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Freezing and Thawing Cycle of Soils at Local, Regional, and Global Scales

NASAThis project is funded by NASA


To improve our understanding of the near-surface soil thermal regime and seasonal freezing/thawing processes of soils, and their interactions and feedbacks to changes in hydrologic and climatic systems at local, regional, and global scales


Tingjun Zhang is the PI and Richard Armstrong is Co-Investigator.

Project Summary

This project will (i) further improve and validate passive microwave frozen soil algorithms and numerical models to detect surface soil freeze/thaw status, (ii) investigate seasonal and interannual variations of seasonally frozen ground, (iii) examine the snow-frozen ground-monsoon hypothesis over the GAPP (GEWEX American Prediction Project) study area associated with the North American monsoon system, and (iv) generate a frozen ground data set as part of a contribution to the NASA/NOAA Land Data Assimilation Systems and to the GEWEX CEOP program.

In 2005, we developed a combined frozen soil algorithm, which we validated to detect the nearsurface soil freeze/thaw cycle over snow-free and snow-covered land areas in the contiguous United States. The combined frozen soil algorithm consists of two parts. For snow-free land areas, we used a passive microwave remote sensing algorithm to detect the near-surface soil freeze/thaw cycle over snow-free land areas, and for snow-covered land areas, we used a one dimensional numerical heat-transfer model with phase change to detect soil freeze/thaw status under snow cover. We applied the validated frozen soil algorithm to investigate near-surface soil freeze/thaw status over the contiguous United States from 1978 through 2003 and the Northern Hemisphere as a whole. The long-term average maximum area extent of seasonally frozen ground, including the active layer over permafrost, is approximately 50.5 percent of the landmass in the Northern Hemisphere. Preliminary results indicate that the extent of seasonally frozen ground has decreased about 15 to 20 percent during the past few decades.

This study also examined the 20th-century variations of active layer thickness (ALT) for the Ob, Yenisey, and Lena River basins over the Russian Arctic. ALT is estimated from historical soil temperature measurements from 17 stations (1956 to 1990, Lena basin only), an annual thawing index based on surface air temperature data (1901 to 2002), and numerical modeling (1980 to 2002). The latter two provide spatial fields. Based on the thawing index, the long-term (1961 to 1990) average ALT is about 1.87 meters in the Ob, 1.67 meters in the Yenisey, and 1.69 meters in the Lena basin. Over the past several decades, ALT over the three basins shows positive trends, but with different magnitudes. Based on the 17 stations, ALT increased about 0.32 meters between 1956 and 1990 in the Lena. To the extent that results based on the soil temperatures represent ground "truth," ALT obtained from both the thawing index and numerical modeling is underestimated. It is widely believed that ALT will increase with global warming. However, this hypothesis needs further refinement since ALT responds primarily to summer air temperature while observed warming has occurred mainly in winter and spring. It is also shown that ALT has complex and inconsistent responses to variations in snow cover.

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