Postscript verison (text) and (pictures)
1 Centre for Atmospheric Science, Department of Chemistry, University of Cambridge, UK. 2 British Antarctic Survey, Cambridge, UK. 3 Centre for Atmospheric Science, Department of Applied Mathematics and Theoretical Physics, University of Cambridge, UK. 4 Department of Meteorology, Edinburgh Univeristy, UK.
E.F.Shuckburgh@damtp.cam.ac.ukAccepted by J. Geophys. Res.