approach wind.

The results reveal overall improvements in air quality at the lower half of the building facades and on the sidewalks in street canyons with hedges compared to the hedge-free case. The reductions are larger for the central hedgerow arrangements than with the sidewise hedgerow arrangements. At the leeward facade of the windward building in the central street canyon zone - where traffic pollutant concentrations are highest for the given approach wind - zone-averaged reductions of up to 61% are achieved with central hedgerows and up to 39% with sidewise hedgerows. At the lateral end zones of the street canyon, reductions in traffic pollutant concentrations are found with central hedgerows but increases with sidewise hedgerows. However, since the concentrations of traffic pollutants in the lateral end zones are considerably lower compared to those in the central zone, in total an improvement in air quality dominates.

The study shows that roadside hedgerows can beneficially affect the dispersion of traffic pollutants in urban street canyons. Overall, they lead to a reduction of concentrations and in particular to a strong reduction in the central street canyon zone were typically the highest pollutant levels occur. Hence, they can therefore be employed in a targeted manner to considerably remedy the exposure of residents and pedestrians to traffic pollutants in urban street canyons. 

How to cite: Gromke, C.: Modification of traffic pollutant dispersion and concentration levels by hedgerows in urban street canyons, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-217, https://doi.org/10.5194/icuc12-217, 2025.

Air pollution from motor vehicles in densely populated cities remains an urgent problem. In order to analyse and predict air quality at city scale, in-situ monitoring systems of pollutant concentrations and meteorological parameters are used, supplemented by mathematical prediction models and numerical simulations. However, local pollution processes in urban areas located close to roads often cannot be studied efficiently by monitoring due to the lack of a detailed sensor network. Numerical modelling at the urban micro-scale can provide detailed information on the pollutant transport around buildings, but requires high resolution building geometry and approaches to describe boundary and initial conditions for meteorological parameters, pollution sources and traffic intensity.

In the present study, the authors focus on the numerical modelling of scenarios of pollutant transport from vehicles in the vicinity of an urban environment fragment in Stuttgart, Germany. The modelling methodology is based on a combination of classical CFD methods, sensor data on traffic intensity and vehicle statistics, and meteorological data. The traffic statistics used in the simulation include the number of vehicles per hour, divided into 11 vehicle classes, and vehicle speed data. The numerical simulation was performed using the 3D Reynolds-averaged Navier-Stokes equations with the k-ε turbulence model, supplemented by gaseous pollutant and particulate matter transport models. Ansys Fluent 2023R1 is used as the main CFD modelling tool.

Non-stationary measured data of traffic on the considered road section were analysed and used to improve the numerical model. This allows the description of the daily air pollution dynamics within the urban fragment. This detailed simulation of unsteady aerodynamics and air quality at the urban micro-scale can be used to focus on indoor/outdoor air exchange problems, to optimise the transport system, and to contribute to the provision of sustainability and climate safety technologies in cities.

How to cite: Valger, S. and Voss, U.: Micro-Scale Modelling of Pollutant Transfer from Vehicles in Urban Area based on Numerical Methods and In-Situ Measurements, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-228, https://doi.org/10.5194/icuc12-228, 2025.

As global warming accelerates and urban areas expand, understanding urban overheating and its local variations is critical for effective heat adaptation strategies. Numerical modeling is essential for diagnosing urban overheating and evaluating mitigation strategies. However, validating urban microclimate models remains a challenge due to data limitations and the complexity of urban systems, highlighting the need for a standardized framework for model qualification.

This paper introduces AppBenchmark, a user-friendly tool designed to evaluate urban microclimate models using an academic benchmark [1]. The benchmark methodology is based on intercomparison of microclimate simulations and, where available, comparison with reference data. Four benchmark cases are defined to analyze individual and coupled heat transfer processes in an idealized street canyon: shortwave radiation, longwave radiation, aeraulics, and their coupling with heat conduction and storage in walls and ground.

AppBenchmark processes surface-related variables (e.g., surface temperatures, heat fluxes) from formatted simulation data for analysis across canyon surfaces. To account for varying spatial and temporal resolutions, the tool provides three levels of analysis: surface averages, 1D profiles, and 2D fields. It also calculates RMSE to quantify deviations, offering a robust framework for evaluating and comparing simulation results.

An initial dataset containing simulation results from four microclimate models applied to the benchmark [1] is provided as a basis for comparison. The tool, dataset, and user documentation will be made publicly available on the DIAMS ANR project website. Expanding the benchmark application to additional models will help establish a standardized framework for urban microclimate model qualification. This initiative represents a first step toward a comprehensive validation methodology, with future efforts focused on incorporating additional physical phenomena and realistic urban configurations.

[1] Gresse, T. et al. Qualification of Microclimate Models and Simulation Tools: An Academic Benchmark. Available at SSRN: http://dx.doi.org/10.2139/ssrn.5036556

How to cite: Gresse, T., Rodler, A., Musy, M., and Merlier, L.: AppBenchmark: A User-Friendly Application for Urban Microclimate Models Qualification, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-230, https://doi.org/10.5194/icuc12-230, 2025.

Urban street canyons are critical areas for air quality concerns, where interactions among wind flow, traffic, thermodynamics, and pollutant dispersion pose significant challenges for microscale climate modeling. In this study, we use high-resolution large-eddy simulation (LES) to investigate the effects of traffic-induced turbulence and exhaust emissions on pollutant transport within street canyons. The Imposed Velocity Method (IVM) for incorporating vehicle-induced effects was developed, implemented, and validated by extending the PALM model system.
Through a comprehensive parameter study, we examine how traffic, thermodynamics, and varying wind conditions interact to influence local flow dynamics and pollutant dispersion. We systematically vary vehicle speed, wind speed, and thermal boundary conditions to represent a range of realistic traffic and diurnal heating scenarios. This parametric approach allows us to assess how changes in the wind-to-vehicle speed ratio and different heating configurations influence the interplay between vehicle-induced turbulence, thermal effects, and overall flow dynamics in the street canyon. We also investigate the impact of emission source representation by comparing a point and line source approach, revealing how these modeling choices influence the pollutant transport. This comparison underscores the importance of accurately modeling vehicular emissions to avoid under- or over-estimation of local concentrations.
In this presentation, we will share our latest results and discuss key insights into the role of vehicle-induced turbulence (VIT) in influencing urban air quality.

How to cite: Motisi, G. and Maronga, B.: Advancing Urban Street Canyon Dispersion Modeling: A Large-Eddy Simulation Study on Vehicle-Induced Turbulence, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-259, https://doi.org/10.5194/icuc12-259, 2025.

Accurately representing urban hydrological processes is essential for understanding energy and water exchanges in cities, improving weather and climate simulations across scales, and informing effective flood and water resource management. Despite notable advancements in urban land surface models since the last international urban model intercomparison project, several challenges persist. Notably, many models still struggle to achieve a closed water balance, and the representation of hydrological processes often remains oversimplified. These issues primarily stem from the inherent complexity and heterogeneity of the urban hydrologic cycle. In this study, we integrated multiple hydrological parameterization schemes into a single-layer urban canopy model to better capture key processes such as canopy interception by urban grass and trees, surface runoff, soil moisture dynamics, and groundwater runoff. These new schemes complement the model’s existing capabilities of resolving root water uptake and evapotranspiration. We evaluated the performance of these new schemes against 16 global urban sites with various background climates and site characteristics. Results demonstrate that employing these new schemes enhances the accuracy of surface energy and water partitioning and improves the characterization of urban hydrological behavior compared to previous versions. Additionally, our findings highlight that different hydrological schemes have greater impact on simulated runoff fluxes (especially in either dry or high precipitation regions) than energy fluxes and near-surface hydrometeorological conditions. Our approach has important implications for urban planners and policymakers, especially in enhancing urban water management and resilience under extreme weather and climate conditions. Furthermore, these multi-parameterization schemes can be coupled into the urban canopy models in mesoscale and global models such as WRF, MPAS, and CESM to improve the representation of urban hydrological processes, ultimately leading to better predictive capabilities and more informed decision-making.  

How to cite: Huang, Y. and Wang, C.: Multi-parameterization of hydrological processes in an urban canopy model: Model development and multi-site evaluation, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-299, https://doi.org/10.5194/icuc12-299, 2025.

Manifestations of climate change and the urban heat island effect increase urban residents' exposure to heat stress, further impacting their physical and mental well-being. These challenges highlight the need for more effective, adaptable, and holistic mitigation and adaptation strategies to reduce heat stress and enhance the urban thermal environment. In this contribution, we introduce outcomes combining two methods: i) high-fidelity simulation of the urban environment with a realistic spatial and temporal setup (1 min and 1 m); ii) thermal walks (aka ‘heat-walk’) conducted during hot summer days in Prague, a central European city. The combination of accurate fine-scale model simulations with a real-time, on-site, human-oriented approach provides comprehensive information about the realistic human thermal environment at the pedestrian level and reveals causes of thermal discomfort. Both methods precisely and consistently identify hotspots that should be improved in terms of thermal comfort. Vulnerable areas are typically open spaces and arterial streets with a lack of greenery and a high proportion of impervious surfaces, unshaded northern parts of streets (typically E-W oriented), or parts of boulevards with inappropriate tree spacing. The results also emphasize the significance of accurately positioned blue-green infrastructure in mitigating urban heat effects. Although both approaches are demanding in terms of obtaining input data, the results are unique regarding time and spatial scale. The findings of these studies can enhance our understanding of the complex spatiotemporal dynamics of human thermal comfort in urban environments, aiding in effective urban planning and urban heat mitigation.

