The proportion of the world’s population living in cities has increased from 37% to 56% over the last 50 years, and it is expected to continue rising further to 60% by 2030 (UN, 2022). As an essential effect of this evolution, urban land cover has expanded rapidly. In the case of Madrid, the increase in urban fraction during the period from 1970 to 2020 has been 20%. It is well known that urbanization reduces the vegetated cover and modifies surfaces properties altering the surface-atmosphere interactions and the different terms of the Surface Energy Balnace (SEB) compared to nearby rural areas. Therefore, analyzing the influence of these changes in urban land cover contributes to understand the potential risks that urban residents might face considering the urban grown and the expected temperatures increases, as this has adverse impacts on human health, livelihoods, and key urban infrastructure.

The aim of the present study is to examine the consequence of Madrid's urban growth on the near-surface air temperature and on the SEB. We conduct a modeling study using WRF-ARW with the multilayer urban parameterization BEP-BEM, in which the land use and the land cover have been modified according to urban expansion in Madrid and its surroundings from 1970 to 2020. Two scenarios of common meteorological conditions of special interest are selected for this study: a period of intense heatwave during the summer season and a short period of strongly stable atmospheric conditions in winter, both observed in 2020. The results show that in areas where the urban fraction become greater an increase in near-surface air temperature is found for both simulated periods, especially during the night, pointing out that the cooling rate decreases in urban areas. The growing of urban land cover over time also modifies the SEB and turbulent transport in Madrid and surroundings, leading to an increase in temperatures, specially for the minima ones.

How to cite: Carbone, J., Sánchez, B., Román-Cascón, C., Martilli, A., Royé, D., and Yagüe, C.: Impacts of urban development on the local weather: A comprehensive analysis from 1970 to 2020 in Madrid., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5608, https://doi.org/10.5194/egusphere-egu24-5608, 2024.

It is well known that cities can modify the rainfall distributions, in particular deep moist convection is more frequently triggered over and downwind urban areas. However, the effect of cities on the most extreme convective events, such as hailstorms, downbursts or tornadoes, is poorly studied. This topic needs further investigation since exposure and vulnerability to severe storm risk is larger in cities than in the surrounding rural area. What happens if a severe convective windstorm impacts a big city? Is the storm modified by the urban land use?

Our analysis focuses on a case study that occurred on 25 July 2023, when a nocturnal downburst affected the city of Milan, in northern Italy, with measured wind gusts up to 30 m/s. The intense wind gusts downed many trees in the public parks and over the streets, blocking urban mobility. The event is investigated in depth using both observations and high-resolution numerical simulations performed with the WRF model.

Observations show that a UHI over Milan before the storm was negligible, while there was a drier air mass over the city than over the surrounding rural area. Consequently, a pool with low values of equivalent potential temperature (theta-e), a quantity that strongly influences deep moist convection, was present over the city.

Four nested WRF simulations are carried out with grid resolution from 9 km up to 333 m, and 64 vertical levels starting from 5m AGL. Two different boundary layer parametrizations are tested, namely MYJ and BouLac schemes, as well as two different microphysics schemes: Thompson and WRF Single-moment 6-class. Moreover, simulations with bulk urban parametrizations are compared with those coupled with the building effect parameterization and the building energy model (BEP-BEM), employing data of the World Urban Database and Access Portal Tools (WUDAPT).  Simulations without the urban land use (no-urban) are carried out to test the effect of the Milan urban area on the convective storm. Results of all these simulations are compared with surface observations and radar data. The simulations have a similar skill, with slightly better results using the BouLac scheme coupled with BEP-BEM. Simulations using urban parametrizations are able to reproduce the pre-storm pool with low theta-e values over Milan, while no-urban simulations do not simulate the low theta-e pool.

All WRF simulations accurately reproduce the violent windstorm, both in terms of simulated wind gusts, rainfalls and radar reflectivity. Removing the city, stronger wind gusts are simulated at the surface due to the significantly reduced drag. However, rainfalls are slightly intensified downwind of the city, as well as the drop of potential temperatures associated with the downdrafts.

