1. Civil and Environmental Engineering Dept., Clarkson University, Potsdam, NY 13699, USA,
2. Dept. of Applied Mathematics and Theoretical Physics, University of Cambridge, Wilberforce Road, Cambridge CB3 0WA, UK;
3. Laboratory for Hydrospheric Processes, NASA Goddard Space Flight Center, Code 971, Greenbelt, MD 20771, USA;
4. Antarctic Cooperative Research Centre, Private Bag 80, University of Tasmania, Hobart, Tasmania 7001, Australia, and Australian Antarctic Division, Box 252-80, Hobart, Tasmania 7001, Australia.