Fragmentation of ground surface freezing processes in Northeast China from 1950 to 2022

Current studies mainly employ the onset and end dates of freezing, freeze duration and freeze depth to describe the freeze state, often overlooking the discontinuous nature of the freezing process. In this study, the dynamics of the ground surface freezing processes in Northeast China (NEC) was systematically analysed using ERA5-Land reanalysis datasets from 1950 to 2022, and a new and fragmentation-focused perspective was adopted. The ground surface (0−7 cm in depth) temperature obtained from the ERA5-Land dataset performed well in detecting freeze days when compared with the observations (5 cm in depth) in NEC. From 1950 to 2022, the freezing processes in NEC became increasingly fragmented, with the number of freeze interruptions increasing at a rate of 0.04 times per decade (p < 0.05) and the freeze fragmentation degree rising at a rate of 0.07 times per 100 d per decade (p < 0.01). Regional differences in the spatial and temporal variations of the freezing process fragmentation were also detected. Further analysis suggested that a significant increase (p < 0.05) in the number of freeze–thaw cycle events and thaw days within the shortened freeze period (−1.8 d per decade; p < 0.01) may partially increase the probability of freezing process fragmentation. Moreover, the freeze fragmentation degree was positively and significantly correlated with air and skin temperatures (p < 0.01), while it was negatively and significantly correlated with snow cover duration and volumetric soil moisture (p < 0.05).