Connectivity of Nocturnal Cold-Air Flows for Urban Heat Island Mitigation: Introduction of the Cold-Air Trajectory Calculator KLATra

Ventilation of cities by local cold-air flows is an important measure in urban heat island mitigation and climate-resilient urban planning. We introduce a cold-air connectivity analysis to identify relevant cold-air formation areas as well as urban quarters ventilated by cold-air flows. The nocturnal cold-air flow trajectories are calculated from numerical model simulations using the single-layer cold-air drainage model KLAM_21 and the newly developed trajectory calculator KLATra. The German city of Freiburg im Breisgau is chosen to demonstrate the cold-air connectivity analysis based on trajectories calculated for two 3-hourly periods during an idealised night. Hydrological catchment boundaries and land use define eight rural cold-air formation areas as starting points for forward trajectories, whereas administrative urban district boundaries and land use data are used to define five built-up quarters potentially prone to overheating as starting points for cold-air backward trajectories. A rate of connectivity is calculated from the ratio of trajectories connecting cold-air formation areas with overheated urban quarters to the total number of trajectories. The analysis reveals the potential of cold-air formation areas to ventilate single or multiple urban quarters at connectivity rates up to 82%. The connectivity analysis therefore supports identification and assessment of the relevance of specific cold-air formation areas for urban heat island mitigation and may serve as a valuable planning tool and data basis for objective decision making.