Six years of ground–air temperature tracking at Malence (Slovenia): thermal diffusivity from subsurface temperature data

To understand the processes which govern the downward propagation of the surface temperature signal and to quantify the relationship between ground and air temperatures, a borehole climate observatory was established at Malence (Slovenia). A substantial (climate?) warming has been observed; surface air temperature from 2003 to 2009 has been increasing by a rate of more than 0.5 K/year. Temperature difference between ground and air temperatures revealed a typical annual course; during the winter months the ground is warmer than the air, in the summer the ground gets slightly colder, the mean yearly offset value amounting to 1.6 K. Here we report on the thermal diffusivity distribution which was determined for a near-surface 10 m deep section using a number of six-year-long temperature–time data series. The series of daily averaged temperatures were processed by spectral analysis. Thermal diffusivity was calculated from the amplitude and phase angle of annual waves corresponding to different subsurface layers using conduction theory as well as the conduction–convection approach. Both methods lead to the generally similar results and revealed a definite heterogeneity of the subsurface medium. The results proved that the annual scale convective heat transfer did not contribute significantly to the temperature–time variations monitored in the uppermost 10 m depth interval.