1. INP-ENSEEIHT, 2, Rue Charles Camichel, BP 7122, CEDEX 7, 31071 Toulouse, France
2. Department of Mathematics and Statistics, University of Victoria, P.O. Box 1700 STN CSC, Victoria, BC V8W 2Y2, Canada
3. Department of Atmospheric Sciences, Texas A&M University, 3150 TAMU, College Station, TX 77843-3150, USA