Research on the temporal-spatial changes of near-surface soil freeze/thaw cycles in China based on Radiometer

Near-surface soil freeze-thaw cycles (NSC) would have dramatic influences on hydrologic processes, ecosystem and engineering operations. Passive microwave remote sensing (PMRS) has been a powerful tool for detecting the changes of NSC. The research on NSC based on large-scale and long-time series PMRS data is still rare. In this research, we used the decision tree to extract daily soil freeze/thaw states derived from Special Sensor Microwave/Imager (SSM/I) data from 1988 to 2008. 7-day moving average method was employed to extract the yearly area-averaged frozen days, freeze/thaw onset date of each pixel and frozen pixels from 1988 to 2008. Through analysis of the results above, we found that about 3/4 of the land surface experienced the NSC every year, and less than 4% was always in frozen states which mainly distributed in snow mountains of Qinghai-Tibet Plateau. The changes of NSC in most part of China were seasonal variations, and changed significantly during a year. The general trends of these changes are later freeze onset dates, earlier thaw onset dates, fewer freeze days and longer growing season. Our research would have contributed to understanding near-surface earth systems and extreme environmental events such as the dust emission in semi-arid and arid regions of East Asia.