Urban Canopy Layer Research Articles

Influence of Urban Morphologies on the Effective Mean Age of Air at Pedestrian Level and Mass Transport Within Urban Canopy Layer

This study adapted the mean age of air, a time scale widely utilized in evaluating indoor ventilation, to assess the impact of building layouts on urban ventilation capacity. To distinguish it from its applications in enclosed indoor environments, the adapted index was termed the effective mean age of air (τ¯E). Based on an experimentally validated method, computational fluid dynamic (CFD) simulations were performed for parametric studies on four generic parameters that describe urban morphologies, including building height, building density, and variations in the heights or frontal areas of adjacent buildings. At the breathing level (z = 1.7 m), the results indicated three distinct distribution patterns of insufficiently ventilated areas: within recirculation zones behind buildings, in the downstream sections of the main road, or within recirculation zones near lateral facades. The spatial heterogeneity of ventilation capacity was emphasized through the statistical distributions of τ¯E. In most cases, convective transport dominates the purging process for the whole canopy zone, while turbulent transport prevails for the pedestrian zone. Additionally, comparisons with a reference case simulating an open area highlighted the dual effects of buildings on urban ventilation, notably through the enhanced dilution promoted by the helical flows between buildings. This study also serves as a preliminary CFD practice utilizing τ¯E with the homogenous emission method, and demonstrates its capability for assessing urban ventilation potential in urban planning.

The Paris low-level jet during PANAME 2022 and its impact on the summertime urban heat island

Abstract. The low-level jet (LLJ) and the urban heat island (UHI) are common nocturnal phenomena. While the UHI has been studied extensively, interactions of the LLJ and the urban atmosphere in general (and the UHI in particular) have received less attention. In the framework of the PANAME (PAris region urbaN Atmospheric observations and models for Multidisciplinary rEsearch) initiative in the Paris region, continuous profiles of horizontal wind speed and vertical velocity were recorded with two Doppler wind lidars (DWLs) – for the first time allowing for a detailed investigation of the summertime LLJ characteristics in the region. Jets are detected for 70 % of the examined nights, often simultaneously at an urban and a suburban site, highlighting the LLJ regional spatial extent. Emerging at around sunset, the mean LLJ duration is ∼ 10 h, the mean wind speed is 9 m s−1, and the average core height is 400 m above the city. The temporal evolution of many events shows signatures that indicate that the inertial oscillation mechanism plays a role in the jet development: a clockwise veering of the wind direction and a rapid acceleration followed by a slower deceleration. The horizontal wind shear below the LLJ core induces variance in the vertical velocity (σw2) above the urban canopy layer. It is shown that σw2 is a powerful predictor for regional contrast in air temperature, as the UHI intensity decreases exponentially with increasing σw2 and strong UHI values only occur when σw2 is very weak. This study demonstrates how DWL observations in cities provide valuable insights into near-surface processes relevant to human and environmental health.

Similarity of the low-occurrence wind profiles within urban turbulent boundary layers of uniform and non-uniform height block arrays

Within urban boundary layers, the safety of pedestrians is markedly affected by wind speed, particularly in urban areas. The characteristics of turbulence above the canopy layers can lead to unpredictable changes in wind speed at the pedestrian level. Therefore, this study analyzes low-occurrence wind speed phenomena above canopy heights for uniform and nonuniform block configurations using wind tunnel experiments (WTE) to understand the background turbulence characteristics which the wind profile is generated by the boundary layer above the canopy. The urban canopy arrays were reproduced using solid blocks arranged in 30 rows in a streamwise direction with a packing density of 25 % at three different heights. An x-type hot-wire anemometer was used to measure the streamwise and vertical velocity components. The low-occurrence values were classified based on wind speed percentiles of 0.1 %, 1.0 %, 99.0 %, and 99.9 % wind speeds. The results demonstrated that above the canopy, there were minor influences of block height variations on the low occurrence factor. The peak factor demonstrated a comparable value between the uniform and nonuniform cases, regardless of the block arrangement. Statistical models based on the Weibull distribution and Gram–Charlier series demonstrating good agreement with the WTE data in predicting the low occurrence and peak factors. This study found that variations in block height have a minor influence on the low occurrence and peak factors within the turbulent boundary layers, implying that we can separate the effect of background turbulence from the local turbulent generation within the urban canopy layers.

