Observations of the convective environment in developing and non-developing tropical disturbances

Analyses of thermodynamic data gathered from airborne dropwindsondes released from the upper troposphere during the Pre-Depression Investigation of Cloud Systems in the Tropics (PREDICT) experiment are presented. The main focus is on two systems that finally became Hurricanes Karl and Matthew and one system that attained Tropical Storm status (Gaston), but subsequently weakened and never redeveloped during five days of monitoring. Data for all events show that the largest values of total precipitable water are collocated with the surface trough and with some high values of convective available potential energy, which coincide roughly with low values of convective inhibition. Vertical profiles of virtual potential temperature show little variability between soundings on a particular day, but the system means from day to day show a slight warming. In contrast, vertical profiles of pseudo-equivalent potential temperature, theta_e, show much more variability between soundings on a particular day on account of the variability in moisture.

In all systems, there is a tendency for the lower troposphere to moisten, but in the system that did not develop, the middle and upper troposphere became progressively drier during the five missions. In the developing systems, the upper levels moistened. The most prominent difference between the non-developing system and the two systems that developed was the much larger reduction of theta_e between the surface and a height of 3 km, typically 25 K in the non-developing system, compared with only 17 K in the systems that developed. Conventional wisdom would suggest that, for this reason, the convective downdraughts would be stronger in the non-developing system and would thereby act to suppress the development. Here we invoke an alternative hypothesis that the drier mid-level air weakens the convective updraughts and thereby weakens the amplification of system relative vorticity necessary for development.