Assessing the Microphysical Characteristics and Updraft Kinematics of Eyewall Convection in an Idealized Simulation of a Tropical Cyclone Undergoing Intensity Vacillation Cycles

Recently, Duran et al. (2021) noted distinct characteristics between two lightning outbreaks that occurred in Hurricane Dorian (2019). During the first lightning outbreak accompanying rapid intensification, eyewall convection was relatively symmetric, and flashes were characterized by relatively high total optical energy and a large average flash area. In contrast, during the second lightning outbreak approaching Dorian’s maximum intensity and subsequent weakening, eyewall convection was asymmetric in the presence of mesovortices, and flashes were characterized by relatively low total optical energy and a smaller average flash area. Furthermore, the second lightning outbreak exhibited a higher number of flashes—a pattern that would generally indicate subsequent intensifying in the absence of additional context. Thus, the authors hypothesized that a symmetric eyewall structure accompanying intensifying TCs favors a charge separation between larger ice particles (i.e., graupel) within the eyewall updrafts and smaller ice particles advected away from the eyewall in the outflow layer. Such a charge separation is conducive to flashes with relatively higher total optical energy and a larger average flash area. Conversely, an asymmetric eyewall is characterized by a relatively larger number of flashes with lower energy and a smaller area in association with bulk updraft volumes concentrated within the asymmetries.

In this study, the hypothesis proposed by Duran et al. is explored by examining the microphysical characteristics and updraft kinematics of eyewall convection in detail with an idealized simulation of an intensifying TC translating in uniform flow. The bulk intensification period of the simulated TC is subdivided into four windows that constitute the intensifying and weakening phases of two vacillation cycles preceding maximum intensity. The degree of azimuthal potential vorticity (PV) asymmetry in the eyewall varies inversely with the azimuthal-mean tangential velocity during the vacillation cycles. Azimuthal-mean and azimuth-height analyses of both ice and graupel mixing ratios and eyewall updrafts are presented to identify the structural evolution of convection during the intensifying and weakening phases of the vacillation cycles. Furthermore, time series of ice and graupel mass and updraft volumes of 1, 5, and 10 m s^-1 are constructed from within the eyewall region to identify potential lag-correlations between the microphysical and kinematic evolution of convection and intensity change. Results will be discussed within the context of relationships between the lightning flash characteristics and intensity change hypothesized in the Duran et al. study.