Urban greening and the UHI: Seasonal trade-offs in heating and cooling energy consumption in Manchester, UK (original) (raw)
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With increasing urbanisation and predictions of increased frequency of heat waves under projected climate change scenarios, one strategy that has been suggested to address both adaptation and mitigation for urban areas is the increased use of greenspace. A number of studies have analysed this strategy through the use of empirical, analytical methods, or numerical methods. These tend to focus on city or regional scale changes in land use with only a broad categorisation of greenspace type. This study tests seven greenspace scenarios that might be applied at a block or neighbourhood level and the resulting microclimate changes that can be achieved through such applications for a temperate city in northwest England. Using a suburban commercial site in Manchester, UK as the case study area, the research utilises the urban microclimate model ENVI-met to compare the changes in air and surface temperatures on a warm summer day in July 2010 (approximately 4 °C above the rural reference July average maximum temperature). The modelling demonstrates that even in suburban areas in temperate cities a 5% increase in mature deciduous trees can reduce mean hourly surface temperatures by 1 °C over the course of a summer's day. A marked increase in air temperature of 3.2 °C at mid-day is modelled for the worst case scenario of replacing all current vegetation with asphalt.
The role of one large greenspace in mitigating London's nocturnal urban heat island
The Science of the total environment, 2014
The term urban heat island (UHI) describes a phenomenon where cities are on average warmer than the surrounding rural area. Trees and greenspaces are recognised for their strong potential to regulate urban air temperatures and combat the UHI. Empirical data is required in the UK to inform predictions on cooling by urban greenspaces and guide planning to maximise cooling of urban populations. We describe a 5-month study to measure the temperature profile of one of central London's large greenspaces and also in an adjacent street to determine the extent to which the greenspace reduced night-time UHI intensity. Statistical modelling displayed an exponential decay in the extent of cooling with increased distance from the greenspace. The extent of cooling ranged from an estimated 20 m on some nights to 440 m on other nights. The mean temperature reduction over these distances was 1.1 °C in the summer months, with a maximum of 4 °C cooling observed on some nights. Results suggest that...
2018
The paper is part of the scientific research sector concerning the government of urban transformations in order to promote efficiency and reduction of energy consumption in urban areas. In this study, urban greenspaces (green areas) are proposed as a strategy for cities to achieve both urban sustainability and resilience while addressing the issues of energy reduction and climate change adaptation. The study investigated the microclimate impact of greenspaces on the cooling energy needs of residential buildings in Naples, Italy, given different urban fabric characteristics by coupling the microclimate model ENVI-met with the building energy model EnergyPlus. The charts resulted from the study could represent an useful decision support tool for urban planners and policy-makers to locate and size greenspaces based on their effectiveness in terms of energy consumption reduction. The study found that—in general—a medium-size green area (4900 m2) would reduce the cooling energy consumpti...
SUMMARY Urban heat island (UHI) is a phenomenon where temperature distribution in the urban area is significantly warmer than its surrounding suburban areas. One of the main causes of UHI is the replacement of natural surfaces by built surfaces through urbanization. Trees and vegetation play vital role to mitigate the UHI effects especially by regulating high temperature in saturated urban areas and their surrounding. This study attempts to evaluate the urban green spaces (UGS) cooling effects on the microclimate of the surrounding areas especially in a hot and humid tropical climate like Malaysia. Shah Alam Lake Garden (Shah Alam), Bandaran Kelana Park (Kelana Jaya) and Subang Ria Recreational Park (Subang Jaya) which are located in the Petaling District, Selangor, Malaysia are selected as the study areas. UGS land cover profile and surface temperature distribution are derived from Landsat 5 Thematic Mapper (TM) image of 2009. Mono-window algorithm is used to generate temperature distribution map of the study areas. Land cover classification and land cover profile of the selected study areas are generated in the digital image processing software. Geographical Information System (GIS) is used to generate the land surface temperature (LST) map, measure the LST of selected points within specified buffer zones, perform overlay and buffer operations. The green space cooling effects intensity and the relationship between intensity and proximity from green space boundary are later determined. Results obtained have indicated that the cooling effects intensity of the surrounding urban areas largely depends on the green space profile and the distance from the park boundary. The introduction of green areas or parks in urban areas can be considered as a good initiative to replace the loss of natural greenery and can potentially reduce the effects of UHI.
