Delimitation of Urban Hot Spots and Rural Cold Air Formation Areas for Nocturnal Ventilation Studies Using Urban Climate Simulations

Due to global warming, the conservation or enhancement of urban ventilation during synoptically calm and hot weather conditions is receiving increasing attention in climate resilient urban and regional planning. The transport of cool air from rural surroundings into the city by local winds during nighttime is important for the alleviation of the urban heat island intensity and heat load in particular. A simple statistical method, which objectively identifies urban thermal hot spots and areas of rural cold air formation from thermodynamic urban climate model simulations is described and applied to Aschaffenburg, a medium-sized town located in hilly terrain in south-central Germany. The delimitated hot spots and nocturnal cold air formation areas are influenced by local land cover, and also by the surrounding landscape heterogeneity, surface energy exchange and atmospheric mixing processes. The results illustrate limitations of hot spot or cool spot estimation methods based purely on the analysis of classified land cover data. Nocturnal backward airflow trajectories from thermal hot spots in the city and forward trajectories from rural areas with substantial cold air formation are calculated to determine which cold air formation areas are contributing to ventilation and advective cooling of thermal hot spots. It is found that nocturnal ventilation mechanisms are not bound to municipal boundaries, which highlights the need for regional cooperation in urban climate adaptation. The described method provides guidance to urban and regional planners in order to protect important cold air formation areas, e.g., from urban sprawl, and it can be applied to study impacts of planning scenarios. Options for improvement or extension of the method are discussed