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Keywords:

  • tropical cyclogenesis;
  • hurricane formation;
  • hurricane spin-up;
  • cyclone intensification;
  • PREDICT;
  • GRIP;
  • IFEX

Abstract

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. We focus on two systems that finally became hurricanes Karl and Matthew, and one system (Gaston) that attained tropical storm status, 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 values of convective available potential energy that seem high enough to support convective organization. These values coincide mostly 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, θe, show much more variability between soundings on a particular day on account of the variability in moisture.

In all systems, there was is a tendency for the lower troposphere to moisten, but in the non-developing system, the troposphere became progressively drier in the height range between approximately 2 and 9 km during the five days of observations. In the developing systems, the troposphere moistened. The most prominent difference between the non-developing system and the two developing systems was the much larger reduction of θe between the surface and a height of 3 km, typically 25 K in the non-developing system, compared with only 17 K in the developing systems. 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 propose an alternative hypothesis in which the drier air weakens the convective updraughts and thereby the convective amplification of absolute vorticity necessary for development. Copyright © 2012 Royal Meteorological Society