Development of a morphology-based wind speed model in the urban roughness sub-layer

Understanding near-surface wind variations in urban environments is essential for a wide range of applications, including urban weather forecasting and wind impact assessment. In this study, we introduced a novel model for mean wind speed profiles within the urban roughness sublayer based on theoretical analysis. To calibrate the model's coefficients, we conducted large-eddy simulations of airflow over a variety of idealized urban configurations. The resulting expression effectively captures wind speed variations across different urban morphologies. Subsequently, a regression modeling was employed to identify the relationships between building morphological parameters and these coefficients. This highlights the pivotal role of using building morphology to predict near-surface velocity profiles. The proposed model also yields significantly more accurate wind speed predictions within the urban canopy layer than traditional exponential profiles. The findings in the present study lay a more robust foundation for assessing urban wind conditions and improving urban-scale weather forecasts.

Geometric Factor Correction Algorithm Based on Temperature and Humidity Profile Lidar

Due to the influence of geometric factors, the temperature and humidity profile of lidar’s near-field signal was warped when sensing the air environment. In order to perform geometric factor correction on near-field signals, this article proposes different correction solutions for the Mie and Raman scattering channels. Here, the Mie scattering channel used the Raman method to invert the aerosol backscatter coefficient and correct the extinction coefficient in the transition zone. The geometric factor was the ratio of the measured signal to the forward-computed vibration Raman scattering signal. The aerosol optical characteristics were reversed using the corrected echo signal, and the US standard atmospheric model was added to the missing signal in the blind zone, reflecting the aerosol evolution process. The stability and dependability of the proposed algorithm were validated by the consistency between the visibility provided by the Environmental Protection Agency and the visibility acquired via lidar retrieval data. The near-field humidity data were supplemented by the interpolation method in the Raman scattering channel to reflect the water vapor transfer process in the temporal dimension. The measured transmittance curve of the filter, the theoretical normalized spectrum, and the sounding data were used to compute the delay geometric factor. The temperature was retrieved and the near-field signal distortion issue was resolved by applying the corrected quotient of the temperature channel. The proposed algorithm exhibited robustness and universality, enhancing the system’s detection accuracy compared to the temperature and humidity data constantly recorded by the probes in the meteorological gradient tower, which have a high correlation with the lidar observation data. The comparison between lidar data and instrument monitoring data showed that the proposed algorithm could effectively correct distorted echo signals in the transition zone, which was of great value for promoting the application of lidar in the meteorological monitoring of the urban canopy layer.

Role of Surface Energy Fluxes in Urban Overheating Under Buoyancy‐Driven Atmospheric Conditions

AbstractUrbanization alters land surface properties in absorbing, reflecting and emitting radiation as well as infiltrating, evaporating and storing water. This consequently modifies surface energy and water fluxes and, thus, urban climates. Weak synoptic flow, clear sky conditions and higher surface temperatures in cities compared to their rural surroundings create a buoyancy‐driven atmospheric circulation, in which surface energy fluxes become the main determinants of urban daytime overheating. Here, we demonstrate the role of surface energy fluxes for warming and cooling processes in the urban canopy layer under buoyancy‐driven atmospheric conditions. We improve and apply an integrated CFD‐GIS modeling approach to provide a detailed analysis of fine‐scale land‐atmosphere interactions and assess the surfaces' profound implications on energy and water exchange. We show that variations in the ratios of the surface energy fluxes to the net radiation can be separated from meteorological conditions (wind speed, air temperature and incoming solar radiation) and emissivity values, varying explicitly with changes in land surface type and water availability for vegetated areas. Based on the energy flux ratios, we introduce an approach to assess the surface‐induced warming and cooling effect and its contribution to urban overheating in the urban canopy layer, under buoyancy‐driven atmospheric conditions, directly applicable to strategic urban planning for climate change adaptation. Independent of meteorological conditions, this approach can be used to evaluate different surface materials (both natural and artificial) and climate adaptation measures, such as urban nature‐based solutions and blue‐green infrastructures, and to monitor changes in the energy and water balance.