How to cite: Geletic, J., Bureš, M., Lehnert, M., and Květoňová, V.: Heat-walk: How do biometeorological indices modeled by modern numerical tools correspond with real human thermal sensation?, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-343, https://doi.org/10.5194/icuc12-343, 2025.

Longwave radiation from the surroundings is an important part of the human comfort in urban areas. While the distribution of longwave radiation from the sky is fairly known, this is not the case with regards to the longwave radiation emitted from the surrounding buildings in complex urban settings. In this study we present observed as well as simulated data from two sites in Gothenburg, Sweden: 1/ a wall with one storey of plastered brick and one of wood and 2/ two brick walls facing southeast (SE) and southwest (SW) respectively.  We also show how wall temperatures can be simulated based on estimations of the radiation balance using a single-layer step heating equation.

Under clear skies in summer, the daytime wall temperatures of the wooden wall exceed air temperature with, on average, 20 °C while the plastered brick wall did not reach 15°C. During the night the wooden wall  was on average 2°C cooler than the air whereas the plastered brick wall  was on average, 2°C warmer. These differences depend on the heat properties of the wall materials but also of its thickness. The heating of the SW-looking brick wall was delayed by four hours compared to the SE-looking but in spite of this the maximum temperature was almost the same.

Net radiation, modelled by the SOLWEIG model, and air temperature are the input variables in simulations of wall temperature. Material parameters are thermal effusivity of the wall and a time constant related to its thickness. In the afternoon, the simulations underestimate wall temperature, probably a result of heat stored within the wall during the warming phase. However, the diurnal march of wall temperatures with max and min are over all well described by the simulation.

How to cite: Holmer, B., Wallenberg, N., Lönn, J., and Lindberg, F.: Observations and model evaluation of building wall temperatures for outdoor thermal comfort applications, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-354, https://doi.org/10.5194/icuc12-354, 2025.

Due to the effects of global warming in recent years, there are concerns that the damage caused by heatstroke will increase in Japan as well. To evaluate the risk of heatstroke, a meteorological model capable of calculating micrometeorology considering the effects of buildings and trees is necessary. The Multi-Scale Simulator for the Geoenvironment (MSSG) developed by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) is a multi-scale atmosphere-ocean coupled model capable of calculating from the global scale to the regional scale and urban scale. Its atmospheric component allows for micrometeorological calculations with a resolution of several meters considering the three-dimensional shapes of buildings and tree canopies.

The Japan Meteorological Agency distributes forecast data called the Meso Spectral Model (MSM). In this study, downscaling was performed using MSSG with this MSM data as the initial and boundary values, and micrometeorological calculations at the urban scale were performed. In addition, WBGT, an index of heatstroke risk, was calculated from the results of the micrometeorological calculations and predicted values of heatstroke risk were provided via a web server.

On the day the prediction calculation was performed, we measured WBGT on-site using a black globe thermometer to confirm the accuracy of the heatstroke risk prediction. In the area behind the building, where the wind is weak, and in the corridor, where the wind is strong, both the measured and predicted WBGT values were lower in the corridor. However, there was a tendency for the predicted WBGT values to be uniformly lower than the measured values. By measuring temperature and relative humidity, it is possible to calculate the WBGT when the measured values are uniformly higher than the predicted values. We believe that by providing this as a predicted WBGT correction value, it can be used for on-site heatstroke crisis management.

How to cite: Honda, K., Nakase, H., Matsuda, K., Sugiyama, T., Kurihara, Y., and Kameda, T.: Providing high-resolution heatstroke risk predictions for an artificial island in Osaka Bay, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-369, https://doi.org/10.5194/icuc12-369, 2025.

Urban areas are characterised by a complex three-dimensional geometry of buildings and trees. Urban climate models can operate at the mesoscale (100 m to 2 km resolution; urban canopy models) and greatly simplify the urban geometry (e.g. by assuming an infinite street canyon), or at the building scale (about 1 m resolution; micrometeorological models). The building-resolving models use the radiosity method in combination with a matrix of view factors to compute the exterior radiative exchange and separate algorithms to compute heat conduction in the buildings. The disadvantages of these methods are their strong dependence on the number of elements in the scene (in terms of computational time and/or memory usage) and the challenges of representing different physical processes such as spectral material reflectivities or specular reflections.

This study presents a new approach to calculate the combined radiative (solar and terrestrial infrared) exchange, heat conduction, and convection in a complex 3D urban geometry. It is based on a Monte Carlo solver (stardis) for combined radiation, conduction, and convection, which is used to calculate the urban surface temperature as a function of the meteorological forcing provided by the meteorological model (Meso-NH) representing the buildings with an immersed boundary method. Stardis solves the physical equations without a computational grid and is therefore very well suited to deal with a highly complex 3D geometry. A very important limitation of the presented model is that moist processes are currently not included. A validation of the sensible heat flux, the near-surface air temperature profile, and the skin surface temperature simulated by Meso-NH-stardis is performed for the COSMO outdoor urban test site for dry heat wave conditions. The new model and the validation results are presented.

How to cite: Schoetter, R., Caliot, C., Nagel, T., Retailleau, F., Kotthaus, S., Inagaki, A., Morrison, W., Forest, V., Eymet, V., and Masson, V.: A new approach to modelling the energy balance of a complex urban geometry, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-396, https://doi.org/10.5194/icuc12-396, 2025.

Air quality deterioration in street canyons is an important issue strongly influencing human health, and the microscale LES models can provide very detailed information about its spatial and temporal structure. However, the proper model configuration and its comprehensive validation in real urban conditions still represent a significant challenge that has not been fully addressed. We present a validation campaign conducted in a traffic-laden area of Prague focusing on the detailed temporal and spatial structure of concentration fields. Almost one year of field measurements were collected with a carefully designed air quality sensor network complemented by reference and remote sensing air quality and meteorological observation tools. All measured data were consequently checked and verified. The detailed data of land cover, emissions, and meteorological conditions were provided to the PALM LES model, which was configured for the studied area in two nested domains. The comparison of the modeled and measured values showed good agreement during some episodes, while significant discrepancies were discovered in other episodes, mainly during stable meteorological situations. The identified reasons include imprecise meteorological boundary conditions, uncertain emission inputs, and insufficiently represented or neglected processes in the model. Also, the imprecision introduced by the advection numerical scheme was identified in some situations. We propose and test selected amends that improve the agreement of the model with observations. This experiment provides a knowledge base applicable to future validation campaigns and for further model development and application.

How to cite: Resler, J., Bauerová, P., Belda, M., Bureš, M., Geletič, J., Eben, K., Fuka, V., Jareš, R., Karel, J., Keder, J., Krč, P., Patiño, W., Radović, J., Řezníček, H., Šindelářová, A., Sühring, M., and Vlček, O.: Challenges of validating microscale LES air quality models in real urban environments: a case study for Prague city centre, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-397, https://doi.org/10.5194/icuc12-397, 2025.

The UrbanGreenEye project represents a transformative approach to enhance urban climate resilience through the integration of advanced modeling and remote sensing techniques. This study focuses on the application of the urban climate model PALM-4U to simulate and analyze urban microclimates across German cities to investigate climate adaptation strategies. As cities face escalating impacts of climate change, the need for high-resolution data to inform urban planning has grown considerably. PALM-4U enables detailed simulations of urban environments to capture the complex interaction between built structures, vegetation, and atmospheric processes. By incorporating satellite-derived indicators such as land surface temperature, urban green volume, and vegetation vitality, the model offers a robust framework for assessing thermal dynamics and identifying areas vulnerable to heat stress. Through the UrbanGreenEye project, PALM-4U has been employed to evaluate the effectiveness of various greening scenarios and infrastructure modifications in mitigating urban heat islands. These simulations not only visualize the potential impacts of adaptation measures but also provide quantitative evidence to support decision-making. Key lessons learned include the importance of localized data for model calibration, the role of green infrastructure in reducing thermal stress, and the challenges of scaling simulations to diverse urban contexts. The collaboration with German municipalities has demonstrated the practical utility of PALM-4U in real-world planning processes. This in turn bridges the gap between scientific research and policy implementation. This talk will highlight the project’s findings and show how PALM-4U can be used to translate simulation results into actionable strategies for climate adaptation. By sharing the lessons we’ve learned through the UrbanGreenEye project, we hope to encourage cities to utilize advanced modeling tools like PALM-4U. Together, we can build greener, cooler, and more resilient urban spaces that are better prepared to face the challenges of a changing climate.

How to cite: Salim, M., Schubert, S., Frick, A., and Churkina, G.: From Simulation to Action: Lessons from Simulating Urban Climate Adaptation Measures Using PALM-4U in the UrbanGreenEye Project, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-419, https://doi.org/10.5194/icuc12-419, 2025.

