Fig. 6; The evolution of vertical velocity (w) in the K1 case during the 1st period; after (a)0.125, (b)0.25, (c)0.375, (d)0.5, (e)0.625, (f)0.75, (g)0.875, (h)1 period. The contour interval is 0.5cms-1. Values in non shaded areas and the thicker shaded areas are positive and negative, respectively, and the absolute Values in the thinner shaded areas are less than 0.1cms-1.
