Climatology of the Timing and Duration of the Near-Surface Soil Freeze-Thaw Status Across China

The near-surface soil is an important interface in ground—atmosphere interactions. The near-surface soil freeze-thaw status is critical for energy, moisture, and carbon exchange between the ground and the atmosphere, plant growth, and the ecosystem as a whole. The main objective of this study is to investigate climatology of the timing and duration of the near-surface soil freeze-thaw status using data from 636 meteorological stations across China for the baseline period from July 1971 through June 2001. The long-term average first date of the near-surface soil freeze is 14 September (30 July–30 October), the last date is 15 May (8 April–21 June), the duration is 245 ± 85 days, and the actual number of the near-surface soil freeze days is 202 ± 90 days over China as a whole. On the Qinghai—Tibetan Plateau, the near-surface soil freeze can occur essentially in any month of a year. The spatial variations of the near-surface soil freeze-thaw status are strongly controlled by latitude in east China, and by elevation in west China. The long-term average 220-day and 260-day contours of the near-surface soil freeze coincide approximately with the southern boundary of high—latitude permafrost regions in northeastern China and the lower boundary of high—altitude permafrost regions in west China, respectively. The number of days and duration of the near-surface soil freeze decreased with increasing long-term mean annual air temperature (MAAT). Variation of the actual number of the near-surface soil freeze days presents nonlinear linkage to the length of the near-surface soil freeze duration and also to the MAAT climatology. The timing and duration of the near-surface soil freeze-thaw status are strongly nonlinearly related to air freezing index, but are nearly linearly related to air thawing index.