Boundary Layer, Urban Meteorology and Land-Surface Processes

Simulated near-surface behavior of tornado-like vortices in a Fiedler chamber with resolved turbulence

Dr.Nathan Dahl

University of Miami

16 September 2016, 2:00 PM

National Weather Center, Room 5600
120 David L. Boren Blvd.
University of Oklahoma
Norman, OK

The Fiedler chamber is an idealized framework developed for the purpose of studying tornado-like vortices within a closed domain. Its configuration allows direct comparison between the wind speeds and the theoretical maximum derived from the environmental potential energy, known as the "thermodynamic speed limit." With recent advances in computing power, this framework can be used to generate realistic, high-resolution flow structures in order to examine the processes governing tornado evolution at the very lowest levels of the atmosphere.

In an effort to bridge gaps between vortex theory and actual observations, we performed several high-resolution, three-dimensional large eddy simulations with fully-developed near-surface turbulence obtained through an "eddy injection" technique, which were used to analyze momentum budgets and the influence of various background conditions on vortex intensity and structure. The time-azimuthal mean winds from these simulations consistently and substantially exceeded the thermodynamic speed limit; furthermore, maximum 3-s wind gusts at 10 m height exceeded the EF-scale threshold for "violent" tornadoes (~74 m/s) even when the background vertical forcing was relatively modest, and gusts well in excess of 100 m/s were common. We also simulated observations from sensors impacted by translating vortex cores in an effort to quantify the errors associated with estimating maximum wind speeds from in situ data and damage surveys. Our current results are qualitatively similar to those obtained by Nolan (2014) for hurricanes but quantitatively show a much larger potential for error (over 40 m/s in some cases) due the transience and structural complexity of tornadoes.

Boundary Layer, Urban Meteorology and Land-Surface Processes Seminar Series website