Monitoring near-surface soil freeze-thaw cycles in northern China and Mongolia from 1998 to 2007

Remote sensing based near-surface soil freeze-thaw cycles detection in middle latitude especially where near-surface changing seriously still rare. In this research, soil temperature and 7-day maximum/minimum combined passive microwave brightness temperature were employed to apply a soil freeze-thaw algorithm in northern China and Mongolia. We proposed a random sampling technique to determine brightness temperature thresholds for 37 GHz vertically polarized radiation: 258.2 and 260.1 K for the morning and evening satellite passes, respectively, and determined the onset, offset, and duration of the phases of the near-surface soil freeze-thaw cycle. During the 10 years from 1998 to 2007, the onset and offset of soil frozen/thawed in spring and autumn progressed from south to north and northwest, and from low elevation to high elevation. The durations of the freeze-thaw transitions in spring, autumn and whole year were longest in the Loess Plateau, Ordos Plateau, and Songnen Plain, where they were 1-3 weeks longer than in other regions. The total annual durations of soil frozen/thawed increased/decreased progressively from the south to both the northwest and northeast. Over the 10 years, changes to both the timing and duration of phases of the freeze-thaw cycle were greater in spring than in autumn. Most of the changes were less than 2 weeks, but there were changes of up to 3-4 weeks on the Northeast Plain, Loess Plateau, and at Mt. Yinshan. Our research has contributed to understanding near-surface earth systems and suggests that changes in earth surface may cause extreme environmental events such as the dust emission in semi-arid and arid regions of East Asia. (C) 2010 Elsevier B.V. All rights reserved.