Elevation-dependent warming in Switzerland: Observed signals and dataset limitations

Since the 1950s, Switzerland has experienced pronounced year-round warming, with the strongest increases occurring in summer and spring. The analysis of homogeneous, time-consistent, high-resolution monthly gridded near-surface temperature data reveals two distinct patterns of elevation-dependent warming (EDW). First, low elevations exhibit stronger warming than higher elevations in autumn and early winter, primarily due to increased sunshine duration resulting from a substantial reduction in fog and low stratus clouds. Second, mid-elevations show slightly enhanced spring warming driven by snow-albedo feedback. Popular gridded climate datasets often fail to capture these patterns accurately or tend to overestimate them. For instance, ERA5 and ERA5-Land reproduce the enhanced spring warming at mid-elevations but overestimate its magnitude and exhibit a time lag. They fail to capture the autumn / early winter lowland excess warming, partly due to an inadequate representation of fog and low stratus. Standard E-OBS versions show erroneous EDW signals, whereas the homogenized version E-OBS HOM reproduces elevation-dependent effects with notable accuracy. Our findings underscore the importance of utilizing time-consistent datasets that can accurately resolve fine-scale features to assess EDW in topographically complex mountain regions.