Spatio-Temporal Analysis of Surface Urban Heat Island and Canopy Layer Heat Island in Beijing

Studying urban heat islands holds significance for the sustainable development of cities. This comprehensive study analyzed the temporal characteristics of a Surface Urban Heat Island and Canopy Layer Heat Island by employing Moderate-Resolution Imaging Spectroradiometer image data spanning from 2003 to 2020 over Beijing, China. Leveraging the Gaussian capacity model, the geometrical characteristics of the Surface Urban Heat Island and Canopy Layer Heat Island, such as intensity, center, direction, and range, were examined among three different timescales of day, month, and year. Results indicate that the intensities of the Surface Urban Heat Island and Canopy Layer Heat Island tend to have bigger seasonal variations during winter nights and summer daytime. In addition, at night the centers of Surface Urban Heat Island and Canopy Layer Heat Island are mainly concentrated in the range of 116.3°~116.4° E in longitude and 39.90°~39.95° N in latitude, while during the daytime they are more scattered, mainly in the range of 116.2°~116.5° E in longitude and 39.7°~40.0° N in latitude. In the hot season, the center of the heat island moves east to north, while in the cold season it moves west to south. Monthly average ellipse areas of Surface Urban Heat Island and Canopy Layer Heat Island vary more during the day than that at night, the maximum daytime differences were 2662 km2 and 2293 km2, while the maximum nighttime differences were 484 km2 and 265 km2. Overall, the average area is increasing, with the heat island center moving eastward and deflecting towards the northeast-southwest direction. The expansion of urban areas will continue to influence the movement and extent of heat islands. The study offers insights to inform strategies for mitigating urban heat islands.

Land use as key element for urban heat island inception in small cities. Case study: Barlad City, Romania

The study investigates the thermal characteristics of the urban area of Bârlad city, Romania, in relation with urban land use (LU) and land cover (LC). This was done using Land Surface Temperature (LST) data provided by Moderate Resolution Imaging Spectroradiometer (MODIS) and mobile measurements conducted during stable atmospheric conditions. To identify important factors that influence Surface Urban Heat Island (SUHI) and Canopy Layer Urban Heat Island (CLUHI) two types of LU/LC classifications were used. At the annual level, during the day, the highest LST values are observed in the most densely built-up areas of the city, with values exceeding 18°C. The lowest LST instead are observed in the northern area of the city, with prevailing natural areas, where LST values reach 17.4°C. During the night, the urban area maintains constantly high LST above 4°C, while the lowest LST values are observed in the forested area in the eastern part of the city, where mean LST drops close to 3°C. The results of this study highlight the significant impact that LU/LC has on the inception of SUHI in small cities. Generally, built-up surfaces from Urban Atlas (UA) and Local Climate Zones (LCZ) classifications, are warmer than natural land cover types, both during day and night, with at least 0.5 to 1.5°C, which is enough to outline a clearly delimited SUHI especially during summer days. During stable atmospheric conditions the CLUHI exhibits large intensities emphasized by local cooling processes manifested especially during thermal inversion conditions.

