\section*{Acknowledgments}
\label{s:acknowledgements}

This research is a joint effort of the MIT Center for Meteorology and
Physical Oceanography and the MIT Laboratory for Computer Science.  The
research of Chris Hill is partially funded by ONR contract
N00014-95-1-0967. The research of Andrew Shaw is partially
funded by ARPA under ONR contract N00014-92-J-1310.

John Marshall and Arvind read drafts of this paper. We would also like to 
thank Digital Equipment Corporation and the Pittsburgh Supercomputing
Center for giving us access to the DEC 8400 SMP.  Experiments on the
CM-5 were done under the MIT SCOUT project, ARPA contract
\#MDA972-92-J-1032.
