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The Spatial and Temporal Variability of the Energy Balance at an Arctic Polygonal Tundra Site

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Langer, M. (2010): The Spatial and Temporal Variability of the Energy Balance at an Arctic Polygonal Tundra Site , PhD thesis, University of Heidelberg, Combined Faculties for Natural Sciences and Mathematics.
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Abstract:

Permafrost soils, which occupy by far the largest fraction of the arctic land area, are expected to be substantially affected by climate warming. The degradation of permafrost is potentially associated with climate feedback mechanisms such as greenhouse emissions and changes in the hydrological cycle, which could magnify future climate warming. The determination of the recent and future permafrost distribution and its thermal condition is therefore an essential issue for the prediction of future climate change. This requires the development of reliable monitoring and modeling schemes, which allow both the future predictions and the validation of the permafrost conditions. Studies of the surface energy balance can significantly contribute to the development of modeling schemes, since they directly measure the processes at the ground-atmosphere interface as they are represented in climate models. This thesis investigates the surface energy balance in a polygonal tundra landscape of the Lena River Delta, Siberia, in a series of extensive field measurements. The controlling factors of the surface energy balance are in particular the snow cover, the presence of a cloud cover and the ground thermal regime. The first two factors mainly inuence the radiation budget by reecting the largest part of the incoming short-wave radiation in spring, and by increasing the incoming long-wave radiation, respectively. The ground heat ux is found to be of remarkable importance for the surface energy balance, especially during the polar winter, when the refreezing active layer provides a strong supply of energy. In addition, the large annual temperature amplitude at the study site contributes to the strong ground heat uxes. Turbulent heat uxes are of great importance particularly during summer, when latent heat uxes consume about half of the net radiation. However, spatially distributed measurements of the turbulent heat uxes suggest distinctly different surface energy balances over scales of ten meters due to the regular pattern of dry and wet areas of the polygonal tundra. This is also confirmed by spatially resolved measurements of the surface temperature with a thermal imaging system during the summer season. Due to different partitioning of energy at dry and wet surfaces remarkable temperature differences on the order of 5 to 10 K can occur. These spatial differences in the surface temperature are found to vanish in temporal averages longer than the diurnal cycle. While this suggests that the summer radiation budget of dry and wet areas is not too different, it also has important implications for permafrost monitoring schemes based on remotely sensed land surface temperatures. The diminished surface temperature variability for temporal averages reduces the requirements on the spatial resolution of satellite-based surface temperature products. The study is a clear indication of the potential of satellite-based monitoring of permafrost landscapes.

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