2019
Urban areas are typically warmer than rural ones. This is mainly due to denser configuration dominated by impermeable surfaces such as buildings and roads, compared to rural areas which are less densely built and mainly dominated by open spaces. Rapid urban expansion in dense cities bares direct impact on surface and air temperature patterns within street canyons; a phenomena which is known as the Urban Heat Island (UHI) effect. Thus, several UK city councils such as Birmingham, Manchester, and London have started to develop strategies aiming at enhancing urban green systems (UGS) through trees, green walls and green roofs. Some of those strategies include considering the green space factor, and increasing green areas within the cities to improve street canyon microclimate and reduce UHI. The Mayor of London has adopted a strategy for London 2050 aspiring to transform it to be the greenest city in the world by increasing the green areas up to 50%. This paper investigates the influence of increasing the UGS percentage which is considered as a key solution to mitigate UHI effect which will, in turn, provide thermally comfortable outdoor environments for pedestrians. The investigation is undertaken by comparing the morphology of precincts and streets in relation to air temperature, mean radiant temperature and surface temperature within Oxford Street canyons in London city centre; being one of the world's busiest streets. The results from this research demonstrate that different UGS interventions with varying percentage are required depending on particular canyon orientations and geometries. The study found that, in general, more trees would have significant thermal comfort effect followed by living façade, while high albedo pavement (HAP) came last. However, HAP had high influence on improving thermal comfort in North-South orientated streets with minor variance to trees and living facades which, changing their percentage levels was insignificant.
Combining a Detailed Building Energy Model with a Physically-Based Urban Canopy Model
Boundary-Layer Meteorology, 2011
A scheme that couples a detailed building energy model, EnergyPlus, and an urban canopy model, the Town Energy Balance (TEB), is presented. Both models are well accepted and evaluated within their individual scientific communities. The coupled scheme proposes a more realistic representation of buildings and heating, ventilation and air-conditioning (HVAC) systems, which allows a broader analysis of the two-way interactions between the energy performance of buildings and the urban climate around the buildings. The scheme can be used to evaluate the building energy models that are being developed within the urban climate community. In this study, the coupled scheme is evaluated using measurements conducted over the dense urban centre of Toulouse, France. The comparison includes electricity and natural gas energy consumption of buildings, building façade temperatures, and urban canyon air temperatures. The coupled scheme is then used to analyze the effect of different building and HVAC system configurations on building energy consumption, waste heat released from HVAC systems, and outdoor air temperatures for the case study of Toulouse. Three different energy efficiency strategies are analyzed: shading devices, economizers, and heat recovery.
