Three-Dimensional Thermodynamic Observations in Supercell Thunderstorms from Swarms of Balloon-Borne Probes
Open Access
- Author:
- Bartos, Elissa
- Graduate Program:
- Meteorology and Atmospheric Science
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- October 29, 2020
- Committee Members:
- Paul Markowski, Thesis Advisor/Co-Advisor
Yvette Pamela Richardson, Thesis Advisor/Co-Advisor
Matthew Robert Kumjian, Committee Member
David Jonathan Stensrud, Program Head/Chair - Keywords:
- supercell thunderstorms
tornadoes
supercell thermodynamics
baroclinic vorticity
observational study - Abstract:
- Supercell thunderstorms are responsible for a variety of weather hazards, including violent tornadoes. In leading theories for tornado formation in supercell thunderstorms, the thermodynamic properties of the downdrafts and associated rain-cooled outflow play a prominent role. In particular, the baroclinic generation of vorticity along descending air parcel trajectories within the forward-flank precipitation region is crucial for the development of near-surface vertical vorticity. Moreover, the buoyancy within the near-surface mesocyclone region has been found to influence the degree to which near-surface vertical vorticity is amplified to tornado strength via stretching. Our understanding to date is the result of numerical simulations (which have imperfect microphysical parameterizations), dual-Doppler wind retrievals, and in-situ thermodynamic observations from instrumented automobiles (which are confined to the surface and roads). This study uses a novel approach by collecting aboveground thermodynamic observations via swarms of small balloon-borne probes. These balloon-borne probes act as ‘pseudo-Lagrangian’ drifters; the storm-relative winds draw the probes through the key forward-flank precipitation region of the storm. In May 2019, one nontornadic and three tornadic supercells were intercepted. Observations from the balloon-borne probes are used to map important three-dimensional (3D) thermodynamic fields (equivalent potential temperature and virtual potential temperature) of these supercells. The 3D gridded fields indicate that the air parcels' origin heights in the environments and their implied trajectories through the supercells are consistent with previous numerical modeling studies, with parcels within much of the forward flank descending from only slightly higher aloft in the environment and parcels closer to the mesocyclone descending from much higher aloft. Additionally, the 3D gridded fields indicate that descending parcels pass through aboveground buoyancy gradients, which can generate substantial baroclinic vorticity to enhance the total horizontal vorticity. Provided that the baroclinically generated vorticity becomes tilted vertically near the surface and is then stretched within vertically accelerating air, as past studies have found, the low-level updrafts would ingest the near-surface vertical vorticity necessary for tornado formation.