Urban Microclimate Model Research Articles

A WRF-UCM-SOLWEIG framework for mapping thermal comfort and quantifying urban climate drivers: Advancing spatial and temporal resolutions at city scale

The capability to evaluate city-scale outdoor thermal comfort is crucial for understanding the public's experience of heat stress, especially during heat waves. Nevertheless, there remains a methodological gap in accurately mapping thermal comfort with high spatial and temporal resolutions in complex urban environment and in quantifying the contributions of various urban climate drivers, such as sky view factors and tree canopy fraction. To bridge this gap, we propose an innovative WRF-UCM-SOLWEIG framework that couples a mesoscale model with an urban microclimate model. Specifically, it integrates the Weather Research and Forecasting model coupled with the urban canopy model (WRF-UCM) and the Solar and Longwave Environmental Irradiance Geometry model (SOLWEIG). This framework is then applied to a densely populated tropical city in China, to quantify the impact of urban climate drivers on the Universal Thermal Climate Index (UTCI). The results reveal a significant diurnal variation in the impact of urban morphology. Notably, the most significant drivers affecting daytime UTCI are the impervious surface fraction and the tree canopy fraction, with maximum Pearson's correlation coefficients of 0.80 and -0.70, respectively. These findings suggest that urban heat mitigation strategies should prioritize on reducing impervious surfaces and enhancing shade management, alongside expanding urban forestry measures.

Modeling the impacts of building energy efficiency on the thermal microclimate in a midsize German city

Increasing building energy efficiency (BEE) is an established mitigation measure against climate change. However, it is currently less understood, if the implementation of increased BEE could have an impact on urban microclimate and therefore potentially interferes with adaptation efforts to climate change. In this modeling study, we tested the hypothesis that decreased thermal diffusivities of building facades due to building retrofitting will increase sensible heat fluxes at the facade surfaces and can therefore lead to an increase in urban heat exposure. Our modeling results conducted with the urban microclimate model ENVI-met for two different neighborhoods in the mid-sized city of Braunschweig, Germany indicate that retrofitting complete neighborhoods might lead to an increase in urban temperatures by up to 2.5 K locally and 1 K when spatially averaged at daytime. However, at night-time a cooling effect of almost the same magnitude is noted that could reduce the maximum of the urban heat island. Overall, it can be expected that a necessary increase in BEE standards due to their role in climate change mitigation lead to an increase in daytime outdoor heat exposure. Methods to alleviate this trade-off between climate change mitigation and adaptation should be studied in the future.

Detailed urban roughness parametrization for anthropogenic heat flux estimation using earth observation data

Anthropogenic Heat (AH) emissions modify the energy balance in urban areas and is crucial for urban microclimate modelling and improved weather forecast modelling. Therefore, the present study conducted on Delhi and its surroundings firstly aims to estimate AH using Earth Observation (EO) data of Landsat 8 then, evaluate the impact of detailed urban roughness parameterization on the estimation of AH and further validate the obtained flux values with ground based observations of Large Aperture Scintillometer (LAS) setup. The study has been conducted over three time periods for October 2017, March 2018 and June 2018 by processing six Landsat tiles. Three methods have been employed on EO data for AH computation i.e. single urban roughness value for entire study area (Method 1), LULC based roughness values adopted from the literature (Method 2) and lastly, detailed pixel-by-pixel varying roughness values calculated from fine scale urban parameterization (Method 3) for each time period, The average AH values are higher for the month of June 2018: 359.91, 368.57 and 359.16 W/m2 as compared to month of March 2018 (322.44, 330.84 and 298.35 W/m2) and October 2017 (318.00, 331.04 and 306.71 W/m2) for method 1, method 2 and method 3 respectively. Net radiation and Sensible Heat Flux shows a good correspondence with in-situ measurements for most of the tiles and method 3 shows better spatial distribution of fluxes as compared to other two methods. However, due to difference in approach for estimation of fluxes (LAS setup computes latent heat flux as residual while EO based approach computes AH as residual), conclusive results could not be drawn with respect to aptness of a single method. Accurate estimation of AH in urban areas can assist further in formulating policies, regulations and action plans related to mitigation and control of heat stress, climate change and improved weather forecasting.