Turbulent flow modification in the atmospheric surface layer over a dense city

Winds in the atmospheric surface layer (ASL) over distinctive urban morphology are investigated by building-resolved large-eddy simulation (LES). The exponential law is applied to urban canopy layers (UCLs) unprecedentedly to parameterize vertical profiles of mean-wind-speed u¯z and examine the influence of morphological factors. The skewness of streamwise velocity Su is peaked at the zero-plane displacement d (drag center) where flows decelerate mostly. The dynamics and intermittency in roughness sublayers (RSLs) are further contrasted. It helps determine the critical strength of the organized structures (ejection, Q2 and sweep Q4) in their contributions to the average momentum transport (i.e., 3<u”w”> to 5<u”w”>). Two key factors of the local-scale dynamics are revealed - building heterogeneity and upstream giant wakes that could amplify turbulence kinetic energy (TKE) and energetic intermittent Q4 by different mechanisms. The former is conductive for large-eddy generation that promotes vertical fluctuating velocity w”, stimulating intermittent, energetic Q2 and Q4. The latter, whose footprints are identified by the two-point correlation of streamwise velocity Ruu with specific size and inclination, facilitates intermittent, fast streamwise fluctuating velocity u”, forming vigorous Q4. Nevertheless, excessive planar density λp (≈ 0.7) is detrimental to both transport processes. These findings contribute to the theoretical and empirical wall models of large-scale roughness that help urban planners and policymakers to improve air quality.

Winds and eddy dynamics in the urban canopy layer over a city: A parameterization based on the mixing-layer analogy

Urban atmospheric flows are vital to the global ecology. This study characterizes urban canopy layer (UCL) dynamics and parameterizes the flows in the atmospheric surface layer (ASL) over heterogeneous urban surfaces. Large-eddy simulations (LESs) are used to transiently calculate the winds over a real, dense city. A linear function of eddy diffusivity of momentum KM is applied to the lower UCL. Analogous to its mixing-layer counterpart, the strong UCL top shear manifests an inflected mean wind speed profile which aligns well with the exponential law. The solutions to the mixing length lm and the turbulent momentum flux are analytically derived by consolidating the mixing-layer type shear and the form drag from the explicitly resolved roughness elements. The behavior of lm in the lower UCL, especially its peaked level, is captured well. Based on the balance between shear and form drag, an aerodynamic effective roof level Hae is designated where the ground effect is alleviated under shear dominance. Results reveal that a rougher urban surface generates eddies with a larger shear length scale, thus enhancing momentum transport. In-canopy turbulence mixing, which slows down wind decay, is also enhanced, resulting in stronger street-level breezes. The newly developed ASL flow model will be beneficial to urban planning by offering reliable predictions, effectuating the management of urban sustainability.

Examining temporally varying nonlinear effects of urban form on urban heat island using explainable machine learning: A case of Seoul

Many empirical studies have examined the relationship between urban form and canopy layer urban heat island (CUHI). However, these studies mostly have used small sample sizes and statistical methods that assume CUHI responds linearly to urban form features. Additionally, the differences in the effect of urban form on CUHI at different time instances during the diurnal cycle have not received sufficient attention. To address these issues, this study employs a relatively large sample of 1,058 microclimate sensors in Seoul, Korea, and analyzes CUHI at 9 a.m., 3 p.m., and 9 p.m. on a typical summer day. Gradient boosting decision tree (GBDT) based regression model and linear regression model and their variants with spatial information were constructed and compared for each time instance. Results show that spatially explicit GBDT models had the highest accuracy. Feature importance analysis shows that built form factors such as mean building height were more important during the afternoon, while surface fractions such as road surface fraction had greater influences during the morning and nighttime. Partial dependence plots (PDPs) show that urban form features influence CUHI in a complex nonlinear manner that varies for each time instance. PDPs were further scrutinized based on their activation and threshold effects. GBDT model findings directionally aligned with linear regression, but they provided nuanced insights into the form-CUHI relationship. These findings help planners to better understand the complexity of urban climate and intervention required to reduce CUHI magnitude across the diurnal cycle.