Several indices have been proposed in the domain of urban climate and biometeorology for the assessment of heat stress or thermal comfort. In this study, the UTCI is selected as the primary index, which incorporates factors such as air temperature, humidity, wind speed and mean radiant temperature (Tmrt), where urban morphology plays a pivotal role. This paper presents the methodology and preliminary results of a heat stress neighbourhood scale model, accounting for the urban effect. To this end, SOLWEIG microscale urban simulations have been done for various LCZs (Local Climate Zones) dispersed across diverse latitudes in Europe, with hourly input data from ERA5-Land.The UTCI index is derived from the Tmrt outcomes of the microscale modelling. An artificial intelligence model is then trained using a deep neural network to estimate the number of hours of strong heat stress (UTCI > 32) using daily climate data, LCZ, day of the year, sun elevation, and sunset and sunrise time. Finally, a 30-year period heat stress database is calculated for different regions of Europe with a resolution of 100 meters and the average number of hours per year of strong heat stress is calculated for each region. Preliminary results show good agreement between the neural network and SOLWEIG results. This approach increases the agility and efficiency of the simulation, once the artificial intelligence model is trained, thereby facilitating its utilisation by urban and territorial planners, and decision-makers.

How to cite: Navarro, D., Simon-Moral, A., Glodeanu, A., Peña, N., Garcia, I., and Feliu, E.: Heat stress neighbourhood scale model combining urban simulations and deep neural network, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-422, https://doi.org/10.5194/icuc12-422, 2025.

As climate change intensifies and urban areas expand, city dwellers become more exposed to heat stress. The built environment plays a significant role in shaping local microclimates through complex interactions between radiation, airflow and materials. Understanding these interactions is essential to developing effective mitigation strategies.

This study presents a two-dimensional urban microclimate model developed using COMSOL Multiphysics to simulate the thermal and radiative dynamics within a street canyon. The street is represented by two façades and one ground. The model couples a radiative model that accounts for multiple reflections, a thermal model solving the heat equation within building façades and ground, and an aeraulic model which solve the flow inside the street while considering buoyancy effects. In addition, a thermal comfort model allows to calculate the radiative flux absorbed by a human by means of Mean Radiant Temperature (Tmrt). Thermal comfort indicator such as the Universal Thermal Climate Index (UTCI) is calculated through Tmrt, temperature and velocity of the air at the pedestrian level. This model establishes heat balances with detailed resolution, making it possible to quantitatively attribute the impact of each physical phenomenon on the urban microclimate and thermal comfort.

The developed model is applied to a case study in Toulouse, France, where multiple streets with varying geometries, orientations and materials are analyzed. The choice of streets varies from a narrow canyon in the city center to more open streets in the outskirts. Meteorological conditions representative of summer heat waves are used to conduct microclimate simulations. Toulouse has a temperate climate and its buildings are mainly made of brick. The results highlight the impact of urban morphology, material properties or meteorological conditions on thermal comfort.

How to cite: Matry, H., Bonhomme, M., Labat, M., and Ginestet, S.: Urban microclimate modeling of a 2D street canyon for pedestrian thermal comfort: a case study in Toulouse, France, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-439, https://doi.org/10.5194/icuc12-439, 2025.

Accurately assessing socioeconomic disparities in human-perceived urban heat exposure and vulnerability is crucial for effective urban planning. However, the lack of high-resolution 2-meter air temperature (T_a) data due to insufficient monitoring stations has led to widespread reliance on land surface temperature (T_s) as a proxy, despite T_s not having direct implications for public health. It remains unclear whether, and to what extent, T_s misrepresents such disparities, particularly at decision-making scales. In this study, we leverage our newly developed 1-km resolution daily urban T_a data from 2013 to 2023 to evaluate discrepancies between T_a- and T_s-based assessments of urban heat disparities. Our preliminary results show that at daytime, T_s significantly exaggerates disparities among racial and socioeconomic groups and misidentifies vulnerable communities. Across the contiguous United States, 40.9% of census tracts classified as vulnerable based on T_s are not identified as such by T_a, while 22.4% of tracts that are vulnerable by T_a are overlooked by T_s, as defined by an energy burden above 6% and temperatures above the locally-defined median. These biases in T_s-based assessments could lead to misguided decision-making and inefficient resource allocation. Our findings underscore the need for high-resolution T_a data to ensure more equitable and effective urban heat mitigation strategies.

How to cite: Zhang, Y. and Zhao, L.: Large discrepancies between air temperature and land surface temperature in assessing urban heat disparities, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-458, https://doi.org/10.5194/icuc12-458, 2025.

There is a strong interaction between the thermal properties of buildings and urban microclimate, which could be seen as a nonlinear feedback loop affecting urban areas' energy consumption. Large eddy simulation (LES) is a reliable model to study the urban microclimate and thermal behavior of buildings. However, the thermal properties of buildings are highly complex and variable, and thus their inventory and representation in LES models is often lacking in detail. This underlines the need for sensitivity studies to improve the efficiency of LES and its application in studies on buildings' energy and their microclimate effects. This paper investigates the sensitivity of the PALM-4U model towards building parameters using the TABULA archetype building framework. In this case study, the effects of varying levels of building detail from homogeneous typologies to very detailed representations on 2 m air temperature, roof, and wall surface temperatures, sensible heat flux, and near-wall (10 cm) air temperature are investigated across three modeling scenarios: (1) a baseline scenario that uses uniform building types, (2) a simplified classification scheme based on PALM’s predefined categories, and (3) a detailed representation that uses 28 TABULA archetypes. Results show 2 m air temperature differences of −0.3 to 0.3°C between scenarios, with the detailed building properties representation exhibiting lower diurnal and higher nocturnal temperatures. Simplified scenarios demonstrate strong surface temperature fluctuations and lower nighttime surface temperatures which indicates fast heat transfer to the environment. In contrast, the TABULA parameterization depicts reduced surface temperature fluctuations and higher nighttime surface temperatures. These findings show a relevant sensitivity of the LES model towards building parameterization and hence detailed and accurate input data on thermal building properties are required to improve the modeling approaches for LES models in urban energy and microclimate studies.

How to cite: Nowzamani, N., Maronga, B., van der Linden, L., and Bechtel, B.: Sensitivity of the LES PALM Model to Building Parameters Using TABULA Archetypes , 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-467, https://doi.org/10.5194/icuc12-467, 2025.

Growing adoption of integrated water management and urban greening strategies provide an opportunity for urban heat amelioration. Planning the use of and optimizing the benefits from these techniques, under present day and future climate conditions, requires suitable modelling tools. However, the Urban-PLUMBER project found that most land surface models fail to properly represent urban water processes (Jongen et al., 2024), including water balance closure, evapotranspiration, water storage, and surface runoff, leading to lower modelling performance. These shortcomings may be overcome by implementing a soil water balance model component into existing land surface models. Here, we adapted the soil water balance model, SIMPEL (Hörmann et al., 2007) for integration into land surface models. SIMPEL's stand-alone performance was evaluated against observations of turf grass irrigation trials conducted in 2021 and 2022 (Cheung et al., 2024) and showed good skill in predicting infiltration and soil moisture storage levels under a variety of precipitation and irrigation scenarios.

After the initial integration of the SIMPEL module into the TARGET local scaled model (Broadbent et al., 2019), TARGET shows increased accuracy compared to Phase 1 of the Urban-PLUMBER project (Lipson et al., 2023), using the Coutts et al. (2007) AU-Preston dataset.

As a standalone and computationally efficient model component, SIMPEL is suitable to be added as a module to a wide range of models to overcome the shortcomings found around water balance representation, ensuring water balance closure and internal water storage consistency. Future work is planned to improve the coupling with TARGET and to add SIMPEL to the VTUF-3D micro-scale model. Work is also planned to improve performance in urban sub-surface runoff representations.

References

Broadbent et al. (2019) 10.5194/gmd-2018-177
Cheung et al. (2024) 10.1016/j.uclim.2024.101914
Coutts et al. (2007) 10.1175/JAM2462.1
Hörmann et al. (2007) 10.1016/j.ecolmodel.2007.07.019
Jongen et al. (2024) 10.1029/2024MS004231
Lipson et al. (2023) 10.1002/qj.4589

How to cite: Nice, K. A., Jongen, H. J., and Förster, K.: Improving urban water process representations in land surface models with the SIMPEL soil water balance module, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-487, https://doi.org/10.5194/icuc12-487, 2025.

This study investigates the relationship between tree age, density, and their cooling effect on the urban canopy layer (UCL) and human thermal comfort at the microscale during a heatwave in Berlin. An empirical tree growth model based on Lalic and Mihailovic (2004) is used to derive leaf area density (LAD) profiles for Tilia trees at various ages. The Larsen and Kristoffersen (2002) tree database of 331 Tilia trees is employed to establish the physiological tree dimensions based on the empirical relationship between tree growth and total leaf area. The PALM canopy generator is used to configure age-specific LAD profiles at a 1m grid resolution for 48 scenarios, varying tree age (10-80 years) and stem-to-stem density (10-35m). The study simulates an extreme heatwave event using PALM in LES mode.

The results are expected to show a linear correlation between tree age, density, and cooling effect, with mature trees at higher densities providing significant air temperature reduction - up to 6°C cooler during the day and 4°C at night beneath the canopy. Trees younger than 20 years will likely provide less cooling due to reduced leaf area and shading. These findings will prove valuable for urban planners implementing tree planting as a thermal buffer against heatwaves. The study offers a wide range of numerical tree parameters demonstrating the cooling effects of urban forest canopies on local microclimate at high spatial-temporal resolution.

