Utilising green and bluespace to mitigate urban heat island intensity

Abstract

It has long been recognised that cities exhibit their own microclimate and are typically warmer than the surrounding rural areas. This 'mesoscale' influence is known as the urban heat island (UHI) effect and results largely from modification of surface properties leading to greater absorption of solar radiation, reduced convective cooling and lower water evaporation rates. Cities typically contain less vegetation and bodies of water than rural areas, and existing green and bluespace is often under threat from increasing population densities. This paper presents a meta-analysis of the key ways in which green and bluespace affect both urban canopy- and boundary-layer temperatures, examined from the perspectives of city-planning, urban climatology and climate science. The analysis suggests that the evapotranspiration-based cooling influence of both green and bluespace is primarily relevant for urban canopy-layer conditions, and that tree-dominated greenspace offers the greatest heat stress relief when it is most needed. However, the magnitude and transport of cooling experienced depends on size, spread, and geometry of greenspaces, with some solitary large parks found to offer minimal boundary-layer cooling. Contribution to cooling at the scale of the urban boundary-layer climate is attributed mainly to greenspace increasing surface roughness and thereby improving convection efficiency rather than evaporation. Although bluespace cooling and transport during the day can be substantial, nocturnal warming is highlighted as likely when conditions are most oppressive. However, when both features are employed together they can offer many synergistic ecosystem benefits including cooling. The ways in which green and bluespace infrastructure is applied in future urban growth strategies, particularly in countries expected to experience rapid urbanisation, warrants greater consideration in urban planning policy to mitigate the adverse effects of the UHI and enhance climate resilience.

Ecosystem-based climate change adaptation for Essenvelt, Middelburg, The Netherlands

“…It suffices to mention that the phenomenon illustrated above conforms to observations and model calculations indicating that water may contribute to heating instead of cooling, in particular during late summer nights (Steeneveld et al, 2014: 92;Jacobs La Rivière & Goosen, 2014: 136;Gunawardena, Wells & Kershaw, 2017: 1040. By autumn, the seawater tends to display relatively high temperatures.…”

The Use of a Decision Support System for Sustainable Urbanization and Thermal Comfort in Adaptation to Climate Change Actions—The Case of the Wrocław Larger Urban Zone (Poland)

“…Metronamica, which was used for the land use change projections, is predesigned for larger areas. The type of data used for such purposes is generalized, and can be used to assess mesoscale climate conditions [71]. Therefore, the suggested solution is to focus on the land use level rather than on the architectural level.…”

“…The next limitation is connected with wind conditions. The present study takes the heat flux [71] and ventilation of linear corridors in urban space [49] into account. However, it does not include the dominant wind direction resulting from regional conditions.…”

“…In the case of high density green areas, these may reduce the thermal stress as the green infrastructure does not heat up as much as the built-up area. Shading by vegetation keeps the air cooler by acting as a solar radiation interceptor that reflects and absorbs radiant energy [71]. As the temperature between neighboring areas tends to equalize, urban areas in a close proximity of green infrastructure-300 m from urban green areas and forests [50]-were also excluded from the UHI exposure assessment.…”

“…Temperature of areas in the transitional zone can be partly reduced by the neighborhood from one side. The temperature in areas at the urban edge is reduced by heat fluxes (so-called city-country breezes) [71]. Therefore, in order to select urban areas exposed to the UHI impact, a core analysis was made in two ranges: 250 m-500 m from the patch edge (m-UHI-moderate exposure to UHI) and more than 500 m from the patch edge (h-UHI-high exposure to UHI).…”