[1] Meteorol Serv Canada, Downsview, ON M3H 5T4, Canada
[2] World Meteorol Org, Environm Div, Atmospher Res & Environm Program, CH-1211 Geneva 2, Switzerland
[3] Univ Quebec, Dept Earth Sci, Montreal, PQ H3C 3P8, Canada
[4] McGill Univ, Dept Atmospher & Ocean Sci, Montreal, PQ H3A 2K6, Canada
[5] Dalhousie Univ, Dept Phys, Atmospher Sci Program, Halifax, NS B3H 3J5, Canada