Delineation of Surface and Near-Surface Melt on the Greenland Ice Sheet using MODIS and QuikSCAT Data

Recently there has been a focus on the Greenland Ice Sheet because of observations of increasing arctic surface temperatures over the last few decades. Furthermore, airborne- and satellite-borne altimeter and satellite gravimetry measurements show net mass loss of ice from Greenland. To quantify the extent of surface melt on the ice sheet which is an important component of the total melt, researchers use data from various remote-sensing instruments, but different instruments are sensitive to different physical features on the ice sheet and they may have different spatial resolutions. Thus there has been ambiguity about the actual extent and location of melt in a given day (or maximum melt in a season). For example, using passive-microwave radiometry employing different algorithms from the same instrument can provide quite different values of total melt extent. To determine the actual amount of surface melt, we use three melt algorithms from two different instruments. The physical basis of the algorithms has been established in previous work. A special melt product derived from QuikSCAT (QS) provides a value of 862,769 km2 ±3% for maximum melt extent on the ice sheet for the 2007 melt season, while the Moderate Resolution Imaging Spectroradiometer (MODIS) land-surface temperature (LST) standard product provides a value of 766,184 km2±8%. The QS product detects about 11% greater melt than is detected by the MODIS LST product; this is because QS is more sensitive to surface melt, and can also detect subsurface melt. In addition, the QS algorithm detects any melt that occurs between 6:00 A.M. and 6:00 P.M. local time, while the MODIS algorithm is derived from a single "snapshot" view in a given day and may thus miss melt if it occurs at another time of the day. In spite of these differences, there is a high degree of consistency in the response of the QS and MODIS products to daily surface melt throughout the 2007 melt season. This demonstrates unequivocally that physically-meaningful melt/freeze boundaries can be detected. The addition of the MODIS daily snow albedo product allows further refinement of the melt extent on the ice sheet on many days. Used together, these products can improve the precision in mapping melt extent on the Greenland Ice Sheet. In the near future, melt season data in all available years from 2000 through at least 2009 will be processed, and QS/MODIS results for each year will be evaluated over Greenland.