Influences of winter climatic conditions on the relation between annual mean soil and air temperatures from central to northern Japan

Annual mean soil temperature (STA) is an important factor to evaluate the potential of the productivity of agricultural fields and the amount of ground thermal resources. To assess the influence of the climate on the STA, we collected and analyzed a field dataset at five suburban sites from central to northern Japan. Additionally, the effects of winter climate on the relation between STA and ATA were investigated using a numerical soil temperature model. Results show that STA was correlated positively with ATA. The difference between STA and ATA (STA/ATA offset) was approximately 1 °C at the southern sites, where monthly mean air temperatures do not drop below 0 °C. The offset, 2–4 °C in the northern and colder sites, was correlated positively with the cumulative freezing degree-days (CFD). Analyses conducted using a numerical soil-temperature model revealed that the estimated STA/ATA offset under a snow-free condition was 1.2 °C on average in the northern sites, which was similar to the offset in the southern snow-free sites. This similarity suggests that the thermal insulation of snowpack was the dominant factor in raising the offset. Additionally, the numerical simulation showed that the offset reached the ceiling when the winter mean snow cover thickness (SCTmean) exceeded 0.2 m under the air temperature in colder northern sites in Japan. In conclusion, although the STA/ATA offset is known to be influenced by both CFD and SCTmean, the offset increases directly with CFD rather than SCTmean when snow cover is sufficiently thick to insulate the soil from cold air.