Passive microwave‐derived snow melt regions on the Greenland Ice Sheet

By comparing data from the Special Sensor Microwave Imager (SSM/I) to field data, a melt threshold of the cross‐polarized gradient ratio (XPGR), which is a normalized difference between the 19 GHz horizontally‐polarized and 37 GHz vertically polarized brightness temperatures, is determined. This threshold, XPGR = −0.025, is used to classify dry and wet snow. The annual areal extent of melt is mapped for the years 1988 through 1991, and inter‐annual variations of melt extent are examined. The results show that the melt extent varied from a low of 38.3% of the ice sheet (1990) to a high of 41.7% (1991) during the years 1988–1991.