VARIATIONS IN SNOWPACK MELT ON THE GREENLAND ICE-SHEET BASED ON PASSIVE-MICROWAVE MEASUREMENTS

A simple microwave-emission model is used to simulate 37 GHz brightnes s temperatures associated with snowpack-melt conditions for locations across the Greenland ice sheet. The simulated values are utilized as t hreshold values and compared to daily, gridded SMMR and SSM/I passive- microwave data, in order to reveal regions experiencing melt. The spat ial extent of the area classified as melting is examined on a daily, m onthly and seasonal (May-August) basis for 1979-91. The typical season al cycle of melt coverage shows melt beginning in late April; a rapid increase in the melting area from mid-May to mid-July, a rapid decreas e in melt extent from late July through mid-August, and cessation of m elt in late September. Seasonal averages of the daily melt extents dem onstrate an apparent increase in melt coverage over the 13 year period of approximately 3.8% annually (significant at the 95% confidence int erval). This increase is dominated by statistically significant positi ve trends in melt coverage during July and August in the west and sout hwest of the ice sheet. We find that a linear correlation between micr owave-derived melt extent and a surface measure of ablation rate is si gnificant in June and July but not August, so caution must be exercise d in using the microwave-derived melt extents in August. Nevertheless, knowledge of the variability of snowpack melt on the Greenland ice sh eet as derived from microwave data should prove useful in detecting cl imate change in the Arctic and examining the impact of climate change on the ice sheet.