How to cite: Brook-Lawson, J., Meier, F., Schubert, S., and Kershaw, T.: Simulating the Cooling Potential of Tilia Trees in Berlin Using PALM-LES: The Impact of Tree Age and Density During a Heatwave, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-488, https://doi.org/10.5194/icuc12-488, 2025.

Given the urban heat island effect and extreme events induced by climate change, studying the urban microclimate and mitigation strategies is essential, particularly to enhance pedestrian thermal comfort.
Vegetation has demonstrated its effectiveness in mitigating urban heat at local scale but its effect on thermal comfort has still to be accurately studied across various climates and urban configurations. Vegetation improves pedestrian thermal comfort through shading and evapotranspiration. However, vegetation can deteriorate thermal comfort by increasing the relative humidity and impeding wind flow and nocturnal heat removal.
In this study, the urban microclimate is simulated using the high-fidelity CFD model urbanMicroclimateFoam based on OpenFOAM. This solver solves successively turbulent air flow, heat and moisture transport in solid materials, and radiation exchanges. Environmental boundary conditions are dynamically downscaled from Weather Research and Forecasting (WRF) mesoscale results. The impact of vegetation is analyzed across three distinct climates: a continental climate in Montreal, Canada, a tropical climate in Singapore, and an arid climate in Morocco. 
Trees yield local improvements in all three cases, while the highest cooling potential is observed for the arid climate context. Conversely, in tropical climate, vegetation air cooling is offset by the increase in humidity, resulting in a reduced thermal comfort impact. On a daytime average, vegetation in Montreal reduces UTCI locally by up to 6°C, with a non-local adverse heating effect of 2°C. In Singapore, the local cooling effect evaluated reaches 5°C UTCI, with non-local increases up to 3°C, while in Morocco, vegetation achieves a local improvement of UTCI by 7°C, with non-local adverse effects limited to 1°C. The primary factor contributing to pedestrian thermal comfort improvement in all climates is shading provided by trees.
Urban planners and stakeholders can integrate such valuable insights to harness the benefits and challenges of vegetation across diverse climatic contexts.

How to cite: Nevers, C., Carmeliet, J., Kubilay, A., and Derome, D.: Climate-dependent impact of vegetation on thermal comfort in urban neighborhoods through resolved CFD simulations , 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-514, https://doi.org/10.5194/icuc12-514, 2025.

Computational Fluid Dynamics (CFD) validations for urban microclimates often rely on full-scale experiments. However, spatial heterogeneities and anthropogenic influences in actual urban environments introduce significant data uncertainties, making it challenging to evaluate the impact of different turbulence, radiation, and heat conduction models on simulation results. Therefore, this study validated CFD simulations using scaled outdoor experiments with more controlled and reliable data. The unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations were conducted by coupling radiative transfer, turbulent convection, and heat conduction. The simulations were evaluated against measured urban surface and air temperatures, as well as wind speeds. Differences among various radiation, turbulence and conduction models were compared. Results demonstrated that the simulations accurately captured diurnal variations in urban microclimates. Among the radiation models, the Discrete Ordinates (DO) and Surface-to-Surface (S2S) models provided better accuracy in predicting surface temperatures compared with the P-1 model. In turbulence modeling, the Standard (STD) and Realizable (RLZ) k- models performed similarly, while the Renormalization Group (RNG) k- and Shear-Stress Transport (SST) k- showed larger discrepancies, particularly in wind speed predictions below the urban canopy height. For heat conduction, the shell conduction model, which accounts for heat transfer in both normal and planar directions, yielded more accurate temperature predictions than the thin wall model with one-dimensional heat transfer. This research advances understanding of how numerical schemes affect CFD simulations of urban microclimates. The findings provide valuable guidance for model selection and improve simulation accuracy for urban planning.

How to cite: Chen, G. and Hang, J.: CFD simulations on urban microclimates: Validated by scaled outdoor experiments, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-516, https://doi.org/10.5194/icuc12-516, 2025.

Effective adaptation planning necessitates a stronger integration of climate science with urban planning, supported by tools that facilitate localized climate assessments. Climate mapping plays a crucial role in bridging this gap by providing insights into microclimatic conditions and informing urban interventions. For instance, Toulouse Metropole employs heat island intensity maps generated through TempMap, a GIS plugin (Touati et al., 2020) designed for modeling near-surface air temperatures, to define adaptation strategies.

In this study, we demonstrate the application of TempMap in São Paulo, Brazil—one of the largest cities in the world—utilizing data from a local meteorological network alongside information on topography and building characteristics. To the best of our knowledge, this represents the first application of this method in a city of such scale.

One of the primary challenges encountered was the limited availability of weather stations (33 stations within an area of 1,500 km²) and GeoClimate data, a key component of the original TempMap method used to extract information on local morphology. Despite this limitation, we successfully adapted the method by integrating a local building dataset and calculating building density, impermeable surface density, and free external surface density in ArcGIS, in accordance with GeoClimate documentation. We then replicated the TempMap process using SAGA GIS.

The mean difference between measured and estimated temperatures was 1.22°C. Nineteen stations exhibited a deviation of less than one degree, while two stations recorded deviations exceeding three degrees. These findings indicate that TempMap provides promising results for estimating air temperatures, even with a relatively sparse network of weather stations. Furthermore, the results underscore the importance of a denser meteorological station network to enhance the accuracy of temperature estimates and have encouraged both the cities of São Paulo and Campinas to propose expanding their monitoring networks.

How to cite: Ferreira, L., Touati, N., and Hidalgo, J.: Exploring Urban Climate Mapping: TempMap Application in São Paulo, Brazil , 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-523, https://doi.org/10.5194/icuc12-523, 2025.

Keywords: Microscale modeling, large-eddy simulations, biometeorology, urban climate, UTCI

Human heat stress and the risk of outdoor thermal discomfort under extreme heat conditions are increasing even in traditionally cold climates due to climate change. Modeling micro-scale urban climate to explore outdoor thermal exposure is a highly topical challenge in modern climate research. PALM-4U is a large-eddy simulation (LES) based high-resolution micrometeorological model that includes a complex radiation scheme and an integrated biometeorology module to assess thermal conditions in complex urban environments. To test the performance of PALM-4U, we compared the model with in-situ measured mean radiant temperature (MRT) and universal thermal climate index (UTCI). Model evaluation was performed using the mobile instrument platform MaRTy. Air temperature, relative humidity, wind speed, and longwave and shortwave radiant flux densities in a 6-directional setup were recorded by the MaRTy cart and compared to PALM-4U. The in-situ measured data were collected across 23 locations in Guelph, Ontario, Canada, during multiple times of day and seasons in 2020 and 2021. Data were collected and aggregated for evaluation of PALM-4U in 10-minute and 1-hour intervals. We found that PALM-4U performed better in simulating UTCI than MRT in summer, especially during periods when the midday surface temperature is high. Furthermore, the MRT simulation result deviates from the measured values under certain shade types. The performance under vegetation is slightly better compared to under engineered shade. This work aids in the improvement of PALM-4U under various urban morphologies and advances the microclimatic modeling research of the field. 

How to cite: Wang, Y., Adebayo, F. F., Crank, P., Krayenhoff, S., and Geletič, J.: Evaluating the PALM-4U representation of outdoor urban thermal exposure in a humid continental climate using MaRTy, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-547, https://doi.org/10.5194/icuc12-547, 2025.

Previous studies have noted that cities enhance cloud cover, but the mechanisms of urban morphological types on cloud formation remain elusive.
Considering basic types of urban land cover land use, we then set up idealized large-eddy simulations (LESs) for varying urban morphological types with different building heights and building plan area fractions under free convective conditions. LES results suggest that cloud patterns closely correspond to the vertical transport of moisture. Clouds over urban-rural interface are controlled by the urban breeze circulation (UBC); while clouds over urban core are controlled by vertical velocity variance. Thus, urban morphologies affect cloud formation via two plausible mechanisms: the roughness-induced convergence at the urban-rural interface increases intensity of UBC while roughness as momentum sinks reduces vertical turbulent transport at the urban core. Two conceptual models are proposed to explain the variation of vertical velocity scales with morphologies. Observations across 44 major US cities corroborate LES results: cloud enhancement increases with street-canyon aspect ratio and decreases with building density. This study demonstrates the importance of capturing vertical motions modified by urban roughness for boundary-layer parameterizations. It also underscores the value of synthesizing long-term observations across distinct cities to understand the effect of urban form on boundary-layer processes.

How to cite: Cui, Y., Chen, S., Xue, L., Munoz-Esparza, D., Sauer, J., Hu, L., Albertson, J., and Li, Q.: Local cloud enhancement in cities depends on urban morphology, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-552, https://doi.org/10.5194/icuc12-552, 2025.

