Vortical hot towers, their aggregate effects and their resolution dependence in the formation of Hurricane Diana (1984)

Abstract

Recent authors have hypothesized that small-scale deep convective towers possessing intense values of cyclonic vertical vorticity in their cores (vortical hot towers) play a critical role in tropical cyclogenesis via a two stage process: (1) preconditioning the local environment by creating small-scale potential vorticity anomalies and humidifying the lower to middle troposphere, and (2) merger, axisymmetrization and collection of these potential vorticity anomalies to generate the larger scale vortex. In this study we further investigate the role played by vortical hot towers in the upscale growth process. We simulate the evolution of Hurricane Diana in a full-physics numerical model with 1km grid spacing and compare our results to previous, coarser resolution simulations. We employ traditional weather analysis techniques and new innovative means of displaying large and complex datasets to investigate the interaction between the cloud scale features and the larger system scale environment. The results are compared to prior studies to assess if simulated vortical hot tower dynamics exhibit a significant dependence on model resolution. We find the basic physics of the vortical hot tower pathway is largely unchanged as grid-spacing decreases from 3km to 1km for simulations of Hurricane Diana. The differences between our high resolution simulation and coarser resolution simulations are mainly associated with fine scale variability. Our 1km simulation represents nearly an order of magnitude more convective towers with smaller spatial scales than what was observed in previous simulations. We find maximum updraft velocities in our 1km simulation typically between 15ms-1 and 20ms-1 with instantaneous maximum values as high as 35ms-1, though these values typically decrease during the simulation. We also find that, while the cores in the vortical hot towers are significantly moistened by the vertical transport of moisture in the updraft, the larger-scale environment actually dries significantly due to horizontal advection. Lastly, we examine a series of vortex merger events and find that merger activity is a ubiquitous and important aspect of the genesis of Hurricane Diana. Our results broadly confirm previous work using coarser numerical resolution and provide new insights into the hypothesized upscale growth process in incipient hurricanes.