Deciphering the sensitivity of urban canopy air temperature to anthropogenic heat flux with a forcing-feedback framework

The sensitivity of urban canopy air temperature () to anthropogenic heat flux () is known to vary with space and time, but the key factors controlling such spatiotemporal variabilities remain elusive. To quantify the contributions of different physical processes to the magnitude and variability of (where represents a change), we develop a forcing-feedback framework based on the energy budget of air within the urban canopy layer and apply it to diagnosing simulated by the Community Land Model Urban over the contiguous United States (CONUS). In summer, the median is around 0.01 over the CONUS. Besides the direct effect of on , there are important feedbacks through changes in the surface temperature, the atmosphere–canopy air heat conductance (), and the surface–canopy air heat conductance. The positive and negative feedbacks nearly cancel each other out and is mostly controlled by the direct effect in summer. In winter, becomes stronger, with the median value increased by about 20% due to weakened negative feedback associated with . The spatial and temporal (both seasonal and diurnal) variability of as well as the nonlinear response of to are strongly related to the variability of , highlighting the importance of correctly parameterizing convective heat transfer in urban canopy models.

Parameterizing building effects on airflows within the urban canopy layer for high‐resolution models using a nudging approach

AbstractIn this study, a new multilayer urban canopy parameterization for high‐resolution (∼1 km) atmospheric models using the nudging approach to represent the impacts of urban canopies on airflow is presented. In our parameterization, a nudging term is added to the momentum equations and a source term to the turbulent kinetic energy equation to account for building effects. The challenge of this parameterization lies in defining appropriate values for the nudging coefficient and the weighting function used to reflect canopy effects. Values of both are derived and the parameterization developed is implemented and tested for idealized cases in the Mesoscale Transport and Stream model (METRAS). Comparison data are taken from obstacle‐resolving microscale model results. Results show that the parameterization using the nudging approach can simulate aerodynamic effects induced within the canopy by obstacles well, in terms of reduction of wind speeds and production of additional turbulent kinetic energy. Thus, models with existing nudging can use this approach as an efficient and effective method to parameterize dynamic urban canopy effects.

Impacts of urban morphology on sensible heat flux and net radiation exchange

Urban morphology affects the sensible heat flux and net radiation exchange which can alter urban heat mitigation plans. This study first parameterized the geometric effects on the net radiation, and then calculated the net radiation and sensible heat flux in the urban landscape of Hong Kong. Considering that the sensible heat flux is the main heat sink in compact urban areas, this study proposes a Normalized Urban Sensible Heat Mitigation Index (NUSHMI) based on the ratio of the net radiation and sensible heat flux. Overall, there is major difference in the dependence of net radiation and sensible heat flux on geometric parameters. Net radiation Rn, reaches an optimal value, either maximum or minimum depending on the parameters of SVF and a standard deviation of building height σh, at intermediate parameter values, which suggests a guideline relevant to urban design targeting the mitigation of urban climate. Contrariwise, sensible heat flux decreases or increases, again depending on SVF and σh, is being considered, with increasing values of the same parameters. For example, Rn, reaches a minimum value for a Sky View Factor (SVF) between 0.5 and 0.6, while it reaches a maximum value for a standard deviation of building height σh between 20 and 30 m. These two results suggest that radiative forcing, i.e. Rn, can be minimized by urban space with SVF around 0.55 and σh around 25 m. The relationships between sensible heat flux and SVF or σh do not show multiple minima or maxima (as with Rn), with the exception of building density, which could also be applied as a guideline in urban design. The results based on the proposed NUSHMI indicated the NUSHMI reaches the highest values when building density is about 0.7 and building height is about 80 m and when the building height standard deviation within an area is about 10 m to 20 m. These findings revealed how the urban morphology affects the surface heat flux exchange between urban canopy and atmosphere boundary layer, and can help to design an efficient urban landscape towards urban heat mitigation for highly compacted cities, e.g. controlling the building density, height, and the height deviation. This combination of urban geometric parameters identifies an urban configuration maximizing the dissipation of absorbed radiant energy as sensible heat. It should be noted, however, that heat load upon buildings would be reduced at the price of maximizing heat dissipation within the built-up space.

