Satellite-Derived Melt-Season Temperature Of The Greenland Ice Sheet - GRL

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<10> In some cases there is only one clear day available to create the 8-day LST map for any given cell, thus the LST of each cell may not represent a true mean for the 8-day period. Figure 1 shows the average number of clear-sky days for each 8-day period during the entire melt season . Data used in Figure 1 were derived from the MODIS LST MOD11C2 product, and provide an indication of the relative cloudiness among the years. Nineteen 8-day-mean LST maps of Greenland were averaged on a cell-by-cell basis for the entire melt season (between days 121 and 272), to develop one map of meansurface temperature along with a standard deviation (SD) map for each melt season. (Only 17 and 18 maps were used for 2001 and 2004, respectively, due to missing and/or
suspected-bad data.) In addition, we also studied the period of most-active melt in each year, May through mid-August . Because freezeup can begin around mid-August, we consider days 121–225 to be the period of most-active melt for the purpose of this work. Surface temperatures during the most-active part of the melt season are more relevant to surface melt on the ice sheet than those derived from the entire melt season, during which time subsurface melt may play a significant role.

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<18> Higher ice-sheet surface temperatures can lead to enhanced ice-sheet disintegration when surface water percolates through great thicknesses of the ice sheet leading to an acceleration of ice-sheet flow . We observed a general expansion of increasing surfacetemperature variability from 2000 to 2005 on the Greenland Ice Sheet, being greatest in 2005, a year cited as being anomalously warm over Greenland by other investigators, and according to our LST data. The SDs are consistent with surface instability caused by airtemperature fluctuations.

<19> We also found that the two warmest years (2002 and 2005) are the same years that experienced the most-extensive melt of the six-year period. Steffen et al. <2004> showed that there was a very large melt extent in 2002, extending over 690,000 km2 of the ice sheet compared to a 1979–1999 average of 455,000 km2 , and 2005 has been cited by Steffen and Huff (http://cires.colorado.edu/science/groups/steffen/greenland/melt2005/) as experiencing melt equal or greater in area than occurred in 2002. This is consistent with the average melt-season LSTs presented herein, and findings by Comiso <2006> showing that those same two years were unusually warm using AVHRR data of the Arctic since 1981.

<20> MODIS-derived LST provides a quantitative assessment of the mean surface temperature and surface-temperature change on the Greenland Ice Sheet. Because the location of the glacier-facies boundaries reflects the ice sheet mass balance, a sustained increase in surface temperature would cause the glacier-facies boundaries to migrate to higher elevations.

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