We present idealized numerical model experiments to investigate the convective generation of vertical vorticity in a tropical depression. The ambient vertical vorticity is represented by a uniform solid-body rotation. The calculations are motivated by observations made during the Pre-Depression Investigation of Cloud-systems in the Tropics (PREDICT) experiment. A specific aim is to isolate and quantify the effects of low- to mid-level dry air on convective cells that form within a depression and, in particular, on the generation of vertical vorticity in these cells. The results do not support a common perception that dry air aloft produces stronger convective downdraughts and more intense, cold-air outflows therefrom. Indeed, we find that dry air aloft weakens both updraughts and downdraughts, corroborating the recent results of James and Markowski.
As in the recent calculations of Wissmeier and Smith, the growing convective cells locally amplify the ambient rotation at low levels by more than an order of magnitude and this vorticity, which is produced by the stretching of existing ambient vorticity, persists long after the initial updraught has decayed. Moreover, significant amplification of vorticity occurs even for clouds of only moderate vertical extent. The maximum amplification of vorticity is relatively insensitive to the maximum updraught strength, or the height at which it occurs, and it is not unduly affected by the presence of dry air aloft. Thus the presence of dry air is not detrimental to the amplification of low-level vorticity, although it reduces the depth through which ambient vorticity is enhanced.
Results for a limited number of different environmental soundings indicate that the maximum amplification of vorticity increases monotonically with the strength of the thermal perturbation that initiates the convection, but the amount of increase depends also on the thermodynamic structure of the sounding.