Ventilation-Sensitive Placement of Nature-Based Cooling for Peak Pedestrian Heat Relief in a Compact Cologne District

Abstract

The heat adaptation strategy of the compact mid-rise districts must reduce pedestrian heat load without affecting the poor air exchange which still persists in such narrow streets and courtyards. In this study, the cooling pathway scores for compact districts are determined based on radiation–ventilation-gated district-level heat adaptation intervention allocation. This case study involves the Volksgartenviertel district, an 16 ha compact district in Cologne, Germany. The scores are assigned based on district characteristics including the land cover, vegetation, 2022 July meteorology, and the air-temperature–physiological equivalent temperature response relationship. The district consists of traffic area, parks, buildings, and inner courtyards covering approximately 25 %, 20 %, 25 %, and 30 % of the district, respectively. Present greening within the district is unbalanced; there are 285 street trees belonging to 18 different species and two green roof areas. There is also one completely covered facade greening and 11 tree–shrub–grass front yards out of 221 front yards surveyed. The intervention set consists of 158 Acer platanoides street trees, 146 facade and roof greening interventions, and 2410 grass grid pavers which correspond to 9640 m² or 0.964 ha permeable grounds. For the hottest days during 18–20 July 2022, the district maximum air temperature is 40.2 °C, and mean air temperature and wind speed are approximately 28.7 °C and 0.29 m s⁻¹, respectively. The mean air-temperature reduction over 72 h is −0.49 K, which is accompanied by −0.91 K of PET reduction. The maximum reduction of both air temperature and PET locally is −5.28 K and −7.66 K, respectively. While PET reduction over 72 h is 3.13 times of air-temperature reduction, locally maximum PET reduction reaches −26.10 K during the hottest hour.