AMSR-E algorithm for snowmelt onset detection in sub-arctic heterogeneous terrain

The onset of snowmelt in the upper Yukon River basin, Canada, can be derived from brightness temperatures (T-b) obtained by the Advanced Microwave Scanning Radiometer for EOS (AMSR-E) on NASA's Aqua satellite. This sensor, with a resolution of 14 x 8 km 2 for the 36.5 GHz frequency, and two to four observations per day, improves upon the twice-daily coverage and 37 x 28 km 2 spatial resolution of the Special Sensor Microwave Imager (SSM/I). The onset of melt within a snowpack causes an increase in the average daily 36.5 GHz vertically polarized T-b as well as a shift to high diurnal amplitude variations (DAV) as the snow melts during the day and re-freezes at night. The higher temporal and spatial resolution makes AMSR-E more sensitive to sub-daily T-b oscillations, resulting in DAV that often show a greater daily range compared to SSM/I. Therefore, thresholds of T-b > 246 K and DAV > 10 K developed for use with SSM/I have been adjusted for detecting the onset of snowmelt with AMSR-E using ground-based surface temperature and snowpack wetness relationships. Using newly developed thresholds of T-b > 252 K and DAV > 18 K, AMSR-E derived snowmelt onset correlates well with SSM/I observations in the small subarctic Wheaton River basin through the 2004 and 2005 winter/spring transition. In addition, the onset of snowmelt derived from AMSR-E data gridded at a higher resolution than the SSM/I data indicates that finer-scale differences in elevation and land cover affect the onset of snowmelt and are detectable with the AMSR-E sensor. On the basis of these observations, the enhanced resolution of AMSR-E is more effective than SSM/I at delineating spatial and temporal snowmelt dynamics in the heterogeneous terrain of the upper Yukon River basin. Copyright (C) 2007 John Wiley & Sons, Ltd.