Autonomous urban topology generation for urban flow modelling

• New framework is developed that integrates the autonomous urban topology generation for urban flow modeling. • A new deep learning model is developed for building footprints extraction from satellite imagery • Deep learning model was meshed with Light Detection and Ranging (LiDAR) data by meshing to generate 3D building models • An illustration pedestrian level wind simulation was carried out for city of London On. One of the challenges in realistic numerical urban micro-climate modelling for wind, heat transfer, and building energy simulation applications is the complexity of urban topology and complex building geometries. This paper presents a deep learning modelling for building footprint polygon extraction from satellite imagery that is integrated with Light Detection and Ranging (LiDAR) data to generate 3D building models. The deep learning model registered a mean squared error of 0.02. The trained deep learning model can then be applied to a new set of input image data to extract building footprint polygons for autonomous application, and it can also be incrementally retrained with good quality data when it becomes available. A framework is developed that integrates the autonomous urban topology generator with urban flow modelling. The modelling steps are explained through an application example of urban flow modelling encountered during a pedestrian level wind assessment for the city of London, Ontario.

A data schema for exchanging information between urban building energy models and urban microclimate models in coupled simulations

Understanding and quantifying the interactions between urban microclimate and urban buildings is essential to improve the urban environment and building performance, especially during heatwaves. Most previous studies used tool or application specific data exchange mechanisms that cannot be generalized for other tools or applications. In this paper, we introduce a new flexible and tool-agnostic data schema to facilitate the exchange of data between urban building energy models and urban microclimate models. The JSON schema was tested using a district of 97 buildings in San Francisco and running simulations with CityBES and CityFFD as the urban building energy and microclimate modeling platforms, respectively. Compared with simulation results using the historical weather data, simulation results considering interactions between two models over a two-day heatwave event showed a 5.3°C higher average peak building facade temperature, an 8.9 °C higher average peak air node temperature, and a 19.5% higher peak cooling energy use.

The potential of facade greening in mitigating the effects of heatwaves in Central European cities

This study investigates the potential of living walls in improving the climatic quality in the urban environment. To achieve this goal, numerical simulations using the urban microclimate modelling tool ENVI-met were conducted to analyse the distribution of air temperature, mean radiant temperature (MRT), relative humidity, and wind speed in a residential block in Stuttgart, Germany. The impact of the living wall on the interior air temperature was investigated as well. Before the simulations were conducted, the model was first validated against measured data collected in Weimar, Germany. The validation showed a fairly good agreement between the measured and simulated air temperature, MRT, relative humidity, and wind speed with an averaged respective mean absolute error (MAE) of 0.72 K, 2.67 K, 3.98%, and 0.30 m/s. The simulation of the residential area in Stuttgart indicated that facade greening has only a little impact on air temperature (−0.57 K). Yet, a reduced MRT by up to 3.04 K and a reduced interior air temperature by up to 3.8 K were observed. The living wall also had an impact on the relative humidity among the buildings, which was increased by a maximum of 5.07%. Only a marginal impact on airflow between buildings was observed when facade greening was implemented. Yet, air velocity directly in front of the foliage was reduced by an average of 0.25 m/s.

Basic Principles, Most Common Computational Tools, and Capabilities for Building Energy and Urban Microclimate Simulations

This paper presents basic principles of built-environment physics’ modelling, and it reviews common computational tools and capabilities in a scope of practical design approaches for retrofitting purposes. Well-established simulation models and methods, with applications found mainly in the international scientific literature, are described by means of strengths and weaknesses as regards related tools’ availability, easiness to use, and reliability towards the determination of the optimal blends of retrofit measures for building energy upgrading and Urban Heat Island (UHI) mitigation. The various characteristics of computational approaches are listed and collated by means of comparison among the principal modelling methods as well as among the respective computational tools that may be used for simulation and decision-making purposes. Insights of coupling between building energy and urban microclimate models are also presented. The main goal was to provide a comprehensive overview of available simulation methods that can be used at the early design stages for planning retrofitting strategies and guiding engineers and technical professionals through the simulation tools’ options oriented to the considered case study.