Large-scale turbulent structures within an urban canopy significantly affect the air exchange and thermal environment in urban areas by enhancing ventilation, dispersing pollutants, and facilitating heat transfer. However, their detailed turbulence characteristics remain poorly understood, partly because of the complex interactions between turbulent flows and urban geometries. In this study, proper orthogonal decomposition (POD) was applied to time-series data of streamwise and spanwise turbulent velocities obtained from large-eddy simulations to capture large-scale turbulence structures around an inline array of three different types of buildings: a cubic building, a vertical rectangular building, and a horizontal rectangular building. In all cases, in the first and second POD modes near the bottom surface, pairs of large-scale high-velocity and low-velocity structures regularly appear in the streamwise direction on both sides of the buildings. These structures are strongly linked to the spanwise velocity between buildings, indicating that air exchange and scalar transfer within the urban canopy are significantly affected by large-scale turbulent structures generated on both sides of the buildings. However, the characteristics of these structures vary with building geometry. Around cubic buildings, pairs of high- and low-velocity structures also appear in both the upper and lower regions on both sides of the buildings. In contrast, such vertical pairs are not observed for both vertical or horizontal rectangular buildings. Instead, large-scale structures extend further in the vertical direction, reaching the top of the buildings.

How to cite: Michioka, T.: Large-scale turbulent structures around various type of buildings, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-553, https://doi.org/10.5194/icuc12-553, 2025.

Human thermal experience is highly influenced by the characteristics of the local environment. Advancing space design and understanding the microclimate are fundamental to achieving a healthy thermal environment. Considering the increasing urbanization and climate change, such concerns will interest a large part of the world population. In this study we used municipal datasets including 3D City Models and Building typology information to build a multi-physics model of the microclimate in Downtown New York City, arriving at temperature and flow fields for the calculation of the human physiological equivalent temperature. A methodology was developed to assess the accuracy of the simulation output by combining weather stations and broadband sensors including airborne and terrestrial infrared imaging systems for surface temperature measurements. We investigated the usefulness of a steady-state versus transient formulation of the flow field and long- and shortwave radiative interactions. We find that the steady-state models, which lack full integration of the thermal capacitance of the urban fabric, do not fully represent how heat is absorbed and stored and released. The steady state formulation of this thermal inertia overestimates pedestrian-level temperatures and overall urban heat retention. It is concluded that transient simulations are required for accurate estimation of climate parameters pertaining to human thermal exposure and the estimation of physiological equivalent temperature.

How to cite: Fitzky, M. and Ghandehari, M.: Modeling Human Thermal Experience in the Urban Microclimate, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-561, https://doi.org/10.5194/icuc12-561, 2025.

While the benefits of trees for thermal comfort in urban street canyons are well established, their impact on air quality remains uncertain, particularly regarding pollutants emitted by heavy traffic at the pedestrian level. High-resolution microscale models of the urban boundary layer, such as Large Eddy Simulation (LES)-based models, provide valuable insights into street-level processes, allowing for precise simulations of air quality and bio-meteorological variables.

To explore this complex interaction, this study employed the LES model PALM to simulate different tree coverage scenarios in two different urban streets under varying atmospheric stratifications. The simulations incorporated real geographic conditions and quasi-real meteorology to ensure realistic representation. Special attention was given to selecting boundary conditions that closely matched the real temperature stratification while allowing for a meaningful comparison between cases. The results revealed significant spatio-temporal variability in thermal comfort and particulate matter concentrations at the pedestrian level, highlighting the intricate relationship between urban greenery, airflow dynamics, and pollution dispersion. As expected, trees notably improved thermal comfort by reducing the biometeorological index UTCI  in shaded areas during maximum irradiance and contributing to cooling effects after sunset (in the case of optimal tree conditions). Oppositely, in narrow street canyons, tree-induced changes in airflow led to a substantial increase in PM₁₀ concentrations by more than 100% compared to tree-free scenarios. This effect was primarily attributed to the slowdown and vertical displacement of the primary vortex, which hindered pollutant dispersion.

The study highlights the need for careful and  responsible urban planning when implementing greening strategies in cities. For example, our results indicate that when the number of trees in the street is halved, the impact on thermal comfort remains comparable to the scenario with the full amount of trees, but the air pollution aggravation is notably lower

How to cite: Řezníček, H., Geletič, J., Belda, M., Beneš, L., Bureš, M., Eben, K., Fuka, V., Krč, P., Radović, J., and Resler, J.: Finding trade-offs between heat stress and air quality in street tree planting scenarios., 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-626, https://doi.org/10.5194/icuc12-626, 2025.

Shading is a key factor for mitigating heat stress on hot summer days. This study evaluates how modelled street tree cooling efficiency (CE) is shaped by interactions between urban morphology and tree characteristics. CE is defined as the reduction in mean radiant temperature (ΔTmrt) relative to street tree canopy coverage, enabling standardized comparisons across diverse urban configurations. Using the SOLWEIG model, Tmrt is simulated at meter-scale resolution during 24 h of the hottest day of 2023 in a modified street canyon in Ghent, Belgium.

A distribution for three urban morphological parameters is derived from city-center inventories (street widths 10 - 20 m; building heights (0 - 20 m);  4 orientations). Similarly, realistic tree structural variability is determined for three parameters: canopy heights (10 - 16 m, derived from an inventory of Terrestrial LiDAR scans of 346 street trees), trunk heights (0 - 8 m), and shortwave transmissivities (0 - 40%). A chosen street and tree were modified across these morphological ranges and repeatedly used as simulation input.

Preliminary results indicate for urban morphology, daily averaged Tmrt decreased exponentially with higher street height-to-width (H/W) ratios. North-South oriented streets exhibit 2.5x the decay rate of East-West streets. Moreover, by adding the same singular tree to different street canyons, the resulting tree’s CE correlated strongly positively with street width (R² = 0.97). Regarding tree morphology, vertical canopy distribution significantly impacts CE: extending the lower canopy (the space between the ground and start of canopy) has a 3.4 larger impact on CE compared to equivalent height extensions of the upper canopy. Finally, increasing vegetation transmissivity (i.e., less dense foliage) by 1% reduced CE by 1.3%.

We demonstrate a sensitivity analysis framework applied to the urban morphology of Ghent, revealing how tree parameterization affects Tmrt and the implications for thermal comfort and urban tree management.

How to cite: Daelman, T., Demuzere, M., Vancoillie, F., D'hont, B., Lindberg, F., and Verbeeck, H.: How the city shapes the shade: the influence of urban and tree morphology on modelled daily mean radiant temperature, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-645, https://doi.org/10.5194/icuc12-645, 2025.

Pedestrian level wind environment is closely related to pedestrian comforts regarding the air quality, thermal comfort, wind load, etc. Now a day, computational fluid dynamic models can successfully simulate the airflows at an urban street scale. However, it still takes much computational costs to run daily similar with the operational weather forecasts.

This study conducts a diagnostic prediction of the microscale wind environment based on the statistical downscaling. This statistical downscaling requires the relationship between the regional meteorological parameters between a coarse grid and local meteorological parameters to be refined. The refined grid is produced by lattice Boltzmann equation model simulations performed on a huge GPU cluster (TSUBAME4.0). Simple linear relationships between the local wind environment and the regional meteorological parameters were observed.

Based on this empirical relationship, the few-meter scale fine velocity distribution within an urban district was reproduced which was downscaled from the regional meteorological observation. The results were compared with the several near-ground observation points in urban districts. A good correlation between the measured and diagnostically estimated velocity distributions was confirmed.

In addition, the regional meteorological data was also replaced by the output of regional weather forecasting model, which the result can also confirm the reproduction of the microscale wind environment

How to cite: Inagaki, A., Nakai, K., Kanda, M., Onodera, N., Hasegawa, Y., I Dewa, J., and Oda, R.: Statistical downscaling into the airflows within urban districts based on a numerical simulation datasets, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-658, https://doi.org/10.5194/icuc12-658, 2025.

uDALES is an open-source, multi-physics microclimate modelling framework specifically developed for studying outdoor urban environments. It performs the large-eddy simulation (LES) of airflow while accounting for the heat transfer (sensible and latent), moisture, and pollutant dispersion within the urban atmospheric boundary layer at microscale resolution. The framework employs a novel conservative immersed boundary method (IBM) to resolve complex urban geometries, represented via a triangulated surface with a resolution independent of the Cartesian computational grid. This offers greater flexibility and accuracy in representing realistic urban structures. The interaction between the urban surfaces and the surrounding airflow is incorporated in the IBM using wall functions for surface shear stresses and heat fluxes. The latter are two-way coupled with a surface energy balance model that takes into account net short and long-wave radiation and ground heat conduction, with the capability to accommodate both man-made and vegetative materials. uDALES is also capable of modelling the drag introduced by trees in urban areas and their influence on modifying the local wind flow and pollutant dispersion. Recent advancements in uDALES have enabled two-dimensional domain decomposition via the 2DECOMP&FFT library, facilitating efficient parallelization and optimized use of supercomputing resources such as ARCHER2, thereby preparing for exascale computing. Several validation exercises have been conducted on idealized cases, including neutral flow over staggered urban-like structures, flow over a single tree and over tree arrays, cross-ventilation of an isolated building, and non-neutral flow over a single cube that is misaligned with the computational grid. The results demonstrate strong agreement with experimental and numerical benchmarks, highlighting the code’s accuracy and efficiency.  With its enhanced functionality, uDALES (https://github.com/uDALES/u-dales) is a robust tool for high-resolution LES studies of urban environments, supporting advanced research in urban atmospheric and environmental sciences.

How to cite: Majumdar, D., Owens, S. O., Wilson, C. E., Bartholomew, P., and van Reeuwijk, M.: uDALES: a high-fidelity multi-physics large-eddy simulation framework for urban microclimate modelling, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-677, https://doi.org/10.5194/icuc12-677, 2025.