Computational wind engineering: 30 years of research progress in building structures and environment

The paper reviews the evolution of computational wind engineering from environmental and structural perspectives, since the inaugural conference of computational wind engineering held in Tokyo 30 years ago (CWE 92). The progress in computational methodologies and important aspects for accurate analysis are discussed. As a groundwork for the application of computational fluid dynamics (CFD) to various environmental issues, the importance of accurate modeling of atmospheric boundary layer, urban boundary layer, and urban canopy layer is pronounced. Environmental applications refer to urban micro-climate, pedestrian level wind, near-field pollutant dispersion, natural and urban ventilation, urban wind energy and snow/sand erosion and accumulation. Structural applications refer to wind loading on low- and high-rise buildings, including wind directionality. The most seminal contributions are examined, and their results are presented. It becomes clear that the engineering community has gained more benefits from environmental than structural computational wind engineering applications, mainly due to the usually less demanding computational needs for the former. Future challenges in CFD applications are thoroughly discussed and the need to bridge the gap between environmental and structural applications is highlighted.

Numerical Investigation of Flow and Dispersion over Two-Dimensional Semi-Open Street Canyon

A semi-open street canyon is able to protect pedestrians from unpleasant situations such as direct sunlight and rain. However, the protruding elements of the two opposite building facades that form the semi-open configuration can affect the air quality of the urban canopy layer (UCL). Therefore, this paper investigated the influence of the eave structures on the flow and pollutant dispersion over an idealized 2D street canyon with a unity aspect ratio. The length of the eaves was varied into 0.25H and 0.5H (H is the building height) and placed either on the leeward wall, the windward wall, or on both building facades located at the same elevation as the street canyon. Numerical simulations were performed using the steady-state Reynolds-averaged Navier-Stokes (RANS) equations in conjunction with Re-Normalization Group (RNG) k-ε as the turbulence closure model. The pollutant was released from a line source in the center of the bottom of the target canyon with uniform flow rate. Six different eave configurations were simulated in the wind direction perpendicular to the canyon axis, representing the worst condition of canyon ventilation. The evolution of the primary vortex, which occupied the entire canyon with the characteristic of skimming flow, showed less dependence on the length and position of the eave, except for the longest eave on the windward wall. However, the position of the vortex center depicted opposite results. The pollutant concentration is always higher near the leeward wall, but for the eave that protrudes from the windward wall with a length of 0.5H, the pollutant accumulates near the windward region. The ratio of pollutant concentration showed higher concentration in the semi-open configurations compared to the fully open layout as a result of limited penetration of shear flow into the canyon, which leads to deterioration of pollutant removal.

Observations of the urban boundary layer in a cold climate city

Cold environment supports a large diversity of local climates. Among them, urban climates in northern cities stand out for their pronounced warm temperature anomaly known as the Urban Heat Island (UHI). UHI in northern cities has been already studies through satellite images and in-situ observations in the urban canopy layer (UCL). Yet, the vertical structure of the urban atmospheric boundary layer (UBL) has not been studied there. This work presents new observations of UBL in Nadym – a sub-Arctic Siberian city. During several intensive observing periods we run simultaneous registration of urban and rural meteorological parameters with unmanned drones, a microwave temperature profiler and a dense network of ground-based sensors. The data analysis reveals details of UHI development in the UCL and UBL, and links together horizontal urban-rural canopy-layer temperature differences, boundary layer stability, and UHI vertical extent. We show that during strong temperature inversions, UBL is less stratified than its rural counterpart, but it still remains very thin and limited in height by a few tens of meters. The observations disclose that the ground-based (50 m – 100 m above ground) temperature inversion is one of the strongest control factors for UHI in cold climate conditions in winter.