A comprehensive indoor–outdoor urban climate model with hydrology: The Vertical City Weather Generator (VCWG v2.0.0)

Urban climate models can predict the environmental impacts of urban development by simulating the exchange processes between the atmosphere and urban surfaces. A comprehensive simulation of urban climate requires adequate representation of the exchanges of momentum, heat, and water between the atmosphere and the impervious, vegetated, or soil surfaces. This study presents the inclusion of hydrological processes in a computationally-efficient urban micro-climate model, the Vertical City Weather Generator (VCWG v2.0.0). VCWG v2.0.0 accounts for not only the interaction between indoor and outdoor environments through parameterizations including building energy, surface energy balance, radiation, and vertical diffusion models, but also the biophysical and ecophysiological behavior of urban vegetation via an advanced hydrology model. VCWG v2.0.0 is evaluated against field measurements from Basel, Switzerland, in 2002, and Vancouver, Canada, in 2008. The model outperforms the previous version by reducing the RMSE of potential temperature, wind speed, and specific humidity by 0.5 K, 0.52 m s−1, and 0.001 kg kg−1, respectively. Inclusion of the hydrology model also improves prediction of sensible/latent heat fluxes with RMSE of 18.1/27.7 W m−2 for the Vancouver case. VCWG v2.0.0 is further assessed by explorations related to seasonal variations, modification of ground vegetation, green and cool roofs, and changes in the Local Climate Zone (LCZ), which are all in reasonable agreement with models and observations in previous studies. VCWG v2.0.0 can be used as a design, prediction, or investigation tool to understand how urban climate variables are influenced as a function of forcing environmental conditions and urban configurations.

Detailed investigation of vegetation effects on microclimate by means of computational fluid dynamics (CFD) in a tropical urban environment

In light of globally increasing temperatures, accentuated in cities by the urban heat island effect, urban planners and designers are looking for new, quantitative methods to assess the performance of their designs in terms of ecosystem services provided by vegetation. Among these ecosystem services, improved microclimate conditions are particularly important for human thermal comfort and health. In this study, an urban scene in the tropical city of Singapore is numerically investigated with a fully-integrated, three-dimensional urban microclimate model implemented in OpenFOAM. Mass and heat transport in air and storage effect in the urban environment are coupled so that the daily turbulent transport in air using steady Reynolds-averaged Navier-Stokes (RANS) can be solved iteratively with the unsteady heat and moisture transfer from urban surfaces. Vegetation is modeled as a porous medium for the flow of moist air and a leaf energy balance model is used to determine the heat fluxes and transpiration at leaf surfaces. The analysis shows the influence of an urban park upon air temperatures and thermal comfort. Cooling intensity of 1 °C is observed downwind of the park within a region of 27 m for an incoming wind speed of 2.3 m s−1, which reduces to 0.6 °C at a distance of 117 m from the park. The Universal Thermal Comfort Index (UTCI) shows a reduction in thermal stress in and around the park. The approach presented here can provide specific guidelines for urban planners and frame expectations on magnitude and spatial extent of local microclimate modifications generated by an urban park in a tropical city.

UDALES: large-eddy-simulation software for urban flow, dispersion, and microclimate modelling

uDALES: large-eddy-simulation software for urban flow, dispersion, and microclimate modelling

Coupling detailed urban energy and water budgets with TEB-Hydro model: Towards an assessment tool for nature based solution performances

Nature-based solutions (NBS) are increasingly promoted to mitigate urbanization effects, such as the urban heat island. The release of latent heat requires the availability of water in the urban soils. Models able to represent both detailed water and energy budgets are needed for a reliable evaluation of NBSs performances. The TEB-Hydro model is a recent hydro-microclimate model that extends the physics of the urban microclimate model TEB-Veg to water processes in urban subsoil in order to represent more realistically coupled water and energy budgets. Hence, the aim of this paper is to evaluate the TEB-Hydro model regarding how the water processes affect the energy balance. The model is applied to an urban French catchment for which both hydrological and microclimate data are continuously collected. The model shows general good performances in both simulating latent and sensible heat fluxes. Nevertheless, soil water contents are slightly underestimated during wet periods and overestimated during dry periods. Compared to the previous version of the model (TEB-Veg) with a simplified water budget, TEB-Hydro tends to more overestimate latent heat fluxes than TEB-Veg during dry periods. During wet periods, however, TEB-Hydro simulates better sensible heat fluxes and latent heat fluxes.