A realistic representation of urban environments at street scale is crucial for accurately capturing local climate dynamics. Large-eddy simulation (LES) models at meter-scale resolutions are a powerful tool for urban studies. FastEddy^®, NCAR-RAL’s GPU-accelerated LES model, offers a computationally efficient approach to high-resolution, turbulence-resolving urban simulations. It explicitly includes building effects and urban heterogeneity, enabling a detailed representation of flow, transport, and exchange processes.

Building effects in FastEddy^® are modeled using an immersed body force method (IBFM), which represents buildings as momentum sinks typically applied together with isothermal boundary conditions when radiation effects and a building energy model are not considered. Therefore, surface heat fluxes between the ground and urban structures cannot directly modify building temperatures, highlighting the need of alternative energy redistribution mechanisms that can lead to realistic urban-atmosphere thermal exchanges.

This study explores different approaches to redistributing surface heat fluxes within the urban environment to achieve a more physically consistent representation of urban energy balance and to better account for urban morphological complexity. Several strategies are tested, evaluating their impact on heat exchange dynamics and urban microscale simulations. The results yield new insights into heat redistribution processes in urban LES and provide improvements for these types of modeling efforts.

Acknowledgements: The authors acknowledge the ECCE project (PID2020-115693RB-I00) of the Ministerio de Ciencia e Innovación/Agencia Estatal de Investigación (MCIN/AEI/10.13039/501100011033). ERL thanks her predoctoral contract FPU (FPU21/02464) to the Ministerio de Universidades of Spain.

How to cite: Raluy-López, E., Muñoz-Esparza, D., Sauer, J., Gil-Guirado, S., and Montávez, J. P.: Enhancing Heat Exchange Forcings in Microscale Simulations of Urban Environments with FastEddy®, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-704, https://doi.org/10.5194/icuc12-704, 2025.

The orthogonal 3D grid used in PALM and other atmospheric models brings challenges in representing resolved obstacles (such as terrain or buildings) with non-orthogonal surfaces. The simple binary form of representation, in which each grid cell is either entirely atmospheric or obstacle, causes aliasing effects (artificial stairs). It has been demonstrated that this leads to significant biases in radiation, surface energy balance and fluid dynamics.

The newly introduced PALM slanted surface option involves new methods in the Radiative Transfer Model (RTM) and the surface-related modules in PALM, and it enables a more faithful representation of non-orthogonal surfaces while preserving the PALM’s Arakawa-C model grid without inducing significant performance penalty. For surface-atmospheric interactions, the cut-cell approach is used. In the RTM, a new representation of surface elements is introduced for both raytracing in the preprocessing phase and radiosity-based interactions in the time-stepping phase of the simulation.

The format of the input data had to be extended to accommodate the representation of the slanted surfaces. The open-source input processor PALM-GeM is able to prepare slanted surface PALM inputs using various geographic data.

Validation and benchmarking results from synthetic and real-case scenarios are presented. The slanted surface model is also being prepared for wind tunnel validation.

How to cite: Krc, P., Suehring, M., Bures, M., and Resler, J.: Full implementation of non-orthogonal surfaces within the PALM model, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-723, https://doi.org/10.5194/icuc12-723, 2025.

Regarding the actual conditions of the heat environment within urban city blocks, mobile meteorological observations in a residential area of Tokyo have pointed out the need to properly understand the dynamics of water vapor content on a city block scale due to significant WBGT fluctuations even within a few kilometers of observation routes. In this study, tracer flow analysis using LBM based on a detailed three-dimensional urban model assuming sea breeze inflow was conducted in order to understand the characteristics of water vapor distribution within a city block. As a result, it was found that the water vapor distribution in the city block is characterized by the distribution of water vapor inflow from the sea area due to sea breezes. The area with low water vapor was located on the leeward side of the high-rise buildings, where the sea breeze was blocked by the buildings and water vapor originating from the sea did not easily flow into the area. It was also found that the advection pattern of water vapor changed depending on the location of the main flow direction and the arrangement of the city block structures. Although we confirmed that the above distributions are generally consistent with the water vapor distributions measured by the mobile observations, there are also some difference from the observation result. This is expected to be evapo-transpiration from water bodies and vegetation which are not considered in the numerical model.

How to cite: Oda, R., Wada, T., Inagaki, A., Hasegawa, Y., and Onodera, N.: Water Vapor Distribution in an Urban District using Three-dimensional Urban Geometry Model Under Sea Breeze Event, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-745, https://doi.org/10.5194/icuc12-745, 2025.

Understanding and modeling conditions at the neighborhood scale is essential for addressing weather and climate impacts on urban communities. Mesoscale weather models provide information at horizontal resolutions of a few kilometers. Such resolutions are still too coarse to represent environmental conditions at neighborhood scales of tens of meters or smaller experienced by individual residents. On the other hand, microscale urban simulations are often limited by stationary boundary conditions from mesoscale weather models and relatively small spatial extents. We evaluate a dynamic downscaling approach that bridges the gap between mesoscale and neighborhood scales using multi-nested WRF-LES model simulations. Our simulations implement recently developed or updated meter-resolution static (land cover, soil, building, topographic, etc.) data to represent the unique urban environments within Baltimore, MD. We then assess model performance using remote sensing, surface weather, surface flux, soil, biophysical, and boundary layer profile observations affiliated with the Baltimore Social-Environmental Collaborative (BSEC) and Coastal Urban-Rural Atmospheric Gradient Experiment (CoURAGE), paying particular attention to surface layer turbulence profiles. We will share lessons learned through our model development and validation efforts, especially the impacts of the description of the complex urban land surface on atmospheric turbulence and neighborhood-level climate conditions. We endeavor to contribute to the ongoing effort to improve urban modeling across scales.

How to cite: Horne, J., Pan, Y., and Davis, K.: Evaluating an Atmospheric Dynamic Downscaling Approach from Mesoscale to the Neighborhood Scale Using Large-Eddy Simulations, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-748, https://doi.org/10.5194/icuc12-748, 2025.

PALM-meteo is an open-source modular tool for creating the so-called dynamic driver – the input file for PALM with initial and boundary conditions (IBC) and other time-varying data. Correctly selected and prepared meteorological inputs are essential for realistic PALM simulations, as will be demonstrated.

PALM-meteo can select relevant data from large input datasets. It performs geographic conversions, horizontal and vertical interpolation, temporal disaggregation and interpolation, pressure adaptation, unit conversions as well as other calculations among meteorological and chemical variables.

The horizontal interpolation phase uses either bilinear interpolation for rectangular grids or Delaunay triangulation together with barycentric interpolation for the icosahedral or unstructured grids. An important part of its processing is the vertical adaptation method, which enables terrain matching at the bottom while avoiding unrealistic vertical shifts in the higher levels. As part of creating boundary conditions it performs the necessary mass balancing.

Currently, PALM-meteo contains input modules for the mesoscale weather models WRF, ICON, Aladin and the chemical transport models CAMx and CAMS, as well as user-defined synthetic inputs. More input modules are coming soon, thanks to its modular and extensible architecture.

The tool will be presented together with input and output data from example PALM simulations. The aspects of selecting and configuring meteorological inputs will be discussed.

How to cite: Krc, P., Bures, M., Belda, M., Resler, J., and Radovic, J.: PALM-meteo: universal meteorological input processor for the PALM model, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-749, https://doi.org/10.5194/icuc12-749, 2025.

Increasing building energy efficiency (BEE) standards in regions where HVAC systems are not prevalent, e.g., most of Europe, can lead to an increase in daytime outdoor heat exposure (Maronga et al. 2022, Heusinger et al. 2023). According to microscale simulation studies with PALM and ENVI-met, retrofitting of old buildings to highly energy efficient standards may increase ambient, near-surface temperatures up to 4 K locally at daytime (Maronga et al. 2022, Heusinger et al. 2023). Increased BEE leads to much lower conductive heat fluxes through building materials, thereby increasing building surface temperatures and sensible heat fluxes during the day. This translates into higher daytime air temperatures, which are especially noticeable in building courtyards, potentially due to lower air exchange compared to street canyons. In this study we analyzed several common heat mitigation measures in their efficacy of remediating the additional ambient heat due to higher BEE. The results demonstrate that only measures that directly target the building surfaces are able to neutralize the daytime heating effect by higher BEE, e.g., building greening. Other common heat mitigation measures such as street trees and sun sails are much less effective in this particular case. The results of these modeling experiments should motivate further observational studies to validate the findings and to get a deeper understanding of the processes involved in counteracting the outdoor heating effects caused by increasing BEE standards.

References:

Heusinger, J., Bruchmann, N., & Weber, S. (2023). Modeling the impacts of building energy efficiency on the thermal microclimate in a midsize German city. Urban Climate, 52. https://doi.org/10.1016/j.uclim.2023.101678

Maronga, B., Winkler, M., & Li, D. (2022). Can Areawide Building Retrofitting Affect the Urban Microclimate? An LES Study for Berlin, Germany. Journal of Applied Meteorology and Climatology, 61(7), 800–817. https://doi.org/10.1175/JAMC-D-21-0216.1

How to cite: Heusinger, J. and Weber, S.: How to counteract increasing daytime ambient heat caused by higher building energy efficiency standards, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-750, https://doi.org/10.5194/icuc12-750, 2025.

