A New Approach to Determining the Melt-Area Extent Over the Greenland Ice Sheet Using MODIS Data

Polar regions are predicted to warm at a rate exceeding the global mean due to anthropogenic greenhouse gas forcing, creating the potential for serious impacts on future Earth system states. There has been considerable interest in the degree of change over the Greenland ice sheet, with parallel studies focusing on regions in Alaska and Antarctica. Observations of decreasing ice sheet mass, accelerating outflow of ice, increasing regions of summer melt area, growing numbers of ice bergs and future model predictions, all point to the possibility of serious and abrupt climate changes. Such analyses are alarming since the Greenland ice sheet alone holds enough water to raise sea levels by several meters. The potential for rapid and extreme response underscores the need for accurate observations of the cryosphere to allow more reasoned and observationally validated predictions of the natural variability and responses of the cryosphere to future climate scenarios. A key parameter needed to monitor the status of the ice sheet is the extent of summer melt area. Currently, melt area is derived principally through passive microwave satellite data, with a coarse spatial resolution of about 625 km2. Here we introduce a new approach for the retrieval of the summer melt area, focusing on a central region of the Greenland ice sheet, using data from the Earth Observing Systems (EOS) Terra and Aqua. Using reflectance data from the MODIS instrument, we have identified calibrated radiances at discrete visible and near infrared wavelengths, that respond differently to ice, water, and melted ice. Through the development of this approach, it is expected that improved knowledge of both inter- and intra-annual melt area change will be gained, and will also provide a means to better understand and distinguish between natural process dynamics and climate induced variability. The MODIS based melt area index provides a vastly improved spatial resolution of 1 km2 and, with 7 years of available MODIS data, will allow identification of seasonal and interannual variabilities to discern any trends in the melting extent of the Greenland ice sheet. This paper will demonstrate the validity of our approach by comparing MODIS observations with available ground-based data. We will present MODIS Greenland melt area results for the cloud free summer season, and interpret the melt statistics and spatial variability by utilizing meteorological and related information.