In conclusion, the urban canopy may have prevented the development of even more violent wind gusts in the city, due to the increased surface roughness. On the other hand, despite the presence of a pool of low theta-e values, the storm likely intensified downwind the city. A possible motivation to that intensification will be proposed in the presentation. 

How to cite: De Martin, F., Zonato, A., and Di Sabatino, S.: Can a city modify a severe convective windstorm?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17845, https://doi.org/10.5194/egusphere-egu24-17845, 2024.

 The horizontal mean wind speed profile in vertical direction within and above urban canopy (UC) is an essential information to drive the exchange of momentum, heat, moisture and pollutants in atmosphere. Well-known profiles in logarithmic and exponential layers, which express upper and lower wind over UC, respectively, are efficient assumptions used to express UC wind profile.

 This study attempted to add the intermediate layer (IL) between those two-layers to include the effect of building height variability on the mean wind speed profile. Large-eddy simulations (LESs) of UC with building height variability were conducted using a wide range of morphology parameters, that is, plan area index, aspect ratio, and the standard deviation of building height.

 A tendency of the bulk drag coefficient of the IL was expressed by the plan area index and the frontal area index at the intermediate layer. The wind speed at IL was modeled linearly by the length- and velocity-scale analysis. By parameterizing the coefficients of these three layers, we attempted to analytically represent an entire wind speed profile by the three-layer wind profiles. The results indicated reasonable consistency in the wind speeds at mean building height and the momentum flux with LES data. Effect of the thermal stratification was investigated by the correction of the length-scale in IL.

How to cite: Nakao, K., Kikumoto, H., Takimoto, H., Hongyuan, J., and Xiang, W.: Representing mean wind speed profile over urban canopy with building height variability, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16420, https://doi.org/10.5194/egusphere-egu24-16420, 2024.

As climate change continues to exert an impact on urban areas, the comprehension of its effects on the urban environment becomes crucial for sustainable urban planning. This study presents a novel approach employing the Building Effect Parameterization (BEP) coupled with a Building Energy Model (BEM) in an offline configuration to simulate urban climates. The multi-layer BEP+BEM model, properly describes the vertical arrangement of urban fabric, accounting for the distribution of heat, moisture, and momentum sources throughout the urban canopy layer. Additionally, energy consumption within buildings for both cooling and heating is estimated by the BEM, providing a comprehensive perspective on the urban energy balance. Coupled with a 1-D column model of urban canopy flow, the BEP+BEM offline model accurately estimates drag coefficients and turbulent length scales based on urban fabric characteristics. In the proposed version, the model has been extended to consider additional factors such as green areas and street trees, along with existing green roofs, photovoltaic panels and the permeability of urban materials. This expansion enhances the model's capability to assess the effectiveness of sustainable infrastructure in mitigating climate change effects on urban areas. In this study, the BEP+BEM scheme is forced by data from climate projections, allowing for the dynamic representation of various Local Climate Zones (LCZs) under distinct climatic conditions. Simulations in different LCZs and under different climatic conditions are compared to evaluate the impact of climate change on urban environment, enabling the exploration of how different urban areas respond to changing meteorological forcings. The sensitivity analysis includes a range of standard urban typologies (i.e. LCZs), capturing the complexity of interactions between the built environment and the atmosphere. This approach offers an assessment of the impacts of climate change on key urban phenomena, such as urban heat islands (UHI), thermal discomfort, and heightened energy consumption by buildings. The outcomes of this study provide valuable insights for the urban climate community, policymakers, and researchers with the aim of enhancing the resilience of cities in the face of a changing climate. By bridging the gap between climate projections and urban climate simulations, a consistent framework is presented in this work for evaluating and adapting various urban environments to future climatic conditions.

How to cite: Pappaccogli, G., Zonato, A., Martilli, A., Buccolieri, R., and Lionello, P.: The Implementation of the BEP+BEM Offline Parameterization Scheme: Exploring Urban Dynamics through Climatic Projections, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6308, https://doi.org/10.5194/egusphere-egu24-6308, 2024.

