have initial speeds comparable to the sound speed of the RHG. In order to examine the possible effects of nonzero initial speeds upon the evolution, we consider in Model 2 an initial condition in which the WFC is already falling at the sound speed at the start of the numerical calculation.

The density of Model 2 is shown in Figure \[fig:mod2rho\]. Due to the more rapid motion of the cloud, as compared to that of Model 1, the simulation is only carried out to $t=12$. The initial motion of the cloud can be seen to drive a weak shock ahead of it. Behind the shock, the leading edge of the cloud continues to move downwards at almost $C_h$, slowing down gradually until the very end of the simulation, when it rapidly decelerates as it breaks up, due to the combined action of Rayleigh-Taylor and Kelvin-Helmholtz instabilities. These results suggest that the infalling WFC’s quickly settle to $V_t$ irrespective of the initial conditions, as we have assumed previously (Lin & Murray 2000). The breakup of the cloud proceeds at nearly the same vertical height as in Model 1. The similarity arises because the models have the same gravitational accelerations and density 