Two-dimensional simulations of tropical squall lines. The simulations are identical except for the mean shear in the stratosphere above 15 km. The top panel shows an easterly shear case and the bottom panel shows a westerly shear case. The thick solid lines show the outline of the cloud water, the thin lines show the potential temperature at 10K intervals, and the shading displays the vertical velocity. The vertical velocity scale saturates at 1.3 m/s to show the detail in the stratosphere, but values as large as 5 m/s occur in the tropopause. From Alexander and Holton (1997).