Application of weather data morphing for calibration of urban ENVI-met microclimate models. Results and critical issues

The detailed analysis of the urban microclimate calls for the use of Computational Fluid Dynamics (CFD) tools able to model complex heat and mass transfer mechanisms. ENVI-met is one of the most used and reliable tools for microclimate analysis if an accurate calibration is performed. The calibration process is fundamental to have a reliable model in order to perform microclimate analysis, but requires accurate on-site measurements of urban weather data and it is usually difficult to have urban meteorological data of the required granularity because of the lack of on-site stationary weather stations. To overcome such an issue, this paper explores the use of weather data retrieved from suburban meteorological stations and morphed for the urban context under study through the Urban Weather Generator (UWG) tool. UWG generates an urban weather file in the hourly .epw format that is eventually used for forcing the ENVI-met model of the investigated area. This approach is validated against on-site measurements at different times for various points located in a district of the city center of Catania (Italy). The validation process presented a very good correlation for most of the study points in the district during nighttime, while during daytime some discrepancies may occur.

The Vertical City Weather Generator (VCWG v1.3.2)

Abstract. The Vertical City Weather Generator (VCWG) is a computationally efficient urban microclimate model developed to predict temporal and vertical variation of potential temperature, wind speed, specific humidity, and turbulent kinetic energy. It is composed of various sub-models: a rural model, an urban vertical diffusion model, a radiation model, and a building energy model. Forced with weather data from a nearby rural site, the rural model is used to solve for the vertical profiles of potential temperature, specific humidity, and friction velocity at 10 m a.g.l. The rural model also calculates a horizontal pressure gradient. The rural model outputs are applied to a vertical diffusion urban microclimate model that solves vertical transport equations for potential temperature, momentum, specific humidity, and turbulent kinetic energy. The urban vertical diffusion model is also coupled to the radiation and building energy models using two-way interaction. The aerodynamic and thermal effects of urban elements, surface vegetation, and trees are considered. The predictions of the VCWG model are compared to observations of the Basel UrBan Boundary Layer Experiment (BUBBLE) microclimate field campaign for 8 months from December 2001 to July 2002. The model evaluation indicates that the VCWG predicts vertical profiles of meteorological variables in reasonable agreement with the field measurements. The average bias, root mean square error (RMSE), and R2 for potential temperature are 0.25 K, 1.41 K, and 0.82, respectively. The average bias, RMSE, and R2 for wind speed are 0.67 m s−1, 1.06 m s−1, and 0.41, respectively. The average bias, RMSE, and R2 for specific humidity are 0.00057 kg kg−1, 0.0010 kg kg−1, and 0.85, respectively. In addition, the average bias, RMSE, and R2 for the urban heat island (UHI) are 0.36 K, 1.2 K, and 0.35, respectively. Based on the evaluation, the model performance is comparable to the performance of similar models. The performance of the model is further explored to investigate the effects of urban configurations such as plan and frontal area densities, varying levels of vegetation, building energy configuration, radiation configuration, seasonal variations, and different climate zones on the model predictions. The results obtained from the explorations are reasonably consistent with previous studies in the literature, justifying the reliability and computational efficiency of VCWG for operational urban development projects.

