In a series of collaborative Russian-American works (Levina and Montgomery, 2009-2015), we developed and applied an original research approach based on idealized cloud-resolving numerical simulation to quantitatively analyze the self-organization of helical moist convective atmospheric turbulence during tropical cyclogenesis. This allowed us to discover a pre-depression large-scale vortex instability and answer a question “When will cyclogenesis commence given a favorable tropical environment?” The instability emerges against the background of seemingly disorganized convection, in the absence of a visible center of the surface circulation and pronouncedly precedes, from a few hours up to several dozens of hours, the formation of a tropical depression. The onset of instability is diagnosed as the moment when primary (tangential) and secondary (transverse) circulation in the forming hurricane vortex become linked by special convective coherent structures - Vortical Hot Towers (VHTs). The generated linkage makes the nascent vortex an integral mesoscale helical system and ensures a positive feedback between the circulations. The mutual intensification of the primary and secondary circulation begins. The feedback is maintained by convective instability and vortical cloud convection. The forming vortex becomes energy-self-sustaining and intensifying. This can be interpreted as the beginning of a tropical cyclogenesis, while the subsequent formation of a tropical depression / storm may mark the completion of the genesis stage.
In the present work, we focus on the formation of secondary circulation due to close coincidence in time of this process with the onset of instability, and we explore in detail the crucial role of VHTs in both phenomena. The undertaken examination is also intended to contribute to a recently initiated development (Levina, 2020) of early and exact operational diagnosis for the beginning of tropical cyclogenesis based on GOES Imagery and combined with high resolution numerical analysis. The work was supported by the research project “Monitoring” No. 01200200164.