Impact of Inner-Core Tropical Cyclone Structure on the Potential for Rapid Intensification

Data from a 22-year sample of Vortex Data Messages (VDMs) reveal that immature tropical cyclones (TCs) possess a wide variety of vortex structures at the time of initial eye formation. Such structures may be categorized into three groups according to the relation between the initial eye radius and the radius of maximum winds (RMW, defined as the time-trended lower bound of the radius of the flight-level maximum wind). In the first group, the immature TC structure closely resembles the canonical structure of more mature TCs, wherein the RMW is of a similar radial scale as the initial eye radius. In the second group, the RMW is more than twice the initial eye radius. In the final, smaller group, the RMW is found to be less than the initial eye radius (a situation that should not happen according to conventional understanding). These results highlight the fact that structure of immature TCs is not well understood. Since the majority of RI events commence when TCs are at relatively weak intensities (25-50 kt), further investigation of this class of storms is warranted.

The structure of immature TCs likely has a profound impact on subsequent intensity and structure change. Past theoretical and observational research suggest that a strong intensification response occurs in the TC-scale vortex when diabatic heating occurs in the high inertial stability region of the TC. In immature TCs, however, the region of high inertial stability develops concurrently during intensification. It is hypothesized that the structure of the wind field in immature TCs functions somewhat like a manifold that controls and modifies the expression of the TC's convective elements back onto the vortex scale. In this view, the evolving shape of the TC's wind field is analogous to DNA, controlling the potential for rapid intensification (RI).

This work will compare intensification rates for the three groups of storms (classified according to their subsequent RMW-eye radius ratio when the aircraft eye appears) as well as a fourth class of storms that have remarkably small RMWs. Further examination will seek to determine whether the TCs that undergo RI have different initial RMW characteristics than those that do not and whether eye formation has any relation to either the beginning or the end of the rapid intensification period.