Comparative analysis of shade and underlying surfaces on cooling effect

Urban greenspaces showed the potential to lessen the urban heat island effect. However, a detailed understanding on the mechanisms of different components of greenspaces such as grass surfaces, trees or a combination of grey and green infrastructure on reducing heat loads at local and city scale and different weather conditions is still limited. We designed a small-scale experiment within the sub-urban area Freising, close to Munich in Germany during hot summer days of the year 2020 including wet and dry spells. We investigated surface energy balance and the human thermal comfort measured in terms of physiological equivalent temperature (PET). Six sites including grass lawns and paved surfaces, with or without the shade of trees and buildings were selected. Significant positive relationships between surface (ST) and air temperature (AT) were detected both for grass and paved surfaces; however, the relationships were stronger during the wet spells compared to dry spells and for grass surfaces compared to paved surfaces. Moreover, PET was more strongly related to ST compared to AT. Overall, shade reduced 15 °C, 2 °C and 13 °C of ST, AT and PET respectively compared to sunny sites. The differences between sun and shade were steeper over the grass surfaces and during the wet spells when the grass surfaces lost more than 1.5 L m⁻² d⁻¹ of water. In contrast, sensible heat fluxes between grass and paved surfaces were not different during the dry spells. Moreover, compared to the building shade, tree shade further reduced AT by 0.6 °C and 0.4 °C during wet and dry spells, but PET by 1 °C and 1.6 °C during wet and dry spells respectively. Our results underline the importance of both shade and grass surfaces in reducing the urban heat loads, in particular, the added benefits of tree shade during the summer droughts.