Near-surface soil thermal regime and land-air temperature coupling: A case study over Spain

Abstract

Understanding the near-surface soil thermal regime and its connection to the atmospheric state is important for the assessment of several climate-related processes. However, the lack of in situ soil temperatures measurements limits the analysis of such processes. In this study, we have developed a quality-controlled soil temperature database for Spain that consists of 39 sites spanning from 1987 to 2018. We have used this database to assess the near-surface soil thermal regime. Likewise, we evaluate at seasonal to multidecadal timescales the land-air temperature coupling over Spain by analysing the structure of the surface air temperature (SAT) and the ground surface temperature (GST) covariance and also their long-term evolution. In addition, we have employed the ERA5-Land reanalysis to test the consistence between observations and reanalysis. The results show that the near-surface soil thermal structure is dominated by conduction despite some influence of hydrology-related processes. Regarding the land-air temperature coupling, we have found a strong connection between SAT and GST. However, in the summer months there is an offset in SAT-GST at some sites due to limited evaporation and enhanced sensible heat fluxes. Furthermore, multidecadal SAT-GST decoupling may exist over some sites as a response to decreasing precipitation. The ERA5-Land represents the observations' climatology well, but it underestimates the summer soil temperature observations and the long-term trends at some sites.