Integrating CFD-GIS modelling to refine urban heat and thermal comfort assessment

Constant urban growth exacerbates the demand for residential, commercial and traffic areas, leading to progressive surface sealing and urban densification. With climate change altering precipitation and temperature patterns worldwide, cities are exposed to multiple risks, demanding holistic and anticipatory urban planning strategies and adaptive measures that are multi-beneficial. Sustainable urban planning requires comprehensive tools that account for different aspects and boundary conditions and are capable of mapping and assessing crucial processes of land-atmosphere interactions and the impacts of adaptation measures on the urban climate system. Here, we combine Computational Fluid Dynamics (CFD) and Geographic Information System (GIS) capabilities to refine an existing 2D urban micro- and bioclimatic modelling approach. In particular, we account for the vertical and horizontal variability in wind speed and air temperature patterns in the urban canopy layer. Our results highlight the importance of variability of these patterns in analysing urban heat development, intensity and thermal comfort at multiple heights from the ground surface. Neglecting vertical and horizontal variability, non-integrated CFD modelling underestimates mean land surface temperature by 7.8 °C and the Universal Thermal Climate Index by 6.9 °C compared to CFD-integrated modelling. Due to the strong implications of wind and air temperature patterns on the relationship between surface temperature and human thermal comfort, we urge caution when relying on studies solely based on surface temperatures for urban heat assessment and hot spot analysis as this could lead to misinterpretations of hot and cool spots in cities and, thus, mask the anticipated effects of adaptation measures. The integrated CFD-GIS modelling approach, which we demonstrate, improves urban climate studies and supports more comprehensive assessments of urban heat and human thermal comfort to sustainably develop resilient cities.

Static downscaling of mesoscale wind conditions into an urban canopy layer by a CFD microscale model

Wind flows inside complex urban environments are determined by the mutual effect of large-scale (i.e., wind above the urban boundary layer, UBL) and local-scale forcing (i.e. constraints into the urban canopy layer, UCL). Usually, when the wind field above the UBL is known (e.g. by on-site measurements, OsM), high-resolution microscale models (e.g. CFD) are used to predict the wind inside the UCL. However, standard procedures to map the wind field from an undisturbed position to the UCL are not available yet and several technical aspects need to be investigated. A downscaling method, so-called “static downscaling”, of the wind from mesoscale to microscale is innovatively adopted here to evaluate the performance of two CFD microscale models when predicting the flow in a UCL. The methodology is based on OsM transferred into the UCL by means of so-called “transfer coefficients” calculated by 3D steady RANS simulations for two different spatial extents of the explicitly modeled urban texture (Case A and B). It is discussed in detail the way the transfer coefficients work, how they can be used to understand the correlations between wind above and within the UCL as well as to identify the major limitations of the RANS approach. Results are discussed in qualitative terms and quantified using standard metrics. It is also shown that too high flow rates can occur at the entrance of the waterway of the area of interest and in the outer part of the explicitly modeled urban area, which can be mitigated by “buffer zones”.

Impact of urban greening on microclimate and air quality in the urban canopy layer: Identification of knowledge gaps and challenges

Growing urbanization leads to microclimate perturbations and in particular to higher temperatures inside the city as compared to its rural surroundings, a phenomenon known as the urban heat island. Although it exists at several scales, this study focused only on the urban canopy layer, where inhabitants live. A bibliometric study was performed to describe and understand the relationships between strategies of urban greening and canopy layer urban heat island modification in terms of air quality and microclimate. Science mapping of 506 bibliographical resources was performed through co-word and co-citation analysis. A subset of forty-four articles related to microclimate and air quality modelling was extracted and synthesized. This analysis showed scientific papers were polarized into microclimate or air quality studies without strong links between both, implying small collaboration between these fields. There is need for studies coupling microclimate and air pollution modelling to assess vegetation’s impacts at city scale.Systematic Review Registration: [website], identifier [registration number].