Is Convection in Developing Tropical Systems Special?

This study seeks a difference between non-developing (ND) and developing (D) tropical vorticity maxima, in the Atlantic and East Pacific basins. The vorticity maxima have been tracked manually using time filtered and spatially smoothed ERA-40 Reanalysis, giving a database of 590 ND and 110 D systems for June-October, 1998-2001. Increase in Convective Activity (ICA) events are defined in terms of a relatively large increase in the area of low IR brightness temperature over 12 hours. For June-October, 1998-2001, over 600 ICA events are found with ND systems and over 150 with D systems. In the climatological cyclogenesis regions, there are many ND and many D systems with associated ICA events. This study is motivated by the hypothesis that the nature of the convection may help determine whether a system develops or not. For example, stronger convection may imply stronger vortex stretching on the convective scale, which is then quickly absorbed into the synoptic-scale. Or, more widespread stratiform rain may lead to the production of more and/or stronger mid-level vortices, which may aid development. The position of the convection relative to the vorticity maximum may also play a role. Does the nature and location of the convection matter for cyclogenesis, or is convection playing a passive role with development dependent on the large-scale? Are the current readily-available datasets sufficient to answer this question? In this study, the change in synoptic-scale vorticity in association with ICA events is statistically quantified, for ND and D systems. The vorticity is just as likely to decrease as increase for both types of systems. This may reflect poorly on the quantitative accuracy of the Reanalysis. However, D systems qualitatively tend to both have larger vorticity before the ICA and experience a greater positive change during the ICA. There is a distinct region in the phase space wherein systems are more likely to develop than not develop. We anticipate that looking at other parameters, especially passive microwave-based proxies of convective intensity and coverage, will reveal even more differences between ICA events within developing and non- developing systems.