Finding the sea level – Investigation of rural references for estimation of the urban heat island

Abstract

The urban heat island (UHI) effect is one of the most studied phenomena in urban climatology. Numerous studies have revealed the heterogeneous nature of air temperature within cities, manifesting as an urban heat “archipelago” with small-scale air-temperature differences and multiple hot and cold spots rather than a single, uniform hot spot in the city core. With the introduction of the local climate zones (LCZs) scheme, close attention has been paid to the definition and description of “urban” and “rural” sites. However, what remains understudied and inconsistent across studies is the “sea level” of the “archipelago,” i.e., defining the air-temperature conditions of the surroundings, unaffected by the city. Here, we compare definitions and requirements of that “sea level,” and investigate multiple possible data sets for UHI calculation. Most typically, single weather stations, often at airports, are used as a “rural” reference. However, these stations are not ubiquitously available and typically suffer from effects such as a high fraction of impervious surfaces or urban heat advection when located downwind of the city. Crowd weather stations (CWS), which have gained attention in recent years in urban climate studies due to their abundance, are often affected by nearby buildings and are mostly located in urban areas. Besides station-based data, reanalysis products such as ERA5-Land could provide an independent reference, as they are available globally and often do not consider urban areas. In this study, all three data sources were compared against an ideal case of multiple professional weather stations placed around the city. We investigate the two temperate European cities of Paris (France) and Berlin (Germany) during six years (2019–2024), focusing on crowdsourced data from CWS. We find that ERA5-Land air-temperature data is a consistent and ubiquitously available reference for the definition of “rural,” providing UHI-calculations most comparable to the ideal case of having multiple professional weather stations. Using it as a universal rural reference allows for comparison between cities and further enables exploiting the potential of CWS, even in regions with few stations and a lack of professionally-operated rural weather stations. Establishing a consistent “sea level” for urban air-temperature and UHI-studies enables comparison between various cities globally and allows for the integration of different data sources, such as local networks of weather stations or CWS, on a larger scale.