1. School of Mathematical Sciences, University of Adelaide, Adelaide, South Australia 5005, Australia
2. Department of Atmospheric Sciences, University of Washington, Seattle, WA, USA
3. Department of Meteorology, University of Reading, Reading, UK
4. National Institute of Water and Atmospheric Research, Christchurch, New Zealand
5. School of Mathematical and Physical Sciences, University of Newcastle, Callaghan, New South Wales 2308, Australia