Appendix 1
Figure A1. . Same as Fig. 4, but for the heton structure. The maximal
value of the three-dimensional vorticity
with the positive vertical vorticity is 9.1*10-4s-1, and the maximal value with the negative
vertical vorticity is 7.1*10-4s-1. The parameters for this experiment are
heat flux of 800 Wm-2,
=1.0*10-3s-1, and 
=1.2 m2s-1, and hence 
=2.0*10-2,and 
=2.3*105. The experiment is denoted by blue rectangle
in Fig. 2.