Analysis of the Swiss urban climate over the past five years

Abstract

Switzerland is home to several medium to small cities, usually situated on lakes or large rivers and always in the proximity of mountains. Both size and location contribute to reducing the dominance of the urban climate over the local weather systems, e.g., lake breezes, mountain and valley winds, but the interaction between Swiss cities and the local climate has long been monitored with measurements [1]. This contribution presents a numerical analysis of the alpine country over the past five years (2017-2022). The focus is on the urban climate and urban heat island (UHI) effect. The simulations are performed with the Consortium for Small-Scale Modeling (COSMO) model at a resolution of 1.1km, with explicit convection, and are validated against measurements by the Federal Office of Meteorology and Climatology. The bulk scheme TERRA_URB [2] is employed for modelling urban areas and is provided with a 100m-resolution state-of-the-art database for the subdivision of the urban landscape into local climate zones (LCZ) [3]. Our findings show that the UHI of Swiss cities (i.e., the temperature delta between city centre and a rural reference) closely follows the weather patterns measured over the same period and does not show a net positive trend, despite the continuous urbanization of the country which is in line with the European average [4]. The number of tropical nights (i.e., nights when the temperature did not drop below 20C) and other “classic” measures show comparable trends. The only year exhibiting a clear variation is 2022, which was a record warm year for large parts of Europe. Regarding UHI the worst affected city is Zurich, with a time averaged maximum UHI of around 3C, while most other large cities have values of 2-2.5C and smaller cities between 0.5 and 2C. Lugano, the only city in our analysis south of the Alps, presents the highest number of tropical nights, followed by Geneva and Lausanne. Simulations allow us to investigate the space-dependent nature of the UHI effect, which is especially insightful for medium-small cities. We present such an analysis for the ten largest Swiss cities, providing an objective quantification of the effects caused by different LCZs and geographical features, as well as a direct intercomparison between similarly built-up areas in different cities. The seasonality of UHI as well as the influence of weather patterns on its magnitude are also analysed, completing the spatiotemporal picture of the country’s current urban climate.