A physically-based model of interactions between a building and its outdoor conditions at the urban microscale

This paper introduces a physically-based model to simulate interactions between a building and its outdoor conditions at the urban microscale. In the model, the building is simulated with EnergyPlus while its outdoor conditions are assessed from OpenFOAM. Furthermore, the model simulates the street pavement and surrounding buildings based on the lumped thermal theory. All components of the model are quasi-dynamically co-simulated. Through simulations of the model, waste heat releases from a cooling system can be observed with a higher resolution than this achieved by most urban microclimate models in the literature. Unlike several methods in the literature, the model evaluates a countermeasure to urban heat islands by considering direct and indirect effects simultaneously. To validate the model, measurements were collected during a field experiment in an university campus. Comparing measurements to estimates, the model seems to properly approximate the outdoor temperature and the air motion. However, a discrepancy is observed between estimates and measurements of the surface temperature. The discrepancy could be minimised if a better consideration of the net-longwave radiation was possible when co-simulating EnergyPlus. After some improvements, the model could become a support tool to mitigate urban heat islands and climate change in different regions of the world.

A Street Graph-Based Morphometric Characterization of Two Large Urban Areas

Urban microclimate modelling, both numerical and in the laboratory, has strong implications in many relevant health and life-style management issues e.g., in studies for assessment and forecast of air quality (for both outdoor and, as boundary conditions, indoor investigations), for thermometric trend analysis in urban zones, in cultural heritage preservation, etc. Moreover, the study of urban microclimate modelling is largely promoted and encouraged by international institutions for its implication in human health protection. In the present work, we propose and discuss an adaptive street graph-based method aimed at automatically computing the geometrical parameters adopted in atmospheric turbulent flow modelling. This method has been applied to two real cases, the Italian cities of Rome and Cagliari, and its results has been compared with the ones from traditional methods based on regular grids. Results show that the proposed method leads to a more accurate determination of the urban canyon parameters (Canyon Aspect Ratio and Building Aspect Ratio) and morphometric parameters (Planar Area Index and Frontal Area Index) compared to traditional regular grid-based methods, at least for the tested cases. Further investigations on a larger number of different urban contexts are planned to thoroughly test and validate the proposed algorithm.

In-situ spectroscopy and shortwave radiometry reveals spatial and temporal variation in the crown-level radiative performance of urban trees

In conventional microclimate environment modelling, and the development of tree planning strategies for urban heat mitigation, tree crown surface albedo for any given species is assumed to be a constant. However, our recent research into urban tree radiative performance at the crown level implied that tree crown surface albedo changes over time. Based on the in-situ spectroscopy protocols established previously to measure tree crown transflectance, variation in the characteristics of tree crown surface albedo was explored combining spectroscopy and solar shortwave radiometry. Three commonly planted native UK tree species, Carpinus betulus, Acer campestre, and Taxus baccata, were sampled. Spatial distribution profiles of tree crown transflectance measured at fixed solar altitudes were normalised by in-situ spectroradiometry. Tree crown transflectance in the near infrared (NIR) region was found to be proportionally linked to tree crown surface albedo. Within each species, mean tree crown transflectance in the NIR region of 800–900 nm was approximately 2.5 times tree crown surface albedo. It was further found that infrared radiation (700–2500 nm) accounted for more than 90% of the total transflected shortwave radiation from tree crowns. The results demonstrate that tree crown surface albedo linearly increases with momentary solar altitude and the maximum tree crown surface albedo corresponds to maximum solar altitude at solar noon on sunny days in summer. Tree crown surface albedo across species tends to be strongly dependent on leaf size if considering visibly dense crown foliage. Our findings provide important insights into tree radiative shading effects resulting from temporal variation in tree crown surface albedo, with consequences for urban microclimate modelling and the development of urban heat mitigation strategies.

