A numerical study of rotating convection during tropical cyclogenesis

We present idealized numerical model experiments to isolate and quantify the effects of low to mid level dry air on the dynamics of rotating deep convection in a tropical disturbance. The ambient vertical vorticity is represented by a uniform solid-body rotation. The calculations are directed towards interpreting observations made during the Pre-Depression Investigation of Cloud-systems in the Tropics (PREDICT) experiment. The results do not support a common perception that dry air aloft produces stronger downdraughts and more intense, cold-air outflows. Indeed, we find that dry air aloft weakens both updraughts and downdraughts, corroborating recent results of James and Markowski. As in recent calculations of Wissmeier and Smith, the growing convective cells amplify locally 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. However, it does reduce the depth to which there is significant amplification of vorticity, an effect that may be important in the dynamics of tropical cyclogenesis. 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 low-level stability of the sounding.