National Severe Storms Laboratory

Barotropic and Baroclinic Contributions to Near-Ground Rotation within a Simulated Supercell

Johannes Dahl

North Carolina State University
Raleigh, North Carolina

11 June 2013, 4:00 PM

National Weather Center, Room 1313
120 David L. Boren Blvd.
University of Oklahoma
Norman, OK

The seminar is focused on the mechanisms by which supercell thunderstorms acquire near-ground rotation, which is a necessary condition for tornadogenesis. In particular, the contributions from barotropic and baroclinic processes are investigated using a high-resolution supercell simulation. Herein, barotropic processes represent the rearrangement of pre-existing horizontal vorticity, and baroclinic processes reflect the generation of vorticity within the storm. Quantifying these different sources of vorticity is important for understanding the genesis, maintenance, and decay of tornadoes.

Supercell thunderstorms develop rotation aloft by reorienting pre-existing horizontal vortex lines within the convective updraft. The development of near-ground rotation, on the other hand, requires a downdraft which not only provides baroclinity for the production of horizontal vorticity, but and also tilts this vorticity into the vertical while the air is still descending. However, downdrafts may also rearrange ambient vortex lines, a process usually neglected in analyses of full-physics simulations and observations. To isolate the roles of storm-generated and ambient vorticity, forward trajectories of parcels that gain appreciable rotation within downdrafts, are analyzed. A new Lagrangian technique is employed to track individual fluid volumes and to infer the evolution of ambient vortex-line segments embedded within these volumes. The contributions from storm-generated and ambient vorticity will be quantified and a detailed picture of the vertical-vorticity production in downdrafts will be presented.

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