Variations and driving factors of annual frequency of ground surface freeze–thaw in China

The annual frequency of ground surface freeze–thaw (AFGSFT) is a crucial indicator of hydrological processes, climate change, and energy balance, as it directly measures the number of times the ground surface freezes and thaws within a year. Unlike other indicators, such as freezing depth and duration, AFGSFT is more closely related to ground surface water heat exchange. However, the mechanism behind the variability of the long-term AFGSFT time series in China remains largely unknown. Based on the observation data from 707 meteorological stations, the AFGSFT in China from 1960 to 2020 was analyzed. The analysis revealed a decreasing trend in AFGSFT from 1960 to 2020, with the highest value observed in the central Qinghai-Tibet Plateau, and the rate of change in AFGSFT increasing with elevation. Despite these changes, the spatial distribution of AFGSFT closely aligns with the two regions divided by the Hu Huanyong Line (Hu Line). Elevation was found to be the primary geographic factor explaining the spatial variation of AFGSFT, while the ground surface temperature was the main environmental factor. Using the autoregressive integrated moving average model (ARIMA), the study projected that the downward trend of AFGSFT will slow down in the next decade. The widespread climate warming was identified as the primary cause of AFGSFT changes, and this effect is expected to contribute significantly to the climate warming process in the future.