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threshold of ±18 K that reflects <strong>the</strong> spring tr<strong>an</strong>sition when large melt-refreeze cycles are occurring.<br />
The larger oscillations in daily Tb are most likely due to <strong>the</strong> timing of data acquisition being closer<br />
to <strong>the</strong> actual maximum <strong>an</strong>d minimum daily Tb. The snowmelt onset algorithm will also be<br />
applicable in most arctic <strong>an</strong>d subarctic l<strong>an</strong>dscapes.<br />
<strong>Snow</strong>melt onset dates from AMSR-E, <strong>as</strong> determined using <strong>the</strong> new Tb <strong>an</strong>d DAV thresholds,<br />
compare very well with SSM/I for <strong>the</strong> Wheaton River b<strong>as</strong>in, differing by a maximum of two days.<br />
The enh<strong>an</strong>ced spatial resolution of <strong>the</strong> AMSR-E data provides <strong>an</strong> improvement over SSM/I in<br />
detecting snowmelt onset in are<strong>as</strong> of mixed terrain. Using this AMSR-E melt onset algorithm, <strong>the</strong><br />
snowpack dynamics of <strong>the</strong> Wheaton River b<strong>as</strong>in, <strong>as</strong> well <strong>as</strong> o<strong>the</strong>r are<strong>as</strong> of <strong>the</strong> upper Yukon River<br />
b<strong>as</strong>in, c<strong>an</strong> be fur<strong>the</strong>r investigated spatially <strong>an</strong>d temporally for <strong>the</strong> record of AMSR-E observations.<br />
The improvements by <strong>the</strong> AMSR-E sensor upon SSM/I allow for a more effective spatial <strong>an</strong>d<br />
temporal examination of snowmelt dynamics in <strong>the</strong> heterogeneous terrain of <strong>the</strong> upper Yukon<br />
River b<strong>as</strong>in. It will be a signific<strong>an</strong>t improvement to our ability to underst<strong>an</strong>d <strong>the</strong> melt processes<br />
<strong>an</strong>d timing in <strong>an</strong> import<strong>an</strong>t, remote, high latitude drainage b<strong>as</strong>in.<br />
ACKNOWLEDGEMENTS<br />
We would like to extend our th<strong>an</strong>ks to Mary Jo Brodzik, Am<strong>an</strong>da Leon, <strong>an</strong>d <strong>the</strong> National <strong>Snow</strong><br />
<strong>an</strong>d Ice Data Center for EASE-grid satellite data <strong>an</strong>d continued <strong>as</strong>sist<strong>an</strong>ce. Th<strong>an</strong>ks to<br />
Environment C<strong>an</strong>ada for ongoing support with field logistics <strong>an</strong>d additional data. We th<strong>an</strong>k Sarah<br />
Kopczynski <strong>an</strong>d Sh<strong>an</strong>non Haight for <strong>the</strong>ir <strong>as</strong>sist<strong>an</strong>ce collecting field data. We are also th<strong>an</strong>kful<br />
for helpful comments provided by Andrew Klein <strong>an</strong>d two <strong>an</strong>onymous reviewers. Additional<br />
th<strong>an</strong>ks to Michael Chupa. Funding w<strong>as</strong> provided by Lehigh University <strong>an</strong>d <strong>the</strong> National<br />
Aeronautics <strong>an</strong>d Space Administration’s Terrestrial Hydrology Program (gr<strong>an</strong>t # NNG04GR31G).<br />
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