In recent years, urban canopy models (UCMs) have been used as fully coupled components of mesoscale atmospheric models as well as offline tools to estimate temperature and surface fluxes using atmospheric forcings. Examples include multi-layer urban canopy models (MLUCMs), where the vertical variability of turbulent fluxes is calculated by solving prognostic momentum and turbulent kinetic energy (TKE, $k$) equations using length scale ($l$) and drag parameterizations. These parameterizations are based on the well-established 1.5-order $k-l$ turbulence closure theory and are often informed by microscale fluid dynamics simulations. However, this approach can include simplifications such as the assumption of the same diffusion coefficient for momentum, TKE, and scalars. In addition, the dispersive stresses arising from spatially-averaged flow properties have been parameterized together with the turbulent fluxes while being controlled by different mechanisms. Both of these assumptions impact the quantification of turbulent exchange of flow properties and subsequent air temperature prediction in urban canopies. To assess these assumptions and improve corresponding parameterization, we conducted 49 large-eddy simulations (LES) for idealized urban arrays, encompassing variable building height distributions and a comprehensive range of urban densities ($\lambda_p\in[0.0625,0.64]$) seen in global cities. We find that the efficiency of turbulent transport (numerically described via diffusion coefficients) is similar for scalars and momentum but 3.5 times higher for TKE. Additionally, the parameterization of the dispersive momentum flux using the $k-l$ closure was a source of error, while scaling with the pressure gradient and urban morphological parameters appears more appropriate. In response to these findings, we propose two changes to MLUCM v2.0: (a) separate characterization for turbulent diffusion coefficient for momentum and TKE; and (b) introduction of an explicit physics-based "mass flux" term to represent the non-Gaussian component of the dispersive momentum transport as an amendment to the existing "eddy diffusivity" framework. The updated one-dimensional model, after being tuned for building height variability, is further compared against the original LES results and demonstrates improved performance in predicting vertical turbulent exchange in urban canopies.

How to cite: Lu, J., Nazarian, N., Hart, M., and Krayenhoff, S.: A one-dimensional urban flow model with an Eddy-diffusivity Mass-flux (EDMF) scheme and refined turbulent transport (MLUCM v3.0), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4829, https://doi.org/10.5194/egusphere-egu24-4829, 2024.

Inserting vegetation within the urban environment mitigates the urban heat island effect, the flooding risk, and improves air quality. However, its aerodynamic effect has remarkable impact on the pollutant transport and, consequently, on human health comfort. Indeed, the presence of vegetation within an urban canyon leads to non-trivial patterns of pollutant concentration and mass fluxes, as a consequence of complex mean and turbulent velocity fields. In addition to the vegetation density, the flow structure within canyons is influenced by their geometry and by the wind direction.

   The aim of the present study is to experimentally investigate the velocity field within a canyon, varying the vegetation density and the wind direction. We measured flow velocity statistics within an indefinitely long street canyon, with unit height-to-width ratio, subject to a neutrally stratified boundary layer modeled in the wind tunnel of École Centrale de Lyon. The aerodynamic impact of vegetation was reproduced by inserting plastic miniatures of trees along the two long sides of the canyon. We considered an empty canyon and a vegetated canyon, whose longitudinal axes are oriented with angles of 0°, 30°, and 60° with respect to the external wind flow.

  Results reveal that when the canyon is inclined with respect to the external wind direction the mean flow follows a complex helicoidal structure. The presence of trees decreases significantly the mean longitudinal velocity and weakens the transversal circulation in the inclined canyon. The dampening effect of the mean longitudinal flow is more marked increasing the inclination angle of the canyon. Turbulent fluctuations are enhanced above the tree crowns, mostly when the wind blows parallel to the canyon axis. On the contrary, turbulent fluctuations decreases at tree trunk and crown levels, in particular when the canyon is inclined of 60° with respect to the external wind direction. Spectra of the velocity signal show that the presence of trees induces an evident shift of the energy peak towards high frequencies.

  The collected data constitute a step forward to understand and modeling the urban microclimate.

How to cite: Del Ponte, A. V., Fellini, S., Marro, M., Salizzoni, P., and Ridolfi, L.: Wind tunnel study on the influence of vegetation density and wind direction on urban canyon ventilation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18319, https://doi.org/10.5194/egusphere-egu24-18319, 2024.

