Means and Trends of Seasonally Frozen Ground Thickness in the Northern Hemisphere

Seasonal freezing and thawing processes of soils have a great impact on surface energy balance, hydrological cycle, carbon exchange, and serious natural hazard at middle and high latitudes. In this study, we will investigate spatial and temporal variability of seasonally frozen ground thickness from 1950 through 2000 in the northern hemisphere. Seasonally frozen ground thickness is estimated by a simplified Stefan solution using the "edaphic factor" and the annual freezing and thawing index of air temperature. Over the permafrost regions, the "edaphic factor" is determined using ground-based active layer thickness from 31 stations from 1950s through 1990 in the Russian Arctic, 103 CALM stations since the early 1990s, and six stations over the Tibetan Plateau from 1996 through 2002. Over the non-permafrost regions, the "edaphic factor" is determined based on ground-based measurements of seasonally frozen ground depth from 350 stations in Russia, 150 stations in China, 40 stations in Mongolia, and 30 stations from the United States. Annual freezing and thawing index were calculated from gridded monthly mean air temperature. After comparing with annual freezing and thawing index obtained using daily air temperature, errors of annual freezing and thawing index obtained from mean monthly air temperature are relatively small. We will present climatology, standard deviation, and trends of the "edaphic factor", annual freezing and thawing index, and seasonally frozen ground thickness in the northern hemisphere using all available data. Seasonally frozen ground thickness obtained from this study will also be compared with ground-based measurements and modeling outputs.