Mauricio I. Oliveira - December 10

School of Meteorology MS Thesis Defense

Cyclic Tornadogenesis and Horizontal Vortex Tubes in High-Resolution Idealized Simulations of Supercells

Mauricio I. Oliveira

December 10^th

1:00 pm

NWC 5600

Despite enormous progress in our understanding of tornadic supercells obtained over the last several decades, many aspects regarding the evolution of these storms require further investigation. High-resolution numerical simulations of tornadic supercells represent an important tool to shed light on several physical processes associated with supercell tornadogenesis. In this dissertation, idealized, high-resolution numerical simulations of a tornadic supercell are performed using the Advanced Regional Prediction System (ARPS) to better understand some important aspects of the complex evolution of tornadoes at fine scales. These analyses focus on two aspects of tornado evolution. First, the cyclic tornadogenesis is investigated, with emphasis on understanding how a supercell evolving in a horizontally homogenous, steady-state environment can produce significantly different tornadoes in each tornado cycle. Second, the evolution and interactions of horizontal vortices sometimes observed near real tornadoes is also addressed. Given that surface friction can have a significant impact on the evolution of simulated tornadoes, the effects of surface friction are included in the simulations.

Cyclic tornadogenesis is investigated in a 50-m grid spacing simulation. The supercell produces four tornadoes in relatively regular periods during its life span, all of which develop under intensifying low-level updrafts and lowering pressure aloft. Nevertheless, individual tornado characteristics vary considerably from cycle to cycle. The first tornado develops when the storm displays a “classic” morphology; after its dissipation, large amounts of precipitation in the rear-flank of the storm cause the subsequent tornadoes to have shorter life spans, as they tend to become “wrapped in rain” and detach from their parent low-level updraft too quickly. All tornadoes are preceded by a low-pressure lobe (LPL) associated with accelerating inflow into the tornado’s parent updraft. A band of enhanced near-surface streamwise vorticity in conjunction with the LPL and enhanced inflow also develops and appears to feed into the low-level updraft, potentially intensifying upward motions dynamically. Midlevel updrafts move rearward relative to the storm but do not decay completely but rather merge with newly developed updrafts and produce convoluted downdraft distribution at middle levels and near the rear-flank of the storm, ultimately causing the larger amounts of precipitation to fall near the tornadoes in subsequent cycles. These characteristics add to existing models of cyclic mesocyclogenesis/tornadogenesis.

Interactions between HV and tornadoes are analyzed in similar 100-m and a 30-m grid spacing simulations. Three-dimensional (3D) visualizations aided by visual observations of HVs in a real tornado reveal that HV usually originate in two key regions at the surface: around the base of the tornado and in the rear-flank downdraft (RFD) outflow and are believed to have been generated via surface friction in regions of strong horizontal near-surface wind. HVs around the tornado are produced in the tornado outer circulation and rise abruptly in its periphery, assuming a variety of complex shapes, while HVs to the south-southeast of the tornado, within the RFD outflow, ascend gradually in the updraft. Using the same methodology and the 30-m experiment, a type of HV that persistently trails the right flank of the tornado is described and referred to as “trailing HV”. Trailing HVs are larger, stronger, and last longer than their small-scale counterparts, occasionally displaying smaller spiral vortices circulating around their periphery, which may evolve into complex structures. The trailing HV arises as an entanglement of vortices along an RFD internal boundary, which also serves as focus for stretching of streamwise. The spiral vortices result from the same entangling processes that gives rise to the trailing HV. Moreover, the analysis suggests that trailing HV may act as a rotor that reinforces the surface wind speed in the right flank of the tornado. More quantitative analysis of the results presented in this dissertation are the focus future work and the implications of the structures highlighted in this study for supercell models are discussed.