The Impacts of Urban Heat Islands on Northwestern European Cities: Characterising the heat island intensity based on land use data

Abstract: Many urban and suburban areas experience higher temperatures than their outlying rural surroundings. This difference in temperature is what constitutes an Urban Heat Island (UHI). UHIs result from man-made modifications of the natural environment. The replacement of vegetation, soil and water with concrete, brick, asphalt and metal reduces evapotranspiration, increases the storage and transfer of sensible heat and decreases air movement. Problems that are associated with the urban heat island phenomenon include atmospheric pollution, increased morbidity and mortality, a decreased thermal comfort and an increase in energy consumption. For these reasons, it is desirable to mitigate the UHI effect.
Models and tools to predict the effect of UHI mitigation measures can assist the urban planner to incorporate these in urban design. A number of numerical (simulations) and empirical (observational) models have been constructed to date. Empirical models are most likely to be of use to urban planners. However, such a model does not yet exist for Northwestern Europe. As a testbed for a potential, more comprehensive study in the future, the aim of this research is to empirically investigate the relationship between the Urban Heat Island Intensity (UHII) and the implementation of mitigation measures, in terms of land use characteristics, of urban environments in Northwestern Europe.
Firstly, in order to be able to select relevant data on which the analyses will be conducted, an inventory of UHI mitigation measures has been provided. Mitigation measures of interest have been divided in five categories: vegetation (urban parks and gardens, street trees, green roofs and walls), open water (rivers, ponds, lakes and fountains), built form (low building density and street design that promotes ventilation), material (high albedo materials and porous paving) and anthropogenic sources (improved thermal insulation of buildings and car-free zones).
Secondly, the required data has been gathered. UHII data has been retrieved from the scientific literature. Seven studies, corresponding to seventeen locations, were found to be of use in the analyses. The corresponding land use data has been collected on two spatial scales: the micro scale and the local scale. For both spatial scales, the percentages of impervious surfaces, vegetation and open water, have been calculated. For the smallest spatial scale, the number of street trees and the width and orientation of the adjacent street have been estimated. On the largest spatial scale, the urban density (inhabitants per ha) of the neighbourhood has been chosen as an indicator for building density. In addition, the urban areas have been classified in Local Climate Zones (LCZ). Finally, data with regard to wind direction, wind speed, temperature, precipitation amount and cloud cover have been retrieved to account for the influence of weather on the UHII.
Thirdly, the relationships between the UHII and land use characteristics were analysed. It was found that the UHIIs of cities in the Northwestern European oceanic climatic zone cannot be generalised based on the LCZ classification. Rather, the LCZ classification should be used on the city scale, to account for the influences of the wider urban environment on the UHII. However, potentially large local sources of (anthropogenic) heat, such as the presence of a busy motorway junction, should still be taken into account when applying the LCZ classification as a means to estimate the UHII. The results of bivariate analyses indicate that generalizing the UHIIs of urban areas in Northwestern Europe based on land use characteristics is not possible. However, the analyses show that a number of UHI mitigation measures may be able to reduce the UHII, such as street trees, a low building density and open water.