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ABSTRACT
63 rd EASTERN SNOW CONFERENCE
Newark, Delaware USA 2006
AMSR-E Algorithm for Snowmelt Onset Detection
in Subarctic Heterogeneous Terrain
J.D. APGAR 1 , J.M. RAMAGE 1 , R.A. MCKENNEY 2 , AND P. MALTAIS 3
Snowmelt onset in the upper Yukon River basin, Canada, can be derived from brightness
temperatures (Tb) 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 Tb as well
as a shift to high diurnal amplitude variations (DAV) as the snow melts during the day and
refreezes at night. The higher temporal and spatial resolution makes AMSR-E more sensitive to
sub-daily Tb oscillations, resulting in DAV that often show a greater daily range compared to
SSM/I. Therefore, thresholds of Tb > 246 K and DAV > ±10 K developed for use with SSM/I
have been adjusted for detecting melt onset with AMSR-E using ground-based surface
temperature and snowpack wetness relationships. Using newly developed thresholds of Tb > 252
K and DAV > ±18 K, AMSR-E determined 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, snowmelt onset 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
snowmelt onset and are detectable with the AMSR-E sensor. Based on 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.
Keywords: snowmelt, passive microwave, SSM/I, AMSR-E, Yukon River, Wheaton River
INTRODUCTION
This Hydrologic and atmospheric processes are closely tied with snow characteristics
throughout the arctic and subarctic regions of the Northern Hemisphere. The extent of snow
cover, snow water equivalent (SWE), and the timing of spring snowmelt influence local hydrology
as well as regional and global water budgets, and the high albedo of snow has an important
influence on the global energy budget (Vorosmarty et al., 2001). The spring snowmelt is an
annual event in snow-covered, high-latitude drainage basins that sends significant quantities of
meltwater downstream during a relatively short period of time. The timing and magnitude of
these events are variable, but they can create a large impact on the basin hydrology and influence
geomorphic change within the basin. The use of passive microwave data to examine the
1 Earth and Environmental Sciences, Lehigh University, Bethlehem, PA 18015, USA
2 Geosciences/Environmental Studies, Pacific Lutheran University, Tacoma, WA 98447, USA
3 Water Survey of Canada, Environment Canada, 91782 Alaska Highway, Whitehorse, Yukon
Territory, Canada Y1A 5B7
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