Regulating the microclimate with urban green in densifiying cities: Joint assessment on two scales

Green spaces fulfill an important role in regulating the urban microclimate, however they are under high pressure in growing cities. As much as densification is a threat towards existing green spaces, it also offers the possibility to redesign residential areas in a climate-responsive way. To do so, urban green infrastructure needs to be incorporated from the outset into the planning process and the urban context and mutual influences between different scales have to be considered. However, information on how to secure ventilation in densifying neighbourhoods while simultaneously enhancing thermal comfort in open spaces at site scale still is limited. Therefore, we compare microclimatological modelling outcomes on district and block level for different densification and green intervention scenarios in a real planning case. Our results suggest that green infrastructure can compensate for negative effects of building densification on daytime thermal comfort, but not for impacts on cold air volume flow (CAVF). CAVF is mainly affected by densification, especially by increasing building heights. Strategic placement of trees prevents worsening the nocturnal ventilation, while providing effective cooling during daytime. A replacement of mature trees by new trees in densification scenarios led to an increase of the physiological equivalent temperature (PET) by 7.5–7.9 °C. Adding green roofs and green facades did not lead to a decrease of heat stress levels, but significantly reduced surface temperature. A coupling of microclimate models operating on different scales and with different spatial resolutions is important to consider mutual influences between ventilation and outdoor thermal comfort.