Helical features of tropical cyclogenesis

In dynamical meteorology investigations of helicity first originated in the study of supercell storms and tornadoes in middle latitudes during the 1980s and continues to be used widely in forecasting of such events. First attempts are now being considered to examine helical features of tropical cyclone organization.

In 2006, Montgomery and co-authors (J. Atmos. Sci., vol. 63, pp. 355-386) proposed a new scenario of tropical cyclogenesis within a kinematically and thermodynamically favorable environment of a mesoscale convective vortex.. Using near-cloud-resolving simulations, this work demonstrated how a mesoscale tropical depression vortex could develop from cumulonimbus convection as a result of system-scale convergence and upscale cascade of vorticity. In those simulations the growth of flow scales occurred by both system convergence and multiple diabatic vortex mergers alongside the more familiar dry adiabatic vortex merger of convectively generated remnants. The simulations first generated a number of so-called ‘vortical hot towers' (VHTs), each of 10-30 km horizontal scale, which eventually resulted in an intense helical vortex on the atmospheric mesoscale.

Our current work presents an analysis of the helicity field during this VHT route to tropical cyclogenesis described by Montgomery et al. (2006). Total helicity, as well as its three spatial components, are diagnosed from these 72 h simulations with 10 min time and 3 km horizontal grid space increments. Starting from a weak mesoscale cyclonic vortex, extreme helicity values up to 2000 m2s-2 (integrated over the 0-6 km atmospheric layer) and even higher are found in vicinity of the simulated vortical hot towers. Although the updrafts are not as strong as their midlatitude counterparts the helicity values are comparable to typical helicity values for supercells in middle latitudes. To analyze this self-organization process of moist convective turbulence during this VHT route we examine a number of helical characteristics as well as some other integral characteristics of the velocity field including volume integrated enstrophy and kinetic energy. Statistical moments of 3D helicity field are studied also.

A noteworthy difference is found between the integral characteristics of the velocity field and the total helicity. During the first 10 hours of gestation period the total helicity is nearly zero. Then, its value becomes non-zero, and after t ≈ 20 h distinctly positive and increasing. According to the theory of turbulence the non-zero helicity means a break of the mirror symmetry of atmospheric turbulence and the potential for a large-scale vortex instability.

On the basis of this work perspectives on the role of helicity in tropical cyclogenesis are discussed.

This work is supported by the Russian Foundation for Basic Research under Projects NN 07-05-00060, 10-05-00100 and by the U.S. National Science Foundation under grant ATM-0733380.