Helical nature of tropical cyclogenesis: WHEN will a nascent vortex become self-sustaining ?

We present a numerical diagnosis for tropical cyclogenesis and offer an answer to the above important question.

Our research approach based on near-cloud-resolving numerical simulation (Montgomery et al., 2006, J. Atmos. Sci.) and developed to quantitatively analyze helical self-organization of moist convective atmospheric turbulence (Levina and Montgomery, 2010, Doklady Earth Sciences) allows a diagnosis of cyclogenesis when the primary and secondary circulations in a forming hurricane vortex become linked by rotating convective cloud structures – dubbed Vortical Hot Towers (VHTs).

We analyze first how helicity emerges on cloud convection scales and highlight a process of vertical vorticity generation from horizontal components and its amplification by VHTs. This is found to be a pathway for development of a velocity field with linked vortex lines of horizontal and vertical vorticity that reveals itself in non-zero and increasing integral helicity in the examined area of potential cyclogenesis. We trace a very initial stage of helical self-organization of the vortical convection of different sizes and intensity that results in the formation of the developed secondary circulation.

We diagnose how and when the secondary transverse circulation becomes linked with the primary tangential circulation, and a process of helicity generation starts on the vortex system mesoscales. We show and discuss how the generated circulation linkage makes the nascent vortex an integral helical system and ensures a positive ‘helical' feedback between the circulations. The feedback is sustained by convective instability and vortical convection in the central region of the circulation. The existence of such rising warm flows suggests a release of potential energy that is converted into kinetic energy of developing large-scale helical vortex. It is shown that with adequate moisture fluxes from the underlying sea surface to maintain a degree of convective instability in the cumulus zone, the active feedback provides energy exchange between the primary and secondary circulation and their further mutual intensification, i.e. a self-sustaining amplification process of the system-scale circulation.

Based on the obtained results, new criteria for diagnosing tropical cyclogenesis are offered using our developed numerical approach. We discuss the potential usefulness of combining the foregoing diagnosis of tropical cyclogenesis with a recent paradigm of ‘Marsupial Pouch' and ‘Marsupial Pouch Tracking' (Dunkerton, Montgomery, Wang, 2009, Atmos. Chem. Phys.; Wang, Montgomery, Dunkerton, 2009, Geophys. Res. Lett.) in applications using operational weather models.