Abstract
A large-eddy simulation (LES) framework with an “eddy injection” technique has been developed that ensures a majority of turbulent kinetic energy in numerically simulated tornado-like vortices is represented by resolved eddies. This framework is used to explore the relationships between environmental forcing mechanisms, 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−1 for most of the simulations. Specifically, {V}max exceeds this speed by values ranging from 21% for a large, high-swirl vortex to 59% for a small, low-swirl vortex. Budgets of mean and eddy angular and radial momentum are used to show that resolved eddies in the tornado core act to reduce the wind speed at the location of {V}max, although they do transport angular momentum downward into the lowest levels of the boundary layer, increasing low-level swirl.
Three measures of tornado intensity are introduced: maximum time–azimuthal-mean surface (10 m) horizontal wind speed ({S10}max), maximum 3-s 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−1, and W10-3s in excess of 100 m s−1. For high-swirl vortices, the extreme gusts are confined closer to the center, well inside the radius of maximum azimuthal-mean surface winds. For the low-swirl vortex, both the strongest mean winds and the extreme gusts are restricted to a very narrow core.
The National Center for Atmospheric Research is sponsored by the National Science Foundation.
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