The TEB urban climate model has recently been improved to more realistically address the radiative effects of trees within the urban canopy. These processes necessarily have an impact on the energy balance that needs to be taken into account. This is why a new method for calculating the turbulent fluxes for sensible and latent heat has been implemented. This method remains consistent with the "bigleaf" approach of the ISBA model which deals with energy exchanges between vegetation and atmosphere within TEB. Nonetheless, the turbulent fluxes can now be dissociated between ground-based natural covers and tree stratum above (knowing the vertical leaf density profile), which can modify the vertical profile in air temperature and humidity in the urban canopy. In addition, the aeraulic effect of trees is added, parameterized as a drag term and an energy dissipation term in the evolution equations of momentum and of turbulent kinetic energy, respectively. This set of modifications relating to the explicit representation of tree stratum in TEB is evaluated on an experimental case study. The model results are compared to micrometeorological and surface temperature measurements collected in a semi-open courtyard with trees and bordered by buildings. The new parameterizations improve the modelling of surface temperatures of walls and pavements thanks to taking into account radiation absorption by trees, and of air temperature. The wind speed is strongly slowed down by trees that is also much more realistic. The universal thermal climate index diagnosed in TEB from inside-canyon environmental variables is highly dependent and sensitive to these variations in wind speed and radiation. This demonstrates the importance of properly modelling interactions between buildings and trees in urban environments, especially for climate-sensitive design issues. 1 Introduction The urban climate commonly refers to the modification of local climate by the urban environment. It results from the establishment of radiative, energetic, dynamic, hydrological surface processes that are pecular to urban covers properties (Oke et al., 2017). This urban climate may however present important spatial variabilities within the city. The street-level meteorological variables, i.e. air temperature, humidity, wind, are modified in by the local environment depending the morphology and arrangement of buildings, the surface properties and more generally the land covers composition (Houet and Pigeon, 2011; 1
The Influence of Urban Green Systems on the Urban Heat Island Effect in London
IOP Conference Series: Earth and Environmental Science
Urban areas are typically warmer than rural ones. This is mainly due to denser configuration dominated by impermeable surfaces such as buildings and roads, compared to rural areas which are less densely built and mainly dominated by open spaces. Rapid urban expansion in dense cities bares direct impact on surface and air temperature patterns within street canyons; a phenomena which is known as the Urban Heat Island (UHI) effect. Thus, several UK city councils such as Birmingham, Manchester, and London have started to develop strategies aiming at enhancing urban green systems (UGS) through trees, green walls and green roofs. Some of those strategies include considering the green space factor, and increasing green areas within the cities to improve street canyon microclimate and reduce UHI. The Mayor of London has adopted a strategy for London 2050 aspiring to transform it to be the greenest city in the world by increasing the green areas up to 50%. This paper investigates the influence of increasing the UGS percentage which is considered as a key solution to mitigate UHI effect which will, in turn, provide thermally comfortable outdoor environments for pedestrians. The investigation is undertaken by comparing the morphology of precincts and streets in relation to air temperature, mean radiant temperature and surface temperature within Oxford Street canyons in London city centre; being one of the world's busiest streets. The results from this research demonstrate that different UGS interventions with varying percentage are required depending on particular canyon orientations and geometries. The study found that, in general, more trees would have significant thermal comfort effect followed by living façade, while high albedo pavement (HAP) came last. However, HAP had high influence on improving thermal comfort in North-South orientated streets with minor variance to trees and living facades which, changing their percentage levels was insignificant.
\vspace{8mm}Inclusion of vegetation in the Town Energy Balance model for modelling urban green areas
Geoscientific Model Development, 2012
Cities impact both local climate, through urban heat islands and global climate, because they are an area of heavy greenhouse gas release into the atmosphere due to heating, air conditioning and traffic. Including more vegetation into cities is a planning strategy having possible positive impacts for both concerns. Improving vegetation representation into urban models will allow us to address more accurately these questions. This paper presents an improvement of the Town Energy Balance (TEB) urban canopy model. Vegetation is directly included inside the canyon, allowing shadowing of grass by buildings, better representation of urban canopy form and, a priori, a more accurate simulation of canyon air microclimate. The surface exchanges over vegetation are modelled with the well-known Interaction Soil Biosphere Atmosphere (ISBA) model that is integrated in the TEB's code architecture in order to account for interactions between natural and built-up covers. The design of the code makes possible to plug and use any vegetation scheme. Both versions of TEB are confronted to experimental data issued from a field campaign conducted in Israel in 2007. Two semi-enclosed courtyards arranged with bare soil or watered lawn were instrumented to evaluate the impact of landscaping strategies on microclimatic variables and evapotranspiration. For this case study, the new version of the model with integrated vegetation performs better than if vegetation is treated outside the canyon. Surface temperatures are closer to the observations, especially at night when radiative trapping is important. The integrated vegetation version simulates a more humid air inside the canyon. The microclimatic quantities (i.e., the street-level meteorological variables) are better simulated with this new version. This opens opportunities to study with better accuracy the urban microclimate, down to the micro (or canyon) scale.