The footprint of heat waves and dry spells in the urban climate of Würzburg, Germany, deduced from a continuous measurement campaign during the anomalously warm years 2018–2020

The present study contributes to the issue of the urban heat island (UHI) effect with its possibly associated thermal stress for city dwellers and its potential mitigation during heat waves and dry spells in Central Europe. It is based on meteorological measurements along an urban transect in the city of Würzburg, Germany. Due to its topographic and structural situation, Würzburg is prone to an intense urban heat island (UHI). The measurements have started in 2018 and, hence, cover a period that was characterized by record high temperatures and long dry spells in Central Europe. Particularly on days with a maximum air temperature of more than 25 °C, an intense UHI was observed with the highest amplitude in the afternoon and, even more, during the evening hours. The highest measured difference between the densely built inner city and the outskirts was 8.2 °C. The UHI during summer is noticeably more pronounced, especially during the evening hours, when the regional background climate is anomalously warm and dry. This can be ascribed to anticyclonic weather types that prevailed over Central Europe during summertime between 2018 and 2020. The cooling effect of urban trees, in this case Tilia cordata, on near-surface air temperature amounts to partly more than 2 °C and, hence, mitigates the UHI locally, especially at noon and in the early afternoon. However, the cooling rate is only half as much when the trees suffer from water stress. Thus, an appropriate management of city's green infrastructure represents a useful strategy to mitigate the strength of the UHI and the heat stress in Central Europe.