This study is a part of the joint ASSURE project involving the Universities of Reading, Bristol, Southampton, Surrey, and Imperial College, funded though the UK NERC Highlight Scheme.  The atmospheric airflows and dispersions over a heterogeneous neighbourhood urban area covering the University of Bristol campus and the nearby residential areas were investigated using the large eddy simulation method with synthetic turbulence inflow boundary conditions. The influence of significant local building features across St Michael’s Park, such as a step-change in building height of approximately 20 m, was specifically studied. Various wind directions and pollutant emission source locations within the campus were tested to capture a wide range of scenarios. The numerical simulation was validated against EnFlo wind tunnel experiments conducted at the University of Surrey, ensuring the reliability of the results. The numerical data were processed to obtain time-averaged velocities, concentrations and second order turbulence statistics. The growth and decay of concentration fluctuations were investigated and compared with field measurements and DNS data in the literature. We are in particular focused on the influence of local building height change on the development of internal boundary layer, turbulent fluctuations and scalar fluctuations. These findings provide valuable insights into urban atmospheric dynamics and will be presented at the conference.

How to cite: Wang, C. and Xie, Z.-T.: Impact of local buildings on flow and dispersion over a heterogeneous neighbourhood urban area, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-766, https://doi.org/10.5194/icuc12-766, 2025.

Typhoon-induced wind hazards within urban neighborhoods are characterized with uncertain and extreme characteristics. The neighbourhood-scale wind fields in typhoon Mangkhut (2018) and Mui-fa(2022) were simulated using the Weather Research and Forecasting (WRF) model coupled with the Parallelized Large-Eddy Simulation Model (PALM). The pedestrian distress indicators (Gust Factor, GF and Speeding-up Efficacy, SE) were calculated for describing local risks. Positive correlations showed between λp (building area fraction) and pedestrian distress indicators. The probability density functions (PDFs) of GF and SE to λp can establish the connections between the extreme at specific probabilities and the average. With the increase of λp, these PDFs exhibit a slow rightward shift of the peak and a rapid broadening of the distributions. Positive correlations were also found between λf (frontal area density) and the vertical expanding coherent vortices. At the windward side of the high-rise, clear positive correlations between vertical momentum downward and the vortices with vertical geometry arise at the mid-high levels, corresponding momentum transfer efficiency and extreme gust exhibits more pronounced inside the vortices, but weak outside the vortices. This indicates vortices with vertical geometry around the high-rise contribute significantly to stabilizing momentum transfer and strengthening the extreme wind speed, revealing the prominent role of vortices on mid-high levels, rather than pedestrian levels and top levels, improving the understanding of the mechanism of wind hazards within urban blocks.

How to cite: Zhang, N. and Yang, D.: Large eddy simulation of the urban morphology impact on wind fields, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-778, https://doi.org/10.5194/icuc12-778, 2025.

Understanding how urban surfaces exchange energy with the atmosphere is crucial for predicting microclimate variability and improving urban resilience. This study investigates the role of shading distribution on the surface energy balance (SEB) in urban environments at both micro- and neighborhood scales. Using the building-resolving large-eddy simulation (LES) code uDALES, we model a realistic vegetated urban setting under convective conditions. Two solar zenith angles (0° and 45°) are considered while maintaining constant incoming shortwave radiation to isolate shading effects.

Our results show that shading significantly influences local SEB fluxes, particularly net shortwave radiation and sensible heat flux, which differ by approximately 424 Wm⁻² and 277 Wm⁻², respectively, between shaded and sunlit surfaces. Despite these variations, domain-averaged SEB fluxes remain largely unchanged, except for latent heat flux, which is 22% higher when the sun is directly overhead. Additionally, while the mean radiant temperature (MRT) varies only slightly between cases (ΔMRT = 0.19 K), localized effects are substantial: unshaded areas at 45° zenith experience MRT values up to 10 K higher due to increased radiative contributions from walls.

These findings highlight the critical influence of shading distribution on urban microclimates, emphasizing the need for high-resolution modeling in climate-sensitive urban planning. By explicitly resolving shading effects, this work advances our understanding of urban thermal dynamics and provides valuable insights for improving thermal comfort and reducing heat stress in cities.

How to cite: Wilson, C., Bohnenstengel, S., Paschalis, A., Schonk, J., and van Reeuwijk, M.: Impact of Shading on Urban Surface Energy Balance and Microclimate, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-784, https://doi.org/10.5194/icuc12-784, 2025.

Assessing urban ventilation plays an increasingly important role in supporting urban planning/design, especially for high-density cities suffering from poor wind conditions and an intensive urban heat island effect. Many design and technical guidelines have been developed to assess the impact of building clusters on outdoor natural ventilation and provide mitigation strategies (Franke, 2006; Tominaga, 2008; Ng, 2009; BCA, 2012).

However, most technical guidelines have limitations in dealing with input wind profiles for numerical simulations. The current ways to estimate wind profiles in the urban boundary layer, e.g., regional scale wind tunnel, weather research and forecasting (WRF) model, and empirical models, are not accurate enough to run CFD simulation, and often lead to significant errors (He, et al., 2022 (a) and (b)).

To address this issue, this study applied Doppler LiDAR technology to measure the incoming wind profiles approaching high density urban areas and used these measured wind profiles as boundary conditions to drive CFD simulation. Hourly-averaged wind profiles were applied, and the CFD simulation model covered the high-density and highly heterogeneous urban areas in Singapore, as shown in Figure 1. Ultra-sonic wind anemometers were deployed to collect near-ground wind velocity data for validation. As shown in Figure 2, the validation results indicate that the Doppler LiDAR driven CFD simulation is accurate, with only very minor deviation compared with real measurement, and much more accurate than other traditional ways, e.g., WRF, Logarithmic law, and power law.

Figure 1. Test bed at downtown areas in Singapore, for LiDAR boundary layer wind measurement, Ultra-sonic near ground wind measurement, and CFD simulations.

Figure 2. Cross-comparison between real near-ground measurements and CFD simulation results driven by different incoming wind profiles (e.g., LiDAR, Power Law, WRF, Logarithmic Law). 

How to cite: Yuan, C., Xing, J., Li, K., and Cheng, J.: Doppler Lidar-Driven CFD Simulation for High-Density Urban Areas: Validation with Near-Ground Measurements in Singapore, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-790, https://doi.org/10.5194/icuc12-790, 2025.

The Campus Sustainability Roadmap 2030 is the National University of Singapore initiative that includes Carbon Neutral NUS, Cool NUS, Zero Waste NUS and Campus in a Tropical Rainforest. The roadmap covers climate mitigation, adaptation, resource efficiency and behavioural change for sustainability. The CoolNUS – BEAM (Baseline-Evaluating-Action-Monitoring) has a significant role through climate sensing to monitor the current condition so mitigation strategies can be administered effectively. Climate simulations are done to validate with the measured data so that the same method can be applied for future masterplans with new or refurbished buildings.

In the realm of urban climate simulation, the most validated 3D CFD solver today is urbanMicroclimateFoam, which runs under the openFOAM.org version 8 framework. Therefore, there is intention to open the workflow so it will not be restricted to a single method of mesher and solver workflow. A stable mesher running under HELYX is selected as the preprocessing for the solver. This ensures that the mesh quality will not contribute to the instability of the solver later down the road. The HELYX mesher is not open-source and therefore, there is a validation required with other meshers in the market, including those under the OpenFOAM family it originates from.

The CFD with solar radiation simulation is done to validate with the measured data of the weather stations in campus. It can also be used to simulate mitigation strategies which include solar photovoltaics, vegetation shading and evapotranspiration, cool paint, and solar film. It acts as a foundation for the future masterplan studies to deal with what-if scenarios. The results can also be used for the walkability agent-based modelling to study the viability of policies such as closing a vehicular road for pedestrian linkage as well as the overall pedestrian thermal comfort route choices in campus with shading impact.

How to cite: Hii, D. J. C., Hasama, T., Ignatius, M., Lim, J., Lu, Y., and Wong, N. H.: Towards the urban climate modeling of the Kent Ridge campus, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-805, https://doi.org/10.5194/icuc12-805, 2025.

Natural cooling offers a sustainable alternative to energy-intensive mechanical cooling by utilizing cooler outdoor air, typically at night, to maintain comfortable indoor temperatures during the day. This process relies on buoyant and wind forces to drive ventilation. In building energy models, natural ventilation is often estimated using one-dimensional (1D) flow models driven by pressure differences. These pressures are typically derived from empirical models based on wind tunnel experiments. However, for accurate predictions, both the pressure estimates and flow models must be sufficiently precise.

A key limitation is that these models often fail to capture the complexity of urban environments, where surrounding buildings significantly influence airflow. Large Eddy Simulations (LES) provide a powerful alternative, offering detailed and accurate representations of wind flow through urban areas. Moreover, LES can explicitly simulate building interiors, enabling a fully coupled analysis of wind-driven natural ventilation. However, simulating building interiors presents challenges. First, interior modeling requires a fine mesh, adding computational expense. Second, resolving building interiors depends on detailed knowledge or assumptions about the indoor layout.

