Figure 7.

Contour plot of the instantaneous vorticity field for cases with $St=0.5,\psi ={90}^{\circ }$ and ${\theta }_0={15}^{\circ }$. (a) $h_0=0.75c$; (b) $h_0=1.25c$; (c) $h_0=2c$; (d) $h_0=3c$.