On the freeze-thaw cycles of shallow soil and connections with environmental factors over the Tibetan Plateau

Changes in the freeze-thaw cycles of shallow soil have important consequences for surface and subsurface hydrology, land-atmosphere energy and moisture interaction, carbon exchange, and ecosystem diversity and productivity. This work examines the shallow soil freeze-thaw cycle on the Tibetan Plateau (TP) using in-situ soil temperature observations in 0-20 cm soil layer during July 1982-June 2017. The domain and layer averaged beginning frozen day is November 18 and delays by 2.2 days per decade; the ending frozen day is March 9 and advances by 3.2 days per decade; the number of frozen days is 109 and shortens by 5.2 days per decade. Altitude and latitude combined could explain the spatial patterns of annual mean freeze-thaw status well. Stations located near 0 degrees C contour line experienced dramatic changes in freeze-thaw cycles as seen from subtropical mountain coniferous forest in the southern TP. Soil completely freezes from surface to 20-cm depth in 15 days while completely thaws in 10 days on average. Near-surface soil displays more pronounced changes than deeper soil. Surface air temperature strongly influences the shallow soil freeze-thaw status but snow exerts limited effects. Different thresholds in freeze-thaw status definition lead to differences in the shallow soil freeze-thaw status and multiple-consecutive-day approach appears to be more robust and reliable. Gridded soil temperature products could resolve the spatial pattern of the observed shallow soil freeze-thaw status to some extent but further improvement is needed.