Simulated change in the near-surface soil freeze/thaw cycle on the Tibetan Plateau from 1981 to 2010

The near-surface freeze/thaw cycle in cold regions plays a major role in the surface energy budget, hydrological activity, and terrestrial ecosystems. In this study, the Community Land Model, Version 4 and a suite of high-resolution atmospheric data were used to investigate the changes in the near-surface soil freeze/thaw cycle in response to the warming on the Tibetan Plateau from 1981 to 2010. The in situ observations-based validation showed that, considering the cause of scale mismatch in the comparison, the simulated soil temperature, freeze start and end dates, and freeze duration at the near-surface were reasonable. In response to the warming of the Tibetan Plateau at a rate of approximately 0.44 A degrees C decade(-1), the freeze start-date became delayed at an area-mean rate of 1.7 days decade(-1), while the freeze end-date became advanced at an area-mean rate of 4.7 days decade(-1). The delaying of the freeze start-date, which was combined with the advancing of the freeze end-date, resulted in a statistically significant shortening trend with respect to the freeze duration, at an area-mean rate of 6.4 days decade(-1). Such changes would strongly affect the surface energy flux, hydrological processes, and vegetation dynamics. We also found that the rate of freeze-duration shortening at the near-surface soil layer was approximately 3.0 days decade(-1) lower than that at a depth of 1 m. This implied that the changes in soil freeze/thaw cycles at the near surface cannot be assumed to reflect the situation in deeper soil layers. The significant correlations between freeze duration and air temperature indicated that the shortening of the near-surface freeze duration was caused by the rise in air temperature, which occurred especially in spring, followed by autumn. These results can be used to reveal the laws governing the response of the near-surface freeze/thaw cycle to climate change and indicate related changes in permafrost.