Helical organization of tropical cyclones

In this work we consider a helical flow organization on small and large space scales in a forming tropical cyclone and offer a quantitative analysis for early stages in evolution of large-scale helical vortex based on diagnosis of a set of integral helical and energetic characteristics. Using the data from a near cloud-resolving numerical simulation by Regional Atmospheric Modeling System (RAMS), a key process of vertical vorticity generation from horizontal components and its amplification by special convective coherent structures Vortical Hot Towers (VHTs) is highlighted. The process is found to be a pathway for generation of a velocity field with linked vortex lines of horizontal and vertical vorticity on local and system scales. This reveals itself in generation of nonzero and increasing integral helicity of the vortex system during tropical cyclone formation. Based on these results, a new perspective on the role of VHTs in the amplification of the system-scale circulation is emphasized. They are the connecters of the primary tangential and secondary overturning circulation on the system scales and are elemental building blocks for the nonzero system-scale helicity of the developing vortex throughout the tropical cyclone evolution from genesis to the mature hurricane state. Calculation and analyses of helical and energetic characteristics together with hydroand thermodynamic flow fields allow the diagnosis of tropical cyclogenesis as an event when the primary and secondary circulations become linked on system scales.