This research was supported in part by the National Science Foundation under Grant CEE-8111240A02;
by the Department of Energy under Grant DE-ACO2-83ER60-170 MODA002;
and by the Massachusetts Institute of Technology under Sloan Basic Research Grant No. 26950 and the Technology and Development Program. The second author was a Visiting Fellow at Princeton University when this work was done. He gratefully acknowledges the facilities provided by the University for his visit and for this work. The authors are grateful to P. C. D. Milly for supplying a computer code based on his mass-conserving numerical scheme;