Finding the Sea Level - A universal rural reference for UHI-estimation

The urban heat island 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 the introduction of the Local Climate Zones (LCZ), close attention has been paid to the definition and description of “urban” sites. However, what remains understudied and inconsistent across studies is the “sea level” of the “archipelago”, i.e., defining the surrounding temperature, unaffected by the city. Here, we compare definitions and requirements of that “sea level”.

Most typically, professional weather stations at airports are used as a “rural” reference. However, these stations are not ubiquitously available and suffer from local effects such as a high fraction of impervious surfaces and meso-scale effects like 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 available in urban areas. ERA5-Land reanalysis data, on the other hand, is available globally and does not consider urban areas; hence, it provides an independent reference.

In this study, all three data sources were compared against an ideal case of multiple professional weather stations placed around the city. We find that ERA5-Land data is a consistent and ubiquitously available reference for the definition of “rural”. Using it as a universal rural reference allows for comparison between cities and further enables the potential of CWS, even in regions with few stations and a lack of professional 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.