The extent and variability of seasonal snow cover are recognized as important parameters in climate and hydrologic systems. During the past three decades much important information on hemispheric-scale snow extent has been provided by satellite remote sensing in the visible wavelengths. Since 1966 the National Oceanic and Atmospheric Administration (NOAA) has produced weekly snow extent charts for Northern Hemisphere land surfaces using visible-band satellite imagery. Limitations associated with the application of visible wavelength data include the inability to acquire information below clouds or during darkness and the fact that data are limited to snow extent only.
Because of the ability to penetrate clouds, provide data during darkness and the potential to provide an index of snow depth or water equivalent, passive microwave satellite remote sensing can greatly enhance snow measurements based on visible data alone. When snow covers the ground, some of the microwave energy emitted by the underlying soil is scattered by the snow grains. Therefore, when moving from snow-free to snow-covered land surfaces, a sharp decrease in emissivity provides a nearly unambiguous indicator of the presence of dry snow. It is now possible to monitor the global fluctuation of snow cover over a 20 year period using passive microwave data (Scanning Multichannel Microwave Radiometer (SMMR) 1978-1987 and Special Sensor Microwave/Imager(SSM/I),1987-present).
The National Snow and Ice Data Center (NSIDC), University of Colorado, is producing an earth-gridded, daily, global, brightness temperature and snow product time series by combining SMMR and SSM/I data in a common format: the Equal Area Scalable Earth Grid (EASE-Grid). The snow product data set includes individual files of global weekly maximum snow extent and water equivalent as well as monthly climatologies describing average snow extent, snow water equivalent, probability of occurrence, and variance. Results from the analysis of spatial and temporal trends in snow extent are presented and compared with the climatology derived from the NOAA weekly snow extent charts. For the Northern Hemisphere, the twenty year trend in snow extent derived from departures from the monthly means shows a decrease in snow extent of approximately 80,000 square kilometers per year for the period 1978 - 1998.
The EASE-Grid was selected by the NASA-EOS Pathfinder Program as the format for the Level 3 SMMR and SSM/I brightness temperatures and associated geophysical products. Within the NASA Polar Pathfinder project the cell size for the passive microwave brightness temperature data is 25 x 25 km, with an additional 12.5 X 12.5 km cell size for the 85 GHz channel. For the AVHRR products, cell size will be 1.25 X 1.25, 5 X 5, and 25 x 25 and for TOVS, 100 X 100 km. This project is supported by the NOAA/NASA Pathfinder Program which is designed to assure that certain key remote sensing data sets are scientifically validated and made readily available to the research community.