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Remote Sensing of Environment
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Spatial and temporal variations of summer surface temperatures of high-arctic tundra on Svalbard — Implications for MODIS LST based permafrost monitoring
Sebastian Westermann ⁎, MoritzLanger, Julia Boike
Alfred-Wegener-Institute for Polar and Marine Research, Telegrafenberg A6, Potsdam, Germany
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The ground surface temperature is one of the key parameters that determine the thermal regime of permafrost soils in arctic regions. Due to remoteness of most permafrost areas, monitoring of the land surface temperature (LST) throughremote sensing is desirable. However, suitable satellite platforms such as MODIS provide spatial resolutions that cannot resolve the considerable small-scale heterogeneity of the surface conditions characteristic for many permafrost areas. This study investigates the spatial variability of summer surface temperatures of high-arctic tundra on Svalbard, Norway. A thermal imaging system mounted on a mastfacilitates continuous monitoring of approximately 100 × 100 m of tundra with a wide variability of different surface covers and soil moisture conditions over the entire summer season from the snow melt until fall. The net radiation is found to be a control parameter for the differences in surface temperature between wet and dry areas. Under clear-sky conditions in July, the differences insurface temperature between wet and dry areas reach up to 10 K. The spatial differences reduce strongly in weekly averages of the surface temperature, which are relevant for the soil temperature evolution of deeper layers. Nevertheless, a considerable variability remains, with maximum differences between wet and dry areas of 3 to 4 K. Furthermore, the pattern of snow patches and snow-free areas duringsnow melt in July causes even greater differences of more than 10 K in the weekly averages. Towards the end of the summer season, the differences in surface temperature gradually diminish. Due to the pronounced spatial variability in July, the accumulated degree-day totals of the snow-free period can differ by more than 60% throughout the study area. The terrestrial observations from the thermalimaging system are compared to measurements of the land surface temperature from the MODIS sensor. During periods with frequent clear-sky conditions and thus a high density of satellite data, weekly averages calculated from the thermal imaging system and from MODIS LST agree within less than 2 K. Larger deviations occur when prolonged cloudy periods prevent satellite measurements. Furthermore, theemployed MODIS L2 LST data set contains a number of strongly biased measurements, which suggest an admixing of cloud top temperatures. We conclude that a reliable gap ﬁlling procedure to moderate the impact of prolonged cloudy periods would be of high value for a future LST-based permafrost monitoring scheme. The occurrence of sustained subpixel variability of the summer surface temperature is acomplicating factor, whose impact needs to be assessed further in conjunction with other spatially variable parameters such as the snow cover and soil properties. © 2010 Elsevier Inc. All rights reserved.
Article history: Received 27 April 2010 Received in revised form 13 October 2010 Accepted 7 November 2010 Available online 31 December 2010 Keywords: Permafrost Thermal imaging Land surfacetemperature Subpixel variability MODIS L2 product Svalbard
1. Introduction Permafrost occurs in about a quarter of the land masses in the Northern Hemisphere (Brown et al., 1997). Particularly in arctic and subarctic regions of Russia, Canada and Alaska, permafrost-related processes play a crucial role in the energy and water cycle and thus determine the hydrology and ecology of these regions. In...