Outdoor heat in urban areas - Model development and applications

Abstract

Heat waves and high outdoor air temperature can lead to heat stress with negative implications for human health and wellbeing such as heat stroke, heat cramps, dehydration and in extreme cases death. The urban population is at higher risk of such outcomes because of the generally warmer urban climate. Daytime outdoor thermal comfort is substantially affected by short- (solar) and longwave (thermal) radiation, i.e. mean radiant temperature (Tmrt). The aim of this thesis is to deepen the knowledge of radiant conditions in complex urban areas and how such knowledge can be utilized in modelling of Tmrt and thermal comfort of humans.

The overall aim is examined in three parts. The first part examines the effects of anisotropic (non-uniform) estimations of sky diffuse shortwave radiation and longwave radiation in the SOlar and LongWave Environmental Irradiance Geometry model (SOLWEIG) and how these influences Tmrt of humans in outdoor urban environments, compared to isotropic conditions. The results show that anisotropic sky diffuse shortwave radiation and longwave radiation are important in estimations of Tmrt. The circumsolar and horizon regions irradiates more diffuse shortwave radiation when the sky is anisotropic, which increases radiant load mainly in sunlit areas. Anisotropic sky longwave radiation increases with zenith angle, reaching its maximum at the horizon, resulting in higher Tmrt in open areas where the horizon region is visible.

The second part focuses on outdoor thermal comfort of preschoolers in Sweden in the present and future climate using SOLWEIG. It is concluded that two thirds of preschool yards in Gothenburg have 50% or more of their yard area exposed to strong heat stress. Heat stress in preschools lead to drowsy, tired and overheated children, with negative consequences for the pedagogical activities, forcing teachers to ensure that children stay cool on the expense of education. Heat stress days are expected to increase in the future, potentially exacerbating already existing heat related issues. However, with abundant tree shade heat stress is limited, both in the present and in the future.

In the third part optimized locations for trees to mitigate excessive Tmrt with regards to the shading effect of trees is analyzed. Tree positions depend on tree size and what time of day when shading is required.

The results of this thesis highlights the significance of realistic models, importance of applied studies to identify heat related problems and how such problems can be mitigated.