Tornado Vortex Structure, Intensity, and Surface Wind Gusts in Large-Eddy Simulations with Fully Developed Turbulence

A large-eddy simulation (LES) framework with an “eddy injection”
technique has been developed that ensures that a significant majority
of turbulent kinetic energy in numerically simulated, tornado-like
vortices is represented by resolved eddies, with subgrid-scale effects
being dominant only in the lowest 10 meters (see Bryan et al., abstract
in this conference). This framework is used to explore the relationships
between environmental forcings, surface boundary conditions, and
tornado vortex structure, intensity, and wind gusts. Similar to previous
LES studies, results show that the maximum time- and azimuthal-mean
tangential winds {V}max can be well in excess of the “thermodynamic
speed limit,” which is 66 m/s for most of the simulations. {V}max
exceeds this speed by values ranging from 21% for a large, high-swirl
vortex to 59% for a small, low-swirl vortex.

Three measures of tornado intensity are introduced: maximum
time-azimuthal mean surface (10 m) horizontal wind speed ({S10}max),
maximum 3-second gusts of S10 (S10-3s), and maximum vertical 3-s gusts
at 10 m (W10-3s). While {S10}max is considerably less than {V}max,
transient features in the boundary layer can generate S10-3s in excess
of 150 m/s, and W10-3s in excess of 100 m/s. For the larger vortices,
the most extreme winds are confined closer to the center, well inside
the radius of maximum azimuthal-mean surface winds.