City-Scale Building Anthropogenic Heating during Heat Waves

More frequent and longer duration heat waves have been observed worldwide and are recognized as a serious threat to human health and the stability of electrical grids. Past studies have identified a positive feedback between heat waves and urban heat island effects. Anthropogenic heat emissions from buildings have a crucial impact on the urban environment, and hence it is critical to understand the interactive effects of urban microclimate and building heat emissions in terms of the urban energy balance. Here we developed a coupled-simulation approach to quantify these effects, mapping urban environmental data generated by the mesoscale Weather Research and Forecasting (WRF) coupled to Urban Canopy Model (UCM) to urban building energy models (UBEM). We conducted a case study in the city of Los Angeles, California, during a five-day heat wave event in September 2009. We analyzed the surge in city-scale building heat emission and energy use during the extreme heat event. We first simulated the urban microclimate at a high resolution (500 m by 500 m) using WRF-UCM. We then generated grid-level building heat emission profiles and aggregated them using prototype building energy models informed by spatially disaggregated urban land use and urban building density data. The spatial patterns of anthropogenic heat discharge from the building sector were analyzed, and the quantitative relationship with weather conditions and urban land-use dynamics were assessed at the grid level. The simulation results indicate that the dispersion of anthropogenic heat from urban buildings to the urban environment increases by up to 20% on average and varies significantly, both in time and space, during the heat wave event. The heat dispersion from the air-conditioning heat rejection contributes most (86.5%) of the total waste heat from the buildings to the urban environment. We also found that the waste heat discharge in inland, dense urban districts is more sensitive to extreme events than it is in coastal or suburban areas. The generated anthropogenic heat profiles can be used in urban microclimate models to provide a more accurate estimation of urban air temperature rises during heat waves.

Smart wetting of permeable pavements as an evaporative-cooling measure for improving the urban climate during heat waves

An urban microclimate model is used to design a smart wetting protocol for multilayer street pavements in order to maximize the evaporative cooling effect as a mitigation measure for thermal discomfort during heat waves. The microclimate model is built upon a computational fluid dynamics (CFD) model for solving the turbulent air, heat and moisture flow in the air domain of a street canyon. The CFD model is coupled to a model for heat and moisture transport in porous urban materials and to a radiative exchange model, determining the net solar and thermal radiation on each urban surface. A two-layer pavement system, previously optimized for maximal evaporative cooling applying the principles of capillary pumping and capillary break, is considered to design a smart wetting protocol answering the questions “when,” “how much,” and “how long” a pavement should be artificially wetted. It was found for the current optimized pavement solutions that a daily amount of 6 mm wetting over 10 min in the morning, preferentially between 8:00 and 10:00, guarantees a maximal evaporative cooling for 24 h during a heat wave.

An integrated multiscale urban microclimate model for the urban thermal environment

An urban microclimate model integrated with the mesoscale, local scale, microscale and building scale models is presented for the urban thermal environment and urban heat island (UHI) mitigation study. This integrated multi-scale and multi-physics urban microclimate model takes the impacts on the urban microclimate of multiple physical processes of regional climate, urban climate, building and pavement material properties, and anthropogenic heat into account. This objective is achieved by coupling weather and energy models, the weather research forecasting (WRF), OpenFOAM and EnergyPlus. This study presents a method of distributing initial and boundary conditions from mesoscale into microscale through the integration of building thermal behaviour and microclimate. We further examine the integrated model by taking the Kent Ridge campus of National University of Singapore (NUS) as a test case. The preliminary application focuses on the assessment of weather conditions at the local scale and the feasibility of using mesoscale WRF outputs as inputs for the CFD model.

CFD-CFD coupling: A novel method to develop a fast urban microclimate model

Computational fluid dynamics (CFD) technique is well-known for its powerful capability of modelling the cross-ventilation. CFD is broadly coupled with energy simulation programmes, such as EnergyPlus, to expand its application in building energy calculation. During the coupling process, iterative calculation is required to improve the accuracy of the coupling method, which means the scale of the model should be designed with extra care so that the simulation can be practical in terms of computational cost. A coarse-resolution mesh (here denoted as CFDc) cannot properly represent the required local environment to study the underlying physics. However, a fine-resolution CFD microclimate model (here denoted as CFDf), which includes both indoor and outdoor spaces, always requires a large number of cells to capture the complexities of environment; this makes the coupling procedure a challenging problem. This study aims to propose an innovative, high-resolution and computationally cost-effective CFDf-CFDc model to overcome this gap. The full-scale CFDf works in an off-line manner at the preliminary simulation stage and generates detailed flow parameters at the interfaces for CFDc. At the dynamic stage of the simulation, CFDc participates in the coupling procedure with the other tools, for example, building energy simulation (BES) tool (e.g. EnergyPlus). The coupling of CFDf-CFDc-BES is about 200 times faster than an ordinary coupled CFDf-BES method with a significantly quicker and more stable achievement in the convergence.