Dynamics of the freeze-thaw front of active layer on the Qinghai-Tibet Plateau

The seasonal dynamics of the freeze-thaw front during the freezing and thawing period strongly affect the hy-drothermal processes in the active layer and the water-energy exchange processes between land and the at-mosphere. Therefore, characterizing variation in the freeze-thaw front and its effect on the water and heat transfer mechanisms in the soil layer is important for enhancing our understanding of change in permafrost. In this study, we used data from field observations to analyze the freeze-thaw front dynamics during the freezing and thawing period in permafrost regions of the Qinghai-Tibet Plateau. The zero-curtain duration time was longer during the freezing period than during the thawing period at all sites. Changes in the freeze-thaw front were characterized by three distinct stages: the downward migration of the thaw front, zero curtain process at the bottom of the active layer, and the bidirectional freezing period. In addition, changes in the thaw front and soil moisture thaw period were consistent, but the freezing period of moisture showed large hysteresis compared with ground temperature. There was a power function and exponential relationship between the negative ground temperature and the unfrozen water content during warming and cooling processes, respectively. Linear and power relationships were observed between the net radiation accumulation and thawing front, and there was a power function relationship between the thawing front and soil heat flux. There was a quadratic polynomial relationship between the thawing front and thawing degree-days. The fitted relationships can better represent the dynamics of the freeze-thaw front during the freezing and thawing period. These findings aid our under-standing of the hydrothermal characteristics of the active layers and will improve future permafrost simulation models.