To address these challenges, we compare LES simulations with and without building interiors. For simulations without interiors, we instead predict ventilation rates using 1D flow models. Preliminary results indicate reasonable agreement between the 1D models and simulated ventilation rates. Predicting ventilation rates from LES simulations without building interiors opens up exciting possibilities. First, we can estimate ventilation rates for various interior layouts using a single simulation of the exterior flow. Second, we can combine 1D interior flow models with exterior flow fields generated from machine learning models trained on LES data—without the need to train these models on interior flow fields.

How to cite: Bachand, N., Salehipour, H., and Gorle, C.: From Canopy Flow to Cooling: Can 1D Ventilation Models Predict Natural Cooling from LES?, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-864, https://doi.org/10.5194/icuc12-864, 2025.

Improving urban heat environments through enhanced ventilation has become a critical strategy for mitigating heat stress in the context of climate change. Taiwan’s high-density urban development and tightly packed buildings create significant variations in ventilation potential across regions. This study develops and validates a model for estimating pedestrian-scale wind speeds, offering a scientific basis for urban ventilation planning and thermal comfort improvement.

Using 10-meter wind speed data from the Taiwan Climate Change Projection Information and Adaptation Knowledge Platform (TCCIP) and observations from the Central Weather Administration, this study incorporates geographic information systems (GIS) to calculate surface roughness lengths and terrain coefficients. Pedestrian-scale wind speeds at a height of 2 meters were estimated using the power law and validated through measurements at three social housing sites in Taipei. Computational fluid dynamics (CFD) simulations were employed to analyze ventilation efficiency at the street-block scale, ensuring consistency with observed data.

Results demonstrate a strong correlation between modeled wind speeds and measured data, confirming the method's applicability under diverse environmental conditions. CFD simulations provided detailed insights into microclimate ventilation potential, highlighting spatial distribution patterns that align with observations. This study establishes a rapid, reliable model for evaluating urban ventilation potential, identifying pathways that reduce heat accumulation, enhance urban thermal comfort, and improve resilience against climate change impacts.

By integrating measurement validation and advanced modeling, this research offers a robust tool for urban planners and policymakers, supporting data-driven strategies to optimize urban ventilation and mitigate the effects of heat in densely built environments.

How to cite: Lin, T.-P., Chen, Y.-L., Wu, P.-E., and Matzarakis, A.: Evaluation and Validation of Urban Ventilation Potential for Heat Mitigation, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-935, https://doi.org/10.5194/icuc12-935, 2025.

In the context of global warming, cities are striving to maintain comfortable public spaces. Wind speed significantly affects pedestrian thermal comfort in urban areas, making localized wind data crucial for urban planners. However, obtaining accurate wind data or calculating it using numerical simulation tools remains complex, limiting their systematic use. This study aims to quantify the impact of wind data accuracy on pedestrian comfort assessments.

The "Ydeal" urban block, located in the Confluence neighbourhood of Lyon, France, serves as a case study. Numerical simulations are conducted using SOLENE-microclimat to compute microclimatic variable fields based on different sources of wind velocity data.

Four types of wind data are considered as inputs:

    1. Continuous wind data from the Bron airport meteorological station, located 9 km southeast of the study area.

    2. Wind data from a weather station located 100 meters north of the urban block.

    3. Wind data from a rooftop weather station on the central building within the urban block.

    4. Wind velocity fields derived from numerical simulations at pedestrian scale.

The first three inputs (1, 2, and 3) provide a uniform wind distribution across the urban block but with different intensities. In contrast, the fourth input (4) offers a heterogeneous wind distribution. In all cases, wind speed intensity varies on an hourly basis.

The results from each method will be compared with one another. Subsequently, each dataset will be used as input for pedestrian thermal comfort modelling, using the Universal Thermal Climate Index (UTCI). The study evaluates how each approach affects UTCI outcomes. Finally, based on the complexity of implementation, the most suitable methodology for urban planning applications is recommended.

How to cite: Gros, A., Morille, B., and Bernard, J.: Wind and Urban Comfort: What Level of Detail is Required for Wind Data in Outdoor Comfort Assessment?, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-937, https://doi.org/10.5194/icuc12-937, 2025.

As urban areas grow, understanding the impact of built environments on aerosol distribution is crucial for accurate monitoring and forecasting of urban air quality and for development of mitigation strategies. In this study we use a modern approach, which combines micro-scale Large Eddy Simulation with Local Climate Zones (LCZ) classification to simulate the transport of Lagrangian aerosol particles in different urban configurations. The study simulates several urban configurations based on LCZ classification, specifically open LCZ types, varying in building height and aspect ratio: LCZ 4 (high-rise), LCZ 5 (mid-rise), LCZ 6 (low-rise). Both regular and randomized urban development configurations were examined to understand the impact of building geometry on particle dispersion. The transport of particulate matter emitted from a linear source within a street canyon has been simulated under neutral atmospheric conditions.

The study reveals that the orientation of buildings significantly influences the distribution of particles. Structures parallel to the wind add horizontal dispersion, while those perpendicular promote vertical mixing. Variations in particle concentrations in randomized configurations highlight the role of architectural heterogeneity in turbulence development and aerosol dispersion. In the absence of regular homogeneous structures, the aggregated block- and district-scale geometry of buildings strongly influences aerosol transport. In randomized urban configurations, the large-scale morphological characteristics of different LCZ types have a significantly greater effect on particle dispersion than the local geometrical differences between configurations within the same LCZ.

Future research is recommended to take into account diverse meteorological conditions and a wider range of LCZ types to enhance the accuracy and applicability of this approach.

The work is supported by RSF grant 24-17-00155.

How to cite: Varentsov, A., Mortikov, E., Stepanenko, V., and Glazunov, A.: Large-eddy simulation of aerosol transport in various types of urban development based on local climate zones classification, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-947, https://doi.org/10.5194/icuc12-947, 2025.

Understanding air flow and pollutant dispersion in heterogeneous urban environments is essential for improving urban climate models and air quality predictions. This study presents results from wind tunnel experiments on investigating dispersion patterns within a scaled physical model (1:200) of an area of central Bristol, UK. The city’s complex urban morphology, including varying building heights and street layouts serves as a case study for heterogeneous urban environments. Experiments were conducted in the EnFlo wind tunnel at the University of Surrey to examine the influence of urban form and source locations on flow dynamics and pollutant transport, utilising a fast-response flame ionisation detector (FFID) and three-component laser Doppler anemometry (LDA) to measure the concentration and velocity fields, respectively. Various wind directions, including southerly, south-westerly, and westerly winds, were tested to capture the variability in pollutant dispersion and momentum transport. Pollutant sources were placed at different locations, including elevated release points (rooftop) and near-ground sources within street canyons, to assess sensitivity to emission height and location. Flow measurements and concentration distributions were analysed to identify key dispersion mechanisms, including building-induced wake effects, recirculation zones, and channelling along major street canyons. The results highlight significant variations in dispersion patterns depending on wind direction and urban geometry. Pollutant retention within deep street canyons and leeward of large buildings was observed under certain conditions, while open intersections and gaps between buildings facilitated its dispersion. This work underscores the importance of integrating wind tunnel experiments alongside computational modelling to enhance predictive capabilities for urban air quality and microclimate assessments. Future research will extend this approach to assess the impact of terrain variations and patterns on dispersion and flow dynamics.

How to cite: Bi, D., Carpentieri, M., Placidi, M., and Robins, A.: Wind Tunnel Modelling of Flow and Pollutant Dispersion in a Heterogeneous Urban Area , 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-1014, https://doi.org/10.5194/icuc12-1014, 2025.

Understanding urban energy fluxes and validating them against observations is crucial for improving urban climate models and their predictive capabilities. In this study, Large Eddy Simulations (LES) were conducted using the PALM4U model to investigate turbulent fluxes over the city of Enschede, The Netherlands, under two distinct meteorological conditions: a windy spring day (10-m wind speed: 4–8 m/s) and a calm, hot summer day (10-m wind speed: 0–3 m/s). The simulations capture the dominant turbulence mechanisms under these conditions, where mechanical shear drives turbulence on the windy day, while land surface heating drives the turbulence on the calm summer day. LES simulations were performed at a spatial resolution of 2 m on a 768 × 768 grid driven by Weather research and Forecasting (WRF) model results as the boundary conditions, covering an area of approximately 1.5 km^2. Model results were evaluated against Eddy covariance tower observations, with half-hourly averaged sensible heat flux showing a good agreement.  While average building height over Enschede city center is approximately 20 m, simulations show strong heterogeneous turbulence structures even at 60 m above ground, influenced by the neighboring built environment and local surface characteristics, affecting observational representativity. A key finding is the successful simulation of nighttime sensible heat flux, which remains a challenge for mesoscale models like WRF, even at hectometric scales. Additionally, the study assessed the impact of Enschede’s sloped terrain on  urban turbulent fluxes, highlighting terrain-induced variations in energy exchange. These results demonstrate the advantages of LES in resolving fine-scale turbulence structures and improving the representation of urban atmospheric processes. The findings emphasize the need for high-resolution modeling to enhance our understanding of urban heat flux dynamics, particularly in heterogeneous environments, and to refine model parameterizations for future urban climate applications. 

How to cite: Gadde, S. and Timmermans, W.: Large eddy simulations of Urban Turbulent Fluxes Over Enschede city, The Netherlands: Evaluation Using Eddy Covariance Tower Observations  , 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-1050, https://doi.org/10.5194/icuc12-1050, 2025.