Urban heat island

Quick links:Climate change mitigation Heat extremes LUCID

The built environment affects local climate due to the thermal and morphological properties of urban areas. In addition, anthropogenic (man-made) heat emissions from buildings and traffic are higher within urban areas. Thus, urban areas typically experience higher temperatures than rural areas, particularly in the late afternoon and early evening in the summer; this phenomena is known as the 'urban heat island' (UHI) effect.

UHIs have benefits in winter, with lower energy usage for heating, but drawbacks in summer including higher energy usage for air conditioning and higher incidences of heat-related illnesses and deaths. The adverse consequences of UHIs are increasingly of concern to national and local planning authorities. Conversely, local governments are interested in the positive implications of 'green infrastructure' (e.g. an increase in green space in urban areas), which include elevated community health and well-being alongside improvements in air quality. Also, new developments can be designed, constructed and operated with minimal impact on the local climate.

The ADMS-Urban Temperature and Humidity model has been developed to assess, at user-defined spatial resolution, the variation of temperature over complex urban areas. The system consists of two components:

• A land use model in which perturbations to the mean temperature are related to the spatial variation of surface type and building morphology; parameters are used to represent the following land use features: surface roughness, albedo, thermal admittance, surface resistance to evaporation and building volume; and
• An anthropogenic heat model, which allows for the dispersion of the heat emitted from anthropogenic sources. The emissions of heat generated from sources such as buildings and traffic can be modelled at a high resolution (tens of metres), including the effects of the hourly variations of emission rates.

Researchers using the model at the University of Bologna have published results of their model evaluation exercise. CERC have presented model evaluation results for London.

The system has a number of applications, including the assessment of the effect of new building developments on the local climate, and investigations into climate change mitigation scenarios, such as cool roofs and green roofs. A system to forecast temperatures over the Olympic Park was developed using previous research; this system was live during the London 2012 Olympics.

Climate change mitigation scenarios

In collaboration with CERC, researchers from The Bartlett (University College London) have used the ADMS Temperature and Humidity model during a study to assess the effectiveness of retrofitting green and cool roofs on reducing energy use within a typical office in central London (Virk et al., 2015). By using the microclimate model output from ADMS as input to a building thermal simulation model, they were able to show that green and cool roofs reduced annual energy use. The same group looked at the impact of retrofitting green and cool roofs on overheating within a naturally ventilated building (Virk et al., 2014). Both studies considered impacts for current and future climate scenarios.

Modelling heat extremes

CERC are partners on the ‘Disaster Resilient Cities: Forecasting Local Level Climate Extremes and Physical Hazards for Kuala Lumpur’ project, funded jointly by the UK and Malaysian governments through the Newton-Ungku Omar Fund. The ADMS-Urban Temperature and Humidity model is being used to predict temperature variations within Kuala Lumpur; model predictions are to be evaluated using measurements recorded at a number of locations within the city during a 12-month field campaign. Once the model has been correctly configured to represent Kuala Lumpur’s tropical urban climate, ADMS-Urban model temperature forecasts will form part of the multi-hazard platform for managing and communicating risks to enhance disaster resilience.

LUCID: the intelligent design of cities

The Development of a Local Urban Climate Model and its Application to the Intelligent Design of Cities project (LUCID) was a 3-year research project investigating how cities may adapt to a changing climate. CERC worked on this project between 2007 and 2010, with partners in the academic sector (University College London, London School of Hygiene and Tropical Medicine, University of Reading and Brunel University) as well as other consultancies.

The role of CERC within the project was to develop a tool for modelling changes in temperature and specific humidity on a neighbourhood scale (i.e. over areas of a few square kilometres) due to changing land use. This led to the initial development of the ADMS Temperature and Humidity model for land use effects on temperature. In addition, for the purpose of full model validation against temperature measurements, anthropogenic heat emissions from traffic and buildings were also modelled.

The project, funded by the UK Engineering and Physical Sciences Research Council (EPSRC), led to a number of publications including:

• ‘The comfort, energy and health implications of London’s urban heat island’ (Mavrogianni et al., 2011), which was an overview of the project; and
• ‘The impact of the London Olympic Parkland on the urban heat island’ (Hamilton et al., 2014), which demonstrated how ADMS can be used to predict temperature changes arising from alterations to land use, such as the development of London’s Olympic Parkland.