Recent years have seen more intense and frequent heatwaves across the globe. Urban overheating phenomenon induced by global warming and urban heat island (UHI) effect has adverse impact on human health. In particular, compact high-rise cities witnessed worsened wind environment, exacerbating the UHI phenomenon. Blue space such as urban lakes may help mitigate the UHI effect and improve citizens’ living environment. Under weak synoptic wind conditions, the temperature difference between built-up areas and lakes can induce wind circulation, known as lake-breeze circulation (LBC). The LBC system can transport cool and fresh air from lake surfaces into built-up areas, reducing urban air temperature and improving urban wind environment, while increasing urban air humidity. In this study, we developed a multi-scale water-energy coupled CFD model to simulate the transport processes of heat and moisture between lake surfaces and built-up areas within the urban boundary layer. The model adopted a porous turbulence model to simulate the entire urban canopy layer, a lake evaporation model and a species transport model to simulate lake dynamics, and a coordinate transformation method to simulate the effect of the background atmosphere. The model features the capability of resolving dynamics of atmospheric temperature, humidity, and wind at both street canyon scale (1 m) and city scale (50 km) with relatively low computational costs. Based on this model, we conducted sensitivity analysis to investigate the impact of urban parameters (e.g., city scale, building height and density, anthropogenic activities) and lake parameters (e.g., lake scale and lake surface temperature) on the spatial variation of temperature, humidity, wind, and thermal comfort index. Our results can provide significant references for urban planning and city design for sake of UHI mitigation.

How to cite: Zhong, Q., Song, J., Wang, X., and Li, Y.: Modelling of urban lake breeze circulation: the implications on urban heat island mitigation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4289, https://doi.org/10.5194/egusphere-egu24-4289, 2024.

The urban canopy layer (UCL) is characterized by a heterogeneous flow pattern that responds to heterogeneous urban geometries. The varying heights and layouts of buildings play a pivotal role in shaping this spatial variability, as they block, divert, and slow wind and determine the exchange of momentum and energy above the urban canopy. When representing these complex dynamics, however, research has conventionally relied on microscale simulations conducted over limited (often idealized) building arrays. Extending the findings to realistic urban neighborhoods and urban parameterizations presents a clear limitation, as evidenced by discrepancies in multi-model comparisons with observational data in cities.

More extensive datasets of urban airflow are needed to cover a range of realistic urban neighborhoods and provide a more holistic analysis of turbulent flow in different urban characteristics. Responding to this gap in the field, we developed a historically extensive and comprehensive dataset of Urban Turbulent Airflow based on state-of-the-art  Large Eddy Simulations (UrbanTALES). The dataset includes 400 urban layouts with both idealized and realistic configurations. Realistic urban neighborhoods were obtained from major cities worldwide, incorporating variations in plan area densities [0.0625-0.64] and height distributions [4-70m]. Idealized urban arrays, on the other hand, include two commonly studied configurations (aligned and staggered arrays), featuring both uniform and variable height scenarios along with oblique wind directions. 

UrbanTALES offers canopy-averaged data as well as 2D and 3D flow fields tailored for different applications in urban climate research. The dataset provides time-averaged wind flow properties, as well as second- and third-order flow moments that are critical for understanding turbulent processes in the UCL. Here, we describe the UrbanTALES dataset and its application, noting the unique opportunity to deploy a comprehensive representation of realistic urban neighborhoods for a) revisiting neighborhood-scale urban canopy parameterizations in various models and b) informing in-canopy flow and turbulent analyses. Furthermore, we discuss the application of this dataset for training Machine Learning algorithms for pedestrian wind speed. 

How to cite: Nazarian, N., Lu, J., Hart, M., and Krayenhoff, E. S.: UrbanTALES: A comprehensive dataset of Urban Turbulent Airflow using systematic Large Eddy Simulations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7011, https://doi.org/10.5194/egusphere-egu24-7011, 2024.

In order to better understand how urban areas modify the regional atmospheric boundary layer (ABL) and to improve and evaluate weather and climate models for urban applications and services, detailed ABL observations are needed. With new instrument technologies and advanced automatic algorithms for detection of aerosols, mixed-layer height (MLH) and boundary-layer clouds, ground-based remote sensing instruments are increasingly used in urban observational networks.

