[1] Changes in active layer thickness (ALT) over northern high-latitude permafrost regions have important impacts on the surface energy balance, hydrologic cycle, carbon exchange between the atmosphere and the land surface, plant growth, and ecosystems as a whole. This study examines the 20th century variations of ALT for the Ob, Yenisey, and Lena River basins. ALT is estimated from historical soil temperature measurements from 17 stations , Lena basin only), an annual thawing index based on both surface air temperature data and numerical modeling . The latter two provide spatial fields. Based on the thawing index, the long-term average ALT is about 1.87 m in the Ob, 1.67 in the Yenisey, and 1.69 m 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 m 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 exhibits complex and inconsistent responses to variations in snow cover.Citation: Zhang, T., et al. (2005), Spatial and temporal variability in active layer thickness over the Russian Arctic drainage basin,

No abstract

[1] Thaw lakes are one of the most obvious manifestations of the hydrological system at work in the tundra regions of the Alaskan Arctic Coastal Plain, but the extent of the role of thaw lakes in Arctic land-atmosphere interactions and feedback has yet to be fully understood. This study uses a two-dimensional heat transfer model with phase change under a cylindrical coordinate system to simulate the long-term influence of shallow thaw lakes on the thermal regime of permafrost and talik development on the Alaskan Arctic Coastal Plain. On the basis of previous studies of permafrost and thaw lakes at Barrow, Alaska, a series of simulation cases was conducted using different combinations of long-term mean lake bottom temperature and lake depth. The simulated results indicate that shallow thaw lakes are a significant heat source to permafrost and talik. For a thaw lake with a long-term mean lake bottom temperature of greater than 0.0°C a talik forms under the thaw lake. The maximum talik thicknesses (vertical distance from the ground surface to the permafrost surface) are 28.0, 43.0, and 53.2 m 3000 years after the formation of a shallow thaw lake with long-term mean lake bottom temperatures of 1.0°, 2.0°, and 3.0°C, respectively. Talik development rate is very high in the first several years after a thaw lake formation and decreases gradually with time. No talik forms below a thaw lake with a long-term mean lake bottom temperature equal to or lower than 0.0°C, but the temperature of permafrost below the thaw lake increases with time. Three thousand years after the formation of a thaw lake with a long-term mean lake bottom temperature of greater than or equal to À2.0°C, ground temperature increases of more than 0.5°C occur as far as 300 m from the lake shore and as deep as about 400 m below the ground surface. It is concluded that variation of long-term mean lake bottom temperature has a significant influence on permafrost thermal regime and talik development. Continued monitoring for thaw lake bottom temperature and ground temperature under shallow thaw lakes is needed to further improve the simulation.

No abstract

Variations in the timing and duration of seasonal snow cover are critical to the ground thermal regime at high latitudes, due to changes in surface conditions and the associated ground surface energy balance. This study uses a one-dimensional heat transfer model with phase change, combined with a surface energy balance equation to analyse the impact of changes in the timing and duration of seasonal snow cover on the thermal regime of the active layer and permafrost. Based on meteorological data collected at Barrow, Alaska, a series of simulation cases was conducted by varying the snowpack onset date in autumn and the disappearance date in spring during 1997-98. Results show that the ground thermal regime is sensitive to variations in the timing and duration of seasonal snow cover. Delaying the snow cover onset date by 10 days in autumn and the disappearance date by 10 days in spring results in a decrease in ground temperature. The maximum ground temperature decreases at depths of 0.0, 0.5, 1.0, and 2.0 m were 9.0, 2.9, 2.0, and 1.1°C, respectively, and the mean annual ground temperature decreases were 0.7, 0.5, 0.4, and 0.4°C. Advancing the snow cover disappearance date by 10 days in spring leads to an increase in ground temperature. The maximum ground temperature increases at depths of 0.0, 0.5, 1.0, and 2.0 m were 6.6, 2.2, 1.5, and 0.7°C, and the mean annual ground temperature increases were 0.2, 0.2, 0.1, and 0.1°C, respectively. Variations in the timing and duration of seasonal snow cover also have an influence on active layer thickness, but the effect is limited.

No abstract

No abstract

[1] Numerical simulations were conducted to investigate the long-term impact of thaw lake drainage on the thermal regime of ground under and around drained thaw lakes on the Alaskan Arctic Coastal Plain. The numerical model used in this study is a twodimensional unsteady finite element model for heat transfer with phase change under a cylindrical coordinate system. The initial conditions are the simulated ground thermal regime and talik thickness data at year 3000 under a thaw lake with long-term mean lake bottom temperatures of 1.0, 2.0, and 3.0°C near Barrow, Alaska. The simulated results indicate that lake drainage leads to a rapid freeze-up of talik and a substantial decrease in permafrost temperatures under the former lake bottom. The initial ground temperature conditions have significant influence not only on the talik freeze-up time, but also on the permafrost temperature decrease processes. After thaw lake drainage, taliks with thicknesses of 28, 43, and 53 m under the lake will freeze-up completely by 40, 106, and 157 years, and the corresponding ratios of upward and downward freeze-up depths for the three simulation cases are 1: 8.0, 1: 7.6, and 1: 6.1, respectively. After completion of talik freeze-up, permafrost temperatures still decreased with time and gradually reached a relatively stable value, depending on the initial ground temperature and the distance to the former lake bottom.