Impacts of diurnal temperature cycles on the geothermal regime on Qinghai-Tibet Plateau

This study investigates whether the diurnal temperature cycle affects the geothermal regime on the Qinghai-Tibet Plateau. To separately characterize this effect, the impact of climatic warming on the ground's thermal regime is eliminated by setting the global warming rate to 0 °C/year. The diurnal temperature cycle at the natural ground surface is denoted as sinusoidal functions with amplitudes of 0, 5, 8, and 12 °C, respectively. A one-dimensional heat conduction model was utilized to compute the geo-temperature under the natural ground surface, eliminating the effect of geometric boundaries, such as the roadway's embankment, on the geothermal regime. The results show that the diurnal temperature cycle does affect the geothermal regime as (1) under the same mean annual ground temperature, the higher diurnal temperature fluctuation amplitude (DTFA) on the ground surface, the thinner the active layer; (2) the higher the DTFA, the colder the underlying soil. An analysis of the heat flow at the ground surface showed that the diurnal temperature cycle resulted in a net negative heat balance at the earth's surface. This heat loss induced by the diurnal temperature cycle cools the underlying soil. The results and analysis suggest that, currently, the documented numerical model which ignores the diurnal temperature cycle overestimates the warming of the underlying soil. This overestimation, if the DTFA at ground surface is 12 °C, would be up to 0.4 °C. Considering that pavement surface usually undergoes high diurnal temperature cycles, the impact of the DTFA on pavement subgrade's frost conditions and on the pavement deformation is simply discussed.