During a one-year measurement campaign in Berlin, Germany (urbisphere-Berlin, Autumn 2021 – Autumn 2022), a variety of ground-based ABL observations were carried out in the greater Berlin region. Berlin as an isolated continental city with approximately 3.8 million inhabitants provides a fairly homogeneous rural background. The urbisphere network included five inner-city, six outer-city and 14 rural sites equipped with continuously-operated Automatic Lidar and Ceilometers (ALC). The measurement network was designed and set up in a systematic and rigorous manner in order to capture intra-urban, urban-rural, and upwind-city-downwind effects of MLH, cloud-base height (CBH), and cloud cover fraction (CCF) along several transects as air masses move over the city. Based on the ALC observations, MLH, CBH and CCF were automatically derived. ALC observations are complemented by measurements of wind and temperature profiles over the city using Doppler-Wind Lidars and radiosondes concurrently released in urban and rural locations during selected days. Surface heat fluxes are continuously measured with six eddy-covariance flux towers and seven path-averaging scintillometers in urban and rural settings.

This contribution highlights the scientific considerations of the systematic measurement network design and the corresponding data analysis. We are proposing a scheme of attributing measurements to rings around the city centre representing the inner city (radius of 6 km), the outer city (radius of 18 km) and rural areas (radius of 90 km), further separated into upwind, downwind and other sectors. A detailed statistical analysis of the year-long dataset finds differences in MLH, CBH and CCF during different seasons and under different weather forcings. Selected case-study days are analysed in more detail to understand the processes controlling the interactions between surface fluxes and mixed-layer dynamics. These days are further used to evaluate the forecasting skill of hectometric dynamical-modelling runs with regard to ABL dynamics, quantifying also the sensitivity of ABL dynamics in the model to surface representation (e.g. soil moisture, heat flux partitioning).

How to cite: Fenner, D., Christen, A., Glazer, R., Grimmond, S., Kotthaus, S., Looschelders, D., Meier, F., Morrison, W., and Zeeman, M.: A systematic investigation of urban modifications of mixed layer height and cloud cover in Berlin, Germany, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16891, https://doi.org/10.5194/egusphere-egu24-16891, 2024.

Atmospheric boundary layer dynamics form in response to synoptic flow and surface-atmosphere exchanges. Over cities, the complex roughness and additional heat from storage and anthropogenic emissions clearly affect atmospheric stability, with implications for heat risk and pollution dispersion. This work examines how the specific dynamics of the Paris region urban atmosphere interact with the synoptic flow using observations from a dense measurement network.

The interdisciplinary PANAME initiative is a framework coordinating the synergy of numerous projects that are studying the Paris atmosphere using both numerical modelling at various scales and novel observations. The measurement network not only includes dense surface station measurements and turbulent flux towers, but also ground-based atmospheric profile remote sensing and additional radiosonde measurements within the city. This work exploits observations from automatic lidars and ceilometers (ALC), Doppler wind lidars (DWL), and microwave radiometers (MWR) that are operated along a suburban-urban transect to collect simultaneous profiles of air temperature, wind, turbulence, and aerosol characteristics at high vertical and temporal resolution. The continuous observations from a network of compact ground-based remote sensing instruments are shown to be extremely valuable for an improved understanding of the complex processes that govern the urban atmosphere as they are highly variable in space and time.

The complex dynamics of the urban atmospheric boundary layer are explored through advanced measurement products, such as low-level jet characteristics and mixed layer heights. We evaluate how different indicators of atmospheric stability from synergy of multiple remote sensing profile data can portray the spatial and temporal variations in urban boundary layer dynamics. The work highlights the importance of atmospheric boundary layer dynamics as a crucial driver for near-surface conditions.

How to cite: Kotthaus, S., Haeffelin, M., Céspedes, J., Ribaud, J.-F., Dupont, J.-C., Drouin, M.-A., Martinet, P., and Lemonsu, A.: Linking synoptic flow and city dynamics: PANAME observations of the Paris urban boundary layer  , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18040, https://doi.org/10.5194